UDYIN ANU CCNUITICNINC Li[UED JOINTS
lcvised August 194S
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin
Digitized by the Internet Archive
DRYING AND CONDITIONING GLUED JOINTS
In order to obtain strong joints and to prevent warping, che'h" r.
sunken joints, and other defects in the finished article, it is essenti,,
that wood after gluing be brought to the moisture content most suitable fr
the subsequent use of the article and that the moisture be evenly distribute'
The moisture content of the wood may be either increased or decreare i
during gluii., by an amount depending on (1) the process used, (2) the foi
and composition of the glue, (5) the amount of glue spread, and (4) the
dimensions of the wood parts glued. In general, hot-p'.-_.s methods reduce -r'
moisture content and room-temperature gluing increases it. Glues of high
water content add more moisture to the wood than glues of low water contel,
and heavy spreads add more than light spreads. More water is added by Lthe
glue to a construction made of thin plies than to one of equal thickness wao
of thick plies. Furthermore, the percentage increase in moisture content
from a given amount of glue spread will be greater in woods of low specific
gravity than in woods of high specific gravity.
The older types of woodworking adhesives such as animal, vegetable,
af.d casein glues add considerably to the moisture content of the wood, and
frequently effect an uneven distribution of moist.r-e within the constructi,.
The moisture added with the more recently developed synthetic-resin glues i
much less and sometimes practically negligible. Table 1 illustrates the
approximate effect on moisture content of gluing with several room-
tenmp-rature-setting glues. The table is based on the assumption hat the
wood absorbs all of the water added by the glue. This is not strictly
correct since some of the glue squeezes out of the joirts and some water
evaporates during the pressure period. rh- calculated percentages, hoee-
are reasonably close to the results obtainred in actual gluing.
It is evident from these values that regardless of the type of roo-
temperature-setting glue used, a panel consisting of pli3 of thin veneer
throughout will require a drying period following the gluing operation, e
though the veneers were previously dried to 5 or 4 percent moisture tote
Far less moisture will need to be removed, however, when one of' the resi
glues is used rather than casein, animal, or vegetable glue.
Conditioning Glued Thick Stock
When the moisture increase in the wood is small, as in thick lamrira-
tions dried to a suitable moisture content before gluing, on-ry condition
to a uniform moisture content is necessary.
Sunken joints are common defects in the manufacture of thick -
,.'ed lumber glued at room temperature. They are caused by srfaci, b
stock too soon after gluing. The wood at the joint absorbs more water '
Rept. No. D475
the glue than the remainder of the piece and therefore swells more. If the
piece is surfaced before this excess moisture is distributed, more wood is
removed along the joints than at intermediate points. Then, during subse-
quent drying and conditioning, greater shrinkage occurs at the joints than
elsewhere, and permanent depressions are formed. Such depressions along the
glue line may show very conspicuously in the finished panel when viewed under
a side light. To avoid sunken joints in edge-glued lumber 1 inch thick glued
with casein, animal, or vegetable glue, it should be piled on stickers and
dried for a period of 2 days in a kiln at 100 F., or for 5 to 7 days at
If the gluing is done with room-temperature-setting urea or resorcinol
resins, the added moisture will be much less and a conditioning period of 1
to 2 days at 70 F. may be sufficient.
It is necessary to dry a part of the glue moisture from plywood and
veneer panels pressed at room temperature. For example, assuming a moisture
content of 3 percent in the veneer and an increase of 14 percent from the
glue, the panels when removed from the clamps or press would contain about
17 percent of moisture. Such percentages are common in many types of plywood
immediately after gluing, especially if made with casein, animal, or
vegetable glue. For use in cabinets, in furniture, or in the interior of
buildings, more than one-half of this moisture should be removed before the
anel3 are ready to be put into the finished article. For use outdoors or in
unheated buildings, plywood containing about 12 percent moisture will
generally prove satisfactory. Where veneer is glued over a lumber core, the
increase in moisture content of the whole panel at the time of removing the
panELs from the press is not so large as with thin plywood. In thick core
panels, niowever, the moisture from the glue is largely confined to the out-
icde of tne core and to the veneer. Therefore, the excess moisture of these
parts -s as great as in thin plywood and must be dried out or allowed to
equalize through the core.
The moisture content of panels made with lumber core is also affected
by the typo of glue used, but to a less extent than those made of thin
veneers throughout. Table 2 shows the amount of moisture added by the glue
to two particular lumber core panel constructions. This table also shows
that the moisture added by the resin glues is considerably less than the
amount added with casein, animal, or vegetable glue, and this fact must be
taken into consideration both in drying the core lumber and in conditioning
the glued panels. If, for instance, the core in the first panel listed in
table 2 is dried to 5 to 6 percent moisture content and a urea or resorcinol
glue is used, final drying will not be required if an opportunity is afforded
for the moisture to become equalized among the plies.
If thick cores (1-1/2 inches) are dried to a low moisture content
before gluing, the water added in gluing the veneer onto the core may not
)ring the whole panel above 7 or 8 percent moisture content, even when
casein, animal, or vegetable glues are used. Under such a circumstance, the
panels are sometimes stacked solid in piles and allowed to condition. This
Rept. No. D475
practice requires a long conditioning period, and the absorptio of mdaistu
by the core after the cross bands have been glued to it subjects e whol
panel to severe stresses.
A better and rapidly increasing practice for conditic' tick-cre
panels that contain excessive moisture, is to place the panels on stickers
and allow them to dry in panel kilns or in factory workrooms, This practi
allows the excess moisture to be dried from the panel faces, where it is
largely concentrated, and does not necessitate drying the thick-core sto.l
to an extremely low mo- ;ture content before gluing. Panel kilns permit u-'
rapid drying than factory workrooms, give a better means for controilI .
conditions during drying, and save factory space.
In p.,,l kilns it is very easy to dry most three-ply and five-ply
panels satisfactorily in 2h hours. Thick stock and low drying t-Liperatl ef
increase the required d-ying time. Results of tests at Zhe Forest Pro-l
Laboratory in panel kilns sLiow that under normal conditions the sture
Added in gluing three-ply panels, 5/16 of an inch thicK, can be dried outi
satisfactorily in from 8 to 16 hours. These tests also indicate that the
desired essentials in drying can be met by maintaining a constant tmf ra?.
and relative humidity throughout the drying. To save time in such kiln
operations, it is advantageous to maintain conditions that corrtfsprDi.I to a
moisture content slightly below that to which the panels aCe to be dried.
Table 5 shows several combinations of temperatures and relative
humidities that will bring the stock to approximately the desired moisture
content, but that will not allow an appreciable amount of drying beyTid t-its
Panels are usually open-piled on strips called stickers. Th cK
should be made from dry, straight-grained wood, entirely free fro _ain or
deca&j, Morecver, the stickers should be dressed to a uni :'orm. ` ic -
Sv',:-n-eignths by 1-1/4-inch stickers should he used in dryinrg the .riai u
In load-i ng a kiln truck, stickers should placed at the extem
of -e panel and the intervening space so divided that the distance bwe
stickers will not exceed 18 inches. Where there is danger of warping, te
stickers may be spaced a foot apart. It is important that the stickers i
each tier be placed in vertical alignment on solid foundations to pre e..*.
panels from sagging. The possibility of virping in the upper pajoel y D
further reduced b-. placing a cover board on stickers on top of the :_ .
Sometimes the piles are weighted, but experiments indicate the qplI ca ii
pressure to rnels during drying does not reduce warp as much as conrorlI
thought Whenever p acticable, plywood should be so piled as to pr)vie
fIes om -che top to the bottom of the load in order that ai a r e Tc-r
move in a vertical plane through it.
Dryirg panels to an excessively low moisture content materiall in-
creases warping, checking, opening of joints, and other it-fects, >ess show
that the amount of warping on three-ply veneer panels is approximately pro-
portional to the percentage of moisture removed from the panel in d..
Rept. No. Dh75
In a few instances, plywood has been dried on mechanical veneer driers
and on hot-plate presses. These methods, however, have been confined to ply-
wood of a high moisture content that was glued with water-resistant glue.
Plywood dried in this way is usually comparatively thin and not of the
highest quality. The use of mechanical driers and hot-plate presses results
in quick drying, but involves more expensive equipment than the other methods.
Plywood and other members glued on hot presses commonly contain some
Sor 5 percent moisture when removed from the press. Such material should be
conditioned to about 8 percent if intended for interior use and to about 12
percent if made for exterior service. This may be done in conditioning rooms
irn which a relative humidity is maintained that is approximately equal to or
slightly in excess of that corresponding to the desired moisture content.
Another method is to apply sufficient water to the hot-pressed panels to
bring them to the required moisture content and then to stack them solidly,
allowing t-he moisture to equalize throughout. Care should be used to apply
only sufficient water to bring the panels to the desired moisture content.
The correct amount of water can be readily calculated after determining the
moisture content and weight of the dry panels. The moisture is conveniently
applied by passing the panels between water-covered rolls, such as in a glue
spreader, or by spraying. By weighing a number of panels before and after
the application of the water, the amount and uniformity of the application
,an be checked. The time required for equalization in the solid piles again
varies with the thickness of the individual panels. While the panels are
usually warm when the water is applied, a circumstance that aids equalization,
-he glue lines, especially of synthetic-resin glues, retard diffusion. Con-
ditioning periods for plywood of different thickness and number of plies
should be based on actual moisture-content determinations of both the
interior and exterior plies.
Rept. No. D475
Table l.--Calculated percentages- of moisture added to wood i.'! & i'f:.
five-ply constructions wi th room-temperature-settig ,j ,*-.
SThickness : Moisture added in gluing with
Each ply : Casein, :Room-temperature-: Room-temperature-
or :animal,or: setting urea- :setting resorcinol-
:lamination :vegetable: resin glues : resin glues
S Inch : Percent Percent Percent
Yellow-poplar: 1/32 : 62.0 23.6 17o4
Yellow-poplar: 1/16 : 51.0 : 11.8 8.7
Yellow birch 1/52 : 40.0 15.1 : l,1
Yellow birch 1/16 20.0 : 7.6 5 6
Sitka spruce 1/8 : 15.5 : 5.9 4.3
Sitka spruce : 5/8 : 5.2 : 2.0 1.5
Sitka spruce : 3: 2.6 : 10 0.7
Yellow birch : 1/8 : 10.0 5.8 : 28
Yellow birch : 5/8 5.5 : 1.5 0.9
Yellow birch : 5/4 1.7 : 06 0,5
-Calculated percentages are based on oven-dry .'eirht of wood and volume
qt 12 percent. In the calculations it is assumed that all the sur-pluS
solvent added by the glue is absorbed by the wood. This assimtion i
'.own to be somewhat in error, but it nevertheless affords a satis-
factory basis for comparison.
-Spreads of 75 pounds of wet casein, animal, or vegetable glue and 47
pounds of wet area- and resorcinol-resin glues per 1,000 square fee+
of single glue line are assumed in these calculations It is assmed
that the casein, animal, or vegetable glue is mixed one part dry .ie
to two parts of water (solids content 55 percent), the cold-setting
urea resin one part dry glue to 0.65 parts of water (solids content
60 percent), and that the mixed resorcinol-resin glue has a solids
content of 70 percent.
Rept. No. D)475
Table 2.--Calculated percentages-1 of moisture added to wood in gluing five-
ply lumber core panels with room-temperature-setting glues
Face : Cross Core : Moisture added in gluing with
and bands :
back : : Casein, :Room-temperature-: Room-temperature-
: animal, : setting urea- :setting resorcinol-
or : resin glues resin glues
: Percent Percent : Percent
1/28-inch: 1/20-inch: 15/16-inch:
black : sweetgum: sweetgum : 7.0 : 2.6 : 2.0
mahogany: yellow- : chestnut : 10.7 : 4.0 5.0
or :poplar :
-Calculated percentages are based on oven-dry weight of wood and volume at
12 percent. In the calculations it is assumed that all the surplus solvent
added by the glue is absorbed by the wood. This assumption is known to be
somewhat in error, but it nevertheless affords a satisfactory basis for
-Spreads of 75 pounds of wet casein, animal, or vegetable glue and 47 pounds
of wet urea and resorcinol-resin glues per 1,000 square feet of single
glue line are assumed in these calculations. It is assumed that the casein.
animaL, or vegetable glue is mixed one part dry glue to two parts of water
(s-.lids content 33 percent), the cold setting urea resin one part dry glue
to 0.65 parts of water (solids content 60 percent), and that the mixed
resorcinol-resin glue has a solids content of 70 percent.
Rept. No. D475
Table 53.--Combinations of temperatures and relative humidities suitable f--
drying plywood panels to moisture-content values of 6 to 12
Moisture : Percentage of relative humidity for use with stated temperatur--'
desired: 70 F. 80 F. 90 F. 100 F. "ll F. 120 F. 140
6 : 19 19 : 20 21 22 24 : 26
7 24 : 26 : 27 : 28 : 29 51 54
8 50 51 52 53 5 57 41
10 : 45 44 45 46 48 50 55
12 55 : 56 57 : 58 : 59 61 65
-The relative humidities shown for the lower temperatures and moisture-
content values are obtainable ordinarily only during the winter season,.
Where a low moisture content is necessary during warm, humid weather,
it can be obtained by raising the temperature.
Rept. No. D475
UNIVERSITY OF FLORIDA
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3 1262 08927 9201