Bag-molding of plywood


Material Information

Bag-molding of plywood
Series Title:
Report ;
Physical Description:
7 p. : ill. ; 26 cm.
Heebink, Bruce G
Forest Products Laboratory (U.S.)
Forest Products Laboratory
Place of Publication:
Madison, Wis
Publication Date:


Subjects / Keywords:
Wood bending   ( lcsh )
Plywood   ( lcsh )
Gums and resins, Synthetic   ( lcsh )
federal government publication   ( marcgt )
non-fiction   ( marcgt )


Statement of Responsibility:
by Bruce G. Heebink.
General Note:
Cover title.
General Note:
"May 1943" -- Cover.
General Note:
Author information taken from p. 1.
General Note:
"In cooperation with the University of Wisconsin" -- Cover.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 029302529
oclc - 314797239
System ID:

Full Text


hay 1943

,D~fST 1


No. 11431

Madison, Wisconsin
In Cooperation with the University of Wisconsin

- /


-... F?;.,'; ..C. HETINK, A3 ociat,? l'ech-nolr.-i -t

.'-molii:.y of plyv.'ood ,rrnd larr.innted veneer mr-bhers Fro'bably had its
or.,in in the vacuum-7-i, process that was introduced in th- furniturp in-
dustr,' several '.,--rs .-o (see fig. 1, A). While the .7iuum- a.- frocer-
dprvnded uron atmosrh:ric pressure -ind ordinarily only room t- peratur'o to
.t the fclue between the plies, the newer t echni;s employ higher fluid
-ssres and vary>.:- -r, es of heat.

"*isnomers, such as "lasti c pl,'ood" m-nj ".i1ti,' lanes," h-ive .
aritedu to --r .icturs of mnold plv'.'ooi that are actually '-.-! fror. wood
bonded with '.'.thetic resin adhesive. ,weit, the -- str. tures Ir"-
probably about ') rrcent v.od and 2' recentt resin. n xcr-t for 7n-laticn.-:
"" ~~~~~..-o ian yte --1 sfatp
in shr, the product is ssentilly the -t as flat-prrss .oo.

'.olded plywood is produced by several t.1hiiqru.1 of't:pr r-'frr'' to
spe cifically as the r., ramold, .'idal, Aeromold, or vacuum-a1-:- o:- ,
Other terms solmtimes used in describlrIn th'- techri iL, r- 'i-. -.7o..
cl ve mol I .; o ' 1
4ocla ve-molc-," or ank-molin, Ferhaps the most inclusive
term "fluid-pressure moldi.- ." A r.umber of rl i-h'= a1scri tior:-. of a-.
mold'- i:, processes are listed at the end of this report.

.":- fundamental rro-edre is the same for all rocreses in
us,. In ,rinciplr the techniii- consists o:' attach'-- 'n..-.rorarily 'V
tapes, e, clips, or some oth:r means, -u.-rlr.-- l.-"4-s o" strir
or sheets of glue-coated veneers to a mold of the desM:.* .., a:.0 ol -
- these into a unit structure by the .,rlication o' heat i'. .'': rrs-
r' thrr a flexible, I..rmeable 1- or blanket. All the recses are
s-7, t vr or lexile, .
relatively s'nre and rroide a means by which 'lywood of sl.le or omrfour.0
curvature, and of constant or vary>-.. thickr ss, in ar. rr.. -ement of r'li.-
can be i.c !,Yced, .turally, flat pl,,wood can also '" made l;.'" y ,.-.clrir.g,
but due to the critical 1-, r-.terials r*-.'-iired in most o- a tic:.F, it i
re :on,-nded that the technien be limid to the prc:-ction of stra"og~ C
molded rts that can be manufacture ",no other practical '.-,"r. I-.
Sneral, rts that fall in this cate orv will have c. or more of .h'
folio. I. characteristics: Appreciable :or.'.r.d curvatures; -7 icl-le thick-
ness; s: .-le curvature bends rroximat-,.r or e r-'x'' "- ",hn rKc.
are too thick to be steam bent from flat rl ,..ood; [',rts too far,-' to b-?
made ractacv bly '" matL-., dies; .'-r.t.. too small to ,'sti"v. mTiatyn- di.-.

.same -rincirles of balanced construction thet ii-"l1.' to flat r]1,-
--*co-: are a rlicable to moli'd pl'" -c']. For may:"m:- resistance to ,;-rring
ail -py/ood should ".-. sy;ru,--trical about the .e:.t,-r- V :., of thickess. I:-
this coxrnction ,,yr.v't- involves ;.* odes, r..r-b, r of lies, of
r ies, and dQ rectic:. of .-rain, .' theor',, a ;. i.*'.,t i-i y ,.y c.,'. ..* r'.itc -
panel itoh alternate pli>- laid at v with resi-ct +o direct on of hr-l
wou] hav uaxjmum dimerxsio:'. stability. In '-',-tice, howe'.',r, a

,- ,.. .

construction with alternate plies at 90 to each other is often ir'.rossible
in pieces of pronounced compound curvature.

It is likewise impossible to make flat strips of veneer conform to
appreciable compound curvature before the application of fluid pressure.
Fluid pressure forces the veneer to conform to the surface of the mold;
therefore, the greater the compound curvature at right angles to the grain
of the strips, the narrower the strips must be to avoid wrinkles.


The manufacture of bag-molded ply,v-ood of aircraft or boat quality
requires the use of considerable equipment. Means of supplying fluid pres-
sure and heat must be provided. Normally, an autoclave or cylindrical
pressure tank 3- to 12-foot diameter and 10 to 60 feet in lerFth, which
will withstand an internal working pressure of 30 to 120 pounds per square
inch, is used. In figure 1, C and E, the use of an autoclave in 1-. ,-mold-
ing is illustrated diairrammatically. Occasionally, the means of supplyinF
heat and pressure is combined with the mold as illustrated in figure 1, B
and D. A mold capable of withstanding the desired internal fluid pressure
is then required and the bag is inflated by the pressure fluid.

All bag-molding of aircraft or boat quality plywood requires heat.
This can often be supplied most economically by steam, either directly or
indirectly by heating water or air, hence a boiler or an adequate surely
of steam from an existing steam line is required. then air is used for a
steam-air mixture, or to provide pressure on hot water, or as a combined
heating-pressure medium, a compressor and receiver are required. The size
and capacity of the compressor and the steam generator decond entirely
upon the size and number of autoclaves or units being operated. Obviously,
to charge a pressure tank of perhaps 1,000 cubic feet capacity to working,
pressure and temperature in about 10 minutes requires a large-sized boiler
and compressor.

A vacuum pump may be used either to induce air pressure or to check
the ba over an assembly for leaks. Vacuum alone produces insufficient
pressure for most bag-molding operations and therefore is used only for
single curvature ven-ering operations in furniture work and is not recom-
mendd for aircraft plywood.

Any operation involving the use of veneer presupposes the use of
some of the ordinary veneer trimming tools, such as a saw, clipper, shaper
or router, as well as a glue spreader, and veneer conditioning racks.

In some bag-molding operations careful control of the moisture con-
tent of the veneer throughout the process is desirable. Adequate air con-
ditioning equipment is then an additional requirement.

Mimeo. No. R1431


The forming of nr.y ice of bai-molded plywood requires a mold of
0o:e tp Mold., .or-tim-es called forms, dies, or mandrels, ire broadly
class]7 ed a,. male or frrale. Male molds aE illustrated in. fi. 1, A,
B, tr: C -ire the d,--ir'd hane on convex s -rfaces, ".-ihile femrlc mold-
(2.. 1, D rind E) have the proper shape on concave -urfac.s.

o-.rnon mold nater.-icis re wood (solid or rlywood), r.:-tR! (3t:l,
cast iron, or lovw-t -7perature illo'/), plastic mat'r.rials, and er.rit-:. Thr
choice of m".old materials will d-epend largely on the shape of' the iter. to
be molded, the juar.tit;,' d-sired, availability, advanta,-es '%n-C disadvantages
of "or.sidered materials.

-..ood molds are normally of the male type and have th-- obvious
advar.tape of Fresentlnr a tackin, surface often r.ecesfary in temporarily
attachinF the strips of veneer. I'ale wood molds may le Fr-oo-e. to allow
the insertion of stiffeners to be glupd to the molded ry.v7ooc in a si:nle
operation. '.'.ood molds are prone to distort .oie_,..'hat aft':r rerer&t.-" hating
and cooling, particularly if they are allowed to -becore wet. A mcld r-ty
1ecor.- wet as a result of leaks in the bag. The ter-.prrature pr-netrationr
in wood molds is relatively slow as the mold is usually several ir.'h-s
thick, and the molded pl-ywood must be heated essentially fror. the 'a: side,

Metal molds, usually of greater cost thar. those of wood, may re less
e>:per.sive to use because of their lonr, life and stability I: ,o.-cn
practice, metal molds are female in type. Th,-- smoother surfa,.- c' tho
piece being molded is on the convex side of th- piece.

Some attempts have been trade to use !onrete,, -. ting
resins in mold construction, but to date these materials hLv've ;,ro-c.
practicable in relatively fe-.-; uses.

Lars or Pla :s

:he purpos-e of the bag is to provide a flexible i-.p.-vi'us barrie-
between the fluid ur.der pressure and the mold. Th' rIl ece bP r.- .oid7 i s
pressed bet.':een this flexible bag and th? rigid surfa: of the mold taxd the
full fluid pressure is aprlied at right angles to the- surface of. Th- bag
regardless of the shipe-. Th pressure at certain clue jon.*. r.-. %"
slightly less than the full flui. pressure by the amounr.t rncersary to
shape the vr'.eer.

Pa-s are classified as full Ias or hrilf bags (blanikete), A full
bag is a cor.plete envelope of' impervious flexible matrial (fig I A ad
C) cilamp-i shut at or.n end or side and havir.C a ionn, usually called
a bl-.'-r, to allow the -rtrripFed air to ocape to the at.-.osphere. it may
be ,o,..letely closed, similar in rrincirl- to a ba:-Ketblll blr.dler (fi.g 1,
B and D), ha1-t:,; only a tube connection for inflation. A half bar, or


blanket, is a sheet which normally fits the mold without wrinklir.,1 and is
sealed by some temporary means to the edges of the mold (fig. 1, E). fh.-
bleeder may be attached to the mold or to the bag. Full bq-s are normally
used over male molds and half bags are used on female metal molds.

Mo3t bag-molding operations at present require bas made of specially
compounded natural or synthetic rubber.- The useful life of a bag depends
largely on the heating medium used, the temperature of the cycle, the size
of the bag, and the care used in handling. It may be as short as 10 hours
or as lon: as 200 hours of operation.

Because of the present critical surely problem with all rubber
products, attempts are being made to find suitable substitute bag materials.
Som- of these show promise and preliminary tests have shown that certain
low-cost materials may be used for one operation.


It is possible to use a variety of glues for bag-molding; however,
typical bag-molded parts require long assembly periods, and have definite
use requirements that normally limit the choice of glues to the hot-setting
resin types. Glues most generally preferr-d are those that are dry at the
time the veneer is assembled on the mold and may be pressed at any time
within 7 days after application.

The molding of curved parts often requires that the flat strips of
veneer slip slightly during molding. Some -lues aid this slipping by act-
ing as a lubricant while passing through the plastic stage when heated.
This peculiar characteristic and the som-nwhct critical relation between
temperature, time, and pressure in the curing of the glue makes close coop-
eration between operator and glue supplier advisable. specially cor-.y'curded
glues are available which have been developed for ;g-molding.- -se
glues have been formulated to j:ro3-ce good bonds at the normal moisture con-
tents and temperatures used in hot-pressing plywood at fluid pressures of
30 to 100 pounds per square inch commonly used in bri '-moldirr.

-See partial list of bar material manufacturers in appendix.
-See appendix for Firtial list of suppliers of glues for V'-.-molding.

Mimeo. Hlo. R1431


' '-,. L.. IX
*i r-.

Firt ia1 Li:t of ..urufa.-trers of
R '. "ao .'te2r Ial1'I or Bags

E. I. dL~c'.t d-e'iro v- Co., Inc., >ibriko1J L'ivision, Fairfiid, Cor->.
Firestone 0 -b-r & Latex products t o., 1 ver,
oVc<.- Tire & .'-r Co., Aro.,
Fo.' ... o~ ro h on rC o ioo
B. F. '.Goo rich Co., &i4 S. '" ;t., Akron, Ohio,
["'r Ru.bber Co., 1 Railroad Avw, .-.:ioer, "'as,
Voit .'-bber Co., Los ..r.,-les, Calif.

0 1 r 1... '.o i 1 .

Americran d"am Co.
f:last lcs Div.
Rockerfeller i aza,
Kce, York City

Pakel1ite o"rp.

Bloofield, l. J.

Carbide & Carbon '- :cals Corp.
; E%. '.: rt.
SYore City

Casein ~o. of America
l /inbri -e Y.

'"t alr CoT: .
1 j ik Ave.'
"- ': York City

:., I. du o'or.t d '..-.ours Co.
Plastics Dept.
;.rl ^ -., ton, 2' J.

- -ez plastics & i."-,cals, Inc'.
1l81 ,,i. k Road
:rth Tornaw nd i, .. Y.

L.uxite o&
Lo0"',." C.t I. Y.

..o0r.nto ?>.:. .al Jo.
Tlastics L'iv.
Spri;.. field, ""e-. *s,

':'kins Glue o.
1":,-'.:; I?' Ie, P"-.

Flaskon Co., Inc.
"12-2" 4 A
.112-2' 3',lvan Ave.
Tole -o, Ch:o

Resinous ]",-,:.. ts .'h --:cal "..
d2 2 1". e- ; .; .;ton :;-i.
E, to.n
):i ladelphia, Pt.

Shawini-'F: -oi.cts 0or
F_-i;th Ave.
iev York City

PRrtial List of Articles on foldedd Plywood

1. Ai-~J'O 'OU.,
1940. :'TCLDED AIRPLAIIE." FCOR DEFES .. .od. Plastics 17(l1):'--,
78,g0,82, illus.

2. _____
PLANE. Aero Digest 38(2):165-166, illus.

3. _____
VIDAL FROCLSS PAT"NTS. Business Week (5'93):36-37, illus

4. _____
1941. T77 '7CRO1 VTCTPY P.ACR. Aero Dip-st 39(6):231,27-,

5. _____
19541. i T yOTCRED PLASTIC PLWATOOD FL' S. Fo. Plastics 19(2):
52-53, 110o, 112.

6. ___
19i42. 2:-C'LDING PLASTIC-PLWOOD. "o.i. Plastics 19(l1):4(- C',
112,114,116, illus.

7. _____
Fro d. 4(4):312-315), illus.
14. ... 4 Mod. Plastics 19(5):42-43, illus.


JOE-:: S.
65(1):17-20, illus.

1942. AIRPL!*..- ,:.:'D BAT'irE-, CCC K.D TO C.:D&. Sat. Evening
Post 215(3):12-13,36,39, illus.

1943.. DLTAILS C? DURAL-,LD FABRICATICT. Aero Digest 42(2):232,
235, illus.

1941. MC.LDI1'r, THI LI'iLEY AIRPL..'.: A IJ LAST BO'-:)- ?LY7COD
PL.. 1E. Aviation 40(11):75-76, 154,1(C, illus.

?.C:imeo. No. R1431


i''.\ List o:" Art:zl'-: on M:olaed Fly)-woo (contlr.uel)

-. L. J.
I :Ei '_'C':. .:,: ...:1 :1 3 FCR FL CT'CL S7LUCTTI3. Aviatior.
'+l(ll).;!! --!! *+'"-O (l ) l c ,'- ,-' ll,.o.

1V -1 AL 7.3X r- ~T L* ': i A' ri a t ion .-(1 );
S'+- 127, .:, illus.

15. L.'' .,, KL rLr T.
1 .. DL '.1._LO : 2: : .I A FLJTS.L i.:CLLZ D AZRFL;.. Av;. ticr i;(l):
44-45, i4o,l *t'-+ +, illus.

1Ib -'. Y, v 7H+,:..... 3.
1 .'l. . '*.. T I-Lr.COD *.... AiL'H^- JV3. .'ci. }lasti~s 1'K):
5 _- "r,: .L. ,-^ illus.


I1 <./ '. S ... 'JXI:- Fr'..S" E ::' 7;-.f.. A. ) T':D " "+ r r.t .d at
S .,- ". et ..:-, '.ood T:..' triess ->vision, .-_.. -. r'-h.
....-rs A. 2 -2. i,.l''3.

-.rtial List of ],tonts on 'old, r 1.""c1 7"-'ncc

!. "" '-D.., G "'- E ;. A.
2 -, *+. .l.C .-- 3 'C ,- .

(U ;.tent :o. 2, ,7.

2. C:. :, V. E.
1 -1 LL:-R T :: S ( S .t o. 2,.7 ": )

3 2 ........ z 5
1 9 : F L :: .; n_ .: ., .* _. .... .^ ? r ...' ( -. S .F r.t 7:o 2 :77 : ";+C ._
4 */.*r : ILL::
l. '-+. :" I '. OFP .....:. :" ..... :: ^ ?-:. ( I S. ;- ,t nt : 2,Z Z 5+ ).

5. V:AL
1 4 .

' "I L i nr '. ". .::, L. J.
.. .';D C : : : : : . R (T. -tent
**Io 2,1..'6 +. ) .

'iI--'. 4 :' -31





ZU 11399 T

Figure l.--Fiwe methods of forming bag-molded plywood.


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