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An Investigation on the printing of metal and polymer powders using electrophotographic solid freeform fabrication

University of Florida Institutional Repository

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AN INVESTIGATION ON TH E PRINTING OF METAL AND POLYMER POWDERS USING ELECTROPHOTOGRAPHIC SOLID FREEFORM FABRICATION By AJAY KUMAR DAS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2004

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Copyright 2004 by Ajay Kumar Das

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Dedicated to my mother.

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ACKNOWLEDGMENTS I extend my sincere gratitude to my advisor and chairman of the thesis committee, Dr. Ashok V. Kumar for his guidance and support during the research work which made this thesis possible. I would also like to thank the thesis committee members Dr. John K. Schueller and Dr. Nagaraj Arakere for their patience in reviewing the thesis and for their valuable advice during the research work. I thank the Design and Rapid Prototyping Laboratory co-workers for being helpful, supportive and making the research a pleasurable experience. Last but not least, I would like to thank my parents for their love and encouragement during my studies abroad. iv

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TABLE OF CONTENTS Page ACKNOWLEDGMENTS .................................................................................................iv LIST OF TABLES .............................................................................................................ix LIST OF FIGURES ...........................................................................................................xi ABSTRACT .......................................................................................................................xv CHAPTER 1 INTRODUCTION........................................................................................................1 Electrophotographic Solid Freeform Fabrication (ESFF)............................................1 Past Research and Motivation for the Present Work....................................................2 Chapter Layout.............................................................................................................3 2 RAPID PROTOTYPING..............................................................................................6 Stereolithography Apparatus (SLA).............................................................................8 Solid Ground Curing (SGC).........................................................................................9 Selective Laser Sintering (SLS)..................................................................................10 Fused Deposition Modeling (FDM)...........................................................................11 Laminated Object Manufacturing (LOM)..................................................................12 3-D Printing................................................................................................................14 3 ELECTROPHOTOGRAPHY.....................................................................................16 The Electrophotographic Process...............................................................................16 Photoconductor Material............................................................................................18 Dark Decay..........................................................................................................18 Charge Acceptance..............................................................................................19 Image Formation Time........................................................................................19 Image Stability.....................................................................................................19 Residual Image....................................................................................................19 Material Selection................................................................................................20 Charging..............................................................................................................21 Corona Charger............................................................................................21 Charge Roller...............................................................................................23 Imaging................................................................................................................23 v

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Development........................................................................................................24 Cascade Development..................................................................................25 Magnetic Brush Development......................................................................26 Conductive Magnetic Brush Development..................................................27 Mono-component Development...................................................................28 Charged and Discharged Area Development...............................................28 Toner Powder Charging...............................................................................30 Transfer................................................................................................................32 Fusing..................................................................................................................33 Cleaning...............................................................................................................33 4 ELECTROPHOTOGRAPHIC SOLID FREEFORM FABRICATION (ESFF)........34 Development of ESFF Test-bed System.....................................................................35 Motion Control System.......................................................................................36 Printing................................................................................................................37 Fusing..................................................................................................................37 Software...............................................................................................................38 Measurement of Charge and Mass of Powder............................................................39 Measurement of Powder Properties............................................................................40 Improvement of Print Quality.....................................................................................40 Limitation on Part Height....................................................................................40 Edge Growth (Solid Area Development)............................................................42 Printing of Powders other than Toner.........................................................................43 Study of Laser Imager System of Printer...................................................................45 5 DESIGN AND TESTING OF IMAGE DEVELOPERS............................................47 Developer Design.......................................................................................................48 Powder Box.........................................................................................................50 Developer Roller.................................................................................................50 Developer Roller Casing.....................................................................................51 Pivoting Blade Powder Developer......................................................................53 Development of Charge and Mass Measurement Test Setup.....................................56 Discussion on Development of First Test Setup and Testing Concepts..............56 Improvement in the Design of Test Setup...........................................................59 Independent Charge and Mass Measurement Test Setup...........................................63 Design Considerations.........................................................................................64 Stages of Charge and Mass Measurement Test Cycle.........................................67 Design and Building of the Charge and Mass Measurement Test Setup............68 Transfer Drum Assembly.............................................................................68 Cleaner Box Assembly.................................................................................70 Motor and Stand Assembly..........................................................................71 Experimental Results..................................................................................................73 Experiments with Iron Powder............................................................................73 Experiments with Nylon 12 Powder.................................................................75 vi

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6 METAL POWDER DEVELOPMENT AND PRINTING.........................................77 Metal Powder Development.......................................................................................78 Metal Development Theory........................................................................................80 Constant Parameters used in Calculations...........................................................80 Charge per Unit Mass (Q/M) Calculations..........................................................81 Maximum Surface Charge Density.....................................................................83 Electric Field Range............................................................................................84 Calculation of Forces Involved During Development.........................................85 Calculation of Maximum Powder Particle Radius for Development..................86 Metal Powder Transfer Process Theory.....................................................................87 Powder Transfer Methods...................................................................................88 Capacitor Method of Powder Transfer (Conceptual)..........................................89 Experimental Results..................................................................................................91 Determination of Discharging Voltage...............................................................92 Determination of Frequency of Discharging Voltage.........................................94 Variation of Powder Development with Voltage................................................95 Calculating Mass of Monolayer Development....................................................97 Variation of Powder Developed with Development Gap....................................99 Powder Transfer Experiments...........................................................................101 Powder transfer on printed toner layer..............................................................103 7 POLYMER POWDER DEVELOPMENT AND PRINTING.................................107 Test of Powder Properties.........................................................................................108 Resistivity Test..................................................................................................108 Permittivity Measurement.................................................................................110 Test of Development Characteristics of Polymers...................................................113 Experimental Setup...........................................................................................114 Polyvinyl Alcohol..............................................................................................115 Nylon 12.........................................................................................................116 Nylon 6...........................................................................................................116 ABS (Acrylonitrile Butadiene Styrene).............................................................117 PVC (Polyvinyl Chloride).................................................................................117 Discussion..........................................................................................................118 8 FLAT PHOTOCONDUCTOR PLATE TEST BED................................................120 Electrophotography Cycle and Test Bed Design Concept.......................................122 Charging............................................................................................................123 Imaging..............................................................................................................123 Developing........................................................................................................124 Printing..............................................................................................................125 Fusing and Compacting.....................................................................................125 Cleaning and Discharging.................................................................................125 Photoconductor Plate Motion............................................................................126 Photoconductor Plate................................................................................................126 vii

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Design and Building of Charger-Imager-Developer Assembly...............................128 Developer Design..............................................................................................129 Developer Design..............................................................................................130 Charger-Imager-Developer Assembly...............................................................132 Experiments and Results...........................................................................................135 Metal Powder Development using Flat Photoconductor Plate Test Bed..........135 Polymer Powder Development using Flat Photoconductor Plate Test Bed......139 Printing of Polymer using Flat Photoconductor Plate Test Bed........................140 9 CONCLUDING DISCUSSION AND FUTURE WORK........................................146 APPENDIX MEASUREMENT WITH KEITHLEY ELECTROMETER..........................................150 Resistivity Measurement using Keithley 6517A Electrometer and Test-point Software...............................................................................................................150 Permittivity Measurement Using Keithley 6517A Electrometer and Test-point Software...............................................................................................................153 Step by Step Procedure for Performing the Tests:....................................................156 Common Steps for All Tests.............................................................................156 Steps for Resistivity Test...................................................................................156 Connections................................................................................................156 Software.....................................................................................................157 Steps for Permittivity Test.................................................................................157 Finding Time Constant...............................................................................157 Finding Resistance.....................................................................................158 Finding Permittivity...................................................................................158 LIST OF REFERENCES.................................................................................................159 BIOGRAPHICAL SKETCH...........................................................................................161 viii

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LIST OF TABLES Table page 5-1 Q/M and M/A calculations of toner using QMM test setup.....................................61 5-2 Variation of the Q/M readings with the number of revolutions of the transfer roller............................................................................................................73 5-3 Variation of Q/M measurement with the increase in voltage supplied for development while the number of revolutions of the transfer drum remains constant.....................................................................................................................74 5-4 Variation of the Q/M readings with the number of revolutions of the transfer roller............................................................................................................75 5-5 Variation of Q/M measurement with the increase in voltage supplied for development while the number of revolutions of the transfer drum remains constant.....................................................................................................................76 6-1 Experiment to determine the effective discharging and neutralizing ac voltage......................................................................................................................93 6-2 Experiment to determine the frequency of the ac voltage for effective discharging...............................................................................................................94 6-3 Experiment to determine the dependence of powder development on the development voltage applied to developing electrode.............................................96 6-4 Experiment to determine the relation of powder development with gap between the two electrodes...................................................................................................100 6-5 Measurement of efficiency of powder transfer from developed electrode to transferred electrode...........................................................................................103 6-6 Iron powder transfer on previously printed and fused toner layer by applying positive voltage to the build platform.....................................................105 6-7 Iron powder transfer on previously printed and fused toner layer by applying negative voltage to the transfer plate.......................................................105 7-1 Resistivity of polymer powders in cm................................................................110 ix

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7-2 Resistivity of polymer powders calculated above in m......................................110 7-3 Resistance values of the polymer powders............................................................111 7-4 Time constants of the RC discharge curve.............................................................112 7-5 Permittivity of polymer powders............................................................................113 8-1 Development of iron powder by providing voltage to the development roller and grounding the photoconductor plate (Data 1)........................................136 8-2 Development of iron powder by providing voltage to the photoconductor plate and grounding the developer roller (Data 2).................................................136 8-3 Development of iron powder by charging the photoconductor surface and grounding both photoconductor and developer roller (Data 3)..............................136 8-4 Amount of powder developed with change of voltage in the positive range.......................................................................................................................139 8-5 Amount of powder developed with change of voltage in the negative range.......................................................................................................................139 x

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LIST OF FIGURES Figure page 2.1 Flowchart of the Rapid Prototyping Process..............................................................7 2.2 Schematic of Stereolithography Apparatus (SLA).....................................................8 2.3 Schematic of Selective Laser Sintering (SLS).........................................................11 2.4 Schematic Representation of Fused Deposition Modeling (FDM)..........................12 2.5 Schematic Representation of Laminated Object Manufacturing (LOM).................13 2.6 Schematic Representation of 3D Printing Process...................................................15 3-1 Schematic of the Electrophotography Print Cycle...................................................17 3-2 Schematic of Corotron Charger................................................................................22 3-3 Schematic of Scorotron Charger..............................................................................22 3-4 Image Formation in Organic Photoconductor Drum by UV Laser..........................24 3-5 Schematic Representation of Developer System......................................................25 3-6 Schematic of Cascade Development........................................................................26 3-7 Magnetic Brush Development System.....................................................................27 3-8 Charged Area Development (CAD).........................................................................29 3-9 Discharged Area Development (DAD)....................................................................30 3-10 Schematic of Transfer of Toner to the Paper...........................................................32 4-1 The ESFF Test-bed...................................................................................................36 4-2 Parts Printed using Corona Charging of the Top Printed Layer before the Next Print.................................................................................................................41 4-3 Parts Printed Using Patterns.....................................................................................43 xi

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4-4 Toner Powder Printed over Insulating Alumina Powder Bed..................................44 4-5 Diagram of Laser Jet 4 Imager Assembly................................................................45 5-1 Solid Model Assembly of Powder Developer..........................................................49 5-2 Front End of the Toner Powder Cartridge................................................................52 5-3 Cross-section of Developer Assembled with the Front End....................................52 5-4 Solid Model of Developer Assembly for Pivoting Doctor Blade Powder Developer (with Cross-sectional View).....................................................................................54 5-5 Cross-sectional View of the Developer Assembly...................................................55 5-6 Cross-section of Developer Front End Assembly with the Pivoted Doctor Blade.........................................................................................................................55 5-7 Schematic of Charge and Mass Measurement Test Setup.......................................56 5-8 Charge Measurement Setup for Direct Charge Measurement.................................57 5-9 Solid Model of the Assembly of Toner Powder Developer and Charge and Mass Measurement Test Setup..........................................................................59 5-10 Cross-section View of Charge and Mass Measurement Test Setup.........................60 5-11 Solid Model of Assembly of Developer and Charge and Mass Measurement Test Setup..........................................................................................62 5-12 Cross-section of Charge and Mass Measurement Test Setup with Improved Developer Assembly................................................................................63 5-13 Schematic Illustration of Behavior of Conductive Powder Particle in the Presence of Conductive and Insulative Electrode Surfaces...............................65 5-14 Different Stages of the Test Cycle with respect to the Cross-sectional view of the Charge and Mass Measurement Test Setup..........................................67 5-15 Solid Model of Transfer Drum and Cleaner Box Assembly (with cross-sectional view)................................................................................................69 5-16 Front view and Side Cross-sectional view of the Transfer Drum Assembly and the Cleaner Box Assembly................................................................................71 5-17 Solid Model of Motor and Stand Assembly.............................................................72 5-18 Complete Assembly of Powder Developer with Charge and Mass Measurement Test Setup.................................................................................................................72 xii

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6-1 Concept of the New Modified Test Bed...................................................................78 6-2 Forces Acting on a Powder Particle During Development......................................81 6-3 Schematic Model of the Capacitor Method of Powder Transfer (Conceptual).............................................................................................................90 6-4 Schematic of the Test Setup for Development.........................................................92 6-5 Plot of Variation of Mass of Powder Developed with respect to the Surface Voltage Available for Development........................................................................97 6-6 Dimensions of the Milled Slot.................................................................................98 6-7 Packing of Iron Powder Particles in a Monolayer....................................................98 6-8 Graph Showing Decrease in Amount of Developed Powder with Increase in the Gap Between the Powder and the Insulated Electrode.................................101 6-9 Schematic for Test Setup for Powder Transfer......................................................102 7-1 Schematic of Resistivity Test Cell.........................................................................108 7-2 Schematic of Capacitance Test Cell.......................................................................111 7-3 Cross-section of Polymer Powder Developer........................................................114 8-1 Schematic of the Concept of Flat Photoconductor Plate Test Bed Assembly.......122 8-2 Schematic of Laser Imager.....................................................................................124 8-3 Solid Model of Photoconductor Belt Assembly.....................................................128 8-4 Solid Model of the Powder Developer Assembly..................................................130 8-5 Cross-sectional view of the Powder Developer Assembly....................................131 8-6 Solid Model of Charger-Imager-Developer Assembly..........................................132 8-7 Cross-section of Charger-Imager-Developer Assembly........................................133 8-8 Cross-sectional Image of Charger-Imager-Developer Assembly with Photoconductor Belt Assembly on top...................................................................134 8-9 Close-up of Solid Model of Test Bed showing the Flat Photoconductor Plate Developer Assembly..............................................................................................135 8-10 Charge Flow Through the Ground during Development.......................................137 xiii

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8-11 Variation of Amount of Powder Developed with Change in Voltage...................138 8-12 Electrodes Shaped in Different Patterns.................................................................141 8-13 Patterned Electrodes after Developed with Nylon 12 Powder............................142 8-14 Build Platform for Printing Nylon 12.................................................................142 8-15 Intended Assembly of Patterns...............................................................................143 8-16 Actual Printing of Assembly of Patterns using Nylon 12...................................143 8-17 Print of Assembly of Patterns on PET Sheet Surface............................................145 A-1 Parallel Plate Powder Resistivity Test Cell............................................................152 A-2 Test Window for Resistivity Measurements..........................................................153 A-3 Parallel Plate Powder Capacitance Test Cell.........................................................154 A-4 Hi-R Step Response Window.................................................................................155 A-5 Step Response Settings...........................................................................................155 xiv

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science AN INVESTIGATION ON THE PRINTING OF METAL AND POLYMER POWDERS USING ELECTROPHOTOGRAPHIC SOLID FREEFORM FABRICATION By Ajay Kumar Das August 2004 Chair: Ashok V. Kumar Major Department: Mechanical and Aerospace Engineering Electrophotographic solid freeform fabrication (ESFF) is a novel method of manufacturing which is under development at the Design and Rapid Prototyping Laboratory of the Department of Mechanical and Aerospace Engineering at University of Florida. Electrophotographic solid freeform fabrication uses the principle used in laser printer to print layers of material, one over the other, to form the final three dimensional objects. In past experiments a test bed was built to test the concept of printing in layers using toner powder. Toner powder was successfully shown to be printed in layers. The material used as the toner powder is not suitable as a structural or functional material, and there was a need to explore the use of other materials including metal and polymer powders as structural material in layer by layer printing using ESFF. Powder developers were designed to develop thin uniform layers of metal or polymer powders on the photoconductor surface. A Charge and mass measurement test setup was built to evaluate these developers on the basis of their ability to charge the powders and the xv

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amount of powder developed. The powder properties and development and transfer characteristics were studied for metal and polymer powders. A simpler version of the test bed to develop and print metal and polymer powders was proposed. This new concept was less dependent on available commercial laser printing technology and was based on development of powder on a flat photoconductor plate. This concept made the design modular for easy setup and debugging purposes. A developer was designed to print upwards against gravity on a photoconductor plate (facing downwards). This developer was assembled with the charge roller and the laser imager assembly. A flexible build platform was assembled which could align itself to the photoconductor plate orientation for effective transfer of powder layers. The developer was tested for its efficiency in charging and developing powders. The developed powder was then transferred on the build platform to test the ability of the test setup to print powders in layers. xvi

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CHAPTER 1 INTRODUCTION Rapid prototyping is a manufacturing technology that produces objects layer by layer without the need of elaborate tooling and clamping. The objects can have simple or complex geometries and can take any freeform shape. Manufacturing of these freeform objects in small quantities was expensive by conventional machining processes, but can now be achieved by rapid prototyping systems at a relatively low cost. Electrophotographic Solid Freeform Fabrication (ESFF) Electrophotographic solid freeform fabrication (ESFF) is a rapid prototyping technology based on electrophotography, which is under development at the University of Florida. Electrophotography is used in copiers and laser printers to print loose dry toner powder on paper and then fuse the powder to get the final permanent print. Electrophotography is based on a special property of some materials (photoconductors), which are generally insulators, but become conductive when light of a characteristic range of wavelengths is shined upon them. In electrophotography, a charge of either polarity is deposited and a laser is then shined on the photoconductor at specific spots to make the material conductive and discharge the surface at those spots. This creates an image of charged or discharged area, which is used to pick up charged powder particles using electrostatic force. This powder layer is then printed on a substrate (in the case of a printer it is paper), and then fused by applying heat to get the final print. ESFF technology is being developed to use this concept to build objects by printing multiple layers of different materials. ESFF has some unique capabilities over other 1

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2 commercially available rapid prototyping options on different counts. It has a higher resolution (600-1200 dpi) and can produce objects having fine details. This technology can produce blended objects with predetermined compositions. This makes it a suitable technology for semiconductor and electronics industry applications to produce miniature circuits and components. Past Research and Motivation for the Present Work Past works on the development of this technology were done mainly to understand electrophotography and study its behavior when used for printing multiple layers of toner powder. A test bed was built using a modified laser printer to print toner on a moving platform. The printed powder layer over the build platform was then moved under a fixed plate heater and compacted and fused to bind the loose powders. Experimental studies were done to find out the physical properties of toner, charge per unit mass (Q/M), mass per unit area (M/A), resistivity, permittivity and mass density. These are the important properties of toner, which affect the printing process and print quality. A number of models were proposed to explain the behavior of powder during development and transfer. Software was written to slice the 3D model of the object and then send the cross-sectional image to the printer to print. The need to study the feasibility of printing powders other than the toner powder was the starting motivation for the work presented in this thesis. The physical properties including the resistivity, permittivity and mass density were measured and reported for polymer powders. To measure the charge per unit mass (Q/M) for powders, a test setup was built and experiments were done on it find out Q/M values for iron and polymer powders. A number of developers were designed and tested for their effectiveness to replace the toner powder developer in the laser printer and print powders directly on to

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3 the photoconductor drum. The difficulties associated with the old ESFF test bed design were identified and a new design based on a flat photoconductor plate was proposed. The design and fabrication of the charger imager and developer assembly were done. A number of tests on the development characteristics of metals and polymers were conducted to identify suitable conditions for maximum development. Theoretical models were developed and analyzed for estimating the parameters and their influence on the development process. To summarize, the thesis work presented in this text presents the effort to develop and print metal and polymer powders. The physical properties of the powder particles were determined and their suitability to be used as a structural material for ESFF applications was evaluated. A test bed was built to make it easier to develop and transfer powders of any kind. This research paves the path for further tests and experiments to determine the feasibility of this technology to print powders other than toner in layers to build three-dimensional objects. Chapter Layout The first chapter is a short introduction to the overall research project and the motivation behind the current research work and its significance in the overall development of the ESFF technology. The second chapter discusses the rapid prototyping technology, how it is different from the other manufacturing technologies and its significance in the manufacturing world. Then different rapid prototyping technologies are discussed in brief with illustration. The third chapter discusses electrophotography and its growth as a technology applied to laser printers and photocopiers. It describes the different components of

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4 electrophotography and the way they influence the quality of print. The chapter also discusses various development techniques and the behavior of powders of different materials during electrophotography. The fourth chapter presents the Electrostatic Solid Freeform Fabrication (ESFF) as an alternative rapid prototyping technology. It starts with an introduction to the concept of ESFF, and then goes on to describe the first design of the test bed. Brief highlights of the past works on this technology are discussed and the results that were obtained from those works are summarized. The fifth chapter describes the evolution in the design of the powder developer and its evaluation at each stage by using charge per mass (Q/M) measurement device. The detailed design of the improved charge and mass measurement device is explained and the functions of the components are discussed. Finally, the results from charge and mass measurements from metal and polymer powders are reported. The sixth chapter discusses the theory behind metal powder development and printing. The forces involved during development and printing of metal powders are identified and force equations are solved to determine critical parameters, including the particle size, and the magnitude of field developed. The methods of charging, developing and transferring powders are discussed with reference to their efficiency in printing powders. Experimental results found by preliminary tests done using iron powders are presented. The seventh chapter discusses polymer powders and their properties, and the effectiveness of development and transfer by using them in the new test bed setup. The test results providing information about the resistivity, permittivity and mass density of

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5 some polymer powders are presented. There is also a discussion on the behavior of these powders when they were developed on a photoconductor drum. The results indicated Nylon 12 is the best suited powder to be used as a sample polymer for all further theoretical and experimental calculations. The eighth chapter introduces the flat photoconductor plate test bed as a better alternative to the test bed based on commercial laser printers. The important features of the new test bed are highlighted and design challenges are discussed. The design of upward-facing developer and the assembly of charger, imager, and developer are described in detail. Experimental results from the use of iron and nylon 12 powders in the improved test bed are presented. Finally, the ninth chapter discusses the conclusions drawn from the theoretical and experimental work presented in this thesis. It also suggests the opportunities for future research on this project to develop it as a successful rapid prototyping technology.

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CHAPTER 2 RAPID PROTOTYPING Rapid prototyping is also known as Solid Freeform Fabrication and Layered Manufacturing. Each of the three names stands for an important characteristic of this technology. Rapid prototyping defines it as a technology to produce prototypes of conceptual models quickly without the need of any elaborate tooling and fixture design. The ability of rapid prototyping units to produce design prototypes quickly, shortens the design iteration cycle time significantly. In comparison conventional machining is faster than rapid prototyping, with tooling and fixtures in place. Machining process is also faster and economical when it comes to mass production of components. Solid freeform fabrication refers to the fact that this technology can be used to produce solid objects with any freeform surface. The last term, Layered Manufacturing, signifies that in this manufacturing technology objects are produced in layers. In rapid prototyping the conceptual 3D solid model is first sliced into two dimensional layers using slicing algorithms. Each of these layers is the cross-sectional image of the object at a particular z level. These layers are created one over another by using methods particular to different rapid prototyping technologies. Support structure is also provided along with the part building process to provide support to any overhanging structure in the part. In rapid prototyping there is no restriction on the structure and shape of the object as long as it can be sliced into layers and built within the space limitation of the machine. This allows the freedom to produce objects with any freeform surface as 6

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7 desired. The activities, which are central to all rapid prototyping technologies, are presented in the form of a flowchart below. 3D model of the object 2D cross-section data at each z-levelSlicing Algorithm (computer) Interface (Rapid prototyping machine) Post processing Building of the object layer by layer Final 3D object Figure 2.1 Flowchart of the Rapid Prototyping Process There are many successful commercial rapid prototyping technologies, which are available in the market and there are others which are in their developmental stage and hold a lot of promise of becoming successful in future. The rapid prototyping technologies are characterized by material used to build the object process of binding layers together speed of build process and post processing requirements limit on the choice of geometry of the object that can be built

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8 The process that is best suited for the given application is chosen from all different options based on the above criteria. Some successful commercial technologies are presented below. Stereolithography Apparatus (SLA) Stereolithography is based on the property of certain polymers, called photopolymers that solidify when UV light is shined upon them. The setup is made of a build platform immersed in a liquid polymer container. UV laser is shined on the polymer after being reflected by a mirror to form a crosshatch pattern on the polymer surface. The laser can penetrate only a small depth into the liquid and so it can harden only a thin layer of liquid polymer on the build platform in one scan of the surface. This is a limitation on the speed at which the object is built. Support structures are also built along the building of part, which are difficult and expensive to remove during post-processing. The schematic below shows the setup for a typical SLA machine. LaserMirrorDrum containing photopolymerPiston (Build platform) Build part Figure 2.2 Schematic of Stereolithography Apparatus (SLA)

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9 SLA is one of the earliest rapid prototyping technology, and first to be commercialized. It popularized the concept of rapid prototyping and was able to build objects, which were difficult to make conventionally. There can be unattended and continuous operation for 24 hours. The expensive and sophisticated process requirements made it prohibitive for wide use. Post processing also takes long time and the surface finish and tolerance is poor when compared to conventional machining due to non-uniformity of solidification of the photopolymer by laser. Solid Ground Curing (SGC) Solid ground curing (SGC) is another technology based on the same principle as SLA, which is hardening of photopolymer by using UV light. The difference is that in this technology hardening of the surface is done through a mask, which helps in hardening of the whole surface at once rather than tracing with a laser as in SLA. The mask is prepared by photocopying technology, which prints each cross-section on a glass plate, which is renewed by erasing the print. This is done in a separate process cycle called Mask Plotter cycle, which is different than the actual build cycle called Model Grower cycle. In the model grower cycle the glass plate from the mask plotter cycle is used and a UV flashlight is shined through the glass plate on the polymer surface. The unused polymer is removed and the area is filled by wax. Wax solidifies and gives a strong support to the build structure. The surface of this layer is milled to make the layer uniform. This produces objects with high dimensional accuracy, as there is uniform hardening and avoidance of material tensions. Removal of support structure is easy as the wax can be melted and removed. An important disadvantage of the method is the

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10 sophisticated equipment, which prohibits unmanned manufacturing process. This process also takes a long time because of the elaborate procedure involved. Selective Laser Sintering (SLS) Selective laser sintering (SLS) produces parts by heating powders on a powder bed to the temperature just below their melting point temperature. At these conditions the particles sinter and form strong bonds to build the object. The powder is stored in a reservoir and brought out by a piston. The powder layer is then spread uniformly on the top of the build platform by a leveling roller. Then a high power carbon dioxide laser is shined on this powder bed tracing the outline of the cross-section. The temperature of the powder inside the cross-section is raised to just below its melting point by scanning of the laser in a cross-hatch pattern so that the powder layers sinter and combine to form a solid mass. As the process is repeated, layers of powders are deposited and sintered to form the final object. The loose powder that was not sintered provides a natural support for the built part. The finished part can be easily taken out of the powder bed and the loose powder on the surface can be removed by blowing off with pressurized air. The schematic below shows the SLS process. The SLS process has its own advantages and disadvantages. The advantage lies in the fact that this technology can be used for a variety of materials including different kinds of polymers and metals. The process is also fast with a build rate up to one inch per hour. Post processing is easier as the parts that come out have full strength and do not require additional processes. The major disadvantage of this method lies in the poor geometric accuracy and grainy surface finish. This is mainly dependent on the powder particle size. There is also the possibility that the neighboring powders, along the part boundary, may get sintered and become unwanted part of the object. The process is done

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11 in a nitrogen chamber, and so nitrogen has to be continuously provided to the chamber. Also during sintering, toxic gases are released and have to be handled carefully. This mostly happens when sintering polymers. Co2 LaserMirrorPiston (Build platform)Build part Residual powder acting as supportPowder ReservoirLeveling roller Figure 2.3 Schematic of Selective Laser Sintering (SLS) Fused Deposition Modeling (FDM) As the name suggests in the Fused Deposition Modeling (FDM) method certain material (plastic filament) is melted and deposited on a build platform in layers to form the final object. The part material is available in the form of filaments, which are coiled in the form of spools. These filaments get heated up when they are passed through a nozzle (FDM head), and the material leaves the nozzle in liquid state. This liquid material solidifies immediately at the ambient temperature. Due to this the nozzle has to be very close to the build platform while tracing the model cross-section. The FDM head is capable of translating in the X and Y directions and the build platform moves in the Z direction to accommodate the layer build height. The FDM head first traces the outline of the cross-sectional area and then fills the area with densely packed crosshatch pattern. In

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12 addition to the part material nozzle, there is another nozzle which deposits the support material where needed. The support material is also a polymer filament, which is melted and deposited on the build platform along with the part material. This is a different polymer than the part material and is deposited to form thin wafer-like structures, to support the part material during building process and can be peeled off very easily using a little mechanical force. The schematic in figure 2.4 shows the concept of FDM technology. Figure 2.4 Schematic Representation of Fused Deposition Modeling (FDM) The simplicity of the procedure makes this technology very popular and easy to setup and use. The procedure requires no cleaning and produces no waste, and no post-curing is required. As the filament has a diameter of 1.27mm, the resolution and dimensional accuracy is affected. Laminated Object Manufacturing (LOM) Laminated object manufacturing (LOM) is based on the simple method of sticking together thin sheets of material each representing the cross-section of the object at a

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13 particular given height. The setup for LOM consists of a thin ribbon of sheet material wrapped around a supply roller and a take up roller passing over the build platform supported by several idler rollers. The sheet material starts from the supply roller and stops on the build platform. Then a heated roller is rolled over the piece of the ribbon on the build platform, which binds the sheet material to the top of the stack. A high power CO 2 laser is shined on the sheet material, after being reflected by mirrors that control the X and Y movement of the laser beam. The movement of the build platform provides the Z-axis movement. After the sheet material is bound to the top of the stack of sheets, the laser traces the boundary of the cross-section cutting it out of the sheet material. The unwanted portion is diced by the laser beam into crosshatched squares that provides support to the part. The remaining material is waste and is rolled around the take-up roller. By binding sheets one on top of the other the final object is produced. Figure 2.5 Schematic Representation of Laminated Object Manufacturing (LOM)

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14 The object comes as embedded inside the cubic structure formed by the bound sheets. The post processing involves removing the unwanted material, which were diced during the building of the part. This process of dicing helps in easy removal of this unwanted material. A schematic of the LOM process is shown in the figure 2.5 above. The ability to use a large variety of organic and inorganic materials is the biggest advantage of LOM technology. The process is also faster than the competitive technologies, as the laser has to only trace the outline of the cross-section and not the whole area. The layers stick to the stack very easily and so the process is faster. This method can be used to produce larger prototypes that are not possible by other technologies. The disadvantage of the method is that it produces parts with low strength in the Z-direction and so the objects produced cannot be used as functional prototypes. The other drawback of this system is that it produces a lot of waste during building of the object and during post-processing. 3-D Printing The concept of 3-D printing is also based on gluing two layers together to form a part, but here the two layers are made of loose powders. The process starts with spreading of a thin uniform layer of powder. The powder layer is then selectively joined by ink-jet printing of binder material. The build platform, which is in the shape of a piston inside a cylinder containing the powder bed, is lowered and the next layer of powder is spread. The ink-jet print head scans the powder bed in the same way as it does while printing on paper. The only difference is that here instead of ink, binder material is used. The loose powder around the built part provides natural support and can be removed very easily during post processing. The part has green strength and can be consolidated by

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15 application of heat, which will evaporate the binder. Figure 2.6 shows the schematic of detailed 3D printing process. This method of rapid prototyping can be used to produce objects made of any material that can be powdered. When metal powder is used, the final part is put in the furnace to join the powder particles by melting. Copper infiltration is done to fill the pores left by evaporated binder to form dense and strong metal part. The disadvantage results from the granular nature of the material and the interactions between the binder and the powder. This has bad effect on the texture of the surfaces. Figure 2.6 Schematic Representation of 3D Printing Process

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CHAPTER 3 ELECTROPHOTOGRAPHY Electrophotography is a method of printing image-wise arranged charged powders on a substrate and subsequently fused to form a permanent image. A special insulator material called photoconductor is used to form image patterns of charged or discharged areas by using its special property that turns it conductive when light of a particular wavelength falls on it. The photoconductor is applied as a coating on a roller or plate and is used for the charge imaging. The details of the working of the photoconductor material are explained later in the chapter. Figure 3-1 shows the electrophotographic cycle with the photoconductor material applied on a drum (photoconductor drum). The Electrophotographic Process The six major steps in electrophotography are Charging: In the charging process charge is deposited on the photoconductor surface. Imaging: In this process light of particular wavelength (in the form of laser) is shined on the charged photoconductor surface to discharge certain areas according to the image and produces either a charged image with discharged neighborhood or a discharged image with charged neighborhood. Development: In this step loose charged powder particles move towards the photoconductor surface due to the electrostatic force created between the photoconductor drum and powder. These powders get developed image-wise on the photoconductor surface due to attraction or repulsion by the charged areas on the surface according to the method used. Transfer: In the transfer process the image-wise developed powders are transferred on to a substrate either electro-statically or physically to form the print. Fusing: In this step heat is applied to the powder to melt it and fix it to the substrate to form the final permanent image. 16

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17 Cleaning: In this final step the photoconductor surface is cleaned off the residual powders remaining on its surface after transfer process. The photoconductor surface is also discharged to avoid the carry over of any charge patterns to the next print. An example of electrophotography process is shown schematically in figure 3-1, which shows the way different steps are performed around a photoconductor drum. Most of the modern printers use photoconductor drums for printing as the drums help in making the design compact. This compact design makes it easy for the printer to be used easily as a desktop printer. All the processes are spread around the photoconductor drum and occur simultaneously in a cycle making printing faster. These processes are explained in detail below. Photoconductor Charge RollerLaser ImagerDeveloperPhotoconductor DrumPaperPaper Charger rollerCleaning BladeCleaning Box Figure 3-1. Schematic of the Electrophotography Print Cycle As can be seen from the electrophotography print cycle, the photoconductor drum is central to the electrophotographic process. Therefore, it is essential to know more about the photoconductor and its characteristics to understand electrophotography.

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18 Photoconductor Material As discussed before, the photoconductor material has the unique property of becoming conductive when light of specific wavelengths falls on it. To form a charged image on the photoconductor drum, the drum is first charged with charge roller. Then the charged photoconductor surface is exposed to UV laser pulses which make the photoconductor material conductive and so the charge from the surface passes to the ground. This creates an image-wise charge distribution which picks up powder selectively. This powder image is then transferred to the build platform and fused to make a permanent image. There are many materials that can be used as photoconductors and some of them were used in early versions of copiers and printers. The ones that were popular and widely used are amorphous selenium and organic photoreceptors. Amorphous selenium was used in the early generation of printers, while organic photoreceptors gained popularity later because of heavy demand of inexpensive compact desktop printers. The important parameters that are used to characterize a photoconductor are explained below. Dark Decay Dark decay is the ability of the photoconductor to retain the charged image when no light is falling on it. The photoconductor material is not a good insulator and even when no light is falling on its surface it allows charge leakage. The charge depletion is exponential in nature and the time in which the photoconductor looses half of its surface charge is called depletion time. Organic photoreceptors typically have shorter depletion time than amorphous selenium (Diamond 1991).

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19 Charge Acceptance The surface charge limit that can be deposited on the photoconductor surface is known as charge acceptance. This is generally decided by the dielectric property of the photoconductor material. The desirable surface charge on the photoconductor is what can be retained by the photoconductor and can create sufficient electric field to attract charged powders to get adhered to its surface. Anything more than this will increase the force of attraction between the powder and the photoconductor surface, which will make powder transfer difficult. Image Formation Time Image formation time is the time taken to discharge the photoconductor surface by imaging light. Image formation energy would have been a more specific term to consider, but in electrophotography it is time and not imaging light intensity, which is considered as an influencing variable. The speed at which printing is done is a more critical parameter to judge the efficiency of a copier or printer, which makes image formation time more critical. Image Stability Image stability is considered as the ability of the photoconductor to maintain a charged image on its surface. Image instability occurs due to the inability of the photoconductor to maintain a highly localized area of discharge on its surface against charge migration. Surface contamination also plays a role in migration and dissipation of charge on the surface itself. Residual Image Residual image occurs when the photoconductor surface is not completely discharged after the image transfer process. This can happen because of various reasons

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20 and can be identified after many prints when images of previous prints, ghost images, begin to appear. To reduce this problem the photoconductor surface is cleaned and discharged after each print, but with high speed of print cycles this problem is not completely corrected. Material Selection As stated earlier, the amorphous selenium was replaced by more popular organic photoreceptor in the modern printers. This might seem counter intuitive since, compared to amorphous selenium, organic photoreceptors have less dark decay time and so cannot hold the image for a long time. Organic photoreceptors are softer and are prone to early wear and tear during operation and suffer gradual breakdown by environmental exposure. The cost of organic photoreceptors is much less than amorphous selenium, even after considering the high frequency of replacing them during the printer lifetime. To help in maintenance of the photoconductor based on organic photoreceptor, it is made a part of the toner cartridge. While replacing or renewing the toner cartridge the organic photoconductor is inspected for any damage and recoated accordingly. This makes organic photoreceptor a popular photoconductor material for modern printers. Dark decay is not much of a concern here because the modern printers and copiers have high-speed electrophotography cycles and the required time to hold charge and image is insignificantly small. Moreover amorphous selenium is sensitive to a wide range of wavelengths of light and has to be kept in a well covered dark area during operation which makes the printer design complicated. This is not a problem with the organic photoreceptor as it is only sensitive to a narrow band of wavelengths in the ultra-violet region, which makes it easier to use and results in a simpler printer design.

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21 As can be seen from the last discussion, photoconductor material is the central to the electrophotography process. The following discussion describes in brief the stages which photoconductor goes through during electrophotography. As said before there are six steps in this electrophotographic print cycle. The cycle starts with charging of the photoconductor surface. Charging Charging is the first step in electrophotography. For a good print quality uniform charging of the photoconductor surface is necessary, which is done by depositing charge particles on the photoconductor surface. There are mainly two ways of charging; by using corona charger and by using charge roller. Corona Charger When high voltage is applied to a conducting body having low radius of curvature, high electric fields are generated locally which causes breakdown of air and ions are generated. This principle is put to practice in corona devices in which high voltage is applied to a thin wire enclosed in a metal shield at the same voltage, generally around 7000V (Schaffert 1975). This kind of device is called as corotron. In this the thin wire generates ions by dielectric breakdown of air and repels ions of the same sign. These ions are again repelled by the metal shield, which generates a steady stream of ions directed towards the body to be charged. There can be non-uniformity of charge generation due to ion winds and impurity on the wire due to toner and paper dust.

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22 Corona WireMetal Shield COROTRONIonized particles Figure 3-2. Schematic of Corotron Charger To make the charge deposition more uniform, a device called as scorotron is used in which a screen is used to cover the opening of the metal shield. The screen restricts any ion that is not traveling perpendicular to the screen holes. Metal Shield Corona Wire Screen Parallel Rays of Ionized ParticlesSCROTRON Figure 3-3. Schematic of Scorotron Charger The corona charging device is associated with many reliability problems. It also generates ozone as a byproduct, which is harmful to health and has to be dissipated. All these problems associated with corona charger made this device less favored over the years.

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23 Charge Roller Roller charging is most popular method of charging in the modern printers. It is more compact and easy to use than the corona charger. The charge roller is made of a central metal rod covered by a thick layer of polymer material. This polymer is a special material due to its higher conductivity than regular polymer. The voltage applied to the charge roller is relatively lower than that applied to corona charger. When DC biased AC voltage is applied to the roller, small discharges occur between the irregular polymer surface and photoconductor surface (Hirakawa 1995). These small discharges deposit charge on the photoconductor surface. There is a uniform line contact between the charge roller surface and the photoconductor surface along the length of the photoconductor, which results in uniform charge deposition. Imaging After the photoconductor surface is charged, latent image is created on the surface by discharging the surface locally using laser of particular wavelength, which is in the UV range for organic photoconductors. The latent image is a charge pattern that mirrors the information to be transferred to the real image. The laser is shined on the photoconductor surface after getting reflected by a rotating polygonal mirror. The laser is turned on and off according to the image to create selective discharge points on the photoconductor surface. The print resolution is determined by the laser wavelength and the rate of switching of the laser. The organic photoreceptor is divided into two layers. One is the charge generation layer (which is over the aluminum drum) and the other layer is the charge transport layer (which is over the charge generation layer). When UV laser falls on it, negative and positive charges separate and the charge which is of the opposite sign as the charge

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24 deposited on the photoconductor travels to the surface and discharges the surface and the other charge flows to the ground through the drum. ++++++++++++++++++++++++++++++ +LightNegative Electrostatic ChargeCharge Transport LayerCharge Generation LayerAluminum Ground Figure 3-4. Image Formation in Organic Photoconductor Drum by UV Laser Development Development is the process of charging the toner powder and then transferring it on to the photoconductor surface. The charged toner powder experiences the electrostatic force due to the field created by charge distribution on the surface of the photoconductor drum and gets adhered to the latent image thus forming the real image. The toner powder is an insulator and is triboelectrically charged to the required polarity. There are charge controlling agents added to the toner and they help in preferential charging of the toner with a specified polarity. The toner is brought into the vicinity of the photoconductor surface with the charged latent image. The amount of powder coming out of the powder reservoir is controlled by a doctor blade, which is a thin polymer or metal strip present at the opening for toner. The toner near the latent image jumps the gap between the developer and the photoconductor surface and gets developed. The schematic of a typical toner developer is shown in figure 3-5.

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25 Toner PowderDoctor BladeDeveloper RollerPowder Box Figure 3-5. Schematic Representation of Developer System Development is by far the most difficult process to understand and control. There has been a lot of research done on this subject resulting in gradual increase in understanding of the subject. The process was improved from cascade development to insulative magnetic brush development and then to the most efficient conductive magnetic brush development (Schein 1988). The toner used for these development techniques is a two-component toner, which has large carrier particles are covered with smaller toner particles. Some of the important development methods are discussed below. Cascade Development In cascade development the carrier beads covered with toner is made to flow over the photoconductor surface under the influence of gravity. The development using this method depends on a lot of factors including speed of fall of carrier beads, angle of the photoconductor plate to the horizontal and the bouncing of the beads. As the toner particles are attracted by the electric field generated by latent image rather than the charge itself, solid area development is very poor in this method, because the field generated by the charge image highest at the edges and decreases rapidly to the center of the image (Schein 1988). This is a very simple method of development and was used in the early printers and copiers.

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26 Carrier Particles Toner PhotoconductorSubstrate Figure 3-6. Schematic of Cascade Development This method depends on a lot of parameters and forces and it is very difficult to control the powder behavior in these conditions. The development is not reliable and there is a lot of waste due to powder spillage. All these problems made this method less popular and there was a need for an alternative method of development and led to development of the magnetic brush development technology. Magnetic Brush Development Insulative magnetic brush development was a significant step in powder development history. In this method a roller rotates around a stationary magnet and carries magnetic carrier particles along with it by magnetic friction. The magnetic force provides a strong counter force for the electrostatic force and eliminates any other small forces that could bring uncertainty in the cascade development process. The magnetic force makes sure that the powders that are developed are well charged. This avoids any low charged powder from getting developed and attached loosely to the photoconductor surface. The carrier particles form a chain on the roller due to the magnetic field and appear as brush, and so the method is called magnetic brush development. The charged

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27 toner was carried across the gap to the photoconductor drum when the electrostatic force exceeds the force that holds the toner to the carriers. The carrier beads were spherical and the development limit was decided by the balance between the charge on the carrier beads and the photoconductor surface (Schein 1988). N SN SN S Carrier Particles TonerDevelopment MagnetRollerPhotoconductor Drum Figure 3-7. Magnetic Brush Development System Conductive Magnetic Brush Development The conductive magnetic brush was the most successful development technique invented for two-component development. The difference between this and the previous method was that the carrier particles are irregular in shape so that toner did not cover the whole carrier surface, that helped in maintaining conductive contact between the adjacent carrier particles and transmit current across development gap. There was no balancing of charge buildup on the carrier particles with the photoconductor surface and so more toner particles can be transferred (Kasper 1978). This resulted in darker prints and more solid area development.

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28 In the dual component development, the large carrier particles take up more space in the toner powder box and so there was requirement of larger powder boxes and frequent refill of the toner powder. Due to this inconvenience, mono-component development was developed, which has toner without the carrier particles. Mono-component Development Mono-component development is the most common method of development in modern printers and copiers. It uses insulative polymer toners, which are transported using the same magnetic roller technique. The toner powder is made magnetic by doping the polymer with iron compounds during toner production. This also gives the black color to the toner powder. This black coloration of toner is not suitable to use in the color printers. So in color printers cascade development is used and that results in minor spillage. The magnetic roller transfer technique has the same advantage of force cancellation and control as it is in two-component magnetic brush development. The toner powder particles are charged and are stripped off the magnetic roller by the force of the electric field produced by the latent image on the photoconductor surface. To help in this process a DC biased AC voltage is applied in the development zone that makes the powder jump to and fro which forms a powder cloud. This process helps in further charging of toner powder, and the sufficiently charged toner powder adheres itself to the image. Charged and Discharged Area Development Depending on the nature of latent image created by the charged areas or the discharged areas, the method of development is named as charged area development (CAD) or discharged area development (DAD).

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29 In CAD the image is created of charged particles by discharging the areas that are not a part of the image. The toner powder is charged to polarity opposite to the charge on the photoconductor surface. The powder is developed by the field created by the latent image made of charge. ++Image-600 VNon-Image-100 VBias-200 VToner Figure 3-8. Charged Area Development (CAD) In DAD the image is created by discharging the areas which are part of the image. The toner powder is charged with similar sign compared to the charge on the photoconductor surface. The photoconductor drum is grounded and there is a field created between the ground and the voltage on the developer roller. This field attracts the powder towards the photoconductor surface. When the powder is repelled by the same sign charge on the non-image areas, it is developed on to the discharged latent image on the photoconductor surface.

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30 --Non-Image-600 VImage-100 VBias-500 VToner Figure 3-9. Discharged Area Development (DAD) In DAD the photoreceptor dielectric strength uniformity is critical; otherwise small fringe fields associated with breakdowns are developed by the toner. In CAD the light source lifetime can be an issue because it is on approximately ten times longer than in DAD, as most of a page is usually white. In most of the modern printers DAD is used due to the high cost of laser imager, making it prohibitive to replace frequently. Toner Powder Charging There are several methods that are used to charge the toner powder. Some of them are specific to the nature of the powder and use the powder properties to charge them. Others charge toner by depositing charge on it externally. Toners may self-charge due to triboelectric effects and chemical charging. Two component toners often charge triboelectrically by friction between the toner and the carrier particles, which charges them oppositely to each other. This makes toner stick to the surface of the carrier particles. Liquid toners are charged chemically, in which the charge is exchanged between the liquid and the toner particles. Liquid toners are used for fine powder prints, are suspended in liquid due to the difficulty in handling such fine

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31 powders. Chemical charging is also done in mono-component toner to some extent due to the addition of charge controlling agents, which makes the toner susceptible to be charged with a predetermined polarity. There are also direct ways of charging toner powder. Corona charger is used to deposit charge particles on the toner powder to charge them. This method is very similar to that of charging photoconductor surface using corona charger and faces similar difficulties of being bulky and non-uniform in charging. The toner powder also can get deposited on the corona wire and render it inoperable. In mono-component development the toner powder gets charged by getting rubbed against the parts of the developer. This is a popular method of charging toner powder because the charge controlling agents make the toner active to be charged preferentially (Schein 1988). In case of conductive powder, the powder gets charged by being present in the electrostatic field between the photoconductor surface and the developer surface. The conductive powder can also be charged by inductive charging in which an electrode at a distance can induce charge of opposite sign on the powder, if the powder is grounded. Insulating toner powders can also get charged by injection charging in which the toner is moved rapidly around a roller in presence of electric field. The actual mechanics of the charging is poorly understood (Nelson 1978). The charging of toner is essential to control, because the amount of charge deposited on the toner particle determines the charge per unit mass of the toner, which is an important parameter for the quality of toner development. If the charge per unit mass is less than required then the electrostatic force will not be enough to strip the toner out of the magnetic roller. If charge per unit mass is higher, then only a small amount of toner

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32 will able to discharge the image and there will be a thin layer of development. Thus the process should be designed to have the powder optimally charged depending on the magnetic field of the developer roller. Transfer After the toner powder is developed on to the latent image on the photoconductor surface, it has to be transferred to the paper to transfer the image on to the paper surface. The transfer is accomplished by using both electrostatic and mechanical methods. During transfer the paper is pressed against the photoconductor surface using force by a charge roller which deposits a charge opposite to that of the toner and creates field across the paper to make the toner powder transfer to the paper. This process also holds the toner temporarily to the paper surface by squeezing the toner and pressing it against the paper. Photoconductor DrumCharge RollerPaperToner Figure 3-10. Schematic of Transfer of Toner to the Paper

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33 Fusing After powder transfer, it has to be fused and fixed to the paper surface to create a permanent image. In early days of electrophotography, specialized powders were used as toners and were fixed on to wax papers or papers with adhesives to make a permanent print. In the modern printers the toners are specially created to have fixing characteristics, which enables the use of ordinary paper. The toner is mostly composed of polystyrene, which has low melting point. The paper with the toner is subjected to heat so that the toner melts and gets fixed to the paper. In some printers radiant heaters are used. Most of the printers now use heat rollers to fuse toner. The problem with the heat rollers is that they tend to pick up some toner during heating which may smudge the image. For this reason non-stick coatings, like Teflon, are used on the heat rollers. Cleaning After the transfer of powder from the photoconductor surface, the remaining powder is cleaned before the next electrophotography cycle. In current printers using photoconductor drums, the cleaning is done as a part of the cycle. For cleaning, a blade which is a thin flexible polymer sheet is used to scrape off the toner from the photoconductor surface. This waste toner is stored in a receptacle, which is emptied periodically. Generally this receptacle is made big enough so that it does not get overfilled before the developer, along with the receptacle, is replaced. After cleaning the next step is to discharge the photoconductor surface to clean any residual charge remaining on the surface that may cause background printing. This discharge is done by using corona charger or a charge roller supplied with DC biased AC voltage.

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CHAPTER 4 ELECTROPHOTOGRAPHIC SOLID FREEFORM FABRICATION (ESFF) Currently research is being conducted on the development of a novel method of solid freeform fabrication at Design and Rapid Prototyping Laboratory of the Mechanical and Aerospace Engineering Department at the University of Florida. This method uses the technology of electrophotography to form the image of the two-dimensional slices of the three-dimensional object and print layers of these cross-sectional images to form the final part. This technology is called electrophotographic solid freeform fabrication (ESFF), named after the underlying technology to produce solid freeform objects. As mentioned earlier, ESFF is not just another way to do rapid prototyping, but it has the advantage of printing thin layers with high resolution using various types of materials, which aims to satisfy some unfulfilled requirements in the rapid prototyping industry. Electrophotography is a fast printing process due to advancement in high speed printing. In electrophotography, the whole image is transferred in one step, compared to the crosshatch scanning of the cross-section area done by some other RP technologies, which makes the formation of individual layers faster. Although the thickness of layer in each print is small, the time taken for each print can be made faster to reduce the overall printing time. Any material which is available in powdered form and can be charged is a potential candidate for ESFF, although the actual print characteristics may vary. The corona charger is shown to deposit charge on any material, which can be used to charge the powders. Even if a powder cannot be fused, it can be used with a binder printed between 34

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35 two layers of the part powder. This suggests that there can be a wide choice of material for building parts using ESFF. The other advantage of this method is in the ability to print materials in pre-estimated gradient percentage. This could give some new properties to materials, which may be used for special applications. The laser printer prints with a high resolution up to 1200 dpi (dots per inch). This makes it possible for ESFF to be able to print materials in finer resolutions and tolerances. The major hurdle in developing this technology has been the extreme complexity and unreliability of electrophotographic process when applied to powders other than the popular toner powder. ESFF deals with complex problems like powder flow characteristics, powder-charging methods, charged powder behavior and problems of adhesion of powder to surfaces and agglomeration. These are not so well understood subjects and are still under active research work. Development of ESFF Test-bed System ESFF research started with a test-bed design and fabrication to conduct the experiments to test the concept of the ESFF technology. The requirements of the system were to have a two-axis motion control of the platform on which the part would be built. There should be a printing system for printing of layers of toner powder, which could be later modified to print powders of other materials. The print also has to be fused to make it permanent on the build platform. All these requirements were taken into consideration while designing and building of a test bed using a modified laser printer (Zhang 2001). The schematic of the printer is shown in figure 4.1.

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36 Figure 4-1. The ESFF Test-bed Dutta (2002) Motion Control System The build platform was required to move in two-axes. One of the motions is in the horizontal direction (X-axis) for the printing process and another in the vertical direction (Z-axis) for part height adjustment. These motions were provided by a combination of servomotors. These servomotors are a part of the Parker automation system, which is controlled by Galil-DMC motion controller. The Galil controller was interfaced with the computer using the Galil software. In this software, interface commands can be written to move the motors with precise speed and acceleration and stop after predetermined number of rotations. A program in C++ was written to generate such motion commands to synchronize the motion of the platform with print cycle. The build platform was an aluminum plate supported by springs to compensate for the error in positioning of the platform surface and photoconductor drum with respect to horizontal plane. This also helps in pressing the build platform against the

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37 photoconductor drum without damaging the drum surface. The other place where springs are helpful is during fusing, by correcting any misalignment of the build platform while getting pressed against the fusing plate. A position sensor mounted below the top plate of the platform sends signal when the platform reaches a particular height during compaction of toner. Printing A modified laser printer was used to achieve the task of printing on the build platform. The printer used for this purpose was the Laser Jet 4 printer made by Hewlett Packard. The paper handling system of the printer was removed to clear the path below the photoconductor drum, which gave access of the photoconductor drum surface to the build platform for printing. There are sensors to detect the passing of the paper so that the events associated with printing can be synchronized and also paper jam can be avoided. These sensors are sent right signals at the right time from the computer to keep the printer from detecting error in the normal operation. This ensures proper operation of ESFF process. Fusing The toner after getting printed on the build platform has to be fused to form a permanent print, which was done initially by a non-contact radiant heater. This radiant heater was made of a heating coil placed at the focal point of a concave mirror for distribution of the heat pattern uniformly on the build platform. This heat distribution was observed to be concentrated at some places in real operation causing differential fusing of toner image. It was also necessary to compact the print to correct any errors due to non-uniform powder deposition. There was also the need of discharging volume charge of the printed toner layer by making it contact with a grounded metal plate during fusing and

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38 compaction. These considerations led to changing the heater to a contact type plate heater. This heater was made of a mica strip heating element sandwiched between two aluminum plates. The lower plate had a uniformly heated smooth surface, which compressed the toner powder layer when the upward moving build platform was pressed against it. This also discharged the volume charge contained in the toner powder layer by contact with the conductive heater surface during fusing and compression. During the fusion of powder the molten toner sticks to the heater surface causing distortion in the print. This distortion in print is also increased due to the spreading of toner layer after compaction. This affects the dimensional accuracy of the object produced by using ESFF. The sticking of toner to the heater surface was later reduced by attaching a Teflon coated plate below the lower aluminum plate to create a nonstick surface. Software Software programs were developed in C++ to generate control commands for the Galil motion controller software interface and Parker automation systems to control motion of the build platform (Dutta 2002). This program also sends signals to the sensors in the modified printer to ensure normal printer behavior. This C++ software code is stored as a dynamic link library, which is called by a Java program (Bhaskarapanditha 2002). This Java program called SolidSlicer has slicing algorithms, which use the data from a solid 3D CAD model of an object stored in STL format and determine the cross-section of each layer at different Z-heights. These cross-sectional images are then sent to the printer for printing. The software has a good user interface that allows positioning of multiple part prints on the build platform. This automates the building process and also creates information which is stored in log files.

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39 The initial ESFF test-bed provided a platform to perform basic experiments on the issues of concern during printing of toner powder as the structural material for producing 3D parts. Various parameters could be varied and the resultant effects measured to compare the change in process due to any change in these process parameters. The quality and the print pattern could be changed by modifying the software. Sensors and actuators can be placed on the frame of the test-bed to collect data during experiments. In this test-bed, the printer was used as a Black Box and the only change made to the printer was to replace the paper handling system with the moving build platform. Measurement of Charge and Mass of Powder Charging of toner powder is very important in the process of printing. It should be high enough so that the toner can get developed on the photoconductor surface, but not very high which can make it difficult for the powder to be transferred to the build platform. The effectiveness of a powder developer depends on the charging characteristics and the amount of powder it can develop on the photoconductor surface. This can be estimated by measuring the charge per unit mass of the developed powder. In situ measurement of charge per mass is difficult to accomplish. It is also difficult to isolate the charge on the powder from other charges present during measurement and moreover the charged powder developed on to the photoconductor drum is difficult to rescue and measure. This made it necessary to develop a stand-alone system that can accomplish the task (Gokhale 2001). The development of powder developer and the development of the experimental setup to measure charge per unit mass of the developed powder are discussed in more detail in chapter 5.

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40 Measurement of Powder Properties The important physical properties of the powder, to be used in ESFF technology, are volume resistivity, permittivity and mass density. These were measured for toner powder using a test cell and the Keithley electrometer (Dutta 2002). In chapter 7, results of similar tests to determine the above properties for polymer powders other than the toner powder are reported. Improvement of Print Quality Limitation on Part Height There are many issues related to the printing of toner which are amplified when multiple layers of toner powder are printed in ESFF. The first observation was that the printing stops after the part height is around one millimeter. By theoretical calculations (Dutta 2002) it was found that multiple layers of insulative toner powder increases the voltage drop across printed layer and the electric field available for development decreases significantly, which in turn decreases the amount of powder transferred. This problem gets worse when charge gets trapped inside the volume of the printed toner part due to inefficient discharging of the toner layer after each print. This volume charge distribution, which is the same as charged toner, repels the toner and tries to prevent it from getting printed. An attempt was made to solve this problem by depositing charge on the top of the printed toner layer using corona discharge, which is of opposite sign to that of trapped volume charge. The field created by this deposited charge layer cancels the repulsive field created by the volume charge and also creates an attractive force for toner powder printing. Moreover the charge deposited on the surface of the top layer is not affected by the thickness of the toner layer.

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41 Figure 4-2. Parts Printed Using Corona Charging of the Top Printed Layer Before the next Print (Dutta 2002) This method has its own limitations. It is known that corona deposition on a surface, when the grounded electrode is not near the surface, is limited by the breakdown strength of air (Gaussian Charge Limit). This is true for the toner powder printing because toner layer is insulating in nature and multiple layers of toner effectively move the grounded electrode far away from the top surface. The trapped volume charge in the printed part increases with every layer deposited and it can reach a value where the repulsion due to volume charge exceeds the attraction due to the fixed surface charge deposited by corona. This again creates a limitation on the part height that can be built using ESFF. In case of high volume charge density the decrease in the rate of increase in print layer thickness is faster. In the case where the fused toner is almost fully discharged the rate of increase in print layer thickness decreases very slowly and the part can be built having more thickness. This observation suggests that consistent complete discharge of the printed toner powder before fusing is necessary for building higher part thickness. Complete discharge of the volume charge of a printed insulator layer is very difficult to attain.

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42 Edge Growth (Solid Area Development) Edge growth is another problem associated with electrophotography, which is amplified due to multiple layer deposition. The cause for this is the nature of the electric field distribution in the image area, which is stronger at the edges and decreases rapidly towards the middle. This causes thicker development at the edges and near to zero development at the center. When the printing is done in many layers, there is a distinct difference between the edges and the solid area development. An attempt to solve this problem was done by printing the solid area in patterns. These patterns create edges throughout the solid area fill and the field inside the solid area can be maintained at a particular level. Finite element analysis of the patterns is done to study the electric field distribution by pattern printing (Bhaskarapanditha 2002, Fay 2003). It was concluded that there was a significant improvement in the electrostatic field distribution in the image area due to pattern printing. A series of experiments were performed to find out the pattern that gives the best print (Fay 2003). From the results it was found out that a broad black line with a broad white line is the best pattern to print. This pattern printing however cannot be used to print smaller parts with finer tolerances. The resolution of print has to be increased to produce finer patterns.

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43 Figure 4-3. Parts Printed using Patterns (Fay 2003). Printing of Powders other than Toner Attempts were made to determine the feasibility of printing powders other than toner using the test-bed. A generic developer was designed to be able to develop powders of any physical property on the rotating photoconductor drum. The powder developers were designed to replace the toner developer in the toner cartridge assembly. The effectiveness of the developer was tested by using the charge and mass measurement setup. A detailed description of this testing is presented in chapter 5. The powder was brought out for development by cascading it using gravity. This had problems of powder leakage during development and so was not considered suitable replacement for the toner powder developer in the printer used in the test bed. The alternative of the above method is to have two-powder development, in which one powder is used as part powder and the other is used as binder. In this method toner was chosen to be used as the binder due to its low melting temperature. For binding purposes, toner has to be printed image-wise on a uniform layer of part powder (Dutta 2002). In the two powder development the photoconductor surface has to contact the

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44 abrasive part powder to print toner image that can damage the soft organic photoconductor drum surface. Due to this reason, the photoconductor drum surface was isolated from the abrasive part powder by introducing a transfer roller in between them, which would collect the image-wise toner powder pattern and transfer it over the part powder (Dutta 2002). This printed toner powder with the part powder would be fused to bind the part powder to form the real image and the surrounding loose powder would act as support. Figure 4-4. Toner Powder Printed over Insulating Alumina Powder Bed (Dutta 2002) This method had problems associated with difficulties in printing a thin and uniform layer of part powder using the developers designed for cascade development. In addition to that, the toner powder becomes brittle when it solidifies after fusing, which makes it a bad adhesive. There is also the concern of keeping the support powder from falling off the flat build platform without proper support at the sides of the platform to

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45 prevent powder spillage. Due to these problems associated with this method it was not pursued further in the research. Study of Laser Imager System of Printer The design of a new test bed configuration was necessary to avoid dependency on the use of complex commercial printers. Most of the basic activities performed by the printer can be replicated by using controllers, switches and sensor mechanisms. It is only the laser imager hardware that cannot be duplicated so easily and therefore an attempt was made to modify the laser imager from the HP Laserjet-4 printer, which can be used as an imaging unit for the new test-bed (Fay 2003). The schematic of the HP Laser Jet 4 imager is shown in Figure 4-5. Mirror Polygonal Mirror on Stepper Motor Beam Detection Sensor Laser Diode 1 2 3 4 5 Feedback Motor Enable Switch 24V Supply Speed Control Ground Supply 123 5V Supply Sensor Feedback Ground Supply Ground Supply PD LD GND Laser Diode Photosensor Diode Figure 4-5. Diagram of Laser Jet 4 Imager Assembly (Fay 2003) The imager is controlled by a formatter and a dc controller. The formatter decodes the image file sent from the computer and sends electrical pulses to the laser to go on and off according to the image data, while the dc controller controls the motors for

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46 the movement of the laser beam. The modifications to the laser imager were done by replacing the formatter by sending electrical pulses directly to the laser from the computer and controlling the beam on and off according to the image data. The functions of the dc controller were replaced by controlling the motors and the sensors externally through Galil motion controller interfaced with the computer. These modifications removed any kind of dependency of the laser imager system on the printer and all the control was done from the computer. The design and development of the Flat Photoconductor Plate Test-bed is explained in detail in Chapter 8.

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CHAPTER 5 DESIGN AND TESTING OF IMAGE DEVELOPERS Toner powder is the only material that has been so far used successfully to print in layers on top of each other to form three-dimensional objects. When printed in multiple layers it suffers from quality and reliability problems as discussed in chapter 4. This is the motivation for exploring the feasibility of printing powders other than toner in multiple layers using ESFF to form 3D objects. These powders, which includes various conductive and insulative materials, have different properties than toner and may not have the limitations posed by toner powder during printing. This also gives us an opportunity to explore the possibility of making parts of different types of materials some of which can be used as functional prototypes. For using variety of powders with different physical characteristics, image developers were designed and built to develop these powders on the photoconductor surface. These developers were based on cascade development system, due to the fact that the cascade development system uses gravitational force to supply powder for development and so is independent of physical properties of the powder. These image developers had to be tested before putting them on the test bed to be used as powder developers. The parameters to be tested in these developers are the effectiveness of charging of powder and the amount of powder developed on the photoconductor surface. These can be tested by measuring the total charge and mass of the powder developed on the photoconductor surface. From these readings we can estimate parameters like charge per unit mass (Q/M) and mass per unit area (M/A) which are important for the quality of 47

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48 print (Schein 1988). The initial discussion is presented on the development in the powder developer design. This powder developer is designed to use variety of powders including both conducting and insulating powders. The later discussion is based on the development of charge and mass measurement test setup to test the powder developer efficiency in charging and developing powders. Developer Design To use powders other than the toner powder, developers were designed and fabricated which could replace the toner developer assembly in the toner cartridge of the printer. For testing the efficiency of these developers they have to be also suitable to be used in the charge and mass measurement test setup. In Chapter 3 there is a detailed description on the developer system and its components. The function of the development system is also discussed with respect to the electrophotographic cycle. Again in Chapter 4 we discussed that to be able to print powders with varied physical properties, the developer should be designed on the basis of cascade development system and should have its own powder box and charging and powder supply mechanism suitable for cascade development. The following discussion describes the basic components of the developer assembly design and the realization of the latest version of powder developer through continuous improvements. The function of the developer assembly is to store, charge, transport, and transfer the powder for development and re-circulate the residual powder. The developer assembly consists of the powder box and the nib assembly. The storage of the powder is done in a powder box which also acts as a powder hopper for maintaining the powder supply for development. The nib assembly consists of the developer roller, the doctor blade and a casing to hold these together. The charging of powder is achieved by

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49 injection charging method. Voltage is supplied from high voltage source to a metal strip attached to doctor blade. As explained in Chapter 3, the doctor blade is a thin long plate which is kept pressed against the developer roller along the length of the roller. When the powder comes out through the gap between the roller surface and the blade surface, they get squeezed and rubbed against the metal plate attached to the doctor blade and get charged due to contact with the voltage supply. Figure 5-1. Solid Model Assembly of Powder Developer The developer roller brings out the powder for development by friction. The roller has a rough surface which is porous and could trap the powder in these pores and bring them out. The powder also gets smeared on the roller due to squeezing action between roller and doctor blade. This helps in bringing out more powder for development. The powders also have Van der Waals force of attraction between each other and help in attracting more powder for transport to the development region. The transfer of powder is due to the field created between the developer roller and the photoconductor drum, which creates the appropriate force on the charged particles. The remaining powder which was

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50 not developed is re-circulated by friction as the roller surface rubs against the inner surface of the developer roller casing. The fabrication and the improvement in the developer design are presented in the following discussion. Powder Box The powder box is made of transparent acrylic plates, which makes it easier to inspect the powder flow and the level of powder. The bottom plate of the powder box is made at an angle to the horizontal, which helps in the flow of powder due to gravity. In order to allow easy flow of powder, this angle should be greater than the angle of repose of the powder. The angle also should not be very steep because this will make all the powders accumulate at the bottom and increase the pressure on the developer roller, which will make it difficult for the powder to re-circulate back into the box. As an initial guess this angle is chosen as 34.1 degrees, which is the angle at which the toner cartridge sits in the laserjet printer. This angle worked well with most of the powders put in the box for development. The front plate of the powder box has a pair of arms attached to it by screws. These arms help in assembling and positioning of the developer with the charge and mass measurement setup, which will be discussed later in the chapter. Developer Roller The developer roller is made of conducting polymer with a metal axis. This enables to apply voltage to the powder to charge the powder and also create a field with the photoconductor surface during development. As said earlier, the rough surface of the polymer of developer roller helps in supplying powder to the development region. It also circulates back unused powder after development into the powder box. The recirculation of powder is made possible by a particular geometry of the lower cup like structure of the developer roller casing.

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51 Developer Roller Casing The casing is designed to accommodate the components of the nib (developer roller and doctor blade) and along with that ensure a smooth flow of powder in and out of the powder box without any leakage. The casing has flanges that help in attaching the nib to the powder box. The top part of the casing provides support to the blade. The bottom part of the casing provides support for the developer roller axis and has a cup like structure called lower lip that protrudes below the developer conforming to the roller surface. This special geometry retains the undeveloped powders for their recirculation back into the powder box. The gap between the lower lip and the photoconductor drum is critical; a large gap will result in powder leakage and a small gap will bring the lip edge closer to the photoconductor drum and may scrape developed powder off the drum surface, which also may cause damage to the surface of the photoconductor drum. These requirements make the shape of the casing complicated and so it was manufactured out of ABS using rapid prototyping (FDM machine). The developer was designed to not only fit the charge and mass measurement test setup, but also to be used as a powder developer replacement for the toner developer in the toner cartridge. The toner cartridge consists of the toner powder developer, which is attached by pivoting arrangement with another component assembly called front-end (Figure 5-2, which includes the photoconductor drum, the charging roller, the cleaner blade and the cleaner box). One of the early designs of the developer assembled with the front end is shown in Figure 5-3.

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52 Figure 5-2. Front End of the Toner Powder Cartridge Figure 5-3. Cross-section of Developer Assembled with the Front End The preliminary designs suffered from some serious drawbacks related to powder leakage. There was also the problem of inefficient powder charging and powder flow and recirculation control. Almost all the problems were related to the way the nib assembly was designed. This nib assembly was modified and improved with each new developer design (Fay 2003). In every next generation of developer design it was taken care that the

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53 positive attributes of the previous version are implemented and the problems associated with it are solved. The last developer that was designed solved most of the above mentioned common problems associated with the designs before that. It considerably reduced the powder leakage to be negligible and increased control over the powder flow. There is still some small amount of leakage associated with this design that can be removed when the developer is manufactured precisely by taking care of the close tolerances in the design. It can also be mentioned here that, as long as cascade development is used as the method for development, the problem of powder leakage will remain. Even in commercial color printers which use non-magnetic toner powder and cascade development, there is a small leakage of powder. The issue of powder leakage may matter when the developer is used directly in the printer, but is not of a concern in the charge and mass measurement test setup. This is because the charge and mass readings are taken related to the powder that is actually developed on the photoconductor drum and so the leaked powder does not affect the readings. Pivoting Blade Powder Developer As mentioned before, the design of the nib assembly is the most challenging and has gone through a lot of modifications till the last one was designed with pivoting blade. The doctor blade has a hole in the middle to allow for pivoting by using pins to act as pivot points. These pins also hold the blade by these holes. The blade is bent at the middle, along the length, to allow the positioning of the developer roller in the compact unit and also enables the blade to be kept pressed against the developer roller. It is made of ABS by rapid prototyping (FDM machine). A copper sheet is glued to the bottom part of the blade (which is pressed against the roller) to charge the powder flowing through

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54 the gap between the blade and the roller surface (by contact, injection charging). The blade also controls the amount of powder supply for development. This pivoting blade design has better control over powder flow as the angle of the pivot can be changed easily to change the gap between the blade and the roller surface. The conductive powders get charged by the field created between the developer roller and the photoconductor drum. Figure 5-4. Solid Model of Developer Assembly for Pivoting Doctor Blade Powder Developer (with Cross-sectional view) The schematic diagrams below show the developer cross-section (Figure 5-5), cross-section of assembly of developer with the front end assembly (Figure 5-6).

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55 Figure 5-5. Cross-sectional view of the Developer Assembly Figure 5-6. Cross-section of Developer Front End Assembly with the Pivoted Doctor Blade

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56 Development of Charge and Mass Measurement Test Setup Discussion on Development of First Test Setup and Testing Concepts The first charge and mass measurement test setup was developed using the principle of Direct Charge measurement (Gokhale 2001). The schematic of the test setup is shown in Figure 5-7. Figure 5-7. Schematic of Charge and Mass Measurement Test Setup (Gokhale 2001) The charge and mass measurement test setup had an organic photoconductor drum driven by a stepper motor (This photoconductor drum and motor assembly was borrowed from the HP laserjet printer). The motor rotates the photoconductor drum with a fixed velocity equal to the velocity at which it originally rotates inside the laser printer. The

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57 photoconductor drum along with the drive system is mounted on a movable platform, which slides on rails. The original toner powder developer taken from the printer was used and a custom made stand was made to hold the developer in place. This stand kept the developer at an angle (34.1 degrees) that is the same as that used for positioning the developer in the commercial printer. The photoconductor and the drive assembly mounted on the sliding platform are moved to engage with the developer assembly. The schematic below explains the method of Direct Charge measurement in the charge and mass measurement test setup. Figure 5-8. Charge Measurement Setup for Direct Charge Measurement (Gokhale 2001) The toner developer is connected to a voltage source producing dc biased ac voltage to charge toner powder and create powder cloud in the development nip region. The photoconductor drum is connected to the ground through an electrometer. The electrometer has the ability of integrating current that passes through it to measure the

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58 charge flow. The charged powders get developed on the grounded photoconductor drum surface. This deposited charge attracts equal and opposite charges from the ground to the aluminum drum through the electrometer, which measures the current flowing through from the ground to the aluminum drum. The electrometer then integrates the current with respect to time and displays the total amount charge flow from the ground. For the mass measurement, the photoconductor drum had to be removed from the set-up and weighed. The drum is rotated for one revolution which allows using the value of the surface area of the drum for calculating mass of powder developed per unit area. Removing the photoconductor drum is a complicated process and this could affect the mass readings due to the possibility of powder loss from spillage. This is solved by covering a layer of Mylar sheet over the photoconductor surface and removing it for mass measurement, without removing the whole photoconductor as before. This setup had problems because of the uncertainty of positioning the photoconductor drum with respect to the developer roller. When the rollers do not have a line contact at the development nip the toner development becomes non-uniform. Moreover, as the developer roller is driven by the meshing of its gear with that of the photoconductor drum the drive of the development roller can be shaky due to misalignment. The stand on which the toner developer sits is customized for the particular geometry of the developer and has to be changed for any change in the developer geometry. To solve the above problems the test setup was modified by focusing on redesigning of the mechanism by which the developer assembly engages with the photoconductor drum. In all the improvements done to the test setup, the basic principle of direct charge measurement is used as the method to measure charge.

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59 Improvement in the Design of Test Setup In the HP LaserJet 4 printer the developer assembly pivots by two pins and gets engaged to the photoconductor drum by its weight. This aligns the surface of the developer roller and the photoconductor drum to have a line contact for uniform development throughout the length of the photoconductor surface. This also provides the force, due to gravity, which keeps the gears of the developer roller and the photoconductor drum engaged. This pivoting concept was used in the improved charge and mass test setup. The solid model of the test setup with the toner powder developer is shown in the figure below. Figure 5-9. Solid Model of the Assembly of Toner Powder Developer and Charge and Mass Measurement Test Setup The photoconductor drive assembly, which used to slide on rails in the last design, was removed from rails and used for this design. Side-supports were designed to hold the photoconductor drum in place. The side supports had circular slots made at a particular

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60 angle to hold the developer assembly by its pins and allow it to pivot around the pins. The developer assembly comes to a halt when the developer gear meshes with the photoconductor gear. This type of engagement placed the developer exactly in the same orientation as that would be in the printer. This was a much simpler design and was easy to disassemble for mass measurements of the developed toner. Figure 5-10. Cross-section View of Charge and Mass Measurement Test Setup The development roller was provided with -570V dc biased ac voltage at 1780V (p-p) and 1754 Hz frequency from the voltage source to create powder cloud in the development nip, simulating similar conditions as there in the development region of the laser printer (Zhang 2000). The photoconductor drum was grounded through electrometer for measurement of charge flowing through the ground to balance the charge on the powder particles developed. The toner was printed on a Mylar sheet wrapped around the photoconductor drum. The stepper motor was started, and stopped when the photoconductor had made one

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61 revolution observed by visual inspection. Then the developer assembly was removed and the photoconductor drum taken out to measure the difference between weight of the photoconductor drum before development and after development to find the mass of the powder developed. The electrometer gave the charge reading and the area was taken to be the surface area of the photoconductor drum due to single rotation of the drum during development. The charge, mass and area readings were used to calculate charge per unit mass (Q/M) and mass per unit area (M/A). Tests were carried out using this setup to find out the Q/M and M/A of toner powder. The results are reported in table 5-1. Table 5-1. Q/M and M/A calculations of toner using QMM test setup M 1 (gm) M 2 (gm) Q (C) M=(M 2 -M 1 ) (gm) Q/M (C/gm) A (cm 2 ) M/A (gm/cm 2 ) 96.8690 97.0367 -1.238 0.167 -7.413 Overrun 96.8678 97.0092 -1.0438 0.1414 -7.382 201.41 7.02x10 -4 96.8676 97.0218 -1.254 0.1542 -8.1328 209.84 7.34x10 -4 96.8653 97.0211 -1.2134 0.1558 -7.7878 209.84 7.42x10 -4 96.8661 97.0251 -1.1401 0.159 -7.1706 209.84 7.57x10 -4 Average -7.5772 7.34x10 -4 The average Q/M was found out to be .5772 C/gm and the average M/A for toner powder was found out to be 7.34x10 -4 gm/cm 2 These values were found to be similar to the values obtained by the test done using QMM setup with fixed cartridge developer assembly and sliding photoconductor driver assembly (Gokhale 2001). This setup successfully replicated the older design results and had less complicated design and test procedure. As discussed before, powder developers were designed and fabricated to be used with both the front end of the toner powder cartridge and the charge and mass measurement test setup. These powder developers enabled the development of powders other than toner. Similar to toner powder cartridge, these powder developers are engaged

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62 with the front-end and the charge and mass measurement setup by the pivoting arrangement. The powder box in the developer assembly has two protruding arms attached to its front plate by screws. These arms have pivoting pins protruding outwards that are hooked into the slots on the support provided in charge and mass measurement test setup. This enables the powder box to pivot around the pins and get positioned on the transfer drum by its weight. As the developer roller gear is driven by the photoconductor drum gear, the weight of the developer assembly helps in keeping the two gears engaged all the time without any extra arrangement. The positioning of the powder box can be adjusted by changing the arm length and the angle that the arm makes with the front plate. The assembly of powder developer with the charge and mass measurement test setup is shown in the figures below (figure 5-11, solid model) (figure 5-12, cross-section). Figure 5-11. Solid Model of Assembly of Developer and Charge and Mass Measurement Test Setup

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63 Figure 5-12. Cross-section of Charge and Mass Measurement Test Setup with Improved Developer Assembly Independent Charge and Mass Measurement Test Setup As seen in the last two charge and mass measurement test setups, both of them were based on developing of powder on the photoconductor drum driven by the stepper motor and gearbox assembly originally used in the Laser printer. The stepper motor rotates the photoconductor drum at a fixed speed and hence there is no speed control. It was also necessary to discharge and clean the photoconductor surface before it goes for the next cycle. This demanded more photoconductor surface area to be available for assembling these new additions which was not possible with the small diameter photoconductor drum available from the Laser printer. Therefore it was decided to build a charge and mass measurement test setup independent of the restrictions of borrowed laser printer mechanisms. This new design

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64 configuration has its own controllable drive motor, transfer roller, developer, charger, and cleaning mechanism. Before going into the detail design of the charge and mass measurement test setup, some basic knowledge about powder behavior and photoconductor cycle is essential. This particular charge and mass measurement test setup was designed to be used with powders of any kind including polymers and metals. Polymers and metals represent the two categories of materials available based on their response to electricity; while polymers represent insulator family, metals represent the conductor family. For the charge and mass measurement tests conducted with the powders, iron and nylon-12 are chosen from the conductor and insulator families respectively. Iron is used due to its easy availability and nylon-12 is used because of its good developing characteristics on photoconductor drum (details presented in chapter 7). Design Considerations The insulators can be charged triboelectrically when they are rubbed against the doctor blade and developer roller. They also get charged when rubbed against a conductor surface connected to the voltage source (injection charging). They have surface and volume charge densities and do not get discharged easily. The charged polymers can develop over conducting as well as insulating surfaces. Consider a charged polymer powder placed on a conducting plate (which provides charge to the polymer by injection). If a grounded electrode is brought near this plate, a field will be created which will produce force on the charged polymer powder to travel to the grounded electrode. Once the polymer powder reaches the grounded electrode, it remains attached to the electrode due to the image forces created by the opposite charges flown to the grounded plate from ground due to the presence of the developed polymer powder. It does not matter if the

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65 grounded plate surface is conductive or insulative, as the insulative polymer powder does not lose charge even when in contact with a conductive grounded surface. This is not the case with conductive powders. The metals on other hand get charged when they are placed in an electric field created by charged surface (induction charging) and also when they are in contact with conducting bodies connected to voltage supply. Metals cannot support volume charge density and all the charges are present on them are on the surface. Conductors cannot get developed on a conducting surface, which can be explained as follows. DC Voltage SourceElectrodes with conductive surfaceElectrode with insulative surfaceConductive powder particles Figure 5-13. Schematic Illustration of Behavior of Conductive Powder Particle in the Presence of Conductive and Insulative Electrode Surfaces Consider a metal particle in an electrostatic field between two metal electrodes. One of the electrodes is provided with positive voltage and the other with negative voltage. If initially the particle is in contact with the negative electrode, it will get charged negative and move in the direction of the positive electrode. When it reaches the positive electrode it will loose its charge and get positively charged and move in the opposite direction towards the negative electrode. Once it touches the negative electrode it loses its charge and gets negatively charged and the cycle repeats. So, when we place

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66 conductive powder particles between two electrodes with conductive surfaces a powder cloud is formed due to bouncing of particles between the electrodes. When the charged metal particle encounters the insulator surface it does not lose its charge to the electrode. The particle is attached to the insulator surface by the electrostatic force of attraction by the opposite charge present in the electrode at the backside of the insulator. So, to develop conductive powder on a surface, the surface must be an insulator. Due to the above considerations, the drum on which the powders would be developed in the QMM setup should be a metal drum with an insulator cover. It is more desirable if the insulator has a high dielectric strength to withstand high potential differences in the developing region. Mylar sheet is suitable for this requirement and so is used as the insulative covering over the metal transfer drum. When one layer of charged powder gets developed and then cleaned off the drum surface to make the surface ready for the next layer of powder, the powders leave some of their charge on the surface. This charge is of the same sign as that of the freshly charged powders. If this is not neutralized before each development, the charge gets accumulated and prevents the development of more powder on the surface due to electrostatic repulsion. To prevent this, a charge roller is placed on the drum which discharges the surface of the transfer drum by getting rubbed against it after it has been cleaned. The charge roller is provided with ac voltage that tries to charge the surface in both signs, which effectively discharges it. The charge roller is placed between the cleaning region and the development region of the developer cycle.

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67 Stages of Charge and Mass Measurement Test Cycle The charge and mass measurement test setup is based on a part of the electrophotography cycle. The test cycle does not include some of the stages of the electrophotography cycle dealing with printing of developed powder on a substrate. The electrophotography cycle is explained in detail in chapter 3. In the charge and mass measurement test cycle, the events are distributed around the transfer drum. Powder is developed on the transfer drum from the developer roller and gets collected in the cleaner box when the cleaning blade cleans the surface of the drum. The drum surface is then discharged to make the surface ready for the next development. Figure 5-14. Different Stages of the Test Cycle with respect to the Cross-sectional view of the Charge and Mass Measurement Test Setup.

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68 Design and Building of the Charge and Mass Measurement Test Setup The discussions above serve as guidelines for the design and building of the charge and mass measurement test setup. In this section detailed design and assembly of the test setup is discussed. As a good design practice the design of the test setup was made in a modular fashion which allows individual parts to be assembled to form subassemblies and these subassemblies are assembled to form the final assembly. Each of these subassemblies is independent of each other and can be easily removed from the setup for experimental or repairing purposes. The charge and mass measurement test setup is made of three major subassemblies. These include the transfer drum assembly, the cleaner box assembly and the motor and frame assembly. Transfer Drum Assembly The transfer drum assembly is made of the transfer drum, the charge roller, drive attachments and support assembly to fix it to the frame of the charge and mass measurement setup. The transfer drum is made of a two-inch diameter aluminum pipe covered with Mylar sheet, which provides an insulative surface for conductive powder development. The end caps of the drum are made of high-density polymer and have interference fit with the drum. These end caps provide low frictional surface against side supports during rotation. A steel rod is used as the axis of the drum, which passes through the end caps and rests in the holes provided on the side support plate. These holes work as bearings for the axis to rotate.

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69 Figure 5-15. Solid Model of Transfer Drum and Cleaner Box Assembly (with Cross-sectional view) On one side of the drum there is a pulley and on the other there is a gear, both tightly assembled with the axis of the drum. A servomotor, which is independently controlled by Galil Motion Controller, is used to drive the pulley through a timing belt. The timing pulley and belt drive provides a positive drive between the servomotor and the transfer drum without any slip. This also gives flexibility in the placement of the motor in the whole assembly, which is possible because the length of the belt can be altered to suit any convenient location for the motor and this is difficult to attain with a gear drive. The transfer roller drives the developer roller by gear arrangement. This gear drive is formed by meshing of gear of the transfer roller with the gear on the developer roller. This arrangement maintains the same surface velocities of the developer roller and the transfer drum at the development zone which is necessary for better development. This is because, if the surface velocity of the developer roller surface is slower than the transfer drum surface then very less powder will get developed and if it is faster then there will be more powder coming out of the box, when not developed, may get accumulated in the lower lip and leak.

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70 The charge roller is placed pressed on the transfer drum surface and is located between the cleaner box assembly and the developer assembly. Its role is to discharge the surface of the development roller after the surface is cleaned off the developed powder. This discharging of the surface removes any unwanted charge buildup on the transfer roller surface which may affect the powder development. The charge roller is a steel rod covered with a conductive polymer and is held pressed against the transfer roller surface by spring-loaded holders. The surface of the charge roller is smooth for uniform discharge of the transfer roller surface. This roller rolls on the transfer drum surface by friction. Cleaner Box Assembly The cleaner box is a rectangular box with a cleaning blade attached to it. The cleaning blade which was originally used to clean the photoconductor drum in the laser printer cartridge is modified to be used to clean the transfer drum in this application. It has a high friction flexible polymer sheet attached at its end to clean the roller surface. The cleaner-box has flanges on both of its sides that have screw holes to fix the assembly with the rest of the charge and mass measurement setup. By tightening or loosening these screws the pressure at which the blade is pressed against the transfer drum surface is varied. This kind of arrangement makes it easier to weigh the developed powder mass by removing the cleaner box from the assembly and weighing it with the powder in it. There is a chance of powder leaking by not getting caught by the bottom lip of the cleaner box during the cleaning process. This is avoided by attaching a thin flexible polymer sheet at the bottom edge of the box that acts as a one way valve as it allows the developed powder to pass through, but does not allow any cleaned powder falling down to leak. The

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71 schematic below shows the transfer drum assembly and the cleaner box assembly with the relative position of the components. Figure 5-16. Front view and Side Cross-sectional view of the Transfer Drum Assembly and the Cleaner Box Assembly. Motor and Stand Assembly The whole test setup is supported by an aluminum stand which has leg supports made of angle brackets. These angle brackets have holes at the base for the stand to be bolted down and fixed to any base plate for extra stability. The stand has holes on the top bridge to bolt the transfer drum assembly to it and hang from it. The cleaner box assembly is screwed onto the side plates of the transfer drum assembly and the developer assembly hangs from the pivot holes provided on the side plates of transfer assembly. The servomotor that drives the setup is attached by screws to one side of the stand. The screws are attached through elliptical slots made to adjust the position of the motor and tighten the timing belt by sliding the screws up or down along the slots. The Galil Motion controller controls the motion of the servomotor by providing signals generated from the commands given by the software interface. These signals are amplified for the motor

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72 drive by using amplification circuitry. The figure below shows the complete assembly of the charge and mass measurement test setup with the powder developer. Figure 5-17. Solid Model of Motor and Stand Assembly Figure 5-18. Complete Assembly of Powder Developer with Charge and Mass Measurement Test Setup

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73 Experimental Results Experiments were conducted to determine the charge per unit mass of iron and nylon powders. The charge and mass measurement values were evaluated and the dependence of these values with the experimental variables was investigated. The following tables present the experiments done and the observations reported. Experiments with Iron Powder The following experiment was done to find out the effect of the number of revolutions of the transfer drum on the value of Q/M when the voltage applied remains constant. Size of powder = 60m diameter Constant Charging Voltage = 500V Voltage on Discharge Roller = 960V A.C. (p-p) 2.5KHz (explained in chapter 6) Table 5-2. Variation of the Q/M readings with the number of revolutions of the transfer roller Number of Revolutions Initial Mass (M i ) Final Mass Initial Charge Final Charge Change in Mass(g) Change in Charge(C) Q/M (C/g) 2 230.931 231.727 83 nC 0.2C 0.796 0.117 0.147 4 231.727 232.151 63 nC 0.15C 0.424 0.087 0.205 6 232.151 233.328 -6 nC -153nC 1.177 0.147 0.125 8 234.811 236.294 -32 nC 0.51C 1.483 0.478 0.322 10 236.294 238.121 -10 nC 0.27C 1.827 0.26 0.142 Other than the reading for eight revolutions, it can be seen that the value of Q/M remains almost constant with a small fluctuation which can be attributed to experimental errors. This can be explained by assuming that iron develops in a monolayer. To find out the dependence of the value of Q/M on the voltage applied to charge the powder, we have the following experiment. In this experiment we increase the

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74 voltage from 500V to 1500V and back to 500V. The number of revolutions of the transfer drum remains constant at 2. Number of revolutions = 2 Voltage on Discharge Roller = 960V A.C. (p-p) 2.5KHz Table 5-3. Variation of Q/M measurement with the increase in voltage supplied for development while the number of revolutions of the transfer drum remains constant. Charging Voltage Initial Mass Final Mass Initial Charge Final Charge Change in Mass Change in Charge Q/M (C/g) 500 231.415 231.873 29nC 137nC 0.458 0.108 0.231 1000 231.873 232.295 81nC 0.53C 0.422 0.449 1.064 1500 232.295 232.812 63nC 1.52C 0.517 1.457 2.818 1500 232.812 233.323 35nC 0.91C 0.511 0.875 1.712 1000 233.323 233.775 19nC 0.22C 0.452 0.201 0.445 500 233.775 234.214 10nC 98nC 0.439 0.088 0.200 This indicates that Q/M depends on the applied voltage to charge the powder. The increase in voltage causes increase in the value of Q/M and vice versa. This can be explained for the metal powders as follows. When we apply higher charging voltage, the charge on the powder particle increases due to the relation Q = CV, where C is the capacitance of the powder and is dependent on the geometry of powder and the dielectric constant of the medium of development. So for a powder particle of a particular size developed in a particular medium, the charge on the powder depends on the voltage applied. When conductive powders develop, they form a monolayer of development. This limits the powder mass developed on the transfer drum surface. Therefore, when voltage increases, Q/M increases due to the increase of charge and constant mass development. These two trends can be observed from the table above.

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75 Experiments with Nylon 12 Powder The following experiment was done to find out the effect of the number of revolutions of the transfer drum on the value of Q/M when the voltage applied remains constant. Size of powder = 25 30m diameter Constant Charging Voltage = -500V Voltage on Discharge Roller = 960V A.C. (p-p) 2.5KHz Table 5-4. Variation of the Q/M readings with the number of revolutions of the transfer roller Number of Revolutions Initial Mass (M i ) Final Mass Initial Charge Final Charge Change in Mass(g) Change in Charge(C) Q/M (C/g) 2 230.269 230.275 134 nC 0.193C 0.006 0.059 9.833 4 230.275 230.293 23 nC 0.258C 0.008 0.235 29.375 6 230.293 230.303 35 nC 0.420C 0.010 0.385 38.5 8 230.303 230.316 47 nC 0.612C 0.013 0.565 43.46 10 230.316 230.342 47 nC 0.763C 0.026 0.716 27.538 It can be seen that the value of Q/M is fluctuating. Except the last reading, we can see an increase in the Q/M value as the number of revolutions is increased. This can be explained by considering that the charge on the developed polymer powders is due to triboelectric charging by rubbing against the roller and the blade. More number of revolutions means more rubbing action for triboelectric charging and so the charge on each particle goes up. To find out the dependence of the value of Q/M on the voltage applied to charge the powder, we have the following experiment. In this experiment we increase the voltage from -500V to -3000V. The number of revolutions of the transfer drum remains constant at 3.

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76 Number of revolutions = 3 Voltage on Discharge Roller = 960V A.C. (p-p) 2.5KHz Table 5-5. Variation of Q/M measurement with the increase in voltage supplied for development while the number of revolutions of the transfer drum remains constant. Charging Voltage Initial Mass Final Mass Initial Charge Final Charge Change in Mass Change in Charge Q/M (C/g) -500 230.342 230.348 4nC 0.220C 0.006 0.196 32.67 -1000 230.381 230.432 25nC 0.156C 0.051 0.131 2.569 -1500 230.432 230.491 49nC 0.126C 0.059 0.077 1.305 -2000 230.491 230.548 12nC 0.078C 0.057 0.066 1.158 -2500 230.548 230.689 13nC 0.024C 0.141 0.011 0.078 -3000 230.689 230.805 15nC 0.056C 0.116 0.041 0.353 This clearly indicates that Q/M depends on the applied voltage to charge the powder. Interestingly, the increase in voltage causes a decrease in charge per unit mass readings, while it is normally expected to be not so when compared to conductive powders. Nylon 12 is an insulator and the charged particles do not carry large charges like conductors. This can be seen by comparing the charge values of nylon with that of iron. The powders on the developer roller contain more number of low charged particles than highly charged particles. When the voltage is low the electric field in the development zone is good for only highly charged particles to get developed. As the voltage increases, the development field also increases and so now it is possible for the low charged particles to get developed. These low charged particles compete with the highly charged particles which results in the marginal decrease in the charge measurement. The development of large number of these low charged particles explains the increase in the mass measurement. Due to the combination of both the effects there is a high rate of decrease in the value of charge per unit mass (Q/M) measurement.

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CHAPTER 6 METAL POWDER DEVELOPMENT AND PRINTING Printing of metal powder using electrophotography has many applications like printing circuit boards and making functional prototypes. As discussed in last chapter, there has been a lot of attempt in designing developer based on cascade development technology to help in printing of powders other than the commercial toner powder. All these developers were designed to replace the toner powder developer from the commercial laser printer. It takes a lot of time and effort to modify a commercial laser printer before it can be used for printing using ESFF. Each new model of printer released to the market becomes obsolete in 2-3 years. This makes the repair of the test bed difficult owing to the less availability of spare parts due to the phasing out of the printer. There was a need to design and build a test bed which is independent of the use of laser printers and is modular so that any debugging of problems can be done easily. Chapter 8 discusses in detail about the design and building of the modified test bed. A simplified schematic in figure 6-1 explains the concept of the design. The concept is based on the movement of a photoconductor plate in a straight line passing through the different stages of electrophotography. The development process is done by first charging the photoconductor plate, imaging with the laser imager and then developing powder upward, against the gravity, on the downward facing photoconductor plate. The photoconductor then reaches the build platform and prints the image of the powder. The discussion in this chapter is on the feasibility of such development and printing processes used for this concept. This will generate essential feedback 77

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78 information on the designing and operating the test bed based on the flat photoconductor plate. Figure 6-1. Concept of the New Modified Test Bed Metal Powder Development Powder development in the new test bed is done against gravity. This may create some limitations on this process as discussed in this chapter. The metal powder has some special properties related to development and transfer using electrostatic force, which is shared by all conductive powders. So, the discussions and experimental results presented in this chapter can be applied to any conductive powder. Iron powder was used for the experiments due to its easy availability, and the results are reported at the end of the chapter. Metal powders can be developed in three ways: Applying voltage to the developer roller and grounding photoconductor plate Applying voltage to photoconductor plate and grounding developer roller Grounding both developer roller and photoconductor plate and charging the photoconductor surface

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79 The first two methods are fundamentally the same as they both create electrostatic field between them by the application of voltage source to either the developer roller or the photoconductor plate while the other one is grounded. In the first method the metal powder will get charged by being in contact with the developer roller when the grounded photoconductor is above the developer roller and an electric field is created between them. Development occurs due to the force created by the presence of the charged particle in electric field. In the second method metal powder gets charged by induction when the photoconductor plate, connected to voltage supply, comes over the grounded developer roller and a field is created between them. The field creates force on the charged particle, which overcomes the gravitational pull to develop the powder on the photoconductor surface. In the third method voltage supply is given to the charge roller, which is used to deposit charge on the photoconductor surface. The powder is charged by induction due to the field created by the charge on the photoconductor surface. In this case also the force due to electric field has to overcome the gravitational pull for the development to occur. This third method is different than the first two methods because in the third method image can be created by discharging the deposited charge by UV laser. The powder gets charged oppositely to the charged areas in the image and gets attracted towards the charged regions, which limits the metal powder development to charged area development (CAD) only. So, this is the only method, which could integrate imaging as a part of the development process. The other two methods are only capable of printing

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80 metal powders in thin uniform layers. In those cases, imaging can be done by the printing of image-wise layers of polymer adhesives between uniformly printed metal layers. The metal powder development processes can be analyzed using standard electrostatic equations to determine the influential variables and the characteristic numbers, important for development process. This analysis is done in the following section. Metal Development Theory In the flat photoconductor plate test bed, development is done against gravity. As only the charged particles would experience the electrostatic force to overcome gravity and get developed on the photoconductor surface, this method of development helps in the separation of the charged and uncharged particles, which in turn helps in creation of sharper images and reduces background printing significantly. During development the powders have to overcome gravitational force, which would require large charge per unit mass. Later during transfer of the developed powder onto build platform the large charge per unit mass may make it difficult for the powder to overcome the electrostatic adhesion force and get printed on to the build platform. Therefore, a careful analysis of these processes has to be done to understand them well and be able to control them better. Before getting on with calculation of different parameters for development, the basic properties of the iron powder used for the experiment are discussed below. The fundamental electrostatic parameters are also stated for reference. Constant Parameters used in Calculations The mass of a single iron powder particle is calculated below assuming the powder particle as a spherical particle with radius r. Diameter of the iron powder particle used in experiment = 60 micron

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81 Volume of sphere = 31336310131.11030141.33434mr (1) Density of iron (approximately) = 7.87 x 10 3 Kg/m 3 Mass of a particle = 1.131 x 10 -13 m 3 x 7.87 x 10 3 Kg/m 3 = 8.9 x 10 -10 Kg (2) Breakdown electric field of air = 3.0 x 10 6 V/m Permittivity of air = 8.854 x 10 -12 C 2 /(N-m 2 ) Charge per Unit Mass (Q/M) Calculations As discussed before, charge per unit mass (Q/M) of the developing powders determines the electrostatic force on them and also the quality of development. In the calculations shown below, the minimum and the maximum allowed Q/M of iron powder particles used in the experiment are calculated. The schematic below displays the simple force model of a single iron particle subjected to electrostatic and gravitational forces during development. Figure 6-2. Forces Acting on a Powder Particle During Development For powder development to occur, the electrostatic force (F e ) should be greater than or equal to the gravitational force (F g ). To calculate the minimum Q/M required for development, we use the condition when the electrostatic force on an iron powder particle is just able to overcome force of gravity. For this condition we have: F g = F e (5) Mg = QE (6)

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82 E g M Q (7) Here, g is acceleration due to gravity and is constant. So, Q/M is controlled by the value of electrostatic field E. By increasing the value of E we can reduce Q/M. The maximum value of E can reach 3.0x10 6 V/m. So, substituting the value of electric field we get the minimum value of Q/M to be, gmnCKgCoulombmVsmMQ/27.3/1027.3/100.3/81.9662min (8) To find the minimum charge on the iron powder particle used in the experiment to allow it to develop, we use the mass of the particle: particleQmin = 3.27x10 -6 Coulomb/Kg x 8.9 x 10 -10 Kg = 2.91 x 10 -15 Coulomb (9) The above calculation shows that minMQ is independent of the material properties of the powder used for development and can be used for any powder. Again from Gauss Law we know that, 20004rQAQE (10) And, 334rM (11) rErErMQ03203344 (12) rKrEMQ max0max3 (14)

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83 maxMQ is the maximum charge per unit mass that a powder particle can have before it produces electric discharge due to breakdown of air. Unlike minMQ maxMQ is dependent on the powder size and is inversely proportional to the radius of the powder particle. Finding maxMQ for iron powder particle used in the experiment we get: 63612max02max0max10301087.7100.310854.8333rErrEMQ KgCMQ4max10375.3 =3.375x10 -7 C/gm = 0.338 C/gm (15) This gives the maximum charge an individual powder particle can have without sparking, which is calculated as follows: KgKgCMMQQparticle104maxmax109.810375.3 = 3.0042 x 10 -13 Coulomb (16) Maximum Surface Charge Density The following calculation estimates the maximum surface charge density that can be sustained by the powder particles in air. The calculation is done considering electrical breakdown field limit of air, = 3.0 x 10 maxE 6 V/m. From Gauss law, 00 AQE 251260maxmax/10656.210854.8100.3mCE

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84 This is the maximum charge density that a surface can have when it is kept in air. This limit is called Gaussian Charge Limit. This is independent of the material properties. Electric Field Range We know that the maximum allowable field limit for powder development in air is 3.0 x 10 6 V/m. The following calculation finds the minimum field limit for powder development considering that the powder particle is charged to its maximum allowed charge limit, and the electrostatic force for development just overcomes the force of gravity. maxminQMgEMgQE For the iron particle used for the experiment, using the maximum charge value from equation 16, we get: mVQMgE4139maxmin109059.2100042.31073.8 This suggests the value of electrical field in the development zone should be within the range of 2.9x10 4 V/m to 3.0x10 6 V/m. The maximum voltage available from the voltage source is 5000V. The gap between the developer roller and the photoconductor surface during development has a minimum limit to avoid breakdown of air. This gap is calculated as follows: mmmmmVVEVheriment67.135100.350006maxmax)min(exp During experiment the gap is taken to be more than or equal to 2mm to avoid any sparking due to air breakdown.

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85 Calculation of Forces Involved During Development The electrostatic force and the gravitational force on a powder particle are dependent on the size of the particle. This relation is evaluated below. 222max0max20maxmaxmaxmax44rKrEErEEQFee maxeF is directly proportional to the square of the radius of the powder particle. where, E42maxeK = permittivity of the medium of development. Here, the medium of development is air so = 0 So, does not depend on powder material but on the medium of development. eK 3333434rKrggrvgMgFgg gF is directly proportional to the cube of the radius of the powder particle. materialpowder ofdensity where34gKg So, K g is dependent on the material used for development and not the medium of development. To determine the ability of the electrostatic force available for development to overcome the gravitational force and develop the powder on the photoconductor surface, comparing these forces. The force due to gravity on the powder particle is, NsmKgMgFg9210107309.881.9109.8 The maximum electrostatic force available for development is found out by multiplying the maximum charge that can be attained by a powder particle (equation 16) and the maximum field limit due to air breakdown.

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86 NmVCoulombEQFe7613maxmaxmax100126.9100.3100042.3 The ratio between these forces is: 23.103107309.8100126.997maxgeFF So, the maximum electrostatic force available for development is 100 times stronger than the gravitational pull. This gives a wide range for the electrostatic force to be less than the maximum value and still be good for development. Calculation of Maximum Powder Particle Radius for Development By considering the last calculations relating electrostatic and gravitational forces to the powder particle size it can be seen that the force of gravity is directly proportional to the cube of the particle radius and its rate of increase with increase in radius is more than the rate of increase of the electrostatic force, which in turn is directly proportional to the square of the radius. This indicates that there is a particular powder particle radius beyond which there can be no development. This radius limit is calculated by equating both the forces. ggKKrrKrKFFgegege2max02max0max32maxE334E4 Calculating this radius for the iron powder developed in air: mmmr1.3101.381.91087.7100.3108542.83332612max 3.1 mm = 3100m= 6200m diameter = 3.5 mesh (ASTM US standard)

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87 This derivation suggests that it will be difficult to develop any iron powder particle with radius above 3.1mm, when air is the medium of development. This argument is applicable to this particular type flat plate development method where development is done against gravity. This analysis does not take into account the other forces acting against the electrostatic force, like the van der waals force of attraction between the iron particles themselves and between an iron particle and the surface of the supply roller. If these forces are also taken into consideration then the maximum limit on the powder radius will be less than the current value. Metal Powder Transfer Process Theory After development, the photoconductor plate is moved over to the build platform where the powder is transferred electrostatically on to the built part as the next build layer. During this transportation the powder should not fall off the photoconductor surface due to gravity. This is avoided by the electrostatic force due to the image charge on the photoconductor, which keeps the powder from falling. This force is calculated as: 222max0max2202022020224424rKrEFrErrErQFimimim maximF is directly proportional to the square of the radius. 3333434rKrggrvgMgFgg It can be observed by comparing the dependence of maximF and on the change in particle size, that there is a maximum radius of powder particle exists more than which gF

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88 the photoconductor plate will not be able to hold the powder on its surface. This maximum radius limit of a particle can be found out by equating both the forces. gEgErrgrEFFgim4334342max02max0max322max0max This value of is 1/4 maxr th of the value of calculated by equating the electrostatic and gravitational forces during development. maxr So, meshmmmmr2520775775.04101.33max The iron powder particle used for experiment has diameter of 60 microns which is less than the above calculated value of powder radius and so it can be developed on to the photoconductor and retained by it until the powder layer is transferred and printed over the build platform. This value of maximum radius of particle to hold on to the photoconductor surface is calculated without taking Van der waals force between the powder particle and the photoconductor surface. If that is taken into consideration then this maximum radius limit will increase. Powder Transfer Methods The developed powder is transferred onto the build platform by creating electrostatic field between the photoconductor plate and the build platform. Field creation for transfer can be done in four ways. Applying voltage to the build platform and grounding the photoconductor plate Applying voltage to the photoconductor plate and grounding the build platform

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89 Grounding photoconductor plate and charging the top surface of the built part on the build platform to attract the developed powder to print Create a field on the backside of the photoconductor plate to repel the developed powder. The first two methods are effective when the part height is small. When the part is made of insulator material, there is a voltage drop across the part height and so with increase in part height the voltage difference for powder transfer decreases leading to decrease in electrostatic field and which finally leads to decrease in powder transfer. Even when metal powder is transferred to form metal parts, there has to be insulated layers between two layers of metal powder (as metal powders do not develop on metal or conductive surfaces as explained in chapter 5) which can cause the voltage drop. The other two methods use the field created by deposited charge on the top surface of the part and on the backside of photoconductor plate respectively. A corona charger is used to charge the top layer of the built part for creating a field. The surface of built part top layer can be charged by passing a corona charger over it (Dutta, 2002). The maximum charge that can be deposited is 2.64x10 -6 Cm -2 (Cross, 1987). This surface charge will have field of 3.0x10 5 Vm -1 which is an order less than that the breakdown field of air. The surface cannot be charged till the field reaches the breakdown field of air because a field around the value shown above repels the incoming ions and they do not get deposited on the surface. This leaves us with the last method of creating field, by accumulating charge on the backside of the photoconductor plate to repel the powder off the surface as the field created by this method can go up to the breakdown field of air. Capacitor Method of Powder Transfer (Conceptual) A capacitor can be created at the backside of the photoconductor plate to accumulate charge. To form the capacitor, a metal plate may be placed on the backside of

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90 the photoconductor plate. This plate will act as one of the electrodes for the capacitor, where the photoconductor plate acts as the other electrode. There is a dielectric layer covering over the conductive layer on the backside of the photoconductive material. This dielectric layer acts as the dielectric of the capacitor. The conceptual arrangement of the capacitor is presented in the schematic below. The capacitor back plate is supplied with voltage and the photoconductor plate is grounded. The charged powder is attached to the photoconductor plate by the attractive force of the image charge and van der waals force. For transfer this attractive force should be cancelled by repulsive force created by the charge accumulated in the above mentioned capacitor. This capacitor is illustrated in the following parallel plate model: Figure 6-3. Schematic Model of the Capacitor Method of Powder Transfer (Conceptual) As the voltage is applied to the capacitor back plate and the photoconductor plate is grounded, there is charge accumulation on either side of the capacitor. The applied voltage polarity is chosen to be opposite to that of the powder, so that the photoconductor plate accumulates charge of the same polarity as that of the developed powder, which

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91 helps in repelling like charged developed powder. Then the connection to the ground from the photoconductor plate is cut off and so the charge is trapped in the photoconductor plate. The field by the charge accumulated in the capacitor still remains between the photoconductor plate and capacitor back plate. Then the voltage polarity of the capacitor back plate is reversed and so all the field lines due to the charge trapped in the photoconductor plate are directed towards the developed powder to repel the powder off the surface. This method can create higher fields than the other methods and does not require a field between the photoconductor plate and the build platform for the powder to transfer. This makes it a very useful technique to build thicker parts, and so it is necessary to understand this concept in detail. To experimentally verify the feasibility of this concept a very well designed setup is required due to the high likelihood of charge leak due to leakage current which can happen easily if the capacitor is not insulated properly. Attempts had been made to try to transfer powder using this method by using simple experimental setups which were not successful due to the sensitivity of the experiment. This concept is not further investigated in this thesis work. Experimental Results Before building the flat photoconductor plate test bed, it was necessary to simulate the development and printing process using a simple setup, to perform feasibility analysis of developing and transferring metal powder for printing using electrostatic forces. The setup consisted of a powder bed, developer electrode and transfer electrode.

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92 Figure 6-4. Schematic of the Test Setup for Development The powder bed was made by depositing iron powder in an elliptical slot made in an aluminum plate. Voltage can be applied to this plate to create the field for development, and to charge the powder. The top surface of this powder bed can be leveled to form a uniform surface for development. For powder transfer experiments, the developer and the transfer electrodes are made similar to each other. They both have a sheet metal plate which is attached to an insulator block using PET. This PET also acts as the insulating cover on the surface of the electrode. The insulator blocks have through holes to allow electrical connection to the electrodes. When the developed powder is wiped from the insulator surface, it leaves some charge behind. This charge buildup has to be neutralized by rolling a charge roller connected to ac voltage supply on the insulated surface. This step is necessary for continuing development on the surface. The first two experiments were done to identify the ideal ac voltage and the frequency to discharge the surface. Determination of Discharging Voltage The following experiment was done using the developer electrode to determine the ac voltage at which there is maximum discharging of the surface resulting in a neutral or

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93 close to neutral surface. Some initial tests were done to narrow down the voltage range for approximate estimation. These tests revealed that a range between 950V ac p-p and 1050V ac p-p is the effective surface neutralizing voltage range. The readings taken between this range is presented. The test is done by first charging the PET surface by rubbing a piece of paper on it (triboelectric charging) then discharging it by rolling over a charge roller supplied with ac voltage. The surface voltage of PET surface is measured by using electrostatic voltmeter, which is a non-contact voltmeter. The frequency of ac voltage was at 500Hz. Table 6-1. Experiment to determine the effective discharging and neutralizing ac voltage AC Voltage Surface voltage before discharge (V 1 ) Surface voltage after discharge (V 2 ) V 1 V 2 950 1540 207 1333 1206 274 932 960 526 170 356 778 230 548 970 591 -86 677 665 -60 725 980 782 -70 852 316 -86 402 990 671 -230 901 428 85 343 1000 916 310 606 660 273 387 1010 490 114 376 450 297 153 1020 535 70 465 486 -145 631 1030 409 112 297 548 22 526 1040 440 98 342 350 45 305 1050 290 168 122 550 166 384

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94 By observing the third and fourth column we see that the combination of discharging of the charged surface to near zero with a large difference between the initial voltage and the final voltage is seen for 970V and 980V. For experimental purposes 970V ac p-p is chosen, as it is smaller between the two values. Determination of Frequency of Discharging Voltage This experiment was conducted to determine the frequency of ac voltage at which the discharging of the charged insulator surface of the developer electrode is the maximum. The procedure of this experiment is similar to that explained in the last experiment, but the only difference here is that the voltage is kept constant at 970V ac p-p and the frequency of the voltage is varied. Table 6-2. Experiment to determine the frequency of the ac voltage for effective discharging Frequency (Hz) Surface Voltage before discharge (V 1 ) Surface voltage after discharge (V 2 ) Voltage difference (V 1 V 2 ) 500 648 186 462 602 282 320 1000 608 132 476 750 247 503 1500 440 346 94 835 276 559 2000 770 389 381 500 373 127 2500 973 173 800 952 116 836 3000 757 214 543 668 168 500 3500 540 245 295 804 251 553 Again for this experiment, looking at the third and the fourth column we observe that the combination of discharge of the surface to near zero with high voltage difference

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95 between the initial and the final voltage readings is for 2500Hz. So, the discharging ac voltage should have a frequency of 2500 Hz for effective discharge. From the above two experiments the discharging voltage that should be applied for discharging the photoconductor surface is 970V ac p-p at 2500 Hz. This value is an approximate experimental estimation and is used in other experiments for discharging developing surface. Variation of Powder Development with Voltage As stated before there are three methods of developing metal powder. This experiment attempts to find the relation between the amount of powder developed on the insulated developing electrode and the voltage applied to the developing electrode for development. In this experiment negative voltage is applied to the developing electrode, which induces positive charge on the iron powder. The positively charged powder gets developed on the PET surface. This charged powder when wiped clean from the surface, would leave positive charge on the PET surface. To discharge this we need a negatively biased ac voltage. The voltage for discharging/charging is taken to be V dc biased 970V ac p-p at 2500 Hz. The surface voltage of PET is measured after cleaning of the developed powder and after the discharging/charging process using electrostatic voltmeter. The electrode is weighed before and after development to find out the amount of powder developed. The experimental observations are presented below. As seen from the table the amount of powder developed has no correlation with the increase in voltage. The amount of powder developed seems to be concentrated around a particular value, which supports the theory of monolayer development of conductive powder. The surface voltage measured after discharging is the combination of applied

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96 voltage and the surface voltage as a result of discharging. The mass of powder developed is plotted with respect to the surface voltage using Matlab. Table 6-3. Experiment to determine the dependence of powder development on the development voltage applied to developing electrode DC voltage applied for development in V Surface voltage before discharging (V 1 ) in V Surface voltage after discharging (V 2 ) in V Weight before development (W 1 ) gm Weight after development (W 2 ) gm Weight of powder developed (W 2 W 1 ) gm 0 270 -302 61.810 61.861 0.051 -460 -440 61.861 61.916 0.055 -100 -600 -418 61.917 61.983 0.066 -433 -320 61.983 62.035 0.052 -200 -340 -395 62.035 62.087 0.052 -439 -523 62.087 62.146 0.059 -300 -370 -345 62.146 62.202 0.056 -256 -570 62.203 62.265 0.062 -400 -343 -407 62.264 62.314 0.050 -312 -659 62.315 62.370 0.055 -500 368 -205 61.171 61.217 0.046 52 -700 61.218 61.277 0.059 -600 -43 -703 61.278 61.331 0.053 17 -320 61.332 61.383 0.051 -700 -20 -162 61.383 61.436 0.053 69 -295 61.435 61.493 0.058 -800 125 -105 61.493 61.544 0.051 -86 -530 61.544 61.596 0.052 -900 102 -193 61.596 61.654 0.058 33 -188 61.653 61.706 0.053 -1000 203 -325 61.707 61.759 0.052 21 -340 61.759 61.813 0.054 The figure below shows that increase in voltage does not have any significant effect on the amount of powder developed. This phenomenon can be explained by considering the theory of monolayer development. The following theoretical analysis evaluates the mass of the developed powder considering a monolayer development. This uses the value of area of development for calculation.

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97 Figure 6-5. Plot of Variation of Mass of Powder Developed with respect to the Surface Voltage Available for Development Calculating Mass of Monolayer Development The mass of powder constituting a monolayer can be estimated if the area of development is known. The powder bed has a definite geometry because it is made by filling of a slot milled on an aluminum plate. The dimension of this slot is smaller than the dimension of the surface of the developing electrode, so that when there is a powder development, the image of the slot is printed on the PET surface due to uniform development. The mass of a monolayer forming this image is calculated by measuring dimension of the slot.

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98 24 mm4 mm Figure 6-6. Dimensions of the Milled Slot Area of the slot = Area of rectangle (20x4) + Area of circle (x4 2 /4) = 80 + 12.566 = 92.566 mm 2 Density of Iron = 7870 Kg/m Iron particle size = 60 microns diameter Volume of iron particle = 4/3 r 3 = 1.131 x 10 -13 m 3 Mass of a particle = Volume x Density = 8.9 x 10 -10 Kg Cross-sectional area of the sphere = x (30x10 -6 ) 2 m 2 = 2.827 x 10 -9 m 2 Let the powder be arranged on the surface in a configuration shown below. d Figure 6-7. Packing of Iron Powder Particles in a Monolayer Number of spheres in the hexagon = 6x(1/3) + 1 = 3

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99 Surface area occupied by the spheres in the square = 3 x x d 2 /4 Surface area of the hexagon = 2322162aaa From the geometry a = d Surface area occupied by spheres =3x x a 2 /4 Ratio of surface area occupied by the spheres to that of the square = 42 Number of particles in the image = 3636910827.210566.92422926 mm particles Mass of particles = 36369 8.9x10 -10 Kg = 3.237x10 -5 Kg = 0.0324 gm The experimental readings show mass of development is greater than this value, which can be explained by cohesive force of attraction between two iron particles, and the probability that some iron particles agglomerate and develop. The mass of dirt and dust can also add to the mass readings. Variation of Powder Developed with Development Gap The following experiment was done to determine the dependence of the developed powder on the development gap. The gap between the PET surface and the powder bed surface is varied by adding paper pieces between them (thickness 0.1mm). The experimental results are presented in the table below.

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100 Table 6-4. Experiment to determine the relation of powder development with gap between the two electrodes Gap (mm) Voltage before charging (V 1 ) Voltage after charging (V 2 ) Mass before development (M 1 ) gm Mass after development (M 2 ) gm Developed mass (M 2 M 1 ) gm No paper -601 -350 62.686 62.754 0.068 -384 -295 62.755 62.813 0.058 0.1 -439 -443 62.466 62.519 0.053 -682 -712 62.519 62.573 0.054 0.2 -606 -827 62.357 62.407 0.050 -705 -335 62.407 62.466 0.059 0.3 -732 -745 62.239 62.293 0.054 -712 -724 62.293 62.355 0.052 0.4 -737 -763 62.131 62.183 0.052 -686 -751 62.184 62.239 0.055 0.5 -621 -628 61.639 61.687 0.048 -499 -562 61.687 61.726 0.039 0.6 -555 -526 61.725 61.764 0.039 -618 -684 61.764 61.814 0.050 0.7 -386 -700 61.803 61.843 0.040 -651 -721 61.843 61.879 0.036 0.8 -665 -527 61.879 61.902 0.023 -681 -711 61.902 61.940 0.038 0.9 -652 -715 61.940 61.952 0.012 -741 -699 61.951 61.968 0.017 1.0 -703 -765 62.001 62.021 0.020 -700 -770 62.021 62.052 0.031 It can be seen clearly that with increase in the gap between the insulated electrode plate and powder level the amount of powder developed is decreased. This can be illustrated well in the following graph.

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101 Figure 6-8. Graph Showing Decrease in Amount of Developed Powder with Increase in the Gap between the Powder and the Insulated Electrode Powder Transfer Experiments After experiments regarding the development of powder, some simple experiments were done for transfer of powder. After development the developing electrode is placed on the transfer electrode for powder transfer. The transfer electrode is placed with its insulating PET cover facing upwards. The developing electrode is placed on its top with the PET cover facing downwards with the developed powder on the PET surface. There is an air gap between the two insulative surfaces. The arrangement is better explained using the schematic.

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102 Figure 6-9. Schematic for Test setup for Powder Transfer For development both the powder and the developing electrode are grounded. The insulative PET surface of the developing electrode is charged negative using a charge roller. The negative charge on the developing electrode charges the powder positive and the powder is developed onto the PET surface. The developing electrode with the developed powder is then placed with the PET surface facing down, on the PET surface of transfer electrode. Then negative voltage is applied to the transfer electrode (-5000V DC) to attract the positively charged powders. This process is repeated a number of times and the results are presented in Table 6-5. For mass measurements, the initial mass of a container is noted (M 1 ). After transfer of powder, the transferred powder on the transfer electrode is poured into the container and weighed (M 2 ). The differential mass (M 2 M 1 ) is the mass of the transferred powder. Then the remaining developed powder, which has not transferred to the transfer electrode, is poured in the container and weighed (M 3 ). Subtracting M 1 from M 3 gives the

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103 total powder developed during the development process. The percentage of powder transferred with respect to total developed powder is given by 100%1312MMMM Table 6-5. Measurement of efficiency of powder transfer from developed electrode to transferred electrode Index of attempt M 1 (gm) M 2 (gm) M 3 (gm) M 2 M 1 (gm) M 3 M 1 (gm) Efficiency % 1 62.888 62.912 62.926 0.024 0.038 63.16 2 62.928 62.956 62.976 0.028 0.048 58.33 3 62.976 63.004 63.022 0.028 0.046 60.86 4 63.022 63.039 63.062 0.017 0.040 42.50 5 63.061 63.089 63.101 0.028 0.040 70.00 6 63.101 63.120 63.136 0.019 0.035 54.59 Normally the percentage transfer of toner powder in commercial laser printing process is near 100%. There is a possibility that during powder transfer mostly the loosely held iron particles developed over the more strongly held powder monolayer got transferred, leaving behind the powder monolayer on the PET surface of the developer. The transfer process has to be improved as the percentage powder transferred is desired to be close to 100%. Powder transfer on printed toner layer To be able to print metal parts, each print of metal has to be alternated with a layer of insulative binding material. Printing of conductive iron powder with alternating layers of toner powder as binder can be considered as a possibility for building iron parts. The following experiment evaluates the possibility of iron powder getting transferred on a layer of toner. In this experiment two methods of transfer are examined; one is by providing voltage to the build platform and attracting the developed powder, the other one is by applying voltage to the developing electrode to repel the developed powder.

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104 There may be the issue of toner powder back developing on the developing electrode due to the field. As the toner is negatively charged and retains some of the charge even after fusing, the build platform has to be provided with positive voltage and the transfer plate with negative voltage in the first and second method respectively. This puts restriction on the charging of the iron powder and it can only be charged negative. This will cause less transfer because of the repulsive force from the negatively charged toner, which is undesirable but preferred than toner getting printed back on the transfer plate. First toner was printed and fused using laser printer based ESFF test bed. Iron powder is developed on the developing electrode by grounding the developing electrode and applying negative voltage (-2500V DC) to the powder. This development process is used for both the methods. For the first method, grounded developing electrode with powder developed on it is placed on the printed and fused toner layer. For the transfer to take place the build platform on which toner is printed is supplied positive voltage. In the second method the build platform is grounded and transfer plate is supplied with negative voltage to repel the negatively charged iron powder. The results for both the methods are presented below. To make sure there is no charge buildup on the developing electrode during development, the PET surface is discharged using discharging ac voltage 970V AC (p-p) at 2500Hz. For mass measurements, first the developer electrode is weighed without any powder on it (M 0 ). After development the developer electrode is weighed along with the developed powder (M 1 ). After transfer the developer electrode is weighed with the remaining developed powder that was not transferred (M 2 ). The developed powder mass

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105 is (M 1 M 0 ) and the transferred mass is (M 1 M 2 ). The percentage of transfer is then defined as 100%0121MMMM M 0 = 57.815 gm Table 6-6. Iron powder transfer on previously printed and fused toner layer by applying positive voltage to the build platform Voltage on platform in V Surface voltage before development -V Mass before print (M 1 ) in gm Mass after print (M 2 ) in gm Percentage transfer (%) 1000 -101 57.892 57.871 27.30 -160 57.881 57.870 16.67 2000 -174 57.893 57.872 26.90 -110 57.873 57.858 25.86 3000 -136 57.883 57.865 26.47 -175 57.880 57.863 26.15 Table 6-7. Iron powder transfer on previously printed and fused toner layer by applying negative voltage to the transfer plate Voltage on transfer plate Surface voltage before development -V Mass before print (M 1 ) Mass after print (M 2 ) Percentage transfer (%) -1000 -139 57.889 57.872 23.90 135 57.900 57.873 32.90 -2000 166 57.890 57.865 30.49 157 57.881 57.869 19.05 -3000 132 57.888 57.872 22.86 171 57.887 57.861 37.68 When compared to the percentage transfer observed in table 6-5 this is a low percentage of transfer. This can happen for many reasons; two of them are discussed below. In the experiment involving powder transfer between two electrodes insulated by PET (Table 6.5), the Van der Waals force between the PET sheet on the developing

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106 electrode and the developed iron powder was balanced due to the presence of PET on the transfer electrode surface. This is not the case when transfer is done on printed toner as iron powder particles may have more Van der Waals attraction towards PET sheet than the toner resulting in less powder transfer while printing on toner. The second reason for low transfer percentage may be because the iron powder is charged negative (to avoid toner from printing back onto the developing electrode surface) which may be repelled by the negative volume charge in the printed toner layer resulting in less powder transfer. After verifying the feasibility of developing iron powder using the simple PET covered electrode plates, the flat photoconductor plate test bed was built and iron was developed from upward facing roller developer to the downward facing photoconductor plate. Experiments were done to study the various methods of powder development and they were compared with respect to different variables. The results are discussed in Chapter 8.

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CHAPTER 7 POLYMER POWDER DEVELOPMENT AND PRINTING Although toner is a polymer and has been widely researched and perfected for charging, developing and printing, it is not the suitable material for structural components. The powder is brittle when printed in multiple layers and it retains charge after fusion as volume charge and repels any additional print. To avoid such problems alternative polymers had to be explored and tested for their suitability to be printed in layers to form three-dimensional structural components. Chapter 3 describes electrophotography which uses the popular magnetic insulative toner powder. Chapter 4 describes in brief the research done on printing toner powder in multiple layers to form three-dimensional objects and the problems associated with it. There are many other polymers available in the market and can be tested for their ability to print in multiple layers. Some of the powders used for testing are Nylon 6 PVA (Polyvinyl Alcohol) ABS (Acrylonitrile Butadiene Styrene) Nylon 12 PVC (Polyvinyl Chloride) In this chapter the results of the experiments done on these powders are discussed and analyzed. These powders are insulators and also do not have any special characteristics which would help them in getting developed. So, the results and conclusions present in this chapter may be applicable to any general insulator. 107

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108 Test of Powder Properties The physical properties those are essential for powder charging, development and printing are (Dutta 2002) Volume resistivity Permittivity Mass density Volume resistivity determines the most effective powder charging method. Permittivity of powder determines the electric field in the development zone. The packing fraction of the powder can be calculated from mass density if the density of the powder is known. Resistivity Test A resistivity test cell was designed and built to test the resistivity of powders (Dutta 2002). Figure 7-1 shows the schematic of the test cell with the components marked. Figure 7-1. Schematic of Resistivity Test Cell (Dutta 2002) The test cell electrode cross-section profile should have a particular shape called Rogowski profile (Cross, 1987). This is to avoid any fringe effects at the edges, which

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109 may affect the results. In the absence of Rogowski profile the effective area of the electrode surface would be greater given by the equation 222gBDAeff Where, D is the outside diameter of the circular electrode, g is the gap between the guarded electrode and the ring electrode and B is the effective area coefficient. B is typically taken to be zero for test cells with small gap between the electrodes. So, in the present case it is approximated as zero. The guard ring is grounded or connected to the low voltage electrode. This prevents any current flow between the electrodes through the surface because any such current generated is passed to the low electrode. The cell is made of Polyacrylate, which is a very good insulator. Volume resistivity is a material property and does not depend on the dimensions of the object. It is calculated as dAIVdRAeffeff Where V is the applied voltage, I is the measured current, is the surface area of the electrode and d is the distance between electrodes. effA Alternating polarity resistance test implemented in Keithley Electrometer is used for measuring volume resistivity. This uses the electrometer interface with PC and computes the resistivity by a weighted average of readings. First few readings are discarded as it takes time for the material to attain steady state. The steps to perform the experiment are described in the Appendix. The test is conducted with an alternating

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110 voltage of 50V and measurements with a time gap of 15 seconds. The first 3 readings are discarded and in total 8 readings are kept which the software uses to find the average. The observed readings are presented in table Table 7-1. Resistivity of polymer powders in cm Number of attempt Toner Powder Nylon 6 Polyvinyl Alcohol ABS Nylon-12 PVC 1 1.0281E15 1.3045E16 6.0241E12 1.2008E10 0.5294E16 1.3695E13 2 1.0146E15 1.7305E16 5.4006E12 1.9268E10 0.7902E16 1.4971E13 3 0.9131E15 2.1759E16 5.9668E12 2.7780E10 1.4709E16 1.6067E13 Mean ( cm) 0.9853E15 1.7369E16 5.7971E12 1.9685E10 0.9301E16 1.4911E13 The resistivity calculated is in ohm-cm. For reporting and calculating purposes they have to be converted to ohm-m. Table 7-2. Resistivity of polymer powders calculated above in m Toner Powder 0.9853 x 10 13 m Nylon 6 1.7369 x 10 14 m Polyvinyl Alcohol 5.7971 x 10 10 m ABS 1.9685 x 10 08 m Nylon 12 0.9301 x 10 14 m PVC 1.4911 x 10 11 m Permittivity Measurement The resistivity test cell was modified and used as capacitance cell. The schematic is shown below. As it can be seen in the figure, there is no guard ring. The electrometer was connected to the HI electrode. The electrometer was interfaced with PC for data collection and manipulation. The measurement process steps are presented in the Appendix.

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111 Figure 7-2. Schematic of Capacitance Test Cell (Dutta 2002) There are two set of experiments that have to be done for evaluating the permittivity of the material. The first one is done to measure resistance using software used for resistivity test, which has an option to measure resistance (see Appendix A). The results are presented in the table below. Table 7-3. Resistance values of the polymer powders Number of attempt Toner Powder Nylon 6 Polyvinyl Alcohol ABS Nylon-12 PVC 1 5.2455E10 6.1880E11 2.2460E11 7.1950E10 11.605E10 6.5505E11 2 5.3845E10 5.5132E11 2.9027E11 5.9442E10 9.5247E10 6.7850E11 3 5.0365E10 6.7544E11 2.0525E11 5.9013E10 2.6855E10 7.0465E11 Mean () 5.2221E10 6.1518E11 2.4004E11 6.3468E10 7.9384E10 6.7940E11 In the other test a step voltage is applied to the test cell for some time. Then the voltage is cut off and the capacitor is allowed to discharge. The characteristic exponential decay curve of an RC circuit was plotted by the test software. The time constant of the RC circuit is calculated from this graph. The user picks two points on the plotted graph

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112 and the software fits an exponential curve passing through the two points. The time constant of the exponential curve is displayed. The step voltage is set to auto and the instrument chooses the suitable voltage for the material. The time constants for the polymer powders are reported in the table below. No time constant has been reported for Nylon 6 and PVC because of the nature of the discharge curve that was not suitable to curve fit an exponential decay curve. Table 7-4. Time constants of the RC discharge curve Number of attempt Toner Powder Nylon 6 Polyvinyl Alcohol ABS Nylon-12 PVC 1 0.43102 1.1054 0.51776 0.75884 2 0.44076 1.0988 0.71383 0.74548 3 0.42942 1.1039 0.71317 0.79746 Mean (sec) 0.43373 1.1027 0.64825 0.76726 The value of the time constant is used to calculate the capacitance of the test cell using the relation RTCRCT Where, T = Time constant of exponentially decaying charge in RC circuit C = Capacitance of the test cell R = Resistance of the test cell The value of capacitance is then used to calculate the permittivity of the powder material by using the test cell dimensions. This is done using the following relation for parallel plate capacitors dACeffr 0 Where, C = capacitance of the test cell

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113 0 = permittivity of air r = relative permittivity of material A eff = Effective surface area of the electrode d = Gap between the two electrodes For circular electrodes, as in the present case, the relation changes to 202044DdCdDCrr Where, D is the diameter of the circular electrode. The values of relative permittivity for the polymer powders were estimated and reported in the table below Table 7-5 Permittivity of polymer powders Powders T (Sec) R (x10 10 ) RTC (picoFarad) 42DA (x10 -4 m 2 ) d 204DdCr Toner 0.4337 5.2221 8.305 5.06451 0.001905 3.529 Nylon 6 61.518 5.06451 0.001905 Polyvinyl Alcohol 1.1027 24.004 4.594 5.06451 0.001905 1.952 ABS 0.6482 6.3468 10.213 5.06451 0.001905 4.341 Nylon 12 0.7672 7.9384 9.644 5.06451 0.001905 4.098 PVC 67.940 5.06451 0.001905 Test of Development Characteristics of Polymers The five polymers were tested for the quality of their development and the visual observation was noted down. The qualitative assessment of powder development is an indicator of the behavior of the powder when they are placed under electrostatic field for development and transfer. This test also finds out if the powder is suitable to be developed by the cascade development method.

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114 Experimental Setup The experimental setup looked like the cross-sectional figure shown below. This is the cross-sectional image of one of the charge and mass measurement test setups. An elaborate discussion on the design of this setup is given in chapter 5. The same setup was used for this experiment by developing the polymer powder over the photoconductor drum. Figure 7-3. Cross-section of Polymer Powder Developer The powder is stored in the powder box. Powder flows to the bottom of the box by gravity and stays near the opening. This puts pressure on the developer roller and the friction between the developer roller surface and powder mass increases which helps in both triboelectric charging of the powder and in bringing the powder out of the box to the development region. The powder particles get caught in the porous surface of the developer roller and squeeze through the doctor blade for development. This squeezing action also adds to the charging of the powder by getting rubbed against both the

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115 developer roller and the doctor blade simultaneously. The doctor blade restricts the output of amount of powder to the development zone. The blade has a metal sheet in its front that is supplied with voltage to help in charging of powder and also to create field for development with the grounded photoconductor drum. The developer roller is not connected either to the voltage source or ground. The observations are listed below. Polyvinyl Alcohol When the setup was run without any voltage applied, there was no development. Then 500V DC voltage was applied to the blade and the test was run. The powder developed in thin layers and there were certain areas where the development was thicker than the neighboring regions. The voltage was increased in steps of 100V till 1500V, which resulted in increase in the development. Powder started dropping from particular spots where the thickness of developed layer was higher. On closer observation the size of powders dropping from these sites were much larger than the ones sticking to the drum. The voltage was switched off and the drum was cleaned. After placing back the photoconductor drum there was development of a layer of powder subsequently without the application of voltage. From the observation it can be concluded that the powder particles get charged by injection charging when voltage is applied to the blade, which then gets developed. There may be certain regions in the blade that allowed more powders to get through and there was high development. These regions also allowed large sized particles to get developed and fall off the developer surface due to high gravitational force compared to electrostatic force. The powder retains charged after the voltage is switched off, which results in development of a layer of powder after voltage was off.

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116 Nylon 12 There was no development without application of voltage. Voltage application started at 500V DC and went up to 2000V DC. Development was started in small patches which were the centers for gradual spread of powder and after some time covered the whole drum with a thick uniform layer of nylon powder. There was no leakage of powder and therefore there was no spillage and drop off of powder. When the voltage was turned off the surface of the photoconductor was cleaned off powders and most of it went back into the reservoir and there were stray drop offs of powder. It can be concluded that the charging of Nylon 12 is highly dependent on the voltage applied. This powder got discharged by contact with the grounded doctor blade after the voltage is turned off and did not remain developed on the photoconductor drum and so the drum got cleaned off powder. The powder has a small size that helps in charging the powder. It also helps the powder to remain on the photoconductor drum after development. The powder gets into the porous surface very easily and so there is no leakage during development. The start of development in patches can be attributed to the unevenness of the doctor blade or any charge concentration at those spots on the photoconductor drum due to external contacts. Nylon 6 There was no development without the application of voltage. Voltage was applied from 300V DC to 1800V DC (stopped due to sparking). There was no development on the photoconductor drum after voltage was applied. The increase in voltage had no effect on the development and the test had to be stopped at 1800V DC due to sparking observed in the first test run.

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117 Nylon 6 was not getting charged properly by the application of voltage and so there was no development. This powder may not a suitable choice for printing using electrophotography. ABS (Acrylonitrile Butadiene Styrene) The sample powder was a mixture of small size and large size particles. The large size powder particles drop off the photoconductor drum during development. The powder started developing on the drum without the application of voltage. It forms a thick coating on the photoconductor drum. Then positive voltage of 500V DC was applied and increased up to 1600V DC. Due to the application of positive voltage, the developed powder started to leave the drum surface into the powder box and the drum got gradually cleaned. The photoconductor drum was cleaned and reassembled in the test setup. Negative voltage starting from V DC to V DC was applied to the doctor blade and a thin layer of powder development was observed. It can be concluded from the above observation that, the powder was maybe previously charged and so developed on the drum surface without any application of voltage. This may also mean that this powder retains charge for a long time. Powder was cleaned off the drum by the application of positive voltage indicates that the powder was charged negative and was attracted by the positively charged blade and went into the powder box. The development of powder due to application of negative voltage confirms that the powder has a bias towards getting charged negatively. PVC (Polyvinyl Chloride) This powder has a large particle size and develops on the drum without any application of voltage, but due to large size falls off the drum surface. Positive voltage was applied to the blade starting from 500V DC to 4000V DC. The powder layer

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118 thickness on the developed region increased. Negative voltage was applied from V DC to V DC. The amount of powder coming out exceeds the amount of powder recirculated, which results in leakage. The powder development spreads all over the photoconductor drum. There is a smooth cascade flow of the powder through the developer roller. This powder is large in size and develops without any application of voltage, which makes this unsuitable for ESFF applications. Discussion The experiment was intended to identify the polymer that can be used for future developing and printing experiments. From the tests it can be seen that although the polymer powders are insulators, they behave differently in similar conditions. This experiment shows how important is the right selection of powder for different applications. From the result presented it is concluded that Nylon 12 is the most favorable powder to be used for ESFF applications. The characteristics of this powder that makes it suitable are highlighted below. The powder does not develop without the application of voltage The powder develops as a thick uniform coating on the photoconductor drum The size of the powder is small and it remains attached to the photoconductor surface without falling off There is no leakage of powder and so no spillage because the powder is small and can get into the pores of the developer roller surface very easily and transported without falling off When the voltage is switched off the powder gets cleaned off from the photoconductor drum and does not develop as the powder gets discharged by contact with the grounded doctor blade after the voltage is turned off.

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119 In chapter 5, the charge and mass measurements for polymers were done on nylon 12. In the flat photoconductor plate test bed the nylon 12 is used to test the suitability of using polymers in the flat photoconductor plate test bed configuration of ESFF. Experiments were conducted on the development of nylon 12 using the flat photoconductor plate test bed and study the variation of amount of powder development (powder mass) with respect to the voltage applied to the development roller and the doctor blade (development voltage). The results are presented in chapter 8.

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CHAPTER 8 FLAT PHOTOCONDUCTOR PLATE TEST BED The research work in the past on ESFF (Electrophotographic Solid Freeform Fabrication) was done on a test bed that used the modified laser printer to print toner powder on paper. The main objective of the past research works had been to determine the feasibility of printing parts in three dimension using electrophotography, which was satisfied by the test bed. Later in the research a need was felt to test powders other than toner powder for their ability to get printed in three dimension using electrophotography. Attempts have been made to replace the toner powder cartridge with powder developer designed to use with any powder. As the developer leaked powder, there was danger that it may damage the test bed by powder spillage. There was also attempt to print toner powder as a binder on a bed of other powder (Dutta 2002), which was not pursued due to the reason that this method also required uniform deposition of powders in thin uniform layers that requires powder developers to print the thin uniform layers. This again suffers with the problem of powder leakage on the test bed, which is not desirable. The laser printer is a complicated piece of equipment and there are so many functionalities for user interface applications that are not required for the printer to be used to print toner powder by electrophotography. It has taken a considerable amount of time and effort to modify the printer and make it suitable to be used in the test bed (Zhang 2001). The maintenance of the printer is also equally difficult. Due to the fixed lifetime of a given model of printer it has to be replaced with another printer, which would take a considerable amount of time and effort to understand and modify for the 120

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121 ESFF application, which is counter productive. So, this has to be replaced by some method by which there still can be electrophotographic printing without the additional work. By analyzing the printer components and the electrophototgraphy process it was inferred that only the laser imaging system has to be borrowed from the printer and the rest can be substituted by designing alternate mechanisms. It was also observed that in the printer the presence of a photoconductor drum mandates the presence of all the components that are a part of the electrophotography cycle in a circular fashion around the drum. The drum has to go through this cycle a number of times to produce an image, which can be generated in one step if a photoconductor plate is used. The use of photoconductor plate also spreads the components for development and printing, in a linear fashion, making the design simpler and easier to build. By using a linear distribution we can make a modular design in which the components can be grouped together in a logical way so that each group performs one of the functions in the electrophotography cycle. Each of these modules can be debugged and repaired separately in case of any problem, which makes the troubleshooting and maintenance of this system easy to perform. As mentioned in chapter 4, the laser imager system was extracted from the printer and it was tried to be controlled externally to produce images (Fay 2003). This chapter discusses the design and building of the rest of the test bed based on the movement of the flat photoconductor plate. The design includes the assembly of the charger, the imager and the developer assembly so that there is a uniform development of the powder on the charged image. The developer assembly consists of the developer roller, the doctor blade

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122 and the powder box. Some results related to the development of powder using this test bed have also been discussed. Electrophotography Cycle and Test Bed Design Concept As explained in chapter 3, the electrophotography cycle consists of the events which help in the development and printing of image-wise uniform layers of powders. This includes charging of the photoconductor surface, imaging the charged surface, developing powder onto the image-wise charged surface, printing the developed powder on the build platform, fusing and consolidating the printed powder layer, cleaning the photoconductor surface and discharging it for the next cycle. These steps are shown in the schematic below to illustrate the events during the linear movement of the photoconductor plate. Figure 8-1. Schematic of the Concept of Flat Photoconductor Plate Test Bed Assembly The individual components that can be used for the test bed are discussed below.

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123 Charging The different types of charging methods and their properties were explained in chapter 3. The corona charger is very bulky and difficult to assemble in the test bed setup. It requires the use of high voltage for better performance and there is a chance of the charger damaging the photoconductor surface by sparking. Due to these reasons a charge roller is used for charging the photoconductor surface. The charge roller is supported on either of its ends by spring loaded holders, which are fixed to a common side plate which also holds the developer assembly. When the photoconductor plate passes over the charge roller by rubbing against the roller surface to get charged, the springs get pressed and orient the axis of the roller parallel to the photoconductor surface for uniform charging. Imaging The laser imager in the printer is reused for imaging the photoconductor plate. The organic photoconductor surface is sensitive to ultraviolet light which can be supplied by the laser imager used in the laserjet4 printer. The controls of the imager were identified and modified for it to work independent of the printer (Fay 2003). In the test bed the opening for the laser imager is placed just after the charge roller and before the developer. The schematic below explains different parts of the laser imaging system and the control sensors and actuators.

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124 Mirror Polygonal Mirror on Stepper Motor Beam Detection Sensor Laser Diode 1 2 3 4 5 Feedback Motor Enable Switch 24V Supply Speed Control Ground Supply 123 5V Supply Sensor Feedback Ground Supply Ground Supply PD LD GND Laser Diode Photosensor Diode Figure 8-2. Schematic of Laser Imager (Fay 2003) The laser has to be turned on or off depending on the image and scan the image line by line using the existing rotating mirror system. The imager modification for independent operation is yet to be perfected so that it can replicate the images sent from computer. The laser imaging system is fixed facing upwards, so that it can discharge the downward facing photoconductor plate. Developing The requirement for the developer is to deliver the powder for upward development. The gravitational force acts in the opposite direction to the powder movement direction during powder development. This system also has to be independent of powder property. This is a challenging task as cascade development cannot work without gravity. The details of the design are explained later in the chapter.

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125 Printing There is no elaborate arrangement designed for the build platform in the test bed setup. Current setup only contains an insulator plate (sometimes replaced by conductor plate for insulator powders) on which the photoconductor plate prints powder by various powder transfer methods. More details on the printing method used were discussed in chapter 6. Fusing and Compacting The fusing and compacting system has not been designed and built, but in the illustration in figure 8-1, a conceptual arrangement is presented in which a flat plate heater is attached to the trailing edge of the photoconductor plate at a higher Z-level than the photoconductor surface. This flat plate heater can be borrowed from the old test bed configuration in which a flat plate contact heater was used to fuse and compact the toner powder. After printing the powder, the photoconductor moves ahead and then lowers itself so that the flat plate heater can come in contact with the printed powder surface and apply heat and pressure to fuse and compact the freshly printed powder layer. This gives green strength to the part for post processing operations. Cleaning and Discharging After fusion and compaction the photoconductor plate proceeds to the next station where the powder that was not printed on to the build platform is cleaned off the plate surface. The surface is then discharged by a charge roller supplied with ac voltage, which removes any charge left on the surface by the charged powders. The cleaning and discharging system is also in the stage of concept as shown in the illustration. The concept cleaning system consists of a box with a flexible polymer blade attached to it at an angle. To clean the photoconductor plate surface the plate is pressed lightly on the

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126 cleaner blade and passed over it so that the surface get rubbed against the blade and get cleaned. Along with the cleaning of the plate it is discharged when it passes over a charge roller present after the cleaner. Photoconductor Plate Motion The available motions on the test bed, horizontal linear motion the x direction and vertical linear motion in the z direction were used to move the photoconductor plate around the linear test bed setup. While the x direction motion helps in moving from one functional module to another, the z direction helps in positioning the plate surface with respect to the charge roller, developer roller, build platform and the cleaner and discharge roller. These two axes of motion were performed by servomotors, which were controlled by signals given from the computer commands through the Galil Motion controller. Photoconductor Plate It is important to discuss photoconductor plate assembly before going into the details about the other components of the test bed as it determines the design and distribution of rest of the test bed. As discussed in chapter 3, organic photoconductor and amorphous selenium are the two popular commercial materials available for photoconductivity applications. Organic photoconductor material is widely used in printers compared to amorphous selenium because they are cheap to produce and sensitive to a narrow spectrum of ultraviolet light reducing the chance of any discharge of the image due to ordinary light falling on it. This makes the design and production of organic photoconductors convenient and economical. Organic photoconductor has a very short dark decay time (time taken for decay of half of charge without exposure to light), which has to be taken into consideration while designing the test bed assembly. This is not much of a concern in the case of organic

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127 photoconductor drum as it will be spinning in high velocity giving enough time to develop and print. This is a problem when the whole photoconductor plate is charged, imaged and developed at the same time that requires a longer time comparable to the dark decay time. On other hand, amorphous selenium can retain charged image for a longer time, which makes it a suitable choice for the photoconductor plate. In addition to that the photoconductor surface made of amorphous selenium is more resistant to abrasions than the softer organic photoconductor, which makes it suitable material for the photoconductor plate to be used on various powders (including abrasive metals). It is the price of the amorphous selenium material which offsets all its favorable characteristics. Due to its high cost it is used in very limited applications like the xero-radiography and digital offset printing and has less availability than the organic photoconductor. This is the reason for using organic photoconductor material for making photoconductor plate. Moreover amorphous selenium is sensitive to a wide range of light wavelengths and requires a more complicated design of the test bed with enclosures protecting the photoconductor surface from ambient light. Another advantage of using the organic photoconductor plate is that the laser imaging system from the Laserjet4 printer is proven to be effective on organic photoconductor and can be used directly in the assembly. Due to the unavailability of a plate coated with organic photoconductor material suited for experimental applications, a flat photoconductor surface was created out of organic photoconductor belt taken from one of the laser printers. The belt assembly is shown below.

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128 Figure 8-3. Solid Model of Photoconductor Belt Assembly In the design for creating a flat photoconductor surface the available belt is wrapped around two rectangular insulator plates with smoothened edges. There are four screws at the corners of the plates, which are tightened to increase the gap between the plates, which in turn put tension in the belt and create a flat surface. The screws are attached to the top plate and push against the lower plate, which moves the lower plate outwards and stretches the belt. This flat surface approximately simulates the photoconductor plate. This photoconductor plate is attached to the moving platform through a cantilever beam. The belt is not as flat as a plate, but it is a good approximation for testing purposes. Design and Building of Charger-Imager-Developer Assembly The initial goal of building the flat photoconductor plate test bed was to determine the possibility of developing powder and printing in uniform layers. The next goal is to print an image using the laser imager. The final stage is to complete electrophotographic

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129 cycle by building the test bed setup with a design for the build platform and a cleaning and discharging assembly. The focus of this thesis is to build an assembly consisting of charge roller, the laser imager and the developer assembly, which can be used to develop uniform layers of powder. The discussion about powder properties, development and transfer theories with quantitative experimental results are presented in the next two chapters. This chapter is mainly concerned with discussion about the design of the charger-imager-developer assembly and some qualitative results from the experiments. Developer Design In the design of the test bed setup, the photoconductor plate is moved around with its photoconductive surface facing downwards. This requires the development of the powders against gravity from the developer placed below. For organic photoconductors, the dark decay time is very small and so the development has to be done immediately after the charging process. This requires that the developer powder bed should be placed adjacent to the imager so that the photoconductor plate passes over powder bed as it gets charged and imaged. This demands that a fresh uniform layer of powder should be available immediately after the laser imaging for the whole travel of the photoconductor plate. The developer discussed in last chapter is modified to build a developer to be used for flat photoconductor plate test bed configuration. As said earlier cascade development is required to develop powders with different properties. For cascade development to work the powder should be at a higher level than the developer roller as can be seen in the developer designs in chapter 5. In the present case the flat photoconductor plate needs vertical access to the developer roller surface and a clear path to travel during the

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130 development process. These requirements are met when the powder box is at a lower level than the horizontal tangential plane through the top of the developer roller. This does not have powder supply through gravity and the powder has to be pushed against the developer roller by mechanical force so that when the roller rotates the powder would come out sticking to the developer surface by friction. Developer Design The developer is divided into two parts, the nib assembly (to supply powder) and powder box assembly (to store powder). The nib assembly design is borrowed from the developer used for charge and mass measurement device (chapter 5) and modified to suit the requirements. The solid model and the cross sectional view of the developer assembly is shown as illustration. Figure 8-4. Solid Model of the Powder Developer Assembly

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131 Figure 8-5. Cross-sectional view of the Powder Developer Assembly The pivoting blade was removed and replaced with a bent brass sheet fixed to the powder box and extends into the powder-box opening to act as doctor blade and also charge the powder. Some top portion of the nib was removed to expose the developer roller above the surface of the nib top. This nib has flanges with screw holes provided to fix it to the flanges of the powder box. The nib attaches with the powder box with some packing of soft polymer sheet in between them that prevents any powder leak at the joint. The powder box is a rectangular box with an opening on one of its sides, where the nib is attached. Opposite to this, the powder box has a cap that can be opened to pour powder inside the box. After the powder is filled a rectangular plate is placed on the top of the powder level to act as a piston to push the powder out of the box and the box is closed by fixing the cap. This plate (acting like a piston) is called pressure plate as it is used to apply pressure on the powder to be pressed against the developer roller and transported to the development region by friction. The pressure plate is pushed by two screws using the screw threads on the cap. As the powder gets used up the powder pressure on the developer roller drops that can be brought back to the normal level by

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132 tightening the screws. The height of the box is lower than that of the developer roller to allow for the horizontal motion of the photoconductor plate during development. The developer roller is the same that was used in the charge and mass measurement test setup. It is conductive and has a high frictional porous surface that helps in bringing the powder out of the box for development. The powder box is made of transparent acrylic sheet, which helps in observing the level of powder and the piston position inside the powder box. The developer roller is driven by a gear mechanism attached to a stepper motor. Charger-Imager-Developer Assembly The solid model and cross-sectional view are shown in the following figures. Figure 8-6. Solid Model of Charger-Imager-Developer Assembly

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133 Figure 8-7. Cross-section of Charger-Imager-Developer Assembly The charger and developer are assembled together by side plates on either side with a gap in between to allow the ultraviolet laser to pass through. The charge roller is supported on both sides by spring-loaded holders. As mentioned earlier, these allow the roller to orient according to the photoconductor surface to charge the surface uniformly. The charge roller is assembled at a higher level than the developer roller because when the photoconductor plate passes over them it should touch the charge roller to get charged and at the same time have a small gap with the developer roller. A gap is required between the photoconductor plate and the developer roller as they move in opposite directions at the development region. This opposite direction of motion of photoconductor plate and the surface velocity of the developer roller is due to functional and geometric constraints. Any contact between them will wipe away the developed powder from the photoconductor surface. This gap should not be larger than a threshold, as the electric field for powder development decreases with increase in gap. The laser imager is assembled to the same frame where it used to be fixed to in the printer. The frame is modified to fit into the whole new test bed assembly. It has the correct angular orientation to shine laser in the vertical direction through the slot provided on the frame. The side plates containing the developer and the charger are

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134 assembled to this frame in such a way that all the components are centered and the gap between the charger and the developer roller is positioned over the opening for the laser and is suitable for imaging by laser. The image below shows the position of the photoconductor plate with charging imaging and developing happening at the same time. Note the relative positioning of the charge roller, imager opening and the developer roller. The other thing to note is the gap between the developer roller and the photoconductor surface while it is getting charged. Figure 8-8. Cross-sectional Image of Charger-Imager-Developer Assembly with Photoconductor Belt Assembly on top, Illustrating the Passing of Photoconductor Plate over the Charger, Imager and Developer Simultaneously during Development The four legs attached to the side plates of the laser imager frame by angle brackets give the whole assembly a steady support during development. The legs have set screws to adjust the height and orientation of the charger-imager-developer assembly. The whole assembly standing on the four adjustable legs is placed on a flat plate attached to the test bed frame. After the height and orientation of the assembly are adjusted it is fixed by screwing one of the legs to the base plate. A close up of the assembly on the test bed is

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135 shown in the following solid model to illustrate the relative position of the various components discussed above. Figure 8-9. Close-up of Solid Model of Test Bed Showing the Flat Photoconductor Plate Developer Assembly Experiments and Results A number of experiments were conducted on this test bed involving the development and printing issues of metal and polymer powders. The results are discussed in the following subsections. Some of the pictures taken of the developed and printed nylon powder are also presented. Metal Powder Development using Flat Photoconductor Plate Test Bed The powder was developed using the three methods of development (discussed in chapter 6) and for each method voltage applied for development was varied to note the change in developed powder mass due to this. The other parameters tested are the charge that flows through ground connection of one of the electrodes during development, which gives an approximate measure of charge on the total iron powder developed and the

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136 surface voltage of the photoconductor plate after development of powder. The respective tables and graphs are shown below. Table 8-1. Development of iron powder by providing voltage to the development roller and grounding the photoconductor plate (Data 1) Voltage Charge Q (nC) Surface Voltage M 1 M 2 M 2 M 1 (M) Q/M 500 7 92 8.772 8.775 0.003 2.333 1000 16 104 8.775 8.782 0.007 2.286 1500 95 280 8.782 8.800 0.018 5.278 2000 522 318 8.800 8.870 0.070 7.457 -2000 -397 -328 8.870 8.938 0.068 -5.838 -1500 -122 -264 8.938 8.983 0.045 -2.711 -1000 -20 -65 8.983 8.997 0.014 -1.429 -500 -8 -3 8.997 9.005 0.008 -1 Table 8-2. Development of iron powder by providing voltage to the photoconductor plate and grounding the developer roller (Data 2) Voltage Charge Q (nC) Surface Voltage M 1 M 2 M 2 M 1 (M) Q/M 500 5 430 9.005 9.009 0.004 1.25 1000 13 886 9.009 9.014 0.005 2.6 1500 86 1156 9.014 9.040 0.026 3.308 2000 298 1538 9.040 9.101 0.061 4.885 -2000 -322 -1736 9.101 9.169 0.068 -4.735 -1500 -92 -1270 9.169 9.193 0.024 -3.833 -1000 -12 -980 9.193 9.195 0.002 -6 -500 -4 -483 9.195 9.196 0.001 -4 Table 8-3. Development of iron powder by charging the photoconductor surface and grounding both photoconductor and developer roller (Data 3) Voltage Charge Q (nC) Surface Voltage M 1 M 2 M 2 M 1 (M) Q/M 500 5 243 9.199 9.201 0.002 2.5 1000 15 126 9.201 9.209 0.008 1.875 1500 23 485 9.209 9.223 0.014 1.643 2000 150 224 9.223 9.260 0.037 4.054 -2000 -70 -553 9.260 9.295 0.035 -2 -1500 -26 -457 9.295 9.312 0.017 -1.529 -1000 -15 -501 9.312 9.325 0.013 -1.154 -500 -2 -314 9.325 9.326 0.001 -2

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137 Figure 8-10. Charge Flow through the Ground during Development, where Data 1 is when Voltage is Provided to the Development Roller, Data 2 is when Voltage is Provided to Photoconductor Plate and Data 3 is when Photoconductor Plate is Charged by Charge Roller It can be seen from the graph in figure 8-10 that the case where voltage is applied to the developer roller the charge of total powder developed is marginally more than the case where voltage is applied to photoconductor plate. The case where the photoconductor plate surface is charged the charge of the developed powder is the least. This amount of charge is the charge of the collective developed powder, which may mean more powder got developed or powder with more charge got developed. Both of these give the same charge flow reading.

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138 Figure 8-11. Variation of Amount of Powder Developed with Change in Voltage, where Data 1 is when Voltage is Provided to the Development Roller, Data 2 is when Voltage is Provided to Photoconductor Plate and Data 3 is when Photoconductor Plate is Charged by Charge Roller The graph displayed in figure 8-11 shows that in the case of development when the voltage is applied to the charge roller the powder development is more compared to other methods. The next best development method is the case when voltage is applied to the photoconductor plate. The least productive method of development is when the photoconductor surface is charged. By these results it can be concluded that the most productive way of developing metal powder is to apply voltage to the developer roller and ground the photoconductor plate.

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139 Polymer Powder Development using Flat Photoconductor Plate Test Bed The powder development experiment is done using nylon 12 to observe the development pattern of nylon powder with change in voltage. The experiment is done for both positive and negative set of voltages. In each set of voltages the magnitude of voltage varies from 1000 to 5000 in steps of 1000. The results of these experiments are presented below. Table 8-4. Amount of powder developed with change of voltage in the positive range Development voltage (V) Mass before Development (M 1 ) gm Mass after Development (M 2 ) gm Mass of developed powder (M) gm 1000 8.5158 8.5167 0.0009 8.5167 8.5219 0.0052 2000 8.5219 8.5232 0.0013 8.5232 8.5251 0.0019 3000 8.5251 8.5363 0.0112 8.5363 8.5426 0.0063 4000 8.5426 8.5483 0.0057 8.5483 8.5565 0.0082 5000 8.5565 8.5635 0.0070 8.5635 8.5683 0.0048 Table 8-5. Amount of powder developed with change of voltage in the negative range Development voltage (V) Mass before Development (M 1 ) gm Mass after Development (M 2 ) gm Mass of developed powder (M) gm -1000 8.5683 8.5692 0.0009 8.5692 8.5712 0.0020 -2000 8.5841 8.5846 0.0005 8.5846 8.5851 0.0005 -3000 8.5851 8.5910 0.0059 8.5910 8.5943 0.0033 -4000 8.5943 8.5993 0.0050 8.5993 8.6043 0.0050 -5000 8.6043 8.6102 0.0059 8.6102 8.6156 0.0054 Table 8-4 shows the variation in the amount of nylon powder developed using flat photoconductor plate test bed setup, with the variation in applied positive voltage in the

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140 development zone. Two readings are reported for each voltage to decrease the chance of experimental error. The next table shows the results from similar experiment but using negative voltage. From these two set of experiments it is noted that the amount of powder developed does not depend on whether the voltage is positive or negative. Although it seems like the amount of development is marginally higher with positive voltage, it may be neglected due to the chance of influence of experimental errors. It can also be noticed that, it is only at 3000V (both positive and negative), that we see a significant increase in the amount of powder developed. It may be concluded from this observation that to develop nylon powder, a voltage greater than 3000V should be applied to the developer roller. Printing of Polymer using Flat Photoconductor Plate Test Bed The last section described the experimental results of polymer powder development with respect to its response to the change in voltage. In this section some sample prints of Nylon 12 are presented. The control of laser imager to produce image patterns on the photoconductor surface is beyond the scope of the presented thesis work. As it was necessary to prove the effectiveness of the designed test bed with the new developer and flat plate photoconductor, specially designed electrodes were used to develop powder from the developer roller. These electrodes were cut in the shape of different patterns and assembled on a common back plate to replicate the latent image of photoconductor due to imaging. The surfaces of the electrodes were made sure to be aligned along the same horizontal plane for uniform development due to equal development gap between all the electrodes and the developer roller.

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141 Figure 8-12. Electrodes Shaped in Different Patterns Development of Nylon 12 powder was done by providing voltage to the developing roller and grounding the patterned electrodes. Copper sheets are cut in pattern shapes and glued to the top surfaces of the patterns and grounded. These sheets are covered with PET sheets which are also cut to size according to the patterns. The PET cover is provided to avoid any sparking that may occur during the powder development process. Using result from the last experiments on Nylon 12 development characteristics, a voltage of 3000V was supplied to the developer roller for better powder development.

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142 Figure 8-13. Patterned Electrodes after Developed with Nylon 12 Powder The build platform was borrowed from the old test bed configuration, which is an aluminum plate supported by four springs at its corners. The platform adjusts itself, due to the springs, to the plane of the electrodes when the electrodes are pressed against it. A sponge is placed on the aluminum plate to help in finer adjustments of the surface orientation of the patterned transfer electrodes and the build platform electrode. A schematic of the build platform is presented below. Figure 8-14. Build Platform for Printing Nylon 12

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143 The printing is done on paper rather than PET on the build platform electrode is due to the fact that after each print the PET surface retains charge of the printed layer of powder which distorts the transfer of powder for printing purpose on the build platform. The pictures of intended assembly of patterns and actual printed patterns are shown below. Figure 8-15. Intended Assembly of Patterns Figure 8-16. Actual Printing of Assembly of Patterns using Nylon 12

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144 The difference in the intended print of assembly of patterns and the actual print can be due to various reasons. Some of the reasons are: The surface of the patterned transfer electrodes may not be at the same level during powder development. This may result in differential powder development due to the differential electric field experienced for each of the electrodes, as electric field decreases with increase in distance when voltage is given to be constant. There may be localized surface undulations which make the development non-uniform. These undulations also affect the transfer pattern due to the non-uniform gaps between the electrodes during transfer. The PET surface may retain some charge on its surface and affect the development and transfer patterns due to the influence of electric field produced by these charged areas. Powder may not come out uniformly from all the regions along the length of the developer roller, which may cause differential development. There may be the case of less powder coming out of the developer that is not enough to develop all the patterned transfer electrodes. Less charged powder may get developed due to high electrostatic field and may get transferred as a lump during printing on to the build platform. There may be the possibility of powder getting developed as agglomerated mass (due to cohesive forces) and during printing get transferred as a thick layer due to self weight. All these possibilities present for us a lot of variables to control and analyze. There need to be done a series of experiments to isolate the parameters which have major influence on the development and transfer of powder, which was not pursued due to the limitation in time and resources. These issues can be dealt in the future to get better prints. One of the major improvements was done to the print quality by replacing the PET sheet on the build platform copper electrode with a sheet of paper. This can be clearly visualized by comparing the print in figure 8-14 (printed on paper) with figure 8-17 (printed on PET).

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145 Figure 8-17. Print of Assembly of Patterns on PET Sheet Surface. Comparing the two figures we see that in case of printing on PET surface there is a lot of background printing and diffused printing. This may be due to the possibility that PET surface may retain some surface charge which have their own electric field lines. These may distort the field pattern during powder transfer and cause the developed powder to scatter and result in a diffused background print. This was avoided by the use of paper, which is not a good insulator and so cannot retain surface charge. The paper has enough insulating capability to avoid any spark during the close contact of the two electrodes for transfer of powder. The transfer of powder was done at 1500V for the paper and 5000V for PET. Printing on paper also improved the visibility of the print as it has a contrasting black color with the white colored nylon 12 powder.

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CHAPTER 9 CONCLUDING DISCUSSION AND FUTURE WORK Throughout this thesis there has been an attempt to understand the behavior of metal and polymer powders and their suitability to be printed as a structural material using Electrophotographic Solid Freeform Fabrication (ESFF). A series of experiments were done with five polymer powders, namely, Polyvinyl Alcohol, Nylon 6, Nylon 12, ABS and PVC. Their physical properties including volume resistivity, permittivity and mass density were found out by using a test cell. The qualitative nature of development of these powders was observed and it was concluded that Nylon 12 is the suitable polymer for ESFF test applications. Charge per unit mass (Q/M) and mass per unit area (M/A) are two important properties of materials which are essential for quality of development and print. The old charge and mass measurement test setup was modified and improved so that it can be used to test developers designed to develop any powder. The developers were improved by solving problems of powder leakage and better flow and charge control of the powders. In the charge and mass measurement setup there was a need of discharging the surface of the drum before it goes for another cycle, which was incorporated in the new design. In the new design the method of powder mass measurement was simplified by wiping the powder off the drum surface using a cleaning blade and collecting them in an attached cleaner box and then weighing the box. Tests were conducted on this setup to estimate the Q/M values of iron and nylon 12 powders. It was also concluded that the Q/M value of metals is directly proportional to the applied voltage and can be controlled 146

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147 by it. In contrast to conductive metal powders, it was found that the Q/M value of nylon 12 decreased with increase in voltage. This is attributed to large number of low charged powder particles getting developed at higher fields created by increase in voltage. The problems associated with the test bed design based on commercial laser printer using toner as the print medium were discussed and a need for a test bed independent of any commercial printer was realized. A conceptual design based on a flat photoconductor plate was proposed and discussed. A developer was designed and fabricated, which could develop powder vertically upwards. This developer was assembled with the charge roller and the laser imaging system to make a complete powder development system. Iron and nylon powders were developed and tested using this new test bed concept. The three methods to develop iron were compared and it was concluded that the method in which the developer roller is supplied voltage is the most suitable development technique. Although the method of charging the photoconductor drum is the only way to image iron powder, this method is mostly unreliable and it can only be used for charged area development. It was also concluded during the discussions that to make metal parts there has to be alternating layers of metal and insulator. This is due to the fact that metal cannot be transferred on to another metallic surface. Powder development in alternate layers solves the problem of imaging, as imaging can be done using the insulator powder layer. This also solves the problem of holding metal powders together during development, as the polymer powders can be fused to hold the metal powders together. The only configuration to develop polymer powders is by providing voltage to the charge roller and grounding the photoconductor drum. This is due to the fact that the

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148 polymer powders are insulator powders and they can only be charged triboelectrically and by injection charging. Triboelectric charging is material dependent and it is difficult to design a system which can effectively charge any polymer powder triboelectrically. The only option left for charging is by injection and for that the powder has to be in contact with a conductor connected to voltage source. This makes it necessary that the charge roller and the doctor blade should be connected to voltage source. The polymer powder can develop in both charge area development (CAD) and discharge area development (DAD). The development of polymer powder was tested and the results presented. Attempt was made to print Nylon 12 powder in patterns using patterned electrodes. The prints of patterns on paper and PET surfaces were shown. It was seen that print on paper is better than PET surface, as PET retains surface charge and distorts the print to create diffused background printing. There are other parameters which could be controlled to improve the print can be explored in future. By theoretical calculations on metal powder development using flat photoconductor plate test bed setup it was concluded that the minimum Q/M on a particle is independent of powder size and is equal to 3.27nC/gm for development in air. The electrostatic force available for development is found out to be more than the gravitational pull on an iron powder particle of 60 microns diameter. The maximum diameter that a particle can have to develop in air is 3.1mm. The four methods of transferring powder were discussed and it was found out that the most suitable method of transferring powder, which does not depend on the field created by potential difference across the transfer zone, is the method of repelling powder from the photoconductor plate. A method which uses a capacitor to store charge and then

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149 use this charge to repel the developed powders is proposed. The experimental analysis of this method is a very sensitive process and a new setup has to be designed to test this concept, which was not possible during the duration of research presented in this thesis. This can be done in the future and the effectiveness of the method can be verified. In the future, the imaging capability of the laser imager has to be improved to be able to create image of the cross-sectional figure of the solid model from PC. The print quality of nylon 12 printing can be improved by careful experimentation and identification of critical parameters controlling it. The rest of the flat photoconductor plate test bed, including the build platform and the cleaning and discharging assembly has to be designed and implemented. The method of fusing the powder has to be chosen and assembled into the test bed. As mentioned earlier, the concept of powder transfer for printing on build platform by repelling powder from the photoconductor surface by creating high electric field using a capacitor arrangement at the backside of the photoconductor plate has to be tested for feasibility using improved experimental setup to handle such sensitive experiment.

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APPENDIX MEASUREMENT WITH KEITHLEY ELECTROMETER Resistivity Measurement using Keithley 6517A Electrometer and Test-point Software The electrometer has to be set from the factory default values to user defined values of the parameters which are measurement cell specific. This is done by accessing the ohms configuration menu. This is done, by pressing CONFIG and then R. In the menu, scroll to the MEAS-TYPE option. This is used to select and configure the measurement type for the ohms function. RESISTANCE: Use this menu item to configure the ohms function to make normal resistance measurements. RESISTIVITY: Use this menu item to configure the ohms function to make surface or volume resistivity measurements. After selecting RESISTIVITY there will be two options SURFACE and VOLUME, for configuring parameters for surface and volume resistivity measurements respectively. In both the options there is MODEL-8009 which is used when using the model 8009 Resistivity Test Fixture, and USER option in which user can define his own text fixture. For SURFACE enter the value of Ks and for VOLUME enter the value of Kv. The values are defined below. Ks = P/g 150

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151 Where: P = The effective perimeter of the guarded electrode (mm) g = Distance between the electrodes (mm). 222gBDKv Where, D is the outside diameter of the circular electrode g is the gap between the guarded electrode and the ring electrode B is the effective area coefficient, which is found by comparing the readings from the designed test cell and the one that is already calibrated. B is typically near to zero for electrodes with relatively small gap, d. It is thus approximated as zero. For measuring volume resistivity thickness has to be specified. Use THICKNESS to specify (in mm) the thickness of the sample. Use EXIT to come out of the menu at all times. The figure below explains the connections and the cross section of the test cell. The voltage source is a part of Kiethley. The HI terminal of the electrode is connected to the top electrode and the HI terminal of the voltage source is connected to the bottom electrode. The low terminals of the voltage source and the electrometer are connected internally and one of the terminals is connected to the metal foil acting as the guard ring. Another metal foil is wrapped around the test cell to protect it from errors induced by external fields. The guard ring is connected to the terminals through this shield. The guard ring with the LO terminals are grounded.

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152 Figure A-1. Parallel Plate Powder Resistivity Test Cell Note: The interlock cable with the Model 8009 resistivity test cell MUST be connected to Kiethley to allow the voltage source to apply the voltage. Then open the 6517 Hi-R Test terminal. It will display a window as shown in the figure. From the menus select the autorange on and measurement type Volume (ohm-cm). Then press the run button to start the experiment. At the end of the experiment the window will display the resistivity value. We can also do surface resistivity and resistance measurements with this setup. The window display is shown in the figure below

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153 Figure A-2. Test Window for Resistivity Measurements. Permittivity Measurement Using Keithley 6517A Electrometer and Test-point Software For this test the metal foil acting as the guard ring is removed from the test cell. The circuit setup is shown in the figure below.

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154 Figure A-3. Parallel Plate Powder Capacitance Test Cell. The HI terminals of both electrometer and Voltage source are connected to top and bottom electrode respectively. The LO terminals are connected internally and one of the LO terminals is connected to the outer metal foil shield. The 6517 Hi-R Step Response window is opened. It looks like the figure below.

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155 Figure A-4. Hi-R Step Response Window Figure A-5. Step Response Settings Then the software is run which interfaces with Keithley and runs the test. During the test a step voltage is applied and the capacitor is allowed to discharge. The capacitor

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156 leakage current through the electrometer is used to plot the exponential graph. The two cursors which are at 0,0 are moved so that they have the exponentially decreasing curve between them. Then a best fit curve is plotted and the time period of this curve is displayed. This time period is equal to R multiplied by C. So, the capacitance is found by dividing the time period by the resistance. Resistance is measured by changing the wire connections and connecting them in the same way as it was done for resistivity measurement. Now we do not have the guard ring and the resistance option selected from Hi R test menu. The value of capacitance is then used to calculate the permittivity of the packed powder in the test cell. Step by Step Procedure for Performing the Tests: Common Steps for All Tests Connect the power cable to the main. Connect the cable with the terminals for measurement to the electrometer, the cable has three wires (red for HI, black for LO, and green for GND). Connect the wires acting as the terminals for the voltage source, (red for HI, black for LO). Connect the electrometer to the computer by connecting to the IEEE-488 bus and RS-232 interface. Connect the interlock cable provided with the Model 8009 Resistivity test fixture. Connect a terminal to ground Steps for Resistivity Test Connections Turn on the electrometer Fill powder inside the cavity on the top of the lower electrode of the test cell. Place a guard ring on the top of the powder in the interface between top and bottom electrodes.

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157 Slide the cylindrical metallic foil on the outside of the test cell to be in contact with the guard ring coming out of the cell through the gap between the top and bottom electrode. Connect one of the electrodes of the test cell to the HI of the electrometer and another to the HI of the voltage source. Connect one of the LO, either of the electrometer or the Voltage source, to the outer metallic shield. (The LO of the electrometer is internally connected to the LO of the voltage source.) Connect the ground terminal to the outer shield. Software Open the Hi-R Test window. Click on the Auto Range ON. Click on the measurement type Volume (ohm-cm), Surface (ohm/sq), and Resistance (ohm) For measuring resistivity the dimensions of the test cell has to be provided. This has to be done every time the Hi-R Test window is opened. This is done, by clicking the Geometries button. This will pop up a window, where entries about the surface area and the thickness can be provided. The test is started, by clicking on the run button. Other parameters that can be changed are measurement time, offset voltage, Alternating voltage, readings to store and readings to discard. Normally all these are provided with default value for each. Once the test run is complete, the value of resistivity or resistance is displayed on the main window accordingly. Steps for Permittivity Test Finding Time Constant The connections for this test are similar to that of the resistivity test except the following changes. The guard ring is removed while preparing the test cell with powder. The ground wire terminal is removed.

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158 To use the software for the experiment we use the following steps Open the Hi-R Step Response Test window. Click on 6517 settings to set the values of Measure time and Step Voltage. It is initially provided with the default Values. Click on the Auto Range ON. Click Run to start the test. After the test is complete, a discharge graph is displayed. Move the cursors on the graph and position them suitably on the graph. The cursor controllers are on the left side of the main window. Then an exponential decay curve is fitted on the graph by clicking on the Best Fit button. If the exponential curve does not cover the range of reading (visual inspection), then move the cursors around to get a better fit. The window below the cursor controllers displays the Time Constant of the exponential curve. Finding Resistance The resistance of the test cell is found out by going through the steps of the resistance measurement as explained under resistivity measurement steps. This is done without connecting the guard ring, but connecting the ground terminal. The only difference from the Step Response Test is that the ground terminal is clipped on the outer shield. Finding Permittivity Calculate capacitance by using formula C = T/R Calculate relative permittivity by using the Value of Capacitance and the Geometry of the test cell.

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LIST OF REFERENCES Bhaskarapanditha, Sivakumar V., Solid Slicing and Electrostatic Analysis for Electrophotographic Rapid Prototyping Machine. MS thesis, University of Florida, Gainesville, Florida, 2003. Cross, J. A., Electrostatics Principles, Problems and Applications. Adam Hilger, Bristol, 1987 Diamond, Arthur S., Handbook of Imaging Materials. Marcel Dekker, New York, New York, 1991. Dutta, Anirban, Study and Enhancement of Electrophotographic Solid Freeform Fabrication. MS thesis, University of Florida, Gainesville, Florida, 2002. Fay, James, Electrostatic Analysis of and Improvements to Electrophotographic Solid Freeform Fabrication. MS thesis, University of Florida, Gainesville, Florida, 2003. Gokhale, Samit, Study and Implementation of Electrophotographic Solid Freeform Fabrication and Charge Measurement Apparatus. MS thesis, University of Florida, Gainesville, Florida, 2001. Hewlett-Packard, LaserJet 4 Printer Manual. Boise, Idaho, 1996 Hirakawa, H., and Y. Murata, Mechanism of Contact Charging Photoconductor and Insulator with DC-biased Conductive Roller. IEEE, 1995. Kasper, G. P., and J. W. May, Conductive Magnetic Brush Development, US Patent 4076847, 1978. Keithley, 6517A Electrometer Manual. Cleveland, Ohio, 1999 Kochan, D., Solid Freeform Manufacturing: Advanced Rapid Prototyping. Elsevier, Amsterdam, New York, 1993. Kumar, A. V., Solid Freeform Fabrication Using Powder Deposition, US Patent 6066285, 2000. Nelson, K, Injection Charging of Toner, US Patent 4121931, 1978. Schaffert, R. M., Electrophotography. Focal Press, New York, New York, 1975. 159

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160 Schein, L. B., Electrophotography and Development Physics. Springer-Verlag, New York, New York, 1988. Zhang, Hongxin, Design and Implementation of a Testbed for Electrophotographic Solid Freeform Fabrication. MS thesis, University of Florida, Gainesville, Florida, 2001.

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BIOGRAPHICAL SKETCH The author was born in March 1980, in Bhubaneswar, India. In 2001, he graduated with a Bachelor of Technology degree in mechanical engineering from Indian Institute of Technology, Madras, India. He entered the Master of Science program in mechanical engineering at University of Florida in Fall, 2001. 161


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Permanent Link: http://ufdc.ufl.edu/UFE0005385/00001

Material Information

Title: An Investigation on the printing of metal and polymer powders using electrophotographic solid freeform fabrication
Physical Description: Mixed Material
Language: English
Creator: Das, Ajay Kumar ( Dissertant )
Kumar, Ashok V. ( Thesis advisor )
Schueller, John K. ( Reviewer )
Arakere, Nagaraj ( Reviewer )
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2004
Copyright Date: 2004

Subjects

Subjects / Keywords: Mechanical and Aerospace Engineering thesis, M.S
Dissertations, Academic -- UF -- Mechanical and Aerospace Engineering

Notes

Abstract: Electrophotographic solid freeform fabrication (ESFF) is a novel method of manufacturing which is under development at the Design and Rapid Prototyping Laboratory of the Department of Mechanical and Aerospace Engineering at University of Florida. Electrophotographic solid freeform fabrication uses the principle used in laser printer to print layers of material, one over the other, to form the final three dimensional objects. In past experiments a test bed was built to test the concept of printing in layers using toner powder. Toner powder was successfully shown to be printed in layers. The material used as the toner powder is not suitable as a structural or functional material, and there was a need to explore the use of other materials including metal and polymer powders as structural material in layer by layer printing using ESFF. Powder developers were designed to develop thin uniform layers of metal or polymer powders on the photoconductor surface. A Charge and mass measurement test setup was built to evaluate these developers on the basis of their ability to charge the powders and the amount of powder developed. The powder properties and development and transfer characteristics were studied for metal and polymer powders. A simpler version of the test bed to develop and print metal and polymer powders was proposed. This new concept was less dependent on available commercial laser printing technology and was based on development of powder on a flat photoconductor plate. This concept made the design modular for easy setup and debugging purposes. A developer was designed to print upwards against gravity on a photoconductor plate (facing downwards). This developer was assembled with the charge roller and the laser imager assembly. A flexible build platform was assembled which could align itself to the photoconductor plate orientation for effective transfer of powder layers. The developer was tested for its efficiency in charging and developing powders. The developed powder was then transferred on the build platform to test the ability of the test setup to print powders in layers.
Subject: cascade, charge, development, electrohotography, fabrication, freeform, gravity, iron, layered, manufacturing, mass, measurement, metal, nylon, permittivity, photoconductor, plate, polymer, powder, printing, prototyping, rapid, resistivity, solid
General Note: Title from title page of source document.
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Thesis: Thesis (M.S.)--University of Florida, 2004.
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Material Information

Title: An Investigation on the printing of metal and polymer powders using electrophotographic solid freeform fabrication
Physical Description: Mixed Material
Language: English
Creator: Das, Ajay Kumar ( Dissertant )
Kumar, Ashok V. ( Thesis advisor )
Schueller, John K. ( Reviewer )
Arakere, Nagaraj ( Reviewer )
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2004
Copyright Date: 2004

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Subjects / Keywords: Mechanical and Aerospace Engineering thesis, M.S
Dissertations, Academic -- UF -- Mechanical and Aerospace Engineering

Notes

Abstract: Electrophotographic solid freeform fabrication (ESFF) is a novel method of manufacturing which is under development at the Design and Rapid Prototyping Laboratory of the Department of Mechanical and Aerospace Engineering at University of Florida. Electrophotographic solid freeform fabrication uses the principle used in laser printer to print layers of material, one over the other, to form the final three dimensional objects. In past experiments a test bed was built to test the concept of printing in layers using toner powder. Toner powder was successfully shown to be printed in layers. The material used as the toner powder is not suitable as a structural or functional material, and there was a need to explore the use of other materials including metal and polymer powders as structural material in layer by layer printing using ESFF. Powder developers were designed to develop thin uniform layers of metal or polymer powders on the photoconductor surface. A Charge and mass measurement test setup was built to evaluate these developers on the basis of their ability to charge the powders and the amount of powder developed. The powder properties and development and transfer characteristics were studied for metal and polymer powders. A simpler version of the test bed to develop and print metal and polymer powders was proposed. This new concept was less dependent on available commercial laser printing technology and was based on development of powder on a flat photoconductor plate. This concept made the design modular for easy setup and debugging purposes. A developer was designed to print upwards against gravity on a photoconductor plate (facing downwards). This developer was assembled with the charge roller and the laser imager assembly. A flexible build platform was assembled which could align itself to the photoconductor plate orientation for effective transfer of powder layers. The developer was tested for its efficiency in charging and developing powders. The developed powder was then transferred on the build platform to test the ability of the test setup to print powders in layers.
Subject: cascade, charge, development, electrohotography, fabrication, freeform, gravity, iron, layered, manufacturing, mass, measurement, metal, nylon, permittivity, photoconductor, plate, polymer, powder, printing, prototyping, rapid, resistivity, solid
General Note: Title from title page of source document.
General Note: Document formatted into pages; contains 177 pages.
General Note: Includes vita.
Thesis: Thesis (M.S.)--University of Florida, 2004.
Bibliography: Includes bibliographical references.
General Note: Text (Electronic thesis) in PDF format.

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 003107642
System ID: UFE0005385:00001


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AN INVESTIGATION ON THE PRINTING OF METAL AND POLYMER POWDERS
USING ELECTROPHOTOGRAPHIC SOLID FREEFORM FABRICATION















By

AJAY KUMAR DAS


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2004

































Copyright 2004

by

Ajay Kumar Das

































Dedicated to my mother.















ACKNOWLEDGMENTS

I extend my sincere gratitude to my advisor and chairman of the thesis committee,

Dr. Ashok V. Kumar for his guidance and support during the research work which made

this thesis possible. I would also like to thank the thesis committee members Dr. John K.

Schueller and Dr. Nagaraj Arakere for their patience in reviewing the thesis and for their

valuable advice during the research work. I thank the Design and Rapid Prototyping

Laboratory co-workers for being helpful, supportive and making the research a

pleasurable experience. Last but not least, I would like to thank my parents for their love

and encouragement during my studies abroad.
















TABLE OF CONTENTS
Page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES .................... .. ............. ............................... ix

LIST OF FIGURES ......... ......................... ...... ........ ............ xi

A B S T R A C T .......................................... ..................................................x v

CHAPTER

1 IN TR OD U CTION ............................................... .. ......................... ..

Electrophotographic Solid Freeform Fabrication (ESFF) ...........................................1
Past Research and Motivation for the Present Work......................................2
C chapter L ay out ................................................ .......................... 3

2 RA PID PR O TO TYPIN G ............................................................................. ........ 6

Stereolithography A pparatus (SLA )..................................... ..................................... 8
Solid G round C during (SG C ) ............................................................................. 9
Selective L aser Sintering (SL S)............................................ ........... ............... 10
Fused D position M odeling (FD M ) .................................. ............... ...................11
Laminated Object Manufacturing (LOM) ......................... .................... 12
3-D Printing ............... ......... ...... ....... ........ ............. ............ 14

3 ELECTROPHOTOGRAPHY .................... ........................ 16

The Electrophotographic Process ........................................ .......................... 16
Photoconductor M material ................................................. ............................... 18
D a rk D e c ay ...........................................................................................1 8
Charge A acceptance ........... ................... .............................. .... 19
Im age Form ation Tim e ............... ............................ ...............................19
Im a g e S tab ility ............................................................................................... 1 9
R esidual Im age ................................................................................... .......... 19
M material Selection.............................................. 20
C charging ................................................................... 2 1
C orona C harger ................................................. ............... 2 1
C charge R oller ...................................... ................... ......... 23
Im aging .................... .......................... ............. ....... .... ... ........ 23


v









Development............................. ..... .......... 24
C ascade D evelopm ent ........................................................ ............... 25
Magnetic Brush Development................................... ............... 26
Conductive Magnetic Brush Development ...............................................27
Mono-component Development.............. ..... ...............28
Charged and Discharged Area Development.....................................28
Toner Pow der Charging ........................................ .......................... 30
T ra n sfe r ................................. ..................................................3 2
F u sin g ................................................................3 3
C le a n in g ...............................3 3............................

4 ELECTROPHOTOGRAPHIC SOLID FREEFORM FABRICATION (ESFF) ........34

Development of ESFF Test-bed System .................................. ............................. 35
M otion C control Sy stem ................................................................................. 36
P rin tin g .................................................................3 7
F u sin g .................................................................3 7
Softw are ................ ................ ...................................................... ........38
Measurement of Charge and Mass of Powder .......................... ...........39
Measurement of Powder Properties ..................................................................40
Im provem ent of Print Q quality .................................................................. 40
Limitation on Part Height ........................................................... .............40
Edge Growth (Solid Area Development) ................. ................. ............42
Printing of Powders other than Toner .................................................. 43
Study of Laser Imager System of Printer ...........................................................45

5 DESIGN AND TESTING OF IMAGE DEVELOPERS ................. ............... 47

D ev elo p er D e sig n ................................................................................................. 4 8
P o w d e r B o x ................................................................................................... 5 0
D ev elo p er R o ller ........................................................................................... 5 0
Developer Roller Casing ................. ............. ...................51
Pivoting Blade Pow der D developer ..................................... .................... .53
Development of Charge and Mass Measurement Test Setup ...............................56
Discussion on Development of First Test Setup and Testing Concepts ...........56
Improve ent in the Design of Test Setup ..........................................................59
Independent Charge and Mass Measurement Test Setup .............. ............. 63
D design Considerations ................. ............ .............................. ............... 64
Stages of Charge and Mass Measurement Test Cycle.........................................67
Design and Building of the Charge and Mass Measurement Test Setup ............68
Transfer D rum A ssem bly ...................................................................... 68
C leaner B ox A ssem bly ......................................................... ...............70
M otor and Stand A ssem bly ................................................................... 71
E xperim mental R results ................................ ................................. ............... 73
Experim ents with Iron Powder...................................................................... 73
Experiments with Nylon 12 Powder....................................................7... 5









6 METAL POWDER DEVELOPMENT AND PRINTING ........................................77

M etal P ow der D evelopm ent ............................................................ .....................78
M etal D evelopm ent T theory ......................................................................... ...............80
Constant Parameters used in Calculations...................... .. ............... 80
Charge per Unit Mass (Q/M) Calculations...................................................81
M maximum Surface Charge Density .......................................... ............... 83
Electric Field Range ....................................... ...... .... ..............84
Calculation of Forces Involved During Development.............................85
Calculation of Maximum Powder Particle Radius for Development.................. 86
M etal Powder Transfer Process Theory ........................................ ............... 87
Pow der Transfer M ethods .................... .... .................. ............... ....88
Capacitor Method of Powder Transfer (Conceptual)........................................89
Experim mental R results .................................... .............. ...... .. .. ........ .... 91
Determination of Discharging Voltage .....................................................92
Determination of Frequency of Discharging Voltage .............. ...................94
Variation of Powder Development with Voltage ............ .............. 95
Calculating Mass of Monolayer Development ..............................................97
Variation of Powder Developed with Development Gap............................... 99
Pow der Transfer E xperim ents .......................................................................... 101
Pow der transfer on printed toner layer .............................................................103

7 POLYMER POWDER DEVELOPMENT AND PRINTING ........... .....................107

T est of P ow der P properties .............................................................. ..................... 108
Resistivity Test .................................... .......................... ... ....... 108
Perm ittivity M easurem ent ......................................................... ............ ...... 110
Test of Development Characteristics of Polymers .................................................. 113
E x p erim ental Setu p ................................................................ ..................... 114
Polyvinyl A alcohol .................. .......................... .... .... ............... ... 115
N ylon 12 .................................... ................................ ........116
N ylon 6 ..................................... ................................ .........116
ABS (Acrylonitrile Butadiene Styrene)............... ................... ....................117
PVC (Polyvinyl Chloride) ............................................................. ............ 117
D iscu ssio n ............................................................. ................... 1 18

8 FLAT PHOTOCONDUCTOR PLATE TEST BED.................. ...................120

Electrophotography Cycle and Test Bed Design Concept ......................................122
Charging ..................................... ................................ ......... 123
Im aging ..................................... ................................ ......... 123
D ev elo p in g .................................................................... 12 4
P rin tin g ..............................................................................................................1 2 5
Fusing and C om acting ......................................................... ............... 125
Cleaning and D ischarging ...........................................................................125
Photoconductor Plate M otion ................................................................ ...... 126
Photoconductor Plate .................. .......................... .. ....... ................. 126









Design and Building of Charger-Imager-Developer Assembly .............................128
D developer D esign .................. .............................. ...... .. .. ........ .... 129
D developer D esign ................................................. ...... .. .. ........ .... 130
Charger-Imager-Developer Assembly ............. .......................... .................132
Experim ents and Results....... .......................... ....... ..... ..... .. ................. 135
Metal Powder Development using Flat Photoconductor Plate Test Bed ..........135
Polymer Powder Development using Flat Photoconductor Plate Test Bed ......139
Printing of Polymer using Flat Photoconductor Plate Test Bed........................140

9 CONCLUDING DISCUSSION AND FUTURE WORK ........................................146

APPENDIX

MEASUREMENT WITH KEITHLEY ELECTROMETER ............... ................150

Resistivity Measurement using Keithley 6517A Electrometer and Test-point
S oftw are .............. ..... ..... .............. ....................................... ............. 150
Permittivity Measurement Using Keithley 6517A Electrometer and Test-point
Software ..................... ........ ............... .......... .......... 153
Step by Step Procedure for Performing the Tests: ......................................... 156
Com m on Steps for A ll Tests ........................................ ......... ............... 156
Steps for Resistivity Test ........... .. ......... .... ............... 156
C connections ........... ......... ......... ............ ............... ............ 156
Softw are ......................................................... 157
Steps for Perm ittivity Test........................................... .......................... 157
Finding Tim e C onstant........................................ ........................... 157
Finding R resistance ....................................... .. .............. ...............158
Finding Perm ittivity ......................... ...... ................ ........ .. ........ .. 158

LIST OF REFEREN CES ........................................................... .. ............... 159

BIOGRAPHICAL SKETCH ............................................................. ............... 161
















LIST OF TABLES


Table page

5-1 Q/M and M/A calculations of toner using QMM test setup.................................61

5-2 Variation of the Q/M readings with the number of revolutions of the
transfer roller ........... ..... .............................. ....... ................... 73

5-3 Variation of Q/M measurement with the increase in voltage supplied for
development while the number of revolutions of the transfer drum remains
constant ......................... ................................... .......... ..... 74

5-4 Variation of the Q/M readings with the number of revolutions of the
transfer roller ........... ..... .............................. .......... ................ 75

5-5 Variation of Q/M measurement with the increase in voltage supplied for
development while the number of revolutions of the transfer drum remains
constant ......................... ................................... .......... ..... 76

6-1 Experiment to determine the effective discharging and neutralizing ac
v o ltag e ............................................................................... 9 3

6-2 Experiment to determine the frequency of the ac voltage for effective
d isc h a rg in g ...............................................................................................................9 4

6-3 Experiment to determine the dependence of powder development on the
development voltage applied to developing electrode .......................................... 96

6-4 Experiment to determine the relation of powder development with gap between
the tw o electrodes ......... ...... .. ..... .............. .. ........ .. .. ............ 100

6-5 Measurement of efficiency of powder transfer from developed electrode
to transferred electrode............ ... .... .......... ................... ............... 103

6-6 Iron powder transfer on previously printed and fused toner layer by
applying positive voltage to the build platform ........................................... 105

6-7 Iron powder transfer on previously printed and fused toner layer by
applying negative voltage to the transfer plate..................................................105

7-1 Resistivity of polymer powders in Q cm ................. ....................... .......... 110









7-2 Resistivity of polymer powders calculated above in Q m ..................................110

7-3 Resistance values of the polymer powders ........... .. ....... ........................ ...111

7-4 Time constants of the RC discharge curve....... ............................................112

7-5 Permittivity of polym er powders................................ ......................... ....... 113

8-1 Development of iron powder by providing voltage to the development
roller and grounding the photoconductor plate (Data 1).....................................136

8-2 Development of iron powder by providing voltage to the photoconductor
plate and grounding the developer roller (Data 2) .............................................136

8-3 Development of iron powder by charging the photoconductor surface and
grounding both photoconductor and developer roller (Data 3)............................136

8-4 Amount of powder developed with change of voltage in the positive
ra n g e .......................................................................................... 1 3 9

8-5 Amount of powder developed with change of voltage in the negative
ra n g e .......................................................................................... 1 3 9
















LIST OF FIGURES


Figure p

2.1 Flowchart of the Rapid Prototyping Process...........................................................7

2.2 Schematic of Stereolithography Apparatus (SLA)................. ............................8

2.3 Schematic of Selective Laser Sintering (SLS) .....................................................11

2.4 Schematic Representation of Fused Deposition Modeling (FDM)....................... 12

2.5 Schematic Representation of Laminated Object Manufacturing (LOM)................. 13

2.6 Schematic Representation of 3D Printing Process ......... ...................................15

3-1 Schematic of the Electrophotography Print Cycle .................................................17

3-2 Schem atic of Corotron Charger........................................ ............................ 22

3-3 Schem atic of Scorotron Charger ........................................ ......................... 22

3-4 Image Formation in Organic Photoconductor Drum by UV Laser..........................24

3-5 Schematic Representation of Developer System..................... ...............25

3-6 Schematic of Cascade Development................................................... ............... 26

3-7 Magnetic Brush Development System........................................... ...............27

3-8 Charged Area Development (CAD)...................................................................... 29

3-9 Discharged Area Development (DAD) ........................................ ............... 30

3-10 Schematic of Transfer of Toner to the Paper ................................. ............... 32

4-1 T he E SFF T est-bed............. ............................................................. ..... .... ..... 36

4-2 Parts Printed using Corona Charging of the Top Printed Layer before the
N ext P rint .....................................................................................................4 1

4-3 Parts Printed Using Patterns ........................... .....................................43









4-4 Toner Powder Printed over Insulating Alumina Powder Bed ...............................44

4-5 Diagram of Laser Jet 4 Imager Assembly ............ .................. .... ...........45

5-1 Solid M odel Assembly of Powder Developer.........................................................49

5-2 Front End of the Toner Powder Cartridge............... ............................................ 52

5-3 Cross-section of Developer Assembled with the Front End ...................................52

5-4 Solid Model of Developer Assembly for Pivoting Doctor Blade Powder Developer
(w ith Cross-sectional V iew ) ......................................................... ............. 54

5-5 Cross-sectional View of the Developer Assembly............................................55

5-6 Cross-section of Developer Front End Assembly with the Pivoted Doctor
B la d e .......................................................................... 5 5

5-7 Schematic of Charge and Mass Measurement Test Setup ....................................56

5-8 Charge Measurement Setup for Direct Charge Measurement ..............................57

5-9 Solid Model of the Assembly of Toner Powder Developer and Charge
and Mass Measurement Test Setup............................... ........................59

5-10 Cross-section View of Charge and Mass Measurement Test Setup......................60

5-11 Solid Model of Assembly of Developer and Charge and Mass
M easurem ent Test Setup ................................................ .............................. 62

5-12 Cross-section of Charge and Mass Measurement Test Setup with
Improved Developer Assembly.....................................................63

5-13 Schematic Illustration of Behavior of Conductive Powder Particle in
the Presence of Conductive and Insulative Electrode Surfaces. ...........................65

5-14 Different Stages of the Test Cycle with respect to the Cross-sectional
view of the Charge and Mass Measurement Test Setup. .............. .....................67

5-15 Solid Model of Transfer Drum and Cleaner Box Assembly (with
cross-sectional view ) ......... ..... .................. ..... ........ .... .......... ........ .... 69

5-16 Front view and Side Cross-sectional view of the Transfer Drum Assembly
and the Cleaner B ox A ssem bly. ......................................................................... 71

5-17 Solid M odel of M otor and Stand Assembly........................................ .................72

5-18 Complete Assembly of Powder Developer with Charge and Mass Measurement
T e st S etu p ........................................................................................7 2









6-1 Concept of the New Modified Test Bed .............. ............... ........... 78

6-2 Forces Acting on a Powder Particle During Development ....................................81

6-3 Schematic Model of the Capacitor Method of Powder Transfer
(C o n cep tu al) ....................................................... ................ 9 0

6-4 Schematic of the Test Setup for Development............... ............. ............... 92

6-5 Plot of Variation of Mass of Powder Developed with respect to the Surface
Voltage Available for Development. ............................................ ............... 97

6-6 D im tensions of the M killed Slot ..... ......... ......... ....................................... .......98

6-7 Packing of Iron Powder Particles in a Monolayer ............... ..... ............... 98

6-8 Graph Showing Decrease in Amount of Developed Powder with Increase
in the Gap Between the Powder and the Insulated Electrode..............................101

6-9 Schematic for Test Setup for Powder Transfer ........... ....................... 102

7-1 Schem atic of R esistivity Test Cell ................................ ...................................... 108

7-2 Schem atic of Capacitance Test Cell............................................... ......... .....

7-3 Cross-section of Polymer Powder Developer ....... ..........................................14

8-1 Schematic of the Concept of Flat Photoconductor Plate Test Bed Assembly .......122

8-2 Schem atic of L aser Im ager......................................................................... ... ... 124

8-3 Solid M odel of Photoconductor Belt Assembly..................................................... 128

8-4 Solid M odel of the Powder Developer Assembly .................................................. 130

8-5 Cross-sectional view of the Powder Developer Assembly ............... ................ 131

8-6 Solid Model of Charger-Imager-Developer Assembly ........................................ 132

8-7 Cross-section of Charger-Imager-Developer Assembly ....................................... 133

8-8 Cross-sectional Image of Charger-Imager-Developer Assembly with
Photoconductor Belt A ssem bly on top............................................................... 134

8-9 Close-up of Solid Model of Test Bed showing the Flat Photoconductor Plate
D developer A ssem bly .......................................... ................ ......... 135

8-10 Charge Flow Through the Ground during Development ......................................137









8-11 Variation of Amount of Powder Developed with Change in Voltage .................138

8-12 Electrodes Shaped in Different Patterns ........................................................... 141

8-13 Patterned Electrodes after Developed with Nylon 12 Powder............................142

8-14 Build Platform for Printing Nylon 12 ...................................... ............... 142

8-15 Intended A ssem bly of Patterns........................................ ........................... 143

8-16 Actual Printing of Assembly of Patterns using Nylon 12 ............................... 143

8-17 Print of Assembly of Patterns on PET Sheet Surface. .........................................145

A-i Parallel Plate Powder Resistivity Test Cell..............................................152

A-2 Test Window for Resistivity Measurements. ........................................... 153

A-3 Parallel Plate Powder Capacitance Test Cell. .............................. ......... ...... .154

A-4 Hi-R Step Response W window ..................................................... ................ 155

A -5 Step R response Settings................................................. .............................. 155















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

AN INVESTIGATION ON THE PRINTING OF METAL AND POLYMER POWDERS
USING ELECTROPHOTOGRAPHIC SOLID FREEFORM FABRICATION

By

Ajay Kumar Das

August 2004

Chair: Ashok V. Kumar
Major Department: Mechanical and Aerospace Engineering

Electrophotographic solid freeform fabrication (ESFF) is a novel method of

manufacturing which is under development at the Design and Rapid Prototyping

Laboratory of the Department of Mechanical and Aerospace Engineering at University of

Florida. Electrophotographic solid freeform fabrication uses the principle used in laser

printer to print layers of material, one over the other, to form the final three dimensional

objects. In past experiments a test bed was built to test the concept of printing in layers

using toner powder. Toner powder was successfully shown to be printed in layers.

The material used as the toner powder is not suitable as a structural or functional

material, and there was a need to explore the use of other materials including metal and

polymer powders as structural material in layer by layer printing using ESFF. Powder

developers were designed to develop thin uniform layers of metal or polymer powders on

the photoconductor surface. A Charge and mass measurement test setup was built to

evaluate these developers on the basis of their ability to charge the powders and the









amount of powder developed. The powder properties and development and transfer

characteristics were studied for metal and polymer powders.

A simpler version of the test bed to develop and print metal and polymer powders

was proposed. This new concept was less dependent on available commercial laser

printing technology and was based on development of powder on a flat photoconductor

plate. This concept made the design modular for easy setup and debugging purposes. A

developer was designed to print upwards against gravity on a photoconductor plate

(facing downwards). This developer was assembled with the charge roller and the laser

imager assembly. A flexible build platform was assembled which could align itself to the

photoconductor plate orientation for effective transfer of powder layers. The developer

was tested for its efficiency in charging and developing powders. The developed powder

was then transferred on the build platform to test the ability of the test setup to print

powders in layers.














CHAPTER 1
INTRODUCTION

Rapid prototyping is a manufacturing technology that produces objects layer by

layer without the need of elaborate tooling and clamping. The objects can have simple or

complex geometries and can take any freeform shape. Manufacturing of these freeform

objects in small quantities was expensive by conventional machining processes, but can

now be achieved by rapid prototyping systems at a relatively low cost.

Electrophotographic Solid Freeform Fabrication (ESFF)

Electrophotographic solid freeform fabrication (ESFF) is a rapid prototyping

technology based on electrophotography, which is under development at the University

of Florida. Electrophotography is used in copiers and laser printers to print loose dry

toner powder on paper and then fuse the powder to get the final permanent print.

Electrophotography is based on a special property of some materials (photoconductors),

which are generally insulators, but become conductive when light of a characteristic

range of wavelengths is shined upon them. In electrophotography, a charge of either

polarity is deposited and a laser is then shined on the photoconductor at specific spots to

make the material conductive and discharge the surface at those spots. This creates an

image of charged or discharged area, which is used to pick up charged powder particles

using electrostatic force. This powder layer is then printed on a substrate (in the case of a

printer it is paper), and then fused by applying heat to get the final print.

ESFF technology is being developed to use this concept to build objects by printing

multiple layers of different materials. ESFF has some unique capabilities over other









commercially available rapid prototyping options on different counts. It has a higher

resolution (600-1200 dpi) and can produce objects having fine details. This technology

can produce blended objects with predetermined compositions. This makes it a suitable

technology for semiconductor and electronics industry applications to produce miniature

circuits and components.

Past Research and Motivation for the Present Work

Past works on the development of this technology were done mainly to understand

electrophotography and study its behavior when used for printing multiple layers of toner

powder. A test bed was built using a modified laser printer to print toner on a moving

platform. The printed powder layer over the build platform was then moved under a fixed

plate heater and compacted and fused to bind the loose powders. Experimental studies

were done to find out the physical properties of toner, charge per unit mass (Q/M), mass

per unit area (M/A), resistivity, permittivity and mass density. These are the important

properties of toner, which affect the printing process and print quality. A number of

models were proposed to explain the behavior of powder during development and

transfer. Software was written to slice the 3D model of the object and then send the cross-

sectional image to the printer to print.

The need to study the feasibility of printing powders other than the toner powder

was the starting motivation for the work presented in this thesis. The physical properties

including the resistivity, permittivity and mass density were measured and reported for

polymer powders. To measure the charge per unit mass (Q/M) for powders, a test setup

was built and experiments were done on it find out Q/M values for iron and polymer

powders. A number of developers were designed and tested for their effectiveness to

replace the toner powder developer in the laser printer and print powders directly on to









the photoconductor drum. The difficulties associated with the old ESFF test bed design

were identified and a new design based on a flat photoconductor plate was proposed. The

design and fabrication of the charger imager and developer assembly were done. A

number of tests on the development characteristics of metals and polymers were

conducted to identify suitable conditions for maximum development. Theoretical models

were developed and analyzed for estimating the parameters and their influence on the

development process.

To summarize, the thesis work presented in this text presents the effort to develop

and print metal and polymer powders. The physical properties of the powder particles

were determined and their suitability to be used as a structural material for ESFF

applications was evaluated. A test bed was built to make it easier to develop and transfer

powders of any kind. This research paves the path for further tests and experiments to

determine the feasibility of this technology to print powders other than toner in layers to

build three-dimensional objects.

Chapter Layout

The first chapter is a short introduction to the overall research project and the

motivation behind the current research work and its significance in the overall

development of the ESFF technology.

The second chapter discusses the rapid prototyping technology, how it is different

from the other manufacturing technologies and its significance in the manufacturing

world. Then different rapid prototyping technologies are discussed in brief with

illustration.

The third chapter discusses electrophotography and its growth as a technology

applied to laser printers and photocopiers. It describes the different components of









electrophotography and the way they influence the quality of print. The chapter also

discusses various development techniques and the behavior of powders of different

materials during electrophotography.

The fourth chapter presents the Electrostatic Solid Freeform Fabrication (ESFF) as

an alternative rapid prototyping technology. It starts with an introduction to the concept

of ESFF, and then goes on to describe the first design of the test bed. Brief highlights of

the past works on this technology are discussed and the results that were obtained from

those works are summarized.

The fifth chapter describes the evolution in the design of the powder developer and

its evaluation at each stage by using charge per mass (Q/M) measurement device. The

detailed design of the improved charge and mass measurement device is explained and

the functions of the components are discussed. Finally, the results from charge and mass

measurements from metal and polymer powders are reported.

The sixth chapter discusses the theory behind metal powder development and

printing. The forces involved during development and printing of metal powders are

identified and force equations are solved to determine critical parameters, including the

particle size, and the magnitude of field developed. The methods of charging, developing

and transferring powders are discussed with reference to their efficiency in printing

powders. Experimental results found by preliminary tests done using iron powders are

presented.

The seventh chapter discusses polymer powders and their properties, and the

effectiveness of development and transfer by using them in the new test bed setup. The

test results providing information about the resistivity, permittivity and mass density of









some polymer powders are presented. There is also a discussion on the behavior of these

powders when they were developed on a photoconductor drum. The results indicated

Nylon 12 is the best suited powder to be used as a sample polymer for all further

theoretical and experimental calculations.

The eighth chapter introduces the flat photoconductor plate test bed as a better

alternative to the test bed based on commercial laser printers. The important features of

the new test bed are highlighted and design challenges are discussed. The design of

upward-facing developer and the assembly of charger, imager, and developer are

described in detail. Experimental results from the use of iron and nylon 12 powders in

the improved test bed are presented.

Finally, the ninth chapter discusses the conclusions drawn from the theoretical and

experimental work presented in this thesis. It also suggests the opportunities for future

research on this project to develop it as a successful rapid prototyping technology.














CHAPTER 2
RAPID PROTOTYPING

Rapid prototyping is also known as Solid Freeform Fabrication and Layered

Manufacturing. Each of the three names stands for an important characteristic of this

technology. Rapid prototyping defines it as a technology to produce prototypes of

conceptual models quickly without the need of any elaborate tooling and fixture design.

The ability of rapid prototyping units to produce design prototypes quickly, shortens the

design iteration cycle time significantly. In comparison conventional machining is faster

than rapid prototyping, with tooling and fixtures in place. Machining process is also

faster and economical when it comes to mass production of components. Solid freeform

fabrication refers to the fact that this technology can be used to produce solid objects with

any freeform surface. The last term, Layered Manufacturing, signifies that in this

manufacturing technology objects are produced in layers.

In rapid prototyping the conceptual 3D solid model is first sliced into two

dimensional layers using slicing algorithms. Each of these layers is the cross-sectional

image of the object at a particular z level. These layers are created one over another by

using methods particular to different rapid prototyping technologies. Support structure is

also provided along with the part building process to provide support to any overhanging

structure in the part. In rapid prototyping there is no restriction on the structure and shape

of the object as long as it can be sliced into layers and built within the space limitation of

the machine. This allows the freedom to produce objects with any freeform surface as










desired. The activities, which are central to all rapid prototyping technologies, are

presented in the form of a flowchart below.


S3D model of the object

Slicing Algorithm
v (computer)
2D cross-section
data at each z-level

Interface (Rapid prototyping
machine)


Building of the object layer
by layer





Post processing




Final 3D object

Figure 2.1 Flowchart of the Rapid Prototyping Process

There are many successful commercial rapid prototyping technologies, which are

available in the market and there are others which are in their developmental stage and

hold a lot of promise of becoming successful in future. The rapid prototyping

technologies are characterized by

* material used to build the object

* process of binding layers together

* speed of build process and post processing requirements

* limit on the choice of geometry of the object that can be built










The process that is best suited for the given application is chosen from all different

options based on the above criteria. Some successful commercial technologies are

presented below.

Stereolithography Apparatus (SLA)

Stereolithography is based on the property of certain polymers, called

photopolymers that solidify when UV light is shined upon them. The setup is made of a

build platform immersed in a liquid polymer container. UV laser is shined on the polymer

after being reflected by a mirror to form a crosshatch pattern on the polymer surface. The

laser can penetrate only a small depth into the liquid and so it can harden only a thin layer

of liquid polymer on the build platform in one scan of the surface. This is a limitation on

the speed at which the object is built. Support structures are also built along the building

of part, which are difficult and expensive to remove during post-processing. The

schematic below shows the setup for a typical SLA machine.

Mirror
Laser







Drum containing
Build part photopolymer
Build part







Piston (Build platform)
Figure 2.2 Schematic of Stereolithography Apparatus (SLA)









SLA is one of the earliest rapid prototyping technology, and first to be

commercialized. It popularized the concept of rapid prototyping and was able to build

objects, which were difficult to make conventionally. There can be unattended and

continuous operation for 24 hours. The expensive and sophisticated process requirements

made it prohibitive for wide use. Post processing also takes long time and the surface

finish and tolerance is poor when compared to conventional machining due to non-

uniformity of solidification of the photopolymer by laser.

Solid Ground Curing (SGC)

Solid ground curing (SGC) is another technology based on the same principle as

SLA, which is hardening of photopolymer by using UV light. The difference is that in

this technology hardening of the surface is done through a mask, which helps in

hardening of the whole surface at once rather than tracing with a laser as in SLA. The

mask is prepared by photocopying technology, which prints each cross-section on a glass

plate, which is renewed by erasing the print. This is done in a separate process cycle

called Mask Plotter cycle, which is different than the actual build cycle called Model

Grower cycle.

In the model grower cycle the glass plate from the mask plotter cycle is used and a

UV flashlight is shined through the glass plate on the polymer surface. The unused

polymer is removed and the area is filled by wax. Wax solidifies and gives a strong

support to the build structure. The surface of this layer is milled to make the layer

uniform. This produces objects with high dimensional accuracy, as there is uniform

hardening and avoidance of material tensions. Removal of support structure is easy as the

wax can be melted and removed. An important disadvantage of the method is the









sophisticated equipment, which prohibits unmanned manufacturing process. This process

also takes a long time because of the elaborate procedure involved.

Selective Laser Sintering (SLS)

Selective laser sintering (SLS) produces parts by heating powders on a powder bed

to the temperature just below their melting point temperature. At these conditions the

particles sinter and form strong bonds to build the object. The powder is stored in a

reservoir and brought out by a piston. The powder layer is then spread uniformly on the

top of the build platform by a leveling roller. Then a high power carbon dioxide laser is

shined on this powder bed tracing the outline of the cross-section. The temperature of the

powder inside the cross-section is raised to just below its melting point by scanning of the

laser in a cross-hatch pattern so that the powder layers sinter and combine to form a solid

mass. As the process is repeated, layers of powders are deposited and sintered to form the

final object. The loose powder that was not sintered provides a natural support for the

built part. The finished part can be easily taken out of the powder bed and the loose

powder on the surface can be removed by blowing off with pressurized air. The

schematic below shows the SLS process.

The SLS process has its own advantages and disadvantages. The advantage lies in

the fact that this technology can be used for a variety of materials including different

kinds of polymers and metals. The process is also fast with a build rate up to one inch per

hour. Post processing is easier as the parts that come out have full strength and do not

require additional processes. The major disadvantage of this method lies in the poor

geometric accuracy and grainy surface finish. This is mainly dependent on the powder

particle size. There is also the possibility that the neighboring powders, along the part

boundary, may get sintered and become unwanted part of the object. The process is done










in a nitrogen chamber, and so nitrogen has to be continuously provided to the chamber.

Also during sintering, toxic gases are released and have to be handled carefully. This

mostly happens when sintering polymers.

Mirror -
Leveling roller Co2 Laser
/
IBuild part








Residual powder acting as
support




Powder Reservoir Piston (Build platform)
Figure 2.3 Schematic of Selective Laser Sintering (SLS)

Fused Deposition Modeling (FDM)

As the name suggests in the Fused Deposition Modeling (FDM) method certain

material (plastic filament) is melted and deposited on a build platform in layers to form

the final object. The part material is available in the form of filaments, which are coiled

in the form of spools. These filaments get heated up when they are passed through a

nozzle (FDM head), and the material leaves the nozzle in liquid state. This liquid material

solidifies immediately at the ambient temperature. Due to this the nozzle has to be very

close to the build platform while tracing the model cross-section. The FDM head is

capable of translating in the X and Y directions and the build platform moves in the Z

direction to accommodate the layer build height. The FDM head first traces the outline of

the cross-sectional area and then fills the area with densely packed crosshatch pattern. In








addition to the part material nozzle, there is another nozzle which deposits the support

material where needed. The support material is also a polymer filament, which is melted

and deposited on the build platform along with the part material. This is a different

polymer than the part material and is deposited to form thin wafer-like structures, to

support the part material during building process and can be peeled off very easily using

a little mechanical force. The schematic in figure 2.4 shows the concept of FDM

technology.



Heater

Molten Material tt
Deposited

Nozzle


Spool


/ \
Prototyped Object
Build Platform

Figure 2.4 Schematic Representation of Fused Deposition Modeling (FDM)

The simplicity of the procedure makes this technology very popular and easy to

setup and use. The procedure requires no cleaning and produces no waste, and no post-

curing is required. As the filament has a diameter of 1.27mm, the resolution and

dimensional accuracy is affected.

Laminated Object Manufacturing (LOM)

Laminated object manufacturing (LOM) is based on the simple method of sticking

together thin sheets of material each representing the cross-section of the object at a









particular given height. The setup for LOM consists of a thin ribbon of sheet material

wrapped around a supply roller and a take up roller passing over the build platform

supported by several idler rollers. The sheet material starts from the supply roller and

stops on the build platform. Then a heated roller is rolled over the piece of the ribbon on

the build platform, which binds the sheet material to the top of the stack. A high power

CO2 laser is shined on the sheet material, after being reflected by mirrors that control the

X and Y movement of the laser beam. The movement of the build platform provides the

Z-axis movement. After the sheet material is bound to the top of the stack of sheets, the

laser traces the boundary of the cross-section cutting it out of the sheet material. The

unwanted portion is diced by the laser beam into crosshatched squares that provides

support to the part. The remaining material is waste and is rolled around the take-up

roller. By binding sheets one on top of the other the final object is produced.



Laser

Mirror-

Block of Previous
Layers \ Heated Roller




Sheet Material




Platform
Take-up Roller Supply Roller



Figure 2.5 Schematic Representation of Laminated Object Manufacturing (LOM)









The object comes as embedded inside the cubic structure formed by the bound

sheets. The post processing involves removing the unwanted material, which were diced

during the building of the part. This process of dicing helps in easy removal of this

unwanted material. A schematic of the LOM process is shown in the figure 2.5 above.

The ability to use a large variety of organic and inorganic materials is the biggest

advantage of LOM technology. The process is also faster than the competitive

technologies, as the laser has to only trace the outline of the cross-section and not the

whole area. The layers stick to the stack very easily and so the process is faster. This

method can be used to produce larger prototypes that are not possible by other

technologies. The disadvantage of the method is that it produces parts with low strength

in the Z-direction and so the objects produced cannot be used as functional prototypes.

The other drawback of this system is that it produces a lot of waste during building of the

object and during post-processing.

3-D Printing

The concept of 3-D printing is also based on gluing two layers together to form a

part, but here the two layers are made of loose powders. The process starts with spreading

of a thin uniform layer of powder. The powder layer is then selectively joined by ink-jet

printing of binder material. The build platform, which is in the shape of a piston inside a

cylinder containing the powder bed, is lowered and the next layer of powder is spread.

The ink-jet print head scans the powder bed in the same way as it does while printing on

paper. The only difference is that here instead of ink, binder material is used. The loose

powder around the built part provides natural support and can be removed very easily

during post processing. The part has green strength and can be consolidated by









application of heat, which will evaporate the binder. Figure 2.6 shows the schematic of

detailed 3D printing process.

This method of rapid prototyping can be used to produce objects made of any

material that can be powdered. When metal powder is used, the final part is put in the

furnace to join the powder particles by melting. Copper infiltration is done to fill the

pores left by evaporated binder to form dense and strong metal part. The disadvantage

results from the granular nature of the material and the interactions between the binder

and the powder. This has bad effect on the texture of the surfaces.













Piston
Spread Powder Print Layer
Figure 2.6 Schematic Representation of 3D Printing Process














CHAPTER 3
ELECTROPHOTOGRAPHY

Electrophotography is a method of printing image-wise arranged charged powders

on a substrate and subsequently fused to form a permanent image. A special insulator

material called photoconductor is used to form image patterns of charged or discharged

areas by using its special property that turns it conductive when light of a particular

wavelength falls on it. The photoconductor is applied as a coating on a roller or plate and

is used for the charge imaging. The details of the working of the photoconductor material

are explained later in the chapter. Figure 3-1 shows the electrophotographic cycle with

the photoconductor material applied on a drum (photoconductor drum).

The Electrophotographic Process

The six major steps in electrophotography are

* Charging: In the charging process charge is deposited on the photoconductor
surface.

* Imaging: In this process light of particular wavelength (in the form of laser) is
shined on the charged photoconductor surface to discharge certain areas according
to the image and produces either a charged image with discharged neighborhood or
a discharged image with charged neighborhood.

* Development: In this step loose charged powder particles move towards the
photoconductor surface due to the electrostatic force created between the
photoconductor drum and powder. These powders get developed image-wise on the
photoconductor surface due to attraction or repulsion by the charged areas on the
surface according to the method used.

* Transfer: In the transfer process the image-wise developed powders are transferred
on to a substrate either electro-statically or physically to form the print.

* Fusing: In this step heat is applied to the powder to melt it and fix it to the substrate
to form the final permanent image.









* Cleaning: In this final step the photoconductor surface is cleaned off the residual
powders remaining on its surface after transfer process. The photoconductor
surface is also discharged to avoid the carry over of any charge patterns to the next
print.

An example of electrophotography process is shown schematically in figure 3-1,

which shows the way different steps are performed around a photoconductor drum. Most

of the modern printers use photoconductor drums for printing as the drums help in

making the design compact. This compact design makes it easy for the printer to be used

easily as a desktop printer. All the processes are spread around the photoconductor drum

and occur simultaneously in a cycle making printing faster. These processes are

explained in detail below.

Photoconductor Charge Laser Imager
Roller




Cleaning Box /

Phot conductor Drum-
S i Developer


Cleaning Blade
SPaper




Paper Charger roller
Figure 3-1. Schematic of the Electrophotography Print Cycle

As can be seen from the electrophotography print cycle, the photoconductor drum

is central to the electrophotographic process. Therefore, it is essential to know more

about the photoconductor and its characteristics to understand electrophotography.









Photoconductor Material

As discussed before, the photoconductor material has the unique property of

becoming conductive when light of specific wavelengths falls on it. To form a charged

image on the photoconductor drum, the drum is first charged with charge roller. Then the

charged photoconductor surface is exposed to UV laser pulses which make the

photoconductor material conductive and so the charge from the surface passes to the

ground. This creates an image-wise charge distribution which picks up powder

selectively. This powder image is then transferred to the build platform and fused to

make a permanent image.

There are many materials that can be used as photoconductors and some of them

were used in early versions of copiers and printers. The ones that were popular and

widely used are amorphous selenium and organic photoreceptors. Amorphous selenium

was used in the early generation of printers, while organic photoreceptors gained

popularity later because of heavy demand of inexpensive compact desktop printers. The

important parameters that are used to characterize a photoconductor are explained below.

Dark Decay

Dark decay is the ability of the photoconductor to retain the charged image when

no light is falling on it. The photoconductor material is not a good insulator and even

when no light is falling on its surface it allows charge leakage. The charge depletion is

exponential in nature and the time in which the photoconductor looses half of its surface

charge is called depletion time. Organic photoreceptors typically have shorter depletion

time than amorphous selenium (Diamond 1991).









Charge Acceptance

The surface charge limit that can be deposited on the photoconductor surface is

known as charge acceptance. This is generally decided by the dielectric property of the

photoconductor material. The desirable surface charge on the photoconductor is what can

be retained by the photoconductor and can create sufficient electric field to attract

charged powders to get adhered to its surface. Anything more than this will increase the

force of attraction between the powder and the photoconductor surface, which will make

powder transfer difficult.

Image Formation Time

Image formation time is the time taken to discharge the photoconductor surface by

imaging light. Image formation energy would have been a more specific term to consider,

but in electrophotography it is time and not imaging light intensity, which is considered

as an influencing variable. The speed at which printing is done is a more critical

parameter to judge the efficiency of a copier or printer, which makes image formation

time more critical.

Image Stability

Image stability is considered as the ability of the photoconductor to maintain a

charged image on its surface. Image instability occurs due to the inability of the

photoconductor to maintain a highly localized area of discharge on its surface against

charge migration. Surface contamination also plays a role in migration and dissipation of

charge on the surface itself.

Residual Image

Residual image occurs when the photoconductor surface is not completely

discharged after the image transfer process. This can happen because of various reasons









and can be identified after many prints when images of previous prints, ghost images,

begin to appear. To reduce this problem the photoconductor surface is cleaned and

discharged after each print, but with high speed of print cycles this problem is not

completely corrected.

Material Selection

As stated earlier, the amorphous selenium was replaced by more popular organic

photoreceptor in the modem printers. This might seem counter intuitive since, compared

to amorphous selenium, organic photoreceptors have less dark decay time and so cannot

hold the image for a long time. Organic photoreceptors are softer and are prone to early

wear and tear during operation and suffer gradual breakdown by environmental exposure.

The cost of organic photoreceptors is much less than amorphous selenium, even

after considering the high frequency of replacing them during the printer lifetime. To help

in maintenance of the photoconductor based on organic photoreceptor, it is made a part of

the toner cartridge. While replacing or renewing the toner cartridge the organic

photoconductor is inspected for any damage and recoated accordingly. This makes

organic photoreceptor a popular photoconductor material for modern printers.

Dark decay is not much of a concern here because the modern printers and copiers

have high-speed electrophotography cycles and the required time to hold charge and

image is insignificantly small. Moreover amorphous selenium is sensitive to a wide range

of wavelengths of light and has to be kept in a well covered dark area during operation

which makes the printer design complicated. This is not a problem with the organic

photoreceptor as it is only sensitive to a narrow band of wavelengths in the ultra-violet

region, which makes it easier to use and results in a simpler printer design.









As can be seen from the last discussion, photoconductor material is the central to

the electrophotography process. The following discussion describes in brief the stages

which photoconductor goes through during electrophotography. As said before there are

six steps in this electrophotographic print cycle. The cycle starts with charging of the

photoconductor surface.

Charging

Charging is the first step in electrophotography. For a good print quality uniform

charging of the photoconductor surface is necessary, which is done by depositing charge

particles on the photoconductor surface. There are mainly two ways of charging; by using

corona charger and by using charge roller.

Corona Charger

When high voltage is applied to a conducting body having low radius of curvature,

high electric fields are generated locally which causes breakdown of air and ions are

generated. This principle is put to practice in corona devices in which high voltage is

applied to a thin wire enclosed in a metal shield at the same voltage, generally around

7000V (Schaffert 1975). This kind of device is called as corotron. In this the thin wire

generates ions by dielectric breakdown of air and repels ions of the same sign. These ions

are again repelled by the metal shield, which generates a steady stream of ions directed

towards the body to be charged. There can be non-uniformity of charge generation due to

ion winds and impurity on the wire due to toner and paper dust.









COROTRON
Metal Shield

Corona Wire









Ionized particles
Figure 3-2. Schematic of Corotron Charger

To make the charge deposition more uniform, a device called as scorotron is used

in which a screen is used to cover the opening of the metal shield. The screen restricts

any ion that is not traveling perpendicular to the screen holes.

SCROTRON


Corona Wire Metal Shield






Screen




Parallel Rays of Ionized Particles
Figure 3-3. Schematic of Scorotron Charger

The corona charging device is associated with many reliability problems. It also

generates ozone as a byproduct, which is harmful to health and has to be dissipated. All

these problems associated with corona charger made this device less favored over the

years.









Charge Roller

Roller charging is most popular method of charging in the modern printers. It is

more compact and easy to use than the corona charger. The charge roller is made of a

central metal rod covered by a thick layer of polymer material. This polymer is a special

material due to its higher conductivity than regular polymer. The voltage applied to the

charge roller is relatively lower than that applied to corona charger. When DC biased AC

voltage is applied to the roller, small discharges occur between the irregular polymer

surface and photoconductor surface (Hirakawa 1995). These small discharges deposit

charge on the photoconductor surface. There is a uniform line contact between the charge

roller surface and the photoconductor surface along the length of the photoconductor,

which results in uniform charge deposition.

Imaging

After the photoconductor surface is charged, latent image is created on the surface

by discharging the surface locally using laser of particular wavelength, which is in the

UV range for organic photoconductors. The latent image is a charge pattern that mirrors

the information to be transferred to the real image. The laser is shined on the

photoconductor surface after getting reflected by a rotating polygonal mirror. The laser is

turned on and off according to the image to create selective discharge points on the

photoconductor surface. The print resolution is determined by the laser wavelength and

the rate of switching of the laser.

The organic photoreceptor is divided into two layers. One is the charge generation

layer (which is over the aluminum drum) and the other layer is the charge transport layer

(which is over the charge generation layer). When UV laser falls on it, negative and

positive charges separate and the charge which is of the opposite sign as the charge









deposited on the photoconductor travels to the surface and discharges the surface and the

other charge flows to the ground through the drum.

Light

----------------------- Negative Electrostatic Charge


Charge Transport Layer

0 G Q j -*-Charge Generation Layer
+++++++++++++++++++++++++++++ Aluminum Ground



Figure 3-4. Image Formation in Organic Photoconductor Drum by UV Laser

Development

Development is the process of charging the toner powder and then transferring it on

to the photoconductor surface. The charged toner powder experiences the electrostatic

force due to the field created by charge distribution on the surface of the photoconductor

drum and gets adhered to the latent image thus forming the real image.

The toner powder is an insulator and is triboelectrically charged to the required

polarity. There are charge controlling agents added to the toner and they help in

preferential charging of the toner with a specified polarity. The toner is brought into the

vicinity of the photoconductor surface with the charged latent image. The amount of

powder coming out of the powder reservoir is controlled by a doctor blade, which is a

thin polymer or metal strip present at the opening for toner. The toner near the latent

image jumps the gap between the developer and the photoconductor surface and gets

developed. The schematic of a typical toner developer is shown in figure 3-5.









Toner Powder
Doctor Blade



EEE Powder Box
Developer Roller ::::


Figure 3-5. Schematic Representation of Developer System

Development is by far the most difficult process to understand and control. There

has been a lot of research done on this subject resulting in gradual increase in

understanding of the subject. The process was improved from cascade development to

insulative magnetic brush development and then to the most efficient conductive

magnetic brush development (Schein 1988). The toner used for these development

techniques is a two-component toner, which has large carrier particles are covered with

smaller toner particles. Some of the important development methods are discussed below.

Cascade Development

In cascade development the carrier beads covered with toner is made to flow over

the photoconductor surface under the influence of gravity. The development using this

method depends on a lot of factors including speed of fall of carrier beads, angle of the

photoconductor plate to the horizontal and the bouncing of the beads. As the toner

particles are attracted by the electric field generated by latent image rather than the

charge itself, solid area development is very poor in this method, because the field

generated by the charge image highest at the edges and decreases rapidly to the center of

the image (Schein 1988). This is a very simple method of development and was used in

the early printers and copiers.












O P Carrier Particles




O Toner


Photoconductor


Substrate

Figure 3-6. Schematic of Cascade Development

This method depends on a lot of parameters and forces and it is very difficult to

control the powder behavior in these conditions. The development is not reliable and

there is a lot of waste due to powder spillage. All these problems made this method less

popular and there was a need for an alternative method of development and led to

development of the magnetic brush development technology.

Magnetic Brush Development

Insulative magnetic brush development was a significant step in powder

development history. In this method a roller rotates around a stationary magnet and

carries magnetic carrier particles along with it by magnetic friction. The magnetic force

provides a strong counter force for the electrostatic force and eliminates any other small

forces that could bring uncertainty in the cascade development process. The magnetic

force makes sure that the powders that are developed are well charged. This avoids any

low charged powder from getting developed and attached loosely to the photoconductor

surface. The carrier particles form a chain on the roller due to the magnetic field and

appear as brush, and so the method is called magnetic brush development. The charged









toner was carried across the gap to the photoconductor drum when the electrostatic force

exceeds the force that holds the toner to the carriers. The carrier beads were spherical and

the development limit was decided by the balance between the charge on the carrier

beads and the photoconductor surface (Schein 1988).

Toner Roller


















S -Carrier Particles Development Magnet

Photoconductor Drum
Figure 3-7. Magnetic Brush Development System

Conductive Magnetic Brush Development

The conductive magnetic brush was the most successful development technique

invented for two-component development. The difference between this and the previous

method was that the carrier particles are irregular in shape so that toner did not cover the

whole carrier surface, that helped in maintaining conductive contact between the adjacent

carrier particles and transmit current across development gap. There was no balancing of

charge buildup on the carrier particles with the photoconductor surface and so more toner

particles can be transferred (Kasper 1978). This resulted in darker prints and more solid

area development.









In the dual component development, the large carrier particles take up more space

in the toner powder box and so there was requirement of larger powder boxes and

frequent refill of the toner powder. Due to this inconvenience, mono-component

development was developed, which has toner without the carrier particles.

Mono-component Development

Mono-component development is the most common method of development in

modern printers and copiers. It uses insulative polymer toners, which are transported

using the same magnetic roller technique. The toner powder is made magnetic by doping

the polymer with iron compounds during toner production. This also gives the black color

to the toner powder. This black coloration of toner is not suitable to use in the color

printers. So in color printers cascade development is used and that results in minor

spillage. The magnetic roller transfer technique has the same advantage of force

cancellation and control as it is in two-component magnetic brush development. The

toner powder particles are charged and are stripped off the magnetic roller by the force of

the electric field produced by the latent image on the photoconductor surface. To help in

this process a DC biased AC voltage is applied in the development zone that makes the

powder jump to and fro which forms a powder cloud. This process helps in further

charging of toner powder, and the sufficiently charged toner powder adheres itself to the

image.

Charged and Discharged Area Development

Depending on the nature of latent image created by the charged areas or the

discharged areas, the method of development is named as charged area development

(CAD) or discharged area development (DAD).









In CAD the image is created of charged particles by discharging the areas that are

not a part of the image. The toner powder is charged to polarity opposite to the charge on

the photoconductor surface. The powder is developed by the field created by the latent

image made of charge.


Bias
-200 V
-F


Toner




Image
-600 V


r


Non-Image
-100 V


Figure 3-8. Charged Area Development (CAD)

In DAD the image is created by discharging the areas which are part of the image.

The toner powder is charged with similar sign compared to the charge on the

photoconductor surface. The photoconductor drum is grounded and there is a field

created between the ground and the voltage on the developer roller. This field attracts the

powder towards the photoconductor surface. When the powder is repelled by the same

sign charge on the non-image areas, it is developed on to the discharged latent image on

the photoconductor surface.


I











Bias
-500 V


Toner




Non-Image
-600 V


Image
-100 V


Figure 3-9. Discharged Area Development (DAD)

In DAD the photoreceptor dielectric strength uniformity is critical; otherwise small

fringe fields associated with breakdowns are developed by the toner. In CAD the light

source lifetime can be an issue because it is on approximately ten times longer than in

DAD, as most of a page is usually white. In most of the modern printers DAD is used due

to the high cost of laser imager, making it prohibitive to replace frequently.

Toner Powder Charging

There are several methods that are used to charge the toner powder. Some of them

are specific to the nature of the powder and use the powder properties to charge them.

Others charge toner by depositing charge on it externally.

Toners may self-charge due to triboelectric effects and chemical charging. Two

component toners often charge triboelectrically by friction between the toner and the

carrier particles, which charges them oppositely to each other. This makes toner stick to

the surface of the carrier particles. Liquid toners are charged chemically, in which the

charge is exchanged between the liquid and the toner particles. Liquid toners are used for

fine powder prints, are suspended in liquid due to the difficulty in handling such fine









powders. Chemical charging is also done in mono-component toner to some extent due to

the addition of charge controlling agents, which makes the toner susceptible to be

charged with a predetermined polarity.

There are also direct ways of charging toner powder. Corona charger is used to

deposit charge particles on the toner powder to charge them. This method is very similar

to that of charging photoconductor surface using corona charger and faces similar

difficulties of being bulky and non-uniform in charging. The toner powder also can get

deposited on the corona wire and render it inoperable.

In mono-component development the toner powder gets charged by getting rubbed

against the parts of the developer. This is a popular method of charging toner powder

because the charge controlling agents make the toner active to be charged preferentially

(Schein 1988). In case of conductive powder, the powder gets charged by being present

in the electrostatic field between the photoconductor surface and the developer surface.

The conductive powder can also be charged by inductive charging in which an electrode

at a distance can induce charge of opposite sign on the powder, if the powder is

grounded. Insulating toner powders can also get charged by injection charging in which

the toner is moved rapidly around a roller in presence of electric field. The actual

mechanics of the charging is poorly understood (Nelson 1978).

The charging of toner is essential to control, because the amount of charge

deposited on the toner particle determines the charge per unit mass of the toner, which is

an important parameter for the quality of toner development. If the charge per unit mass

is less than required then the electrostatic force will not be enough to strip the toner out of

the magnetic roller. If charge per unit mass is higher, then only a small amount of toner









will able to discharge the image and there will be a thin layer of development. Thus the

process should be designed to have the powder optimally charged depending on the

magnetic field of the developer roller.

Transfer

After the toner powder is developed on to the latent image on the photoconductor

surface, it has to be transferred to the paper to transfer the image on to the paper surface.

The transfer is accomplished by using both electrostatic and mechanical methods. During

transfer the paper is pressed against the photoconductor surface using force by a charge

roller which deposits a charge opposite to that of the toner and creates field across the

paper to make the toner powder transfer to the paper. This process also holds the toner

temporarily to the paper surface by squeezing the toner and pressing it against the paper.



Photoconductor Drum









Toner

Paper




Charge Roller

Figure 3-10. Schematic of Transfer of Toner to the Paper









Fusing


After powder transfer, it has to be fused and fixed to the paper surface to create a

permanent image. In early days of electrophotography, specialized powders were used as

toners and were fixed on to wax papers or papers with adhesives to make a permanent

print. In the modem printers the toners are specially created to have fixing characteristics,

which enables the use of ordinary paper. The toner is mostly composed of polystyrene,

which has low melting point. The paper with the toner is subjected to heat so that the

toner melts and gets fixed to the paper. In some printers radiant heaters are used. Most of

the printers now use heat rollers to fuse toner. The problem with the heat rollers is that

they tend to pick up some toner during heating which may smudge the image. For this

reason non-stick coatings, like Teflon, are used on the heat rollers.

Cleaning

After the transfer of powder from the photoconductor surface, the remaining

powder is cleaned before the next electrophotography cycle. In current printers using

photoconductor drums, the cleaning is done as a part of the cycle. For cleaning, a blade

which is a thin flexible polymer sheet is used to scrape off the toner from the

photoconductor surface. This waste toner is stored in a receptacle, which is emptied

periodically. Generally this receptacle is made big enough so that it does not get

overfilled before the developer, along with the receptacle, is replaced.

After cleaning the next step is to discharge the photoconductor surface to clean any

residual charge remaining on the surface that may cause background printing. This

discharge is done by using corona charger or a charge roller supplied with DC biased AC

voltage.














CHAPTER 4
ELECTROPHOTOGRAPHIC SOLID FREEFORM FABRICATION (ESFF)

Currently research is being conducted on the development of a novel method of

solid freeform fabrication at Design and Rapid Prototyping Laboratory of the Mechanical

and Aerospace Engineering Department at the University of Florida. This method uses

the technology of electrophotography to form the image of the two-dimensional slices of

the three-dimensional object and print layers of these cross-sectional images to form the

final part. This technology is called electrophotographic solid freeform fabrication

(ESFF), named after the underlying technology to produce solid freeform objects.

As mentioned earlier, ESFF is not just another way to do rapid prototyping, but it

has the advantage of printing thin layers with high resolution using various types of

materials, which aims to satisfy some unfulfilled requirements in the rapid prototyping

industry. Electrophotography is a fast printing process due to advancement in high speed

printing. In electrophotography, the whole image is transferred in one step, compared to

the crosshatch scanning of the cross-section area done by some other RP technologies,

which makes the formation of individual layers faster. Although the thickness of layer in

each print is small, the time taken for each print can be made faster to reduce the overall

printing time.

Any material which is available in powdered form and can be charged is a potential

candidate for ESFF, although the actual print characteristics may vary. The corona

charger is shown to deposit charge on any material, which can be used to charge the

powders. Even if a powder cannot be fused, it can be used with a binder printed between









two layers of the part powder. This suggests that there can be a wide choice of material

for building parts using ESFF. The other advantage of this method is in the ability to print

materials in pre-estimated gradient percentage. This could give some new properties to

materials, which may be used for special applications. The laser printer prints with a high

resolution up to 1200 dpi (dots per inch). This makes it possible for ESFF to be able to

print materials in finer resolutions and tolerances.

The major hurdle in developing this technology has been the extreme complexity

and unreliability of electrophotographic process when applied to powders other than the

popular toner powder. ESFF deals with complex problems like powder flow

characteristics, powder-charging methods, charged powder behavior and problems of

adhesion of powder to surfaces and agglomeration. These are not so well understood

subjects and are still under active research work.

Development of ESFF Test-bed System

ESFF research started with a test-bed design and fabrication to conduct the

experiments to test the concept of the ESFF technology. The requirements of the system

were to have a two-axis motion control of the platform on which the part would be built.

There should be a printing system for printing of layers of toner powder, which could be

later modified to print powders of other materials. The print also has to be fused to make

it permanent on the build platform. All these requirements were taken into consideration

while designing and building of a test bed using a modified laser printer (Zhang 2001).

The schematic of the printer is shown in figure 4.1.



















'H ding yat-m












Figure 4-1. The ESFF Test-bed Dutta (2002)

Motion Control System

The build platform was required to move in two-axes. One of the motions is in the

horizontal direction (X-axis) for the printing process and another in the vertical direction

(Z-axis) for part height adjustment. These motions were provided by a combination of

servomotors. These servomotors are a part of the Parker automation system, which is

controlled by Galil-DMC motion controller. The Galil controller was interfaced with the

computer using the Galil software. In this software, interface commands can be written to

move the motors with precise speed and acceleration and stop after predetermined

number of rotations. A program in C++ was written to generate such motion commands

to synchronize the motion of the platform with print cycle.

The build platform was an aluminum plate supported by springs to compensate for

the error in positioning of the platform surface and photoconductor drum with respect to

horizontal plane. This also helps in pressing the build platform against the









photoconductor drum without damaging the drum surface. The other place where springs

are helpful is during fusing, by correcting any misalignment of the build platform while

getting pressed against the fusing plate. A position sensor mounted below the top plate of

the platform sends signal when the platform reaches a particular height during

compaction of toner.

Printing

A modified laser printer was used to achieve the task of printing on the build

platform. The printer used for this purpose was the "Laser Jet 4" printer made by Hewlett

Packard. The paper handling system of the printer was removed to clear the path below

the photoconductor drum, which gave access of the photoconductor drum surface to the

build platform for printing. There are sensors to detect the passing of the paper so that the

events associated with printing can be synchronized and also paper jam can be avoided.

These sensors are sent right signals at the right time from the computer to keep the printer

from detecting error in the normal operation. This ensures proper operation of ESFF

process.

Fusing

The toner after getting printed on the build platform has to be fused to form a

permanent print, which was done initially by a non-contact radiant heater. This radiant

heater was made of a heating coil placed at the focal point of a concave mirror for

distribution of the heat pattern uniformly on the build platform. This heat distribution was

observed to be concentrated at some places in real operation causing differential fusing of

toner image. It was also necessary to compact the print to correct any errors due to non-

uniform powder deposition. There was also the need of discharging volume charge of the

printed toner layer by making it contact with a grounded metal plate during fusing and









compaction. These considerations led to changing the heater to a contact type plate

heater. This heater was made of a mica strip heating element sandwiched between two

aluminum plates. The lower plate had a uniformly heated smooth surface, which

compressed the toner powder layer when the upward moving build platform was pressed

against it. This also discharged the volume charge contained in the toner powder layer by

contact with the conductive heater surface during fusing and compression. During the

fusion of powder the molten toner sticks to the heater surface causing distortion in the

print. This distortion in print is also increased due to the spreading of toner layer after

compaction. This affects the dimensional accuracy of the object produced by using ESFF.

The sticking of toner to the heater surface was later reduced by attaching a Teflon coated

plate below the lower aluminum plate to create a nonstick surface.

Software

Software programs were developed in C++ to generate control commands for the

Galil motion controller software interface and Parker automation systems to control

motion of the build platform (Dutta 2002). This program also sends signals to the sensors

in the modified printer to ensure normal printer behavior. This C++ software code is

stored as a dynamic link library, which is called by a Java program (Bhaskarapanditha

2002). This Java program called "SolidSlicer" has slicing algorithms, which use the data

from a solid 3D CAD model of an object stored in STL format and determine the cross-

section of each layer at different Z-heights. These cross-sectional images are then sent to

the printer for printing. The software has a good user interface that allows positioning of

multiple part prints on the build platform. This automates the building process and also

creates information which is stored in log files.









The initial ESFF test-bed provided a platform to perform basic experiments on the

issues of concern during printing of toner powder as the structural material for producing

3D parts. Various parameters could be varied and the resultant effects measured to

compare the change in process due to any change in these process parameters. The

quality and the print pattern could be changed by modifying the software. Sensors and

actuators can be placed on the frame of the test-bed to collect data during experiments. In

this test-bed, the printer was used as a "Black Box" and the only change made to the

printer was to replace the paper handling system with the moving build platform.

Measurement of Charge and Mass of Powder

Charging of toner powder is very important in the process of printing. It should be

high enough so that the toner can get developed on the photoconductor surface, but not

very high which can make it difficult for the powder to be transferred to the build

platform. The effectiveness of a powder developer depends on the charging

characteristics and the amount of powder it can develop on the photoconductor surface.

This can be estimated by measuring the charge per unit mass of the developed powder. In

situ measurement of charge per mass is difficult to accomplish. It is also difficult to

isolate the charge on the powder from other charges present during measurement and

moreover the charged powder developed on to the photoconductor drum is difficult to

rescue and measure. This made it necessary to develop a stand-alone system that can

accomplish the task (Gokhale 2001). The development of powder developer and the

development of the experimental setup to measure charge per unit mass of the developed

powder are discussed in more detail in chapter 5.









Measurement of Powder Properties

The important physical properties of the powder, to be used in ESFF technology,

are volume resistivity, permittivity and mass density. These were measured for toner

powder using a test cell and the Keithley electrometer (Dutta 2002). In chapter 7, results

of similar tests to determine the above properties for polymer powders other than the

toner powder are reported.

Improvement of Print Quality

Limitation on Part Height

There are many issues related to the printing of toner which are amplified when

multiple layers of toner powder are printed in ESFF. The first observation was that the

printing stops after the part height is around one millimeter. By theoretical calculations

(Dutta 2002) it was found that multiple layers of insulative toner powder increases the

voltage drop across printed layer and the electric field available for development

decreases significantly, which in turn decreases the amount of powder transferred. This

problem gets worse when charge gets trapped inside the volume of the printed toner part

due to inefficient discharging of the toner layer after each print. This volume charge

distribution, which is the same as charged toner, repels the toner and tries to prevent it

from getting printed.

An attempt was made to solve this problem by depositing charge on the top of the

printed toner layer using corona discharge, which is of opposite sign to that of trapped

volume charge. The field created by this deposited charge layer cancels the repulsive

field created by the volume charge and also creates an attractive force for toner powder

printing. Moreover the charge deposited on the surface of the top layer is not affected by

the thickness of the toner layer.
























Figure 4-2. Parts Printed Using Corona Charging of the Top Printed Layer Before the
next Print (Dutta 2002)

This method has its own limitations. It is known that corona deposition on a

surface, when the grounded electrode is not near the surface, is limited by the breakdown

strength of air (Gaussian Charge Limit). This is true for the toner powder printing

because toner layer is insulating in nature and multiple layers of toner effectively move

the grounded electrode far away from the top surface. The trapped volume charge in the

printed part increases with every layer deposited and it can reach a value where the

repulsion due to volume charge exceeds the attraction due to the fixed surface charge

deposited by corona. This again creates a limitation on the part height that can be built

using ESFF. In case of high volume charge density the decrease in the rate of increase in

print layer thickness is faster. In the case where the fused toner is almost fully discharged

the rate of increase in print layer thickness decreases very slowly and the part can be built

having more thickness. This observation suggests that consistent complete discharge of

the printed toner powder before fusing is necessary for building higher part thickness.

Complete discharge of the volume charge of a printed insulator layer is very difficult to

attain.









Edge Growth (Solid Area Development)

Edge growth is another problem associated with electrophotography, which is

amplified due to multiple layer deposition. The cause for this is the nature of the electric

field distribution in the image area, which is stronger at the edges and decreases rapidly

towards the middle. This causes thicker development at the edges and near to zero

development at the center. When the printing is done in many layers, there is a distinct

difference between the edges and the solid area development.

An attempt to solve this problem was done by printing the solid area in patterns.

These patterns create edges throughout the solid area fill and the field inside the solid

area can be maintained at a particular level. Finite element analysis of the patterns is done

to study the electric field distribution by pattern printing (Bhaskarapanditha 2002, Fay

2003).

It was concluded that there was a significant improvement in the electrostatic field

distribution in the image area due to pattern printing. A series of experiments were

performed to find out the pattern that gives the best print (Fay 2003). From the results it

was found out that a broad black line with a broad white line is the best pattern to print.

This pattern printing however cannot be used to print smaller parts with finer tolerances.

The resolution of print has to be increased to produce finer patterns.






















Figure 4-3. Parts Printed using Patterns (Fay 2003).
Printing of Powders other than Toner
Attempts were made to determine the feasibility of printing powders other than

toner using the test-bed. A generic developer was designed to be able to develop powders

of any physical property on the rotating photoconductor drum. The powder developers

were designed to replace the toner developer in the toner cartridge assembly. The

effectiveness of the developer was tested by using the charge and mass measurement

setup. A detailed description of this testing is presented in chapter 5. The powder was

brought out for development by cascading it using gravity. This had problems of powder

leakage during development and so was not considered suitable replacement for the toner

powder developer in the printer used in the test bed.

The alternative of the above method is to have two-powder development, in which

one powder is used as part powder and the other is used as binder. In this method toner

was chosen to be used as the binder due to its low melting temperature. For binding

purposes, toner has to be printed image-wise on a uniform layer of part powder (Dutta

2002). In the two powder development the photoconductor surface has to contact the


~a~i~e~Pr










abrasive part powder to print toner image that can damage the soft organic

photoconductor drum surface. Due to this reason, the photoconductor drum surface was

isolated from the abrasive part powder by introducing a transfer roller in between them,

which would collect the image-wise toner powder pattern and transfer it over the part

powder (Dutta 2002). This printed toner powder with the part powder would be fused to

bind the part powder to form the real image and the surrounding loose powder would act

as support.















. :....
.. .







Figure 4-4. Toner Powder Printed over Insulating Alumina Powder Bed (Dutta 2002)

This method had problems associated with difficulties in printing a thin and

uniform layer of part powder using the developers designed for cascade development. In

addition to that, the toner powder becomes brittle when it solidifies after fusing, which

makes it a bad adhesive. There is also the concern of keeping the support powder from

falling off the flat build platform without proper support at the sides of the platform to











prevent powder spillage. Due to these problems associated with this method it was not


pursued further in the research.


Study of Laser Imager System of Printer

The design of a new test bed configuration was necessary to avoid dependency on


the use of complex commercial printers. Most of the basic activities performed by the


printer can be replicated by using controllers, switches and sensor mechanisms. It is only


the laser imager hardware that cannot be duplicated so easily and therefore an attempt


was made to modify the laser imager from the HP Laserjet-4 printer, which can be used


as an imaging unit for the new test-bed (Fay 2003). The schematic of the HP Laser Jet 4


imager is shown in Figure 4-5.

Feedback
1 Motor Enable Switch
2
3 24V Supply
4
5 Speed Control
Ground Supply


Polygonal
Mirror on Photosensor Diode
Stepper PD
MStoPr LD LaserDiode
GND
Ground Supply

5V Supply

2 Sensor Feedback

Ground Supply



Beam
Detection
Sensor
Mirror Laser Diode

Figure 4-5. Diagram of Laser Jet 4 Imager Assembly (Fay 2003)

The imager is controlled by a formatterr" and a "dc controller". The formatter


decodes the image file sent from the computer and sends electrical pulses to the laser to


go on and off according to the image data, while the dc controller controls the motors for









the movement of the laser beam. The modifications to the laser imager were done by

replacing the formatter by sending electrical pulses directly to the laser from the

computer and controlling the beam on and off according to the image data. The functions

of the dc controller were replaced by controlling the motors and the sensors externally

through Galil motion controller interfaced with the computer. These modifications

removed any kind of dependency of the laser imager system on the printer and all the

control was done from the computer. The design and development of the Flat

Photoconductor Plate Test-bed is explained in detail in Chapter 8.














CHAPTER 5
DESIGN AND TESTING OF IMAGE DEVELOPERS

Toner powder is the only material that has been so far used successfully to print in

layers on top of each other to form three-dimensional objects. When printed in multiple

layers it suffers from quality and reliability problems as discussed in chapter 4. This is the

motivation for exploring the feasibility of printing powders other than toner in multiple

layers using ESFF to form 3D objects. These powders, which includes various conductive

and insulative materials, have different properties than toner and may not have the

limitations posed by toner powder during printing. This also gives us an opportunity to

explore the possibility of making parts of different types of materials some of which can

be used as functional prototypes.

For using variety of powders with different physical characteristics, image

developers were designed and built to develop these powders on the photoconductor

surface. These developers were based on cascade development system, due to the fact

that the cascade development system uses gravitational force to supply powder for

development and so is independent of physical properties of the powder. These image

developers had to be tested before putting them on the test bed to be used as powder

developers. The parameters to be tested in these developers are the effectiveness of

charging of powder and the amount of powder developed on the photoconductor surface.

These can be tested by measuring the total charge and mass of the powder developed on

the photoconductor surface. From these readings we can estimate parameters like charge

per unit mass (Q/M) and mass per unit area (M/A) which are important for the quality of









print (Schein 1988). The initial discussion is presented on the development in the powder

developer design. This powder developer is designed to use variety of powders including

both conducting and insulating powders. The later discussion is based on the

development of charge and mass measurement test setup to test the powder developer

efficiency in charging and developing powders.

Developer Design

To use powders other than the toner powder, developers were designed and

fabricated which could replace the toner developer assembly in the toner cartridge of the

printer. For testing the efficiency of these developers they have to be also suitable to be

used in the charge and mass measurement test setup.

In Chapter 3 there is a detailed description on the developer system and its

components. The function of the development system is also discussed with respect to the

electrophotographic cycle. Again in Chapter 4 we discussed that to be able to print

powders with varied physical properties, the developer should be designed on the basis of

cascade development system and should have its own powder box and charging and

powder supply mechanism suitable for cascade development. The following discussion

describes the basic components of the developer assembly design and the realization of

the latest version of powder developer through continuous improvements.

The function of the developer assembly is to store, charge, transport, and transfer

the powder for development and re-circulate the residual powder. The developer

assembly consists of the powder box and the "nib" assembly. The storage of the powder

is done in a powder box which also acts as a powder hopper for maintaining the powder

supply for development. The nib assembly consists of the developer roller, the doctor

blade and a casing to hold these together. The charging of powder is achieved by









injection charging method. Voltage is supplied from high voltage source to a metal strip

attached to doctor blade. As explained in Chapter 3, the doctor blade is a thin long plate

which is kept pressed against the developer roller along the length of the roller. When the

powder comes out through the gap between the roller surface and the blade surface, they

get squeezed and rubbed against the metal plate attached to the doctor blade and get

charged due to contact with the voltage supply.


Powder Box

















Figure 5-1. Solid Model Assembly of Powder Developer
The developer roller brings out the powder for development by friction. The roller










has a rough surface which is porous and could trap the powder in these pores and bring

them out. The powder also gets smeared on the roller due to squeezing action between

roller and doctor blade. This helps in bringing out more powder for development. The

powders also have Van der Waals force of attraction between each other and help in

attracting more powder for transport to the development region. The transfer of powder is

due to the field created between the developer roller and the photoconductor drum, which

creates the appropriate force on the charged particles. The remaining powder which was









not developed is re-circulated by friction as the roller surface rubs against the inner

surface of the developer roller casing. The fabrication and the improvement in the

developer design are presented in the following discussion.

Powder Box

The powder box is made of transparent acrylic plates, which makes it easier to

inspect the powder flow and the level of powder. The bottom plate of the powder box is

made at an angle to the horizontal, which helps in the flow of powder due to gravity. In

order to allow easy flow of powder, this angle should be greater than the angle of repose

of the powder. The angle also should not be very steep because this will make all the

powders accumulate at the bottom and increase the pressure on the developer roller,

which will make it difficult for the powder to re-circulate back into the box. As an initial

guess this angle is chosen as 34.1 degrees, which is the angle at which the toner cartridge

sits in the laserjet printer. This angle worked well with most of the powders put in the box

for development. The front plate of the powder box has a pair of arms attached to it by

screws. These arms help in assembling and positioning of the developer with the charge

and mass measurement setup, which will be discussed later in the chapter.

Developer Roller

The developer roller is made of conducting polymer with a metal axis. This enables

to apply voltage to the powder to charge the powder and also create a field with the

photoconductor surface during development. As said earlier, the rough surface of the

polymer of developer roller helps in supplying powder to the development region. It also

circulates back unused powder after development into the powder box. The recirculation

of powder is made possible by a particular geometry of the lower cup like structure of the

developer roller casing.









Developer Roller Casing

The casing is designed to accommodate the components of the nib (developer roller

and doctor blade) and along with that ensure a smooth flow of powder in and out of the

powder box without any leakage. The casing has flanges that help in attaching the nib to

the powder box. The top part of the casing provides support to the blade. The bottom part

of the casing provides support for the developer roller axis and has a cup like structure

called 'lower lip' that protrudes below the developer conforming to the roller surface.

This special geometry retains the undeveloped powders for their recirculation back into

the powder box. The gap between the lower lip and the photoconductor drum is critical; a

large gap will result in powder leakage and a small gap will bring the lip edge closer to

the photoconductor drum and may scrape developed powder off the drum surface, which

also may cause damage to the surface of the photoconductor drum. These requirements

make the shape of the casing complicated and so it was manufactured out of ABS using

rapid prototyping (FDM machine).

The developer was designed to not only fit the charge and mass measurement test

setup, but also to be used as a powder developer replacement for the toner developer in

the toner cartridge. The toner cartridge consists of the toner powder developer, which is

attached by pivoting arrangement with another component assembly called "front-end"

(Figure 5-2, which includes the photoconductor drum, the charging roller, the cleaner

blade and the cleaner box). One of the early designs of the developer assembled with the

"front end" is shown in Figure 5-3.




















Charge
Rollerf ff
Photoconductor

Figure 5-2. Front End of the Toner Powder Cartridge


Figure 5-3. Cross-section of Developer Assembled with the Front End

The preliminary designs suffered from some serious drawbacks related to powder

leakage. There was also the problem of inefficient powder charging and powder flow and

recirculation control. Almost all the problems were related to the way the "nib" assembly

was designed. This nib assembly was modified and improved with each new developer

design (Fay 2003). In every next generation of developer design it was taken care that the









positive attributes of the previous version are implemented and the problems associated

with it are solved.

The last developer that was designed solved most of the above mentioned common

problems associated with the designs before that. It considerably reduced the powder

leakage to be negligible and increased control over the powder flow. There is still some

small amount of leakage associated with this design that can be removed when the

developer is manufactured precisely by taking care of the close tolerances in the design.

It can also be mentioned here that, as long as cascade development is used as the

method for development, the problem of powder leakage will remain. Even in

commercial color printers which use non-magnetic toner powder and cascade

development, there is a small leakage of powder. The issue of powder leakage may

matter when the developer is used directly in the printer, but is not of a concern in the

charge and mass measurement test setup. This is because the charge and mass readings

are taken related to the powder that is actually developed on the photoconductor drum

and so the leaked powder does not affect the readings.

Pivoting Blade Powder Developer

As mentioned before, the design of the nib assembly is the most challenging and

has gone through a lot of modifications till the last one was designed with pivoting blade.

The doctor blade has a hole in the middle to allow for pivoting by using pins to act as

pivot points. These pins also hold the blade by these holes. The blade is bent at the

middle, along the length, to allow the positioning of the developer roller in the compact

unit and also enables the blade to be kept pressed against the developer roller. It is made

of ABS by rapid prototyping (FDM machine). A copper sheet is glued to the bottom part

of the blade (which is pressed against the roller) to charge the powder flowing through










the gap between the blade and the roller surface (by contact, injection charging). The

blade also controls the amount of powder supply for development. This pivoting blade

design has better control over powder flow as the angle of the pivot can be changed easily

to change the gap between the blade and the roller surface. The conductive powders get

charged by the field created between the developer roller and the photoconductor drum.



Powder Box
Top Cover
Pivoting Arm






Pivoting
Pin






Doctor Blade

PivotAxis Pinr Developer Roller
Doctor Blade Ca i
Developer Roger Axis
Figure 5-4. Solid Model of Developer Assembly for Pivoting Doctor Blade Powder
Developer (with Cross-sectional view)

The schematic diagrams below show the developer cross-section (Figure 5-5),

cross-section of assembly of developer with the "front end" assembly (Figure 5-6).












Powder Box


,Pivot Arm


Pivot Pin


;-Doctor Blade (Pivot Point)


-Developer Roller


Lower Lip


Figure 5-5. Cross-sectional view of the Developer Assembly


Figure 5-6. Cross-section of Developer Front End Assembly with the Pivoted Doctor
Blade












Development of Charge and Mass Measurement Test Setup

Discussion on Development of First Test Setup and Testing Concepts

The first charge and mass measurement test setup was developed using the

principle of Direct Charge measurement (Gokhale 2001). The schematic of the test setup

is shown in Figure 5-7.


Drive
system


Pou der
Cmitidge


drum


Sliding
PI atform


Figure 5-7. Schematic of Charge and Mass Measurement Test Setup (Gokhale 2001)

The charge and mass measurement test setup had an organic photoconductor drum

driven by a stepper motor (This photoconductor drum and motor assembly was borrowed

from the HP laserjet printer). The motor rotates the photoconductor drum with a fixed

velocity equal to the velocity at which it originally rotates inside the laser printer. The








photoconductor drum along with the drive system is mounted on a movable platform,

which slides on rails. The original toner powder developer taken from the printer was

used and a custom made stand was made to hold the developer in place. This stand kept

the developer at an angle (34.1 degrees) that is the same as that used for positioning the

developer in the commercial printer. The photoconductor and the drive assembly

mounted on the sliding platform are moved to engage with the developer assembly. The

schematic below explains the method of Direct Charge measurement in the charge and

mass measurement test setup.



Magnetic Powder
Deueloper Cartridge "-.

--" I


Photoconductor

? D. C biased AC
development voltage


O Electrometer





Figure 5-8. Charge Measurement Setup for Direct Charge Measurement (Gokhale 2001)

The toner developer is connected to a voltage source producing dc biased ac

voltage to charge toner powder and create powder cloud in the development nip region.

The photoconductor drum is connected to the ground through an electrometer. The

electrometer has the ability of integrating current that passes through it to measure the









charge flow. The charged powders get developed on the grounded photoconductor drum

surface. This deposited charge attracts equal and opposite charges from the ground to the

aluminum drum through the electrometer, which measures the current flowing through

from the ground to the aluminum drum. The electrometer then integrates the current with

respect to time and displays the total amount charge flow from the ground. For the mass

measurement, the photoconductor drum had to be removed from the set-up and weighed.

The drum is rotated for one revolution which allows using the value of the surface

area of the drum for calculating mass of powder developed per unit area. Removing the

photoconductor drum is a complicated process and this could affect the mass readings

due to the possibility of powder loss from spillage. This is solved by covering a layer of

Mylar sheet over the photoconductor surface and removing it for mass measurement,

without removing the whole photoconductor as before.

This setup had problems because of the uncertainty of positioning the

photoconductor drum with respect to the developer roller. When the rollers do not have a

line contact at the development nip the toner development becomes non-uniform.

Moreover, as the developer roller is driven by the meshing of its gear with that of the

photoconductor drum the drive of the development roller can be shaky due to

misalignment. The stand on which the toner developer sits is customized for the

particular geometry of the developer and has to be changed for any change in the

developer geometry. To solve the above problems the test setup was modified by

focusing on redesigning of the mechanism by which the developer assembly engages

with the photoconductor drum. In all the improvements done to the test setup, the basic

principle of direct charge measurement is used as the method to measure charge.









Improvement in the Design of Test Setup

In the HP LaserJet 4 printer the developer assembly pivots by two pins and gets

engaged to the photoconductor drum by its weight. This aligns the surface of the

developer roller and the photoconductor drum to have a line contact for uniform

development throughout the length of the photoconductor surface. This also provides the

force, due to gravity, which keeps the gears of the developer roller and the

photoconductor drum engaged. This pivoting concept was used in the improved charge

and mass test setup. The solid model of the test setup with the toner powder developer is

shown in the figure below.


Toner CQ ndge Developer



















Stepper Motor

Figure 5-9. Solid Model of the Assembly of Toner Powder Developer and Charge and
Mass Measurement Test Setup

The photoconductor drive assembly, which used to slide on rails in the last design,

was removed from rails and used for this design. Side-supports were designed to hold the

photoconductor drum in place. The side supports had circular slots made at a particular









angle to hold the developer assembly by its pins and allow it to pivot around the pins. The

developer assembly comes to a halt when the developer gear meshes with the

photoconductor gear. This type of engagement placed the developer exactly in the same

orientation as that would be in the printer. This was a much simpler design and was easy

to disassemble for mass measurements of the developed toner.



Toner Cartridge
Deveoper Assembly

0















Figure 5-10. Cross-section View of Charge and Mass Measurement Test Setup

The development roller was provided with -570V dc biased ac voltage at 1780V (p-

p) and 1754 Hz frequency from the voltage source to create powder cloud in the

development nip, simulating similar conditions as there in the development region of the

laser printer (Zhang 2000). The photoconductor drum was grounded through electrometer

for measurement of charge flowing through the ground to balance the charge on the





The stepper motor was started, and stopped when the photoconductor had made one
S0_. 0 D opef








Figure 5-10. Cross-section View of Charge and Mass Measurement Test Setup

The development roller was provided with -570V dc biased ac voltage at 1780V (p-

p) and 1754 Hz frequency from the voltage source to create powder cloud in the

development nip, simulating similar conditions as there in the development region of the

laser printer (Zhang 2000). The photoconductor drum was grounded through electrometer

for measurement of charge flowing through the ground to balance the charge on the

powder particles developed.

The toner was printed on a Mylar sheet wrapped around the photoconductor drum.

The stepper motor was started, and stopped when the photoconductor had made one









revolution observed by visual inspection. Then the developer assembly was removed and

the photoconductor drum taken out to measure the difference between weight of the

photoconductor drum before development and after development to find the mass of the

powder developed. The electrometer gave the charge reading and the area was taken to be

the surface area of the photoconductor drum due to single rotation of the drum during

development. The charge, mass and area readings were used to calculate charge per unit

mass (Q/M) and mass per unit area (M/A). Tests were carried out using this setup to find

out the Q/M and M/A of toner powder. The results are reported in table 5-1.

Table 5-1. Q/M and M/A calculations of toner using QMM test setup
M1 (gm) M2 (gm) Q (QtC) M=(M2-M1) Q/M A (cm2) M/A
(gm) ([C/gm) (gm/cm2)
96.8690 97.0367 -1.238 0.167 -7.413 Overrun
96.8678 97.0092 -1.0438 0.1414 -7.382 201.41 7.02x10-4
96.8676 97.0218 -1.254 0.1542 -8.1328 209.84 7.34x10-4
96.8653 97.0211 -1.2134 0.1558 -7.7878 209.84 7.42x10-4
96.8661 97.0251 -1.1401 0.159 -7.1706 209.84 7.57x10-4
Average -7.5772 7.34x10-4


The average Q/M was found out to be -7.5772 [tC/gm and the average M/A for

toner powder was found out to be 7.34x10-4 gm/cm2. These values were found to be

similar to the values obtained by the test done using QMM setup with fixed cartridge

developer assembly and sliding photoconductor driver assembly (Gokhale 2001). This

setup successfully replicated the older design results and had less complicated design and

test procedure.

As discussed before, powder developers were designed and fabricated to be used

with both the "front end" of the toner powder cartridge and the charge and mass

measurement test setup. These powder developers enabled the development of powders

other than toner. Similar to toner powder cartridge, these powder developers are engaged









with the front-end and the charge and mass measurement setup by the pivoting

arrangement.

The powder box in the developer assembly has two protruding arms attached to its

front plate by screws. These arms have pivoting pins protruding outwards that are hooked

into the slots on the support provided in charge and mass measurement test setup. This

enables the powder box to pivot around the pins and get positioned on the transfer drum

by its weight. As the developer roller gear is driven by the photoconductor drum gear, the

weight of the developer assembly helps in keeping the two gears engaged all the time

without any extra arrangement. The positioning of the powder box can be adjusted by

changing the arm length and the angle that the arm makes with the front plate. The

assembly of powder developer with the charge and mass measurement test setup is shown

in the figures below (figure 5-11, solid model) (figure 5-12, cross-section).





















Figure 5-11. Solid Model of Assembly of Developer and Charge and Mass Measurement
Test Setup































Figure 5-12. Cross-section of Charge and Mass Measurement Test Setup with Improved
Developer Assembly



Independent Charge and Mass Measurement Test Setup

As seen in the last two charge and mass measurement test setups, both of them

were based on developing of powder on the photoconductor drum driven by the stepper

motor and gearbox assembly originally used in the Laser printer. The stepper motor

rotates the photoconductor drum at a fixed speed and hence there is no speed control. It

was also necessary to discharge and clean the photoconductor surface before it goes for

the next cycle. This demanded more photoconductor surface area to be available for

assembling these new additions which was not possible with the small diameter

photoconductor drum available from the Laser printer.

Therefore it was decided to build a charge and mass measurement test setup

independent of the restrictions of borrowed laser printer mechanisms. This new design









configuration has its own controllable drive motor, transfer roller, developer, charger, and

cleaning mechanism. Before going into the detail design of the charge and mass

measurement test setup, some basic knowledge about powder behavior and

photoconductor cycle is essential.

This particular charge and mass measurement test setup was designed to be used

with powders of any kind including polymers and metals. Polymers and metals represent

the two categories of materials available based on their response to electricity; while

polymers represent insulator family, metals represent the conductor family. For the

charge and mass measurement tests conducted with the powders, iron and nylon-12 are

chosen from the conductor and insulator families respectively. Iron is used due to its easy

availability and nylon-12 is used because of its good developing characteristics on

photoconductor drum (details presented in chapter 7).

Design Considerations

The insulators can be charged triboelectrically when they are rubbed against the

doctor blade and developer roller. They also get charged when rubbed against a

conductor surface connected to the voltage source (injection charging). They have surface

and volume charge densities and do not get discharged easily. The charged polymers can

develop over conducting as well as insulating surfaces. Consider a charged polymer

powder placed on a conducting plate (which provides charge to the polymer by injection).

If a grounded electrode is brought near this plate, a field will be created which will

produce force on the charged polymer powder to travel to the grounded electrode. Once

the polymer powder reaches the grounded electrode, it remains attached to the electrode

due to the image forces created by the opposite charges flown to the grounded plate from

ground due to the presence of the developed polymer powder. It does not matter if the









grounded plate surface is conductive or insulative, as the insulative polymer powder does

not lose charge even when in contact with a conductive grounded surface. This is not the

case with conductive powders.

The metals on other hand get charged when they are placed in an electric field

created by charged surface (induction charging) and also when they are in contact with

conducting bodies connected to voltage supply. Metals cannot support volume charge

density and all the charges are present on them are on the surface. Conductors cannot get

developed on a conducting surface, which can be explained as follows.

Electrode with insulative
surface




Voltage ++ Conductive powder
Source DC particles _





Electrodes with conductive
surface
Figure 5-13. Schematic Illustration of Behavior of Conductive Powder Particle in the
Presence of Conductive and Insulative Electrode Surfaces

Consider a metal particle in an electrostatic field between two metal electrodes.

One of the electrodes is provided with positive voltage and the other with negative

voltage. If initially the particle is in contact with the negative electrode, it will get

charged negative and move in the direction of the positive electrode. When it reaches the

positive electrode it will loose its charge and get positively charged and move in the

opposite direction towards the negative electrode. Once it touches the negative electrode

it loses its charge and gets negatively charged and the cycle repeats. So, when we place









conductive powder particles between two electrodes with conductive surfaces a powder

cloud is formed due to bouncing of particles between the electrodes.

When the charged metal particle encounters the insulator surface it does not lose its

charge to the electrode. The particle is attached to the insulator surface by the

electrostatic force of attraction by the opposite charge present in the electrode at the

backside of the insulator. So, to develop conductive powder on a surface, the surface

must be an insulator.

Due to the above considerations, the drum on which the powders would be

developed in the QMM setup should be a metal drum with an insulator cover. It is more

desirable if the insulator has a high dielectric strength to withstand high potential

differences in the developing region. Mylar sheet is suitable for this requirement and so is

used as the insulative covering over the metal transfer drum.

When one layer of charged powder gets developed and then cleaned off the drum

surface to make the surface ready for the next layer of powder, the powders leave some

of their charge on the surface. This charge is of the same sign as that of the freshly

charged powders. If this is not neutralized before each development, the charge gets

accumulated and prevents the development of more powder on the surface due to

electrostatic repulsion. To prevent this, a charge roller is placed on the drum which

discharges the surface of the transfer drum by getting rubbed against it after it has been

cleaned. The charge roller is provided with ac voltage that tries to charge the surface in

both signs, which effectively discharges it. The charge roller is placed between the

cleaning region and the development region of the developer cycle.










Stages of Charge and Mass Measurement Test Cycle

The charge and mass measurement test setup is based on a part of the

electrophotography cycle. The test cycle does not include some of the stages of the

electrophotography cycle dealing with printing of developed powder on a substrate. The

electrophotography cycle is explained in detail in chapter 3. In the charge and mass

measurement test cycle, the events are distributed around the transfer drum. Powder is

developed on the transfer drum from the developer roller and gets collected in the cleaner

box when the cleaning blade cleans the surface of the drum. The drum surface is then

discharged to make the surface ready for the next development.

Discharging
Discharge Roller

II I I Transfer Roller


1I
II I
__ __ Cleaning (Cleaning
.---- Blade)



I


Development
Figure 5-14. Different Stages of the Test Cycle with respect to the Cross-sectional view
of the Charge and Mass Measurement Test Setup.









Design and Building of the Charge and Mass Measurement Test Setup

The discussions above serve as guidelines for the design and building of the charge

and mass measurement test setup. In this section detailed design and assembly of the test

setup is discussed. As a good design practice the design of the test setup was made in a

modular fashion which allows individual parts to be assembled to form subassemblies

and these subassemblies are assembled to form the final assembly. Each of these

subassemblies is independent of each other and can be easily removed from the setup for

experimental or repairing purposes. The charge and mass measurement test setup is made

of three major subassemblies. These include the transfer drum assembly, the cleaner box

assembly and the motor and frame assembly.

Transfer Drum Assembly

The transfer drum assembly is made of the transfer drum, the charge roller, drive

attachments and support assembly to fix it to the frame of the charge and mass

measurement setup. The transfer drum is made of a two-inch diameter aluminum pipe

covered with Mylar sheet, which provides an insulative surface for conductive powder

development. The end caps of the drum are made of high-density polymer and have

interference fit with the drum. These end caps provide low frictional surface against side

supports during rotation. A steel rod is used as the axis of the drum, which passes through

the end caps and rests in the holes provided on the side support plate. These holes work

as bearings for the axis to rotate.










-- -- -- ,j P n -- __ __ __ __ _
I V- T ", Polle.
T- iot P2 IN









Cleaner Box /
Figure 5-15. Solid Model of Transfer Drum and Cleaner Box Assembly (with Cross-
sectional view)

On one side of the drum there is a pulley and on the other there is a gear, both

tightly assembled with the axis of the drum. A servomotor, which is independently

controlled by Galil Motion Controller, is used to drive the pulley through a timing belt.

The timing pulley and belt drive provides a positive drive between the servomotor and

the transfer drum without any slip. This also gives flexibility in the placement of the

motor in the whole assembly, which is possible because the length of the belt can be

altered to suit any convenient location for the motor and this is difficult to attain with a

gear drive. The transfer roller drives the developer roller by gear arrangement. This gear

drive is formed by meshing of gear of the transfer roller with the gear on the developer

roller. This arrangement maintains the same surface velocities of the developer roller and

the transfer drum at the development zone which is necessary for better development.

This is because, if the surface velocity of the developer roller surface is slower than the

transfer drum surface then very less powder will get developed and if it is faster then

there will be more powder coming out of the box, when not developed, may get

accumulated in the lower lip and leak.









The charge roller is placed pressed on the transfer drum surface and is located

between the cleaner box assembly and the developer assembly. Its role is to discharge the

surface of the development roller after the surface is cleaned off the developed powder.

This discharging of the surface removes any unwanted charge buildup on the transfer

roller surface which may affect the powder development.

The charge roller is a steel rod covered with a conductive polymer and is held

pressed against the transfer roller surface by spring-loaded holders. The surface of the

charge roller is smooth for uniform discharge of the transfer roller surface. This roller

rolls on the transfer drum surface by friction.

Cleaner Box Assembly

The cleaner box is a rectangular box with a cleaning blade attached to it. The

cleaning blade which was originally used to clean the photoconductor drum in the laser

printer cartridge is modified to be used to clean the transfer drum in this application. It

has a high friction flexible polymer sheet attached at its end to clean the roller surface.

The cleaner-box has flanges on both of its sides that have screw holes to fix the assembly

with the rest of the charge and mass measurement setup. By tightening or loosening these

screws the pressure at which the blade is pressed against the transfer drum surface is

varied. This kind of arrangement makes it easier to weigh the developed powder mass by

removing the cleaner box from the assembly and weighing it with the powder in it. There

is a chance of powder leaking by not getting caught by the bottom lip of the cleaner box

during the cleaning process. This is avoided by attaching a thin flexible polymer sheet at

the bottom edge of the box that acts as a one way valve as it allows the developed powder

to pass through, but does not allow any cleaned powder falling down to leak. The









schematic below shows the transfer drum assembly and the cleaner box assembly with

the relative position of the components.

PSc l la s Iharge Roller FSprlng Loaded Suppori






If )rlopr DruC- -









made of angle brackets. These angle brackets have holes at the base for the stand to be






bolted down and fixed to any base plate for extra stability. The stand has holes on the top
bridge to bolt the transfer drum assembly to it and hang from it. The cleaner box

Figurassembly is screwed onto the side Cross-sectional view of the transfer drum assembly and the developer
assembly hangs from the pivot holes provided on the side plates of transfer assembly.Assembly.
Motor and Stand Assembly








The servomotor that drives the setup is supporattached by screws to one side of thealuminum stand which has leg supports

screws angle attached through elliptical slobrackets made to adjust the position ofe for the stand tor and
bolted down and fixed to any base plate for extra stability. The stand has holes on the top

bridge to bolt the transfer drum assembly to it and hang from it. The cleaner box

assembly is screwed onto the side plates of the transfer drum assembly and the developer

assembly hangs from the pivot holes provided on the side plates of transfer assembly.

The servomotor that drives the setup is attached by screws to one side of the stand. The

screws are attached through elliptical slots made to adjust the position of the motor and

tighten the timing belt by sliding the screws up or down along the slots. The Galil Motion

controller controls the motion of the servomotor by providing signals generated from the

commands given by the software interface. These signals are amplified for the motor







72


drive by using amplification circuitry. The figure below shows the complete assembly of

the charge and mass measurement test setup with the powder developer.


hLrnf m1n1u SLJp Fp'i


T, oo., ,c, f- RItl






rL' B L r' ,.::l, ) ,:.r ','F-:;rr


E4Iwri:.ll iSlicr. *: Adiju
.,uIo ,:i,, Po: iu,::,rj


Figure 5-17. Solid Model of Motor and Stand Assembly


Bg" &::t hi:_, i : jk-i P::1]


$a


P:- :_.@ B,:,.


"~t-


D.l *.;:pr i P ::.UAr G~i M; durr
i ij Tiii,_,r, Pp::1.,?- G _rM


Figure 5-18. Complete Assembly of Powder Developer with Charge and Mass
Measurement Test Setup









Experimental Results

Experiments were conducted to determine the charge per unit mass of iron and

nylon -12 powders. The charge and mass measurement values were evaluated and the

dependence of these values with the experimental variables was investigated. The

following tables present the experiments done and the observations reported.

Experiments with Iron Powder

The following experiment was done to find out the effect of the number of

revolutions of the transfer drum on the value of Q/M when the voltage applied remains

constant.

Size of powder = 60[tm diameter

Constant Charging Voltage = 500V

Voltage on Discharge Roller = 960V A.C. (p-p) 2.5KHz (explained in chapter 6)

Table 5-2. Variation of the Q/M readings with the number of revolutions of the transfer
roller
Number of Initial Final Initial Final Change Change in Q/M
Revolution Mass Mass Charge Charge in Charge(pC) (iC/g)
s (Mi) Mass(g)
2 230.931 231.727 83 nC 0.2iC 0.796 0.117 0.147
4 231.727 232.151 63 nC 0.15pC 0.424 0.087 0.205
6 232.151 233.328 -6 nC -153nC 1.177 0.147 0.125
8 234.811 236.294 -32 nC 0.51 C 1.483 0.478 0.322
10 236.294 238.121 -10 nC 0.27iC 1.827 0.26 0.142


Other than the reading for eight revolutions, it can be seen that the value of Q/M

remains almost constant with a small fluctuation which can be attributed to experimental

errors. This can be explained by assuming that iron develops in a monolayer.

To find out the dependence of the value of Q/M on the voltage applied to charge

the powder, we have the following experiment. In this experiment we increase the









voltage from 500V to 1500V and back to 500V. The number of revolutions of the transfer

drum remains constant at 2.

Number of revolutions = 2

Voltage on Discharge Roller = 960V A.C. (p-p) 2.5KHz

Table 5-3. Variation of Q/M measurement with the increase in voltage supplied for
development while the number of revolutions of the transfer drum remains
constant.
Charging Initial Final Initial Final Change Change Q/M
Voltage Mass Mass Charge Charge in Mass in (GC/g)
Charge
500 231.415 231.873 29nC 137nC 0.458 0.108 0.231
1000 231.873 232.295 81nC 0.53uC 0.422 0.449 1.064
1500 232.295 232.812 63nC 1.52iC 0.517 1.457 2.818
1500 232.812 233.323 35nC 0.91 C 0.511 0.875 1.712
1000 233.323 233.775 19nC 0.22iC 0.452 0.201 0.445
500 233.775 234.214 10nC 98nC 0.439 0.088 0.200


This indicates that Q/M depends on the applied voltage to charge the powder. The

increase in voltage causes increase in the value of Q/M and vice versa. This can be

explained for the metal powders as follows. When we apply higher charging voltage, the

charge on the powder particle increases due to the relation Q = CV, where C is the

capacitance of the powder and is dependent on the geometry of powder and the dielectric

constant of the medium of development. So for a powder particle of a particular size

developed in a particular medium, the charge on the powder depends on the voltage

applied. When conductive powders develop, they form a monolayer of development. This

limits the powder mass developed on the transfer drum surface. Therefore, when voltage

increases, Q/M increases due to the increase of charge and constant mass development.

These two trends can be observed from the table above.









Experiments with Nylon 12 Powder

The following experiment was done to find out the effect of the number of

revolutions of the transfer drum on the value of Q/M when the voltage applied remains

constant.

Size of powder = 25 30[tm diameter

Constant Charging Voltage = -500V

Voltage on Discharge Roller = 960V A.C. (p-p) 2.5KHz

Table 5-4. Variation of the Q/M readings with the number of revolutions of the transfer
roller
Number of Initial Final Initial Final Change Change in Q/M
Revolutions Mass Mass Charge Charge in Charge(pC) (iC/g)
(Mi) Mass(g)
2 230.269 230.275 134 nC 0.193 C 0.006 0.059 9.833
4 230.275 230.293 23 nC 0.258iC 0.008 0.235 29.375
6 230.293 230.303 35 nC 0.420C 0.010 0.385 38.5
8 230.303 230.316 47 nC 0.612pC 0.013 0.565 43.46
10 230.316 230.342 47 nC 0.763 C 0.026 0.716 27.538


It can be seen that the value of Q/M is fluctuating. Except the last reading, we can

see an increase in the Q/M value as the number of revolutions is increased. This can be

explained by considering that the charge on the developed polymer powders is due to

triboelectric charging by rubbing against the roller and the blade. More number of

revolutions means more rubbing action for triboelectric charging and so the charge on

each particle goes up.

To find out the dependence of the value of Q/M on the voltage applied to charge

the powder, we have the following experiment. In this experiment we increase the

voltage from -500V to -3000V. The number of revolutions of the transfer drum remains

constant at 3.









Number of revolutions = 3

Voltage on Discharge Roller = 960V A.C. (p-p) 2.5KHz

Table 5-5. Variation of Q/M measurement with the increase in voltage supplied for
development while the number of revolutions of the transfer drum remains
constant.
Charging Initial Final Initial Final Change Change Q/M
Voltage Mass Mass Charge Charge in Mass in (GC/g)
Charge
-500 230.342 230.348 4nC 0.220C 0.006 0.196 32.67
-1000 230.381 230.432 25nC 0.156uC 0.051 0.131 2.569
-1500 230.432 230.491 49nC 0.126uC 0.059 0.077 1.305
-2000 230.491 230.548 12nC 0.078uC 0.057 0.066 1.158
-2500 230.548 230.689 13nC 0.024iC 0.141 0.011 0.078
-3000 230.689 230.805 15nC 0.056uC 0.116 0.041 0.353


This clearly indicates that Q/M depends on the applied voltage to charge the

powder. Interestingly, the increase in voltage causes a decrease in charge per unit mass

readings, while it is normally expected to be not so when compared to conductive

powders. Nylon 12 is an insulator and the charged particles do not carry large charges

like conductors. This can be seen by comparing the charge values of nylon with that of

iron. The powders on the developer roller contain more number of low charged particles

than highly charged particles. When the voltage is low the electric field in the

development zone is good for only highly charged particles to get developed. As the

voltage increases, the development field also increases and so now it is possible for the

low charged particles to get developed. These low charged particles compete with the

highly charged particles which results in the marginal decrease in the charge

measurement. The development of large number of these low charged particles explains

the increase in the mass measurement. Due to the combination of both the effects there is

a high rate of decrease in the value of charge per unit mass (Q/M) measurement.














CHAPTER 6
METAL POWDER DEVELOPMENT AND PRINTING

Printing of metal powder using electrophotography has many applications like

printing circuit boards and making functional prototypes. As discussed in last chapter,

there has been a lot of attempt in designing developer based on cascade development

technology to help in printing of powders other than the commercial toner powder. All

these developers were designed to replace the toner powder developer from the

commercial laser printer. It takes a lot of time and effort to modify a commercial laser

printer before it can be used for printing using ESFF. Each new model of printer released

to the market becomes obsolete in 2-3 years. This makes the repair of the test bed

difficult owing to the less availability of spare parts due to the phasing out of the printer.

There was a need to design and build a test bed which is independent of the use of

laser printers and is modular so that any debugging of problems can be done easily.

Chapter 8 discusses in detail about the design and building of the modified test bed. A

simplified schematic in figure 6-1 explains the concept of the design.

The concept is based on the movement of a photoconductor plate in a straight line

passing through the different stages of electrophotography. The development process is

done by first charging the photoconductor plate, imaging with the laser imager and then

developing powder upward, against the gravity, on the downward facing photoconductor

plate. The photoconductor then reaches the build platform and prints the image of the

powder. The discussion in this chapter is on the feasibility of such development and

printing processes used for this concept. This will generate essential feedback









information on the designing and operating the test bed based on the flat photoconductor

plate.



Heater Photocondu ctor pl ate


Supply Roller Build Platforn Cleaner





Charge Roller
Powder Box Z

Laser Imager __X




Figure 6-1. Concept of the New Modified Test Bed

Metal Powder Development

Powder development in the new test bed is done against gravity. This may create

some limitations on this process as discussed in this chapter. The metal powder has some

special properties related to development and transfer using electrostatic force, which is

shared by all conductive powders. So, the discussions and experimental results presented

in this chapter can be applied to any conductive powder. Iron powder was used for the

experiments due to its easy availability, and the results are reported at the end of the

chapter.

Metal powders can be developed in three ways:

* Applying voltage to the developer roller and grounding photoconductor plate
* Applying voltage to photoconductor plate and grounding developer roller
* Grounding both developer roller and photoconductor plate and charging the
photoconductor surface









The first two methods are fundamentally the same as they both create electrostatic

field between them by the application of voltage source to either the developer roller or

the photoconductor plate while the other one is grounded.

In the first method the metal powder will get charged by being in contact with the

developer roller when the grounded photoconductor is above the developer roller and an

electric field is created between them. Development occurs due to the force created by

the presence of the charged particle in electric field.

In the second method metal powder gets charged by induction when the

photoconductor plate, connected to voltage supply, comes over the grounded developer

roller and a field is created between them. The field creates force on the charged particle,

which overcomes the gravitational pull to develop the powder on the photoconductor

surface.

In the third method voltage supply is given to the charge roller, which is used to

deposit charge on the photoconductor surface. The powder is charged by induction due to

the field created by the charge on the photoconductor surface. In this case also the force

due to electric field has to overcome the gravitational pull for the development to occur.

This third method is different than the first two methods because in the third

method image can be created by discharging the deposited charge by UV laser. The

powder gets charged oppositely to the charged areas in the image and gets attracted

towards the charged regions, which limits the metal powder development to charged area

development (CAD) only. So, this is the only method, which could integrate imaging as a

part of the development process. The other two methods are only capable of printing









metal powders in thin uniform layers. In those cases, imaging can be done by the printing

of image-wise layers of polymer adhesives between uniformly printed metal layers.

The metal powder development processes can be analyzed using standard

electrostatic equations to determine the influential variables and the characteristic

numbers, important for development process. This analysis is done in the following

section.

Metal Development Theory

In the flat photoconductor plate test bed, development is done against gravity. As

only the charged particles would experience the electrostatic force to overcome gravity

and get developed on the photoconductor surface, this method of development helps in

the separation of the charged and uncharged particles, which in turn helps in creation of

sharper images and reduces background printing significantly. During development the

powders have to overcome gravitational force, which would require large charge per unit

mass. Later during transfer of the developed powder onto build platform the large charge

per unit mass may make it difficult for the powder to overcome the electrostatic adhesion

force and get printed on to the build platform. Therefore, a careful analysis of these

processes has to be done to understand them well and be able to control them better.

Before getting on with calculation of different parameters for development, the

basic properties of the iron powder used for the experiment are discussed below. The

fundamental electrostatic parameters are also stated for reference.

Constant Parameters used in Calculations

The mass of a single iron powder particle is calculated below assuming the powder

particle as a spherical particle with radius r.

Diameter of the iron powder particle used in experiment = 60 micron










Volume of sphere = 4 = x3.141x(30x106 )3 = 1.13x10 13m3 (1)
3 3

Density of iron (approximately) = 7.87 x 103 Kg/m3

Mass of a particle = 1.131 x 10-13 m3 x 7.87 x 103 Kg/m3 = 8.9 x 10-10 Kg (2)

Breakdown electric field of air = 3.0 x 106 V/m

Permittivity of air = 8.854 x 10-12 C2/(N-m2)

Charge per Unit Mass (Q/M) Calculations

As discussed before, charge per unit mass (Q/M) of the developing powders

determines the electrostatic force on them and also the quality of development. In the

calculations shown below, the minimum and the maximum allowed Q/M of iron powder

particles used in the experiment are calculated.

The schematic below displays the simple force model of a single iron particle

subjected to electrostatic and gravitational forces during development.



F9 = Mg F, = F QE



Figure 6-2. Forces Acting on a Powder Particle During Development

For powder development to occur, the electrostatic force (Fe) should be greater than

or equal to the gravitational force (Fg). To calculate the minimum Q/M required for

development, we use the condition when the electrostatic force on an iron powder particle

is just able to overcome force of gravity. For this condition we have:

Fg= Fe (5)

Mg = QE (6)









Q g (7)
M E

Here, 'g' is acceleration due to gravity and is constant. So, Q/M is controlled by the

value of electrostatic field 'E'. By increasing the value of E we can reduce Q/M. The

maximum value of E can reach 3.0x106 V/m. So, substituting the value of electric field

we get the minimum value of Q/M to be,

IQI = 9.81m/s2
S9.8m 3.27 10-6Coulomb/Kg = 3.27nC/ gm (8)
M 3.0O106V/m

To find the minimum charge on the iron powder particle used in the experiment to allow

it to develop, we use the mass of the particle:

Qmparc = 3.27x10-6 Coulomb/Kg x 8.9 x 10-10 Kg = 2.91 x 10-15 Coulomb (9)


The above calculation shows that Q is independent of the material properties of the


powder used for development and can be used for any powder.

Again from Gauss Law we know that,

S /QIA Q
E---- (10)
So o So (4m2

And,

3
M =,, p (11)


Q 4 7rEOEr 2 3E (12)

3

M max prK
MM)a Pr Pr









SQ is the maximum charge per unit mass that a powder particle can have before it
M) max


produces electric discharge due to breakdown of air. Unlike is


dependent on the powder size and is inversely proportional to the radius of the powder

particle. Finding for iron powder particle used in the experiment we get:
pMc )max

Q 3coEmaxr _3oEmax 3*8.854 x1012 3.0x 106
M max pr2 pr 7.87 x103 30 x10-6

= 3.375x10 4 =3.375x10-7 tC/gm= 0.338 [tC/gm (15)
max Kg

This gives the maximum charge an individual powder particle can have without sparking,

which is calculated as follows:

Qmax/ =- M 3.375 x10-4 C *8.9x1010 Kg = 3.0042 x 10-13
parhcle M)max Kg

Coulomb (16)

Maximum Surface Charge Density

The following calculation estimates the maximum surface charge density that can

be sustained by the powder particles in air. The calculation is done considering electrical

breakdown field limit of air, Emax= 3.0 x 106 V/m.

From Gauss law,

E=
AEo Eo


Omax = Emax o =3.0x106 *8.854 x1012 = 2.656 x105 C/m2









This is the maximum charge density that a surface can have when it is kept in air.

This limit is called Gaussian Charge Limit. This is independent of the material properties.

Electric Field Range

We know that the maximum allowable field limit for powder development in air is

3.0 x 106 V/m. The following calculation finds the minimum field limit for powder

development considering that the powder particle is charged to its maximum allowed

charge limit, and the electrostatic force for development just overcomes the force of

gravity.

QE = Mg

E Mg
Qmax

For the iron particle used for the experiment, using the maximum charge value from

equation 16, we get:


E =M 8.73 x109 = 2.9059 10 V
mm Q 3.0042 x 10-13 m

This suggests the value of electrical field in the development zone should be within the

range of2.9x104 V/m to 3.0x106 V/m.

The maximum voltage available from the voltage source is 5000V. The gap

between the developer roller and the photoconductor surface during development has a

minimum limit to avoid breakdown of air. This gap is calculated as follows:

Vma 5000V 5
h max mm = 1.67am .
mm(expenment) E 3.0 06 V 3
max 3.0 x 106
m

During experiment the gap is taken to be more than or equal to 2mm to avoid any

sparking due to air breakdown.