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Hot Tack Seal Test on Golden Flake VFFS Machines Report and Activities
Mentor: Bruce Welt
College of Agricultural and Life Sciences
Hot Tack and Peel strength performance tests were performed on samples of sealed and unsealed
thermoplastic potato chip pouches from the Golden Flake Potato Chip factory located in Ocala Florida.
Comparisons were made against competitive salty snack packages (Frito-lay) purchased at a local
supermarket. Results suggest that virtually all samples exhibited heat-seal problems that could be mitigated
by modifying sealing temperatures and dwell times.
Key Words: Heat seal, hot tack, impulse, seal strength
Hot Tack is defined as the ability of a heat seal to withstand stress while the seal is still hot
(Soroka, 1999). Hot Tack performance is important in today's fast-paced snack food packaging
world. The higher a heat seal's hot tack strength the faster products can be packaged.
Hot Tack is one of the most important characteristics of thermoplastic films used in bag-
making applications, particularly in vertical-form-fill-seal (VFFS) machines. Vertical-form-fill-
seal machines utilize hot tack strength when products fall on newly formed seals within fractions of
a second after the seal is formed. Soroka, (1999) describes a good hot-tack seal as one that will be
able to resist peeling apart, even though it is still hot.
"The Hot Tack performance of the seal will directly influence the speed at which the VFFS
packaging machine can be operated and/or the maximum weight of product that can be packed
per packing unit" (Anonymous, 2003).
Several companies make Hot Tack testing devices. Solvay Polyolefins Europe, H. W. Theller LLC,
and Brugger are just a few of the companies that have developed machines to test the Hot Tack of
seals using the ASTM F1921-98 guidelines.
The strength of heat seals is often determined by measuring the force required to pull apart the pieces
of film which have been sealed together, either in a dynamic load test or a static load test
(Robertson, 1993). Different laboratory tests have been developed to evaluate hot tack performance
of thermoplastic resins in the form of films:
. The spring test method, according to ASTM D 3706
. The instrumented hot tack testing method, according to ASTM F1921-98
. The falling weight method, according to industry common practice
. The practical test on a VFFS packaging machine
All these methods consist of submitting the hot seal to a tensile force shortly after it has been
formed (Internet, 2003). Work performed in this study involved the use of a dynamic load-
INTRO TO THIS STUDY
. * Realization of defects in Golden Flake bag seals
. * Research Hypotheses: Defects in Golden Flake heat seals are attributable to improperly defined
heat seal settings on Golden flakes VFFS machines.
. * It is expected that analysis of hot tack parameters of Golden flake seals will help to define
more appropriate heat sealing parameters that will significantly reduce heat seal defects
The objective of this study was to determine appropriate seal characteristics for the Golden Flake
chip bags. The manufacturer noted several complaints about the strength and completeness of
these seals and, in particular, the rear seam seal. Golden Flake uses two types of VFFS machines. Lines
A, B, and E are Woodman Co. 1992 models, while lines C and D are 1985 Write models. Observation
of operating lines revealed the following operational parameters (May 27, 2003):
Typical operating parameters observed on Golden Flake VFFS machines
Machine Line Upper Jaw Lower Jaw Seam Temp. Dwell Time Seating Pressure
Temp. (F) Temp. (F) (F) (s) (PSI)
A 295 294 320 0.45 42
B 265 270 314 0.45 42
C No Data No Data No Data No Data No Data
D No Data No Data No Data No Data No Data
E 280 278 317 0.45 42
Mrs. B's 285 285 310 0.34 46
METHODS AND MATERIALS
. Dynisco/Theller Hot Tack Heatsealer with Computer
. * Ruler and scalpel
. * 14 sample unsealed strips (Hot Tack Test)
. 12 Sample bags of chips (3 from each running line samples at 15 min. intervals)
. 3 seals per bag with 4 test strips per seal (144 test strips)
. * 3 Mrs. B's Sample Bags with 12 samples each (36 Samples)
. 10 Mrs. B's Sample shot Tack Strips
. 9 Sample seals from competitor Frito Lay
Test Methods (Theory)
The Theller Hot Tack Heatsealer was used to perform Hot Tack tests on twelve unsealed strips of
source film to identify appropriate sealing parameters. Temperature, dwell time, seal pressure,
and cooling time were evaluated. Ideally, a heat seal should provide uniform strength throughout
the seal, resulting in a profile similar to Figure 1.
1ee8 SEAL PROFILE
.l mp_ -_.|er tiFn - Mr,|, SI
_ ________________lk * sapï¿½ arat og [n
Figure 1. Desirable uniform heat seal profile
The Peel strength test was used to determine the seal profiles of sample bags formed under
conditions outlined in Table 1. "Peel Strength" refers to strength of a heat seal after it has cooled
to ambient temperature and achieved strength stability.
Initially, each VFFS machines sealing parameters was programmed into the Theller Hot tack tester
using the Peel Strength test option. Testing began with Machine Line A, Bag 1 but continued
randomly thereafter. Each seal area (Top, Bottom, and Seam) was averaged from all three bags.
Fourteen virgin film test strips were sealed and tested for Hot Tack. Initial parameters from the 4
VFFS machines were used as the test parameters to eliminate the possibility of sensor readout error
on the actual machines. Next, depending on the seal profiles obtained from these initial tests,
the variables (dwell time, temp, cooling time, and pressure) will be altered slightly to obtain a
normal seal strength profile as shown above.
Chip density testing was performed on sample bags of chips. The contents of each bag were placed into
a container of known volume. This container was then lightly tapped three times to ensure that the
chips settled naturally. The contents were then measured with a ruler by measuring the height of
the container not occupied by chip materials. Once the volume of chips was estimated, bag volume
was estimated using measured bag dimensions. Chip volume density was estimated by dividing
bag volume by chip volume. Experienced potato chip manufacturers suggest that chips should not
take up more than 80 percent of available space in order to minimize catching chip material within
seals (Anonymous, 2003).
RESULTS AND DISCUSSION
Test Data & Results
Golden Flake Potato Chip Line. About 65% of all the standard Golden Flake bags had successful peels
and only 69% of all bags had acceptable peel profiles. Seal strengths and peel profiles for sealed
bags coming off different lines varied in a seemingly random fashion. On several instances, chip
and seasoning matter was located between the seal crimps creating a bumpy profile as can be seen
in Figure 2.
3S- SEAL PROFILE OTIT
0 5 le a5 20 25 3B 35 40 45
Cl-ap Sep4ration il"crease. nn
Figure 2. Chip matter between seals
This problem was not specific to Golden Flake, since it was observed in virtually all samples tested.
Hot Tack tests were initiated with same parameters as the Golden Flake lines. Initially the first
variable changed was a temperature increase from 2650F to 2750F. After several tests, this proved to
be the cause of an increasing arch forward in the profiles. The test temperatures were then lowered
to 2570F and then again to 2440F. These changes caused a peak at the end of the profile indicating
that the last part of the seal was stronger than the interior areas of the seal, which may result in
a bursting effect when bags are opened (Figure 3).
*- HOT SEAL STRENGTH vs COOLING TIMB
ABDee re jae 141
Figure 3. Seal strength peaks at edge of seal
Seal temperature was set back to 2650F and dwell time was increased from 0.45 to 0.50 seconds.
This resulted in a bumpy profile. Dwell time was increased again to confirm this trend. Increase in
dwell time also created a peak at the end of the profile similar to the increased temperatures. Dwell
time was then reduced to 0.40 seconds, which resulted in an acceptable profile, but seal strength
was reduced by as much as 30 percent. This trend was confirmed by reducing dwell time to 0.38 and
0.36 seconds. However at 0.38 seconds and 2650F, seal profile appeared optimal, but with a
strength reduction of about 18%.
Mrs. B'sï¿½ Line. The Mrs. B's line did not perform as well as the Golden Flake potato chip line. Only 11%
of all tests resulted in peel behavior, while none of these provided desirable peel
performance. Delamination of seals accounted for about 88% of all test profiles. Chip and
seasoning matter was present between many of the seals as can be seen in Figure 4. Initial
force required to pull seals apart was fairly high. An average of 26.18 N (+/- 3.3 N) was needed
to separate each seal in all three seal areas. This initially high force, followed by an abrupt decline
in force, indicated that seal would open in a tug and burst manner.
28- SEAL PROFILE
/f5 -?LOT IDFIT
9S~iZ i's 1 l Z 3@n
Figure 4. Delamination and chip debris
Initial hot tack testing parameters were sealing temperature of 3100F, and dwell time of 0.34 seconds
at 46 PSI. This resulted in a flat curve with a large peak at the end as evident by Figure 5.
-l HOT SEAL STRENGTH v COOLING TIME
S8 a' 4e 14
Figure 5. Improved seal profile for Mrs. B's Brand chips
Initially dwell time was increased to 0.38 sec, which evened out the curve but resulted in a
slightly melted seal. Temperature was then dropped to 2850F at 0.38 sec dwell time and 46 PSI.
This profile became closer to ideal except for a larger peak at the end of the test as seen in Figure 6.
le- HOT SEAL STRENGTH v8 COOLING TINE
FI Fir.- rr-.1
2 age 4"B 60B 8so le00 1260 14
Figure 6. Improving seal profile for Mrs. B's Brand chips
Dwell time was reduced to 0.34 sec and then increased to 0.420 sec, which resulted in an
unfavorably curved profile and then flat profile with a peak, respectively. Dwell time was increased
again to 0.45 sec, which melted the seal. To compensate the temperature was reduced to 275ï¿½F
with 0.45 sec dwell time and 46 PSI. This produced the best profile, however, the seal was still
slightly melted. Temperature was reduced again to 2650F, which resulted in a near ideal profile.
Seal strength remained an issue, as this seal was relatively weak. The force required to separate the
seal was about 2.15 N. Dwell time was increased to compensate to 0.46 sec, which increased
seal strength to about 2.59 N. This profile was not as even, as can be seen in Figure 7.
'S - HOT SEAL STRENGTH ve COOLING TIME
I 35K 4ie 6RI Goa loaa laos 141
Figure 7. Best seal performance found Mrs. B's Brand chips
Therefore, it appears that significant improvements to Mrs. B's seal characteristics are possible
by running at a lower temperature of 2650F, and slightly longer dwell time of about 0.46 sec.
Frito Lay@ Line. Results showed that only about 55% of all seals resulted in a smooth peel.
Remaining seals resulted in delamination and seal breaks. No Frito-Lay samples displayed ideal
seal profiles. Profiles indicated several problems including uneven seal temperatures and chip
and seasoning matter between seal crimps as can be seen in Figure 8.
as- SEAL PROFILE
P1 PLOT TDIT
3 i e A 5 2S a 3S 35 s 4U '4
Clani Separatt on increaseA W I
Figure 8. Bumpy seal performance from Frito-Lay due to chip debris
Bulk Density Tests
Industry practice suggests that chip density should not exceed 80% of pouch volume. There are
two ways to determine the volume of a pouch. First, a pouch can be filled with a known volume of
water until full. Another approach is to calculate the volume of a pouch by using a calculation
involving the length, width, height, and air fill space. The latter can be very involved and time
consuming, so for the purposes of these experiments the water fill method was used. The following
are the results of the Bulk Density tests.
Chip volume as percent of total bag volume
Measurement Golden Flakeï¿½ Line Mrs. B'sï¿½ Line Frito-Layï¿½
Pouch Volume (cm3) 4750.00 3700.00 1000.00
Container Volume (cm3) 8390.17 8390.17 1191.79
Chip Head (cm3) 6030.44 6817.02 786.05
Chip Volume (cm3) 2359.73 1573.15 405.74
Chip Density (%) 49.68 42.52 40.57
The Golden Flake potato chip line sealing specifications need to be checked and maintained
more rigorously. Trials should be done with temperatures approaching 2650F and dwell time of
about 0.38 seconds at 42 PSZ. Line B is running closest to these specifications and is indicated by
its nearly 92% peel success rate.
The Mrs. B's lines sealing specifications need to be considerably altered. Trials should be run
with temperatures being reduced from 310 F to about 2650F. As temperature is reduced, dwell
times should be increased from 0.34 s to between 0.45s and 0.46s at 46 PSI.
Bulk density testing results were inconclusive in determining a reason for chip matter being
found between the seals. All brands tested had a bulk density of well under the 80% rule of thumb.
Other causes for this phenomenon may include drop height of more than 6-8 feet, too much
seasoning being added, and/or not enough air pressure to blow past the seal area just prior to sealing.
1. Internet. 2003. http://www.brugger-feinmechanik.de/Englisch/Html/P-S-HotTack.html.
2. Robertson, Gordon L.. 1993 Food Packaging Principles and Practices. New York: Marcel Dekker, Inc. 137P
3. Saroka, Walter. 1999. Fundamentals of Packaging Technology. Naperville, Illinois: Institute of
Packaging Professionals. 204P, 270P 346P-348P
4. http://www.brugger-feinmechanik.de/Englisch/Html/P-S-HotTack.html July 25, 2003
5. http://www.film-bopp.com/key_properties/hot_tack_prac.htm July 25, 2003
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