Notes on the manufacture of flat plywood


Material Information

Notes on the manufacture of flat plywood
Physical Description:
United States -- Forest Service
Forest Products Laboratory (U.S.)
University of Wisconsin
U.S. Dept. of Agriculture, Forest Service, Forest Products Laboratory ( Madison, Wis )
Publication Date:

Record Information

Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 29616581
oclc - 758374273
System ID:

Full Text
, IL
F "-,/ '



lcviscd April 1943

Lyk s~

Ll I


1 3

No. !i43

Madison, Wisconsin
In Cooperation with the University of Wisconsin


Plywood is manufactured by bonding thin sheets of wood (veneers) to-
gether with adhesives in such a way that the mechanical and physical proper-
ties of the wood are redistributed. The manufacture of plywood requires
special equipment, knowledge, and technique. It is the purpose of this paper
to outline the important steps in the process of making flat plywood. Those
notes on manufacture are based upon observations of factory practice and upon
extensive experiments at the Forest Products Laboratory.

The Components of Plywood

The essential components of plywood are veneer and adhesives. 2oth
are available to the plywood manufacturer in several grades and types.


Veneer consists of thin sheets of wood, ranging in thickness from
1/100 inch to occasionally more than 1/4 inch. It is cut from many kinds of
wood, both softwood and hardwood species, and is classified by species and

Veneer must be smoothly cut, uniform in thickness, flat, and uniformly
dried, for plywood manufacture. The desirable moisture content for veneer at
the time it is glued varies with the type of glue used and the conditions the
finished plywood will encounter in service, but in any case the distribution
of moisture should be uniform throughout the veneer.

Veneer for most kinds of plywood should be low in moisture at the time
of gluing so that, when it is removed from the press, the moisture content,
increased by the moisture from the glue, will be near to the optimum for ulti-
mate use. For fancy, cross-grained veneer, gluing at low moisture content is
of particular importance, since drying of the panel from high moisture content
frequently results in checking of the face ply. Glues not water-resistant
also require the use of low moisture content veneer, but -,ith water-resistant
glues it is possible to use veneer of somewhat higher moisture content. Tith
the syntnetic resin glues, best results are obtained with veneer at a moisture
content ranging from 2 to 12 percent, depending upon the particular glue being

-he moisture content of veneer is controlled by (1) drying the veneer
in a related venoer drier shortly before gluing, (2) storing the veneer in
teLaperatre and humidity controlled rooms, or (3) running the veneer through
redrier2 before gluirg.

imeo v ed rril 1943)
(i-lev:] rned Apri! 1943)

The t ear'era ure of veneer t: tne t;;.e cf :lu. .
veneer s e dire ctly ro.: di:'s cro"r :eariers a.d ar:;.bX lea C p '.
at too i a te:.perature frc' 'c Ia, .. iacr 0 ii
qualityy of tne pl-vood panel. -r -. cz aditicr. -an ccc.-ro -0 c;.l, by kn..-
,, the upper litsr, of te..r eraturoc d a.d r olL ra a
veneer t Lnat exceeds t at tc c r..a.t. re -*i.l -a 0 clcd.

.elet or. of Voaeer

Th7 0 ality anad usef-,lness of plyv.ood dep-eld ]ryl "cr t;. ii. '"
of to veac' fro- w it i aade. ta p 'oo for ai,"rafT cr r.ta" :*'. r"
rural purp 0cec, it. is a.ece'ary to avoid cr 7to1 'e ly i ,,X.. (-Ie 'jc
affect z.c Zrre 0th or dra, lity of 1:.e '-.cod. ae :tre .; a rc1 ."0:' p
the Wod :; ec e; usaed 07ast a0so be take Co-:tcra .. a '
the visi'cle ),parts of frir: t are, n aerocr tri, L-na e:, ...
cipal coasiderar,...tior- is appearaacc. ard ndddcer. dole't:; r'r: i :.o ;:^;i!r
.......C .... 02'1O ar} ......'e uA a: e dxu2 LV et .4 .L.' .0:
applearar (7. re 0ra~rl7 a' "eptado.

Adhe s ive s

h-e aduC sves lvailable fcr -odi:y. veo n c-r.- t... o e o ::.a':t py'o
panels are co-ve tieai ls c :i. fied accordir. 0 to t:o ... . .. ,' t-r '.. ..-. C,-.
set a.d degree of ,:a-aer reictar. '.eic:. -they c:.a:."f a:T 'vt. a,' .

O- the baSi of 'et. rertue, e aa. fall t 4 ttr
-;? '.cral roups: (i) Most p.oacl-fors .ehy, ad ;:.el .:i ... a -'o::.a l1 ia de ].' ,c,
which reiurire tei:perat:r.res of `2C0 to 3CC0 F. an-d are -:l; I :t eteea
heated latest; (2) coiPe u-rea-foraldhyde an-d locv:-t' ..ien '' a i e' .ori yl oc,
which reqAire moderate te2Ja r' twre: of abc'ut 90 to 16C0 F. 'i:.d 'a. be Co i
a rooi or il'. ''ith conrtrclled t,:meratu"re and (?) cold-r':. n. ara ,l"e.
that set at 751 F. or above and casein, so-1oa., and strc e, ',.:c *ll
set at ord inary roo a e ra ures.

The extremes of water rcu,:t :P are *ll rstrated ,y ..r. ;ue, '.rih
no .reable after ressa..ce,~c used for interior o.'or; a. pneol --ca
a-dnesives, .used for pE .'-oo. '1.'.. ... .. r.tca.J co d:i 1dc .o : o e."
o.sure und or exterior +on o..d.o:s.

The hlulia.. ard zresthi:a Cperat ioas

he problem of appin, : -le.: e ll -l 1"...'.... f aa
even spread o- the desired thickoec, o. ":.e :o:'-f'e of t.c v..eer ;l:;
en.ou.-, so that te. voceer and :;lu ace,;.l' :.a be pla'ced ' p.".- be-
fore the *lue begins Co set.

., **n usini; 6!ue iL. liquid fora::, the core or cr.?;'a.d- of ":.o pa:.el;
are usually coated or booh faces w, 'a' n oe lue : '-" . a :..e-. 1
spreader. .'ra:er-, idler roll2, or tr :e p ,re. o -. e ;:.. .. '.:- ',- a e

I,.i10eo. il o. .-. .-.

the thickness of the glue layer, according to the character of the spreader
:ein, u&sed. Spreaders for vegetable and casein lues usually have rorr-. 'ted
iron rolls, while rubber covered rolls with fine corrugations are ordinarily
reTqired for resin glues in liquid form. Most liquid glues canr. alsc be spread
with b.ruses or scrapers in small gluing operations and some resin glues can
cc applied 'cy ,:rraing.

Somce resin glues are available in the form of dry films. 3uch luees
do not need a spreader for the dry film cut to proper size can be inserted
betwee- tIe sheets of veneer.

As the glue is spread, the veneers are assembled in relationship to
each other as required in the finished panel.

It is standard practice in plywood manufacture to place the grain of
adjacent plies perpendicular to each other and to use an odd number of plies
so that corresponding plies are located at the same distance from, but on
opLoYite sides of, the center or core ply. It is also important that opposite
plies be of the same thickness and same species; or be of species having simi-
lar shrinking, swelling, and strength characteristics.

Flyrood for special pur-aoses, such as certain aircraft uses, ..* be
laid with the grain of alternate layers at angles other than 9C degrees with
each other or witn the ed-,;es of the panel. Such special plywood does not
always have a- odd .umber of plies. these constructions, however, must be
regarded as ifreuent exceptions.

In three-ply panels the outside plies are referred to as faces and the
center ply s a core. In five-ply construct zon the outside layers are faces,
the first inside plies are crocsbands, and the -enter ply is the core. in
panols with1 larger suibers of o lies, there is no special name for the plies
that lie between the center ply (core) and the crossbands that are adjacent
to tne faces.

"-n the glu e and veneer are properly assembled, they are put under
rrossu-ire. -.ere are two general types of pressing eauipmelt in co irm.on use --
the Lot-plate press and the cold press.

Hot Pressing

"" ci ply:wood gluig is .,ow being done i hot pTresses, particularly
whnr. ..igh w'ter reLsta-e s req -ired. At the present time all gluing with
fil.m Flues, practi cally '.i1 guini g with phenolic-res.n glues, and smuh gluing
wv t urea-r i.n glues is done i hnot presses. Soy-bcan glues, and blends of
soye n anc. blood -:lues, are used extensi-vely in hot-plate presses for
oL rre-reo Jstart plywood production in tho Douilas-fir p,: cod indust'.
,'.I.,gn lco:, glu "s iso give t, er best results whe used i.n hot :-5sscs.

InOT, preyiFn. tnin pl. ood, several panels may be played together
bew''ee ,he .eated p lat es. eith thick panels, only- one panel is pressed in
each opei 0 1ot presses usually, have ma<, plates, so that a number of


panels can be ;lued ir cne pressin. operation. 1.en: .-luln,;; pa:rel" vwit uhi
p o 'rol a::'. u o~ ix u-n elr dr~ i~c n p xCc
faces, tepes nsbelsepruplyatrisri.:tcpa-licrr
0o -avoid partial set tin of the e:lue brefcre io apI)..

.he til.e re .ir.-d in : e o prh'ess de ond.i u o.n t t.i e: :' c ..
uatcriatl fet:, .-Lued a.nd .upon the .har:'ajer of t-e ;,lue I c'h. sed. G..
2CC reiee.. cf C.. ...C. (0 --2. *.T?'-. i[e c*..,
.an cured aat 2127' F. or lteer. Sice the neros 7e line ..:t re C':-
to the required temiper.tu.rc, the T pressing; tlir, e depe:.ds upo. T.e cta:.e *:
heat i.ust travel from the plates to reach tlis tue line. nC te. :.a vr...
Iroi. 2 or 3 -intos for verv, tiin panels "o ai. :our or ,ocre for : '.el' :"
4ince tick. e reired cn'. be calculated tb' :- of t ..m.te.t u"ao
forzualas o'; cn-sIder wood thc '
peraturo, a'nd etti' t poerature of the 0, l7e. Ca 't,, 4/ ; *Ci *'re- "1 .
of heatir02 of ia pan-el :.ay be neteri.ed, are available. -rl ....a et*'-
can s': all furnirh cific re o:.;.oe::dat ions for o'c >1.. r "*/'i':}
t.e ir resf ect ive lues.

Te .. anist of pressuree required in hot-presst .;l.i. vai.e, 't. re.o
kind of vood bein<1 glued.. .oary, dense 'aoodr can s d1. ( 1.rew Tr:'i're ,
lihter, sof'or woods. For *..o03 species of wood, vres:'re,. f 1;.i' to ICC
pounLC per square inch are adequate. ... he pressure e niced :hculd :.'al'l ce
below the crush:ing stotreo of tne wood.

In hot--rOtsi M. plywood, tse veneer is likely 'o lcse .oa'ri, er;;ble
moisture durin 2.: .' im;cdi'-ely after pressing ;, wh: a. : .... C..k..': ad
,warpi. It is customary, terefore, to add moictucre af.7e p'oI.Eir: at at -r
te-oeratIures, cither t'y ap li after r to h pa. i ein ater r+
moval froi the ot pres.- and stac to 'ontrolled uC.i:.di ties. Other e.e1enods, zo.e of the:-, :rc a so0

aold Pressifr-

"".h ?;lues tha*t a:'e pressed v.'t-hout :ati. ., tn pa-.e 'ret d
and a1 ceod hcr.. pr ss at, a ) o' D :,1 e a T" I '' r'. .. C
actual t:ime perl.,it ted betweeL spreadin, glue oe th-e f ir vc:-e,, r ;t ,e ap-
p7 0 f to t-o--f : '
pl.ica Iion of Ce'ure to tc stack of : C els ;::.n: h def tel, ... .- ,-<, a. -
tnouh i vari2 v"'1h -"r'; :7 les frco. several ...",: 'C .n" l cr2
longer. T ee panelt are a...nulated n a p0le u o -? .c

l..orc pnesls. -s-.. a co ( ;, pre. boardtr;s a or 0. tOC '.i ',2 0. "
..... .. rr' v! n-, .... .... .. ct oi ... .. .....l .1 O. t
stac. of panels and tne asseibly is pla-ed p norcure.

I r cold ressi ,:, t''o et cds are -C:-d ......exte .v l ..... ':- .c

f CI I -L e
-o .....e ar-d ...ainta i:.i : it on. the p~anels. T c0 0 ;o.oc: 'c:..i.c:.l, u
n',ts of" : 1lyinN tt-e p~rossure byj a lj, y r~aulic pres aOS &:.aZ t:nrn :.i'ii.. rotai:.i:'

--crs j"o."'' Q' "rnI ;*; '
r rroauct s Iabora. tor:' C ,-icoo:y'cpn i -i' 0 "Tne Rae 0f -,T ...... ture C. ;\:.'c
i. n "';O & o r 13 -R s eated 3et wee !k '- irF 1 a e .'

'C, ~, ''7

- 4-

clanps to keep the panels under pressure. h}'e hydraulic press is usually
e ICuipped with a tgage to show the amount of pressure applied.. Tr. panels are
left in the hydraulic press just long enough to apply the proper load and fix
IeI retain!-n clamps in place. The bundles of panels are then removed on a
truck to an out-of-the-way place in the factory where they are usually left
until the next day.

P the other method, the panels are placed in presses and left until
-he glue is set_ These presses are usually of the hand screw type with no
means for accurately measuring the amount of pressure applied.

Control of pressure is important in all pressing operations. The exact
amount of pressure to apply per square inch of panel surface varies with a
number of conditions. However, under average factory conditions good results
may be obtained with most glues at pressures of 1C00 to 250 pounds per square
"nch of prnel surface,

The determination of the amount of pressure applied per square inch of
panel on a hydraulic press ea4uipped with a pressure gage is simply a matter
of calculation. principal factors that determine the amount of pressure
applied are: the area of the panel, the area of the piston or ram of the
press, and the pressure gage reading. The area of the piston in square inches
multiplied by the pressure gage reading in pounds is approximately equal to
tae total load exerted by the plates. The total load exerted divided by the
area of the panel in square inches gives the approximate pressure secured on
the panel in pounds per square inch. To obtain exact pressures on the panels
it s necessary to correct the above calculations for the weight of the mov-
able parts of the press, which may increase or decrease the pressure applied,
depending on the design of the press. For large presses and small panels
_ hi:- is iu n.

A table, showing -age readings to be used for all sizes of panels
manufactured and for the different pressures used, can be computed and placed
Sear the p1res where tne operator ..,-v see at a glance the am.oui.t of pressure
re 'ired on the gagje reading for each run of panels.

Conditioning and Finishing of Plywood Panels

".n thp glue is set, the pressure is released and the plywood panels
arc removed. The paLels may be ready t trim and sand at once or may require
redryirg before further work is done on theom.

Pase] : glued cold with the common types of aqueous luos take up a
,ood deal. of moisture in gluing and, after coming from the press, are often
placed on nt`iccrs and rua into a kiln or loft st room conditions for final
rd! 4;.; 1ar' C. J'OT beca s of
dr- -; a ryi1 under conditions is slow ad is expensive because of
'.... rsp c 'e r hired.

e.;. 1. of experiments in kiln dr'i g panels have indicated that the
essential r'q1 re:,fnts of minimum iniurv to the material and convenience and

E, eo. jo. .543

economy of o ration an be me1 n ontant
N( IT. cii tL.." ^ i cx i t i IriA~i 2. coo *vc .. 1 I :. -,
to dr c<,r- ,o the desired moit,10 cor. .t i,. a rely:, i : ;:. f ';."
,,w~ich wtill 0or allow apopreo icle drying r( elcw t:nis oir.T. U;.^ '.re of .'- -..c ;.
. .e..i er '.i. hrui:.. tions w:hi'h will ci rj .c.. -r arcl. to '. :ie.:..l '
moisturee ,: coitenr makes the drying iii..- le tii ;;afc. cet parel:. on h-; .......... I d.

ve-pl; veneer, orC of veneer faces rrosszbaiin, and a 1 'ik 'ore, .l,,d a
0n ri t Q the o e r tXe- c',--sar. au2
.o'." moif v 1re content, c n ,: a. ey -t be IC- sn ed "n a few hor.s cr ovrc, i MO.... Te er aicve I:(I i. 'x.avo :
*~. ;cuar :e of dpcra' i^: -; 'e d.& !. time, t't they are ;;o;"e "xbL c "o o'. .c
te j'alit cf ", parol by uiduci.. cr ne k.i w'arp ,., 0... oper c.. e:.-
!os te ta)uity ic"ref-ully ccntrclled. .-tols dried f:oC, a i :o .. Ic'. 0.clsure J are lia1le to 'a'arp u:.lcs ,- a'c did '-I;-
. vely, *lovly.

-";:mi 'r : and .... .,'.

ohe p ood Pa-eli re i tried on sta:dard i,:: a:.d a".-off emr-'-
met. t... p equipent, :muist 1e in ood codltion ai.d atura:ly -et '", ot.e.a-
wise the pas el vill otic bes, .

The trinied panels are usually sa'de, a too C i
o. ratio, -.ost of th, care ,,ive, to :-.kin41 a per-fect i 1: ','ced ,K1
;eleo tin,- veneer of nr-ifor:. t. dibnes moisture o;.te:., : .iita'le .,ece0,
is asted if imp', oer f:.ichi, 1:a.i4 oie faco thin e' .na *h O'r.

3tor. e _

..nised plweeod panels ould he stored uder e.a io ... "
..or '"r;preria'l c'r. the panels dire tly over earch other and a solid cover over "n top of a c-.
pile ts tie panels aa, inx> rapid ca.a. es ix moisture Noo:e: v'ar- i,0
d-ctt a( numulation, and discoloratior. due to lil.t.

Factcrv Affe t-' the yrr. i of Al -'ocp

A p]". -od pa.ixel to ret, i- It. form, th mo( s.:'e : :. t be,
P.i..." tri ll o..r. ea. 3Ji;.mot'r y o t' aied bP '""l:.' c. i -a n er oo
plies. 'ihe plies sxxould be so s'ra0:.:ed tnat fo: r. r, pl of a par"iuln.-:'
thickness -.ero is a parallel ply cf .h. same ti.iekness and of ::.e same spe-
cies or propedtes o0: 'he ppoite side of t. -ere .*.d eua ly lis''n i'rc.
the 'ore.

A h-' in mo isturo content of (','ood ill C 1cvL 'a i r ;':t'e-
dluCe or relieve ite-,- s' ss eeu e f 'x'c .. aif: re'' e i ..r: -
ie 0 of ',.cod ': n he direcio. par' il l to r -ai ''0(.. pc 0'e;.a! ... o ia

,'," '1' -


A three-ply construction subjected to a low humidity so that the moisture
content of the plywood is lowered illustrates how internal stresses na-- act,
",n the grain of the core is at right angles to the grain of the faces,
the normal shrinkage of all plies across the grain is largely prevented by
a very small change in dimensions of the adjacent ply or plies in the di-
rection of the grain. If the faces are of exactly the same thickness, of
like density, and otherwise balanced, the stresses are symmetrically dis-
tributed and no cupping will ensue.

If one face of a three-ply panel has been glued with the grain in the
same direction as the core, and the moisture content of the panel is reduced,
tne internal stresses are no longer symmetrically distributed iinasmuch as
the one face ply does not restrain the core ply from shrinking while the
other does. Cupping takes place as a result.

Titig is another form of warping that may be encountered in the
anufacture of plywood. Tests have shown that deviations as small as 5 de-
*rees between the gr-in of any two corresponcding plies, such as cros.-a'nds,
.aVy introduce -onsiderable twisting. One method of eliminating twist: is
t'o cut tile ve..,-er sheets so that th _D
o cut te r sheets so that e direction of the in is parallel to
the edges of sheets. :he direction of grain may be tested by splitting
the veneer or by other suitable means. It is not always convenient or pos-
sible to cut the veneer in the exact direction of the grain. In such cases,
the tendency to twist may be elir.. nated if the veneers are so glued that the
grain of opposing plies is parallel even though its direction is not exactly
perpendicular to that of the core. This matchiing of plies may ce acccio-
plished :*ost easily when sliced veneer is used and -ieces which were a ja-
cent in the flitch are glued on opposite sides of the core so that they have
the same relative position as they had in the flitch. When maximum freedom
from war-ping is required and rotary-cut veneer is used, it may be necessary
o examine each sheet to make sure it is laEid in the correct position.

If veneered panels are being built up of five or more plies, the di-
rection of the grain of the crossbands is the most important factor in pre-
venting twisting. The faces of five-ply, veneered stock may exert some
influence in causing or preventing twisting, but it is not so :marked as the
fifluence of the crossbands.

oi 3ture Control

The previo-is discussion broIjht out the fact that a change in moisture
(content of a panel nay introd.c-e cupping and twisting- in the panel if it is
-o D carefully constructed. Hence, it is highly desirable that all plies,
p rtic2l >rly the faces anid crossbar-ds, be at about the same mol ture content
before gluing. The moisture content of t-e panel as it leaves the during
room nould be about the same as it will average when in use. A moisture
"on.-e.t of 10 to 1l percent i tie fi rned panel will usually yive satis-
f'ctory roe;tc for a panel to be in service in the open air. For uasc witni
ilci he-t.d at least a part of tbE year, as in furniture, a somewhat
oer .oi. tre contest of 7 to 8 percent will ordinarily give best rectvu.ts.

.~ 5.00. 0. _1<54g

:.merous? test" :.avo (hc''u '.a2 the arpl.a,- of plywood l -ia'' ,

bject A to v2r :l Oa; o io -ure o t;-. : o l 2: for :he p'0 1o .. aC o L --
- s. s i t e:e-c, rr-ch as iba rC WOod, pop 1.. t 1- d cedar, t hat, i :xe; .V ai,

-foet on "acr:. of ta b -c >.. d

0 4/
Ratio o0 the Core ;o t cotal F-y1'.tod.

,:."a fd s-rfac. a' o f a, a.re0-
'C1 to 7/1 of tho tal thia '1eh of -e .'her. :.
ain imporT,<..:. cor-i :iratior. Increa'u-f th o ''ufber of plicr in. a par-ol o:
*ve. c> r!ss also recceo t e c-o to warp.

nhe u alityl of ply,.ood aw d of tIo .re:eer fro1: v1 :- t ,.;.ae ': V fI 'o
;ome p':." oses covered by specI' -ario: s. Aircraft clwood, for exa,.piL, I
covered cy a. Arm.y-v. y A re&o;-. 1:tic l :o -' a_ "Io'. .[ a
standard .eri'ic f lo.s -- o-e 0 over0:.; ou-las-fir p*-n.ood a;-. the o-h r
over..- "C'rdood pl. cod. Gome pjrchaoer: or p'od& r. .. w ,e o' r
20peciflcati o:.s. rurre;^ raft .tse -r. the co;;.ercial e fafeCrds i: efec (AprI' l:1 19-^ C, a .;':

Armj--l:-vy Aeoroa,..i<.al ^po I f,;at,. o:, A--::p--511u" -- Pl yood
SVe-eer;A r cral t Fla t ae1

Cooer E )

.o Sta.dardc C 7-.4. 2 -- ly'. 'ood (ardJccJ a- 02r
l d Cedar)

111I II I l Ii I 1 ll ll //11/1111 I! Il
Rf-- -_3 126208929 1750

Th:is mimeoraph necessarily contains only a limited amount of the in-
formation- available on the manufacture of plywood. A more thorough ir:tro-
a..t"o to t"e subject an be obtained by consult; : the publications listed
' Plywood, Thomas D. Porry published by the Pitman
Pubrlisin 2Company, 2 West 45th Street, -ew York City,
pri e >.t5C. (142)

"Pl ,oods, Taeir DevelopLent, kanufacfcure, and -- plication,'
A ccd ad -. Linn, published by W. and .Jo histon,
Ed i bo0rou S, cotland, price ?6.50 (1942)
''ec'i ue of Plywood, C, .p orri?, publise d bj T.
auc"-:, :c., Seattle, -i. in ton, price 2.5 (12-..)

Soi..e o>f in Ply,,iLood and Veneered Products,
7on BCoucc, Forest Products Laboratory M1imeograph R1242.

T'T lin% of Woo7," T. -. Truax, U. S. Dept. of L Bulletin
( 1.CO0

"Ti :t of Publiations oin le aand Plyvood,` Forest Products
L-'borF-ory i-ieo-raph o. 1. (190)



- ). C. -~