M ING AND CONDITIONIN G
GLUED JOINT S
Information Reviewed and Reaffirme d
July 196 1
No. 475
I111111
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FOREST PRODUCTS LABORATOR Y
UNITED STATES DEPARTMENT OF AGRICULTUR E
FOREST SERVICE
MADISON . 5, WISCONSIN
DRYING AND CONDITIONING GLUED JOINTS By
Forest Products Laboratory, 2 Forest Servic e
U .S . Department of Agricultur e
In order to maintain quality of product and prevent warping, checking, sunke n
joints, and other defects in the finished article, it is essential that woo d
after gluing be brought to the moisture content most suitable for the sub sequent use of the article and that the moisture be evenly distribute d
throughout .
The moisture content of the wood may be either increased or decreased durin g
gluing, by an amount depending on (1) the process used, (2) the form an d
composition of the glue, (3) the amount of glue spread, and (4) the dimensions
of the wood parts glued . In general, hot-press methods reduce the moistur e
content and room-temperature gluing increases it . Glues of high water content
add more moisture to the wood than glues of low water content, and heav y
spreads add more than light spreads . More water is added by the glue to a
construction made of thin plies than to one of equal thickness made of thic k
plies . Furthermore, the percentage increase in moisture content from a give n
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, and casei n
glues add considerably to the moisture content of the wood, and frequentl y
effect an uneven distribution of moisture within the construction . The
moisture added with the more recently developed synthetic-resin glues is much
less and sometimes practically negligible . Table 1 illustrates the approxi mate effect on moisture content of gluing with several room-temperature-setting
glues . The table is based on the assumption that the wood absorbs all of the
water added by the glue . This is not strictly correct, since some of the glu e
squeezes out of the joints and some water evaporates during the pressure
period . The calculated percentages, however, are reasonably close to the
results obtained in actual gluing .
1
"Original report dated May 1955 .
2
-Maintained at Madison, Wis ., in cooperation with the University of Wisconsin .
Report No . 475
It is evident from these values that regardless of the type of room-temperature setting glue used, a panel consisting of plies of thin veneer throughout will
require a drying period following the gluing operation, even though the
- veneers were previously dried to 3 or 4 percent moisture content . Far les s
moisture will need to be removed, however, when one of the resin 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 lamination s
dried to a suitable moisture content before gluing, only conditioning to
uniform moisture content is necessary .
A
', i
-1 . .
Sunken joints are common defects in the manufacture of thick edge-glued lumber
glued at room temperature . They are caused by surfacing the stock too soo n
after gluing . The wood at the joint absorbs more water from the glue than the
N
.remainder of the piece and therefore swells more . If the piece is surface d
before this excess moisture is distributed, more wood is removed along th e
joints than at intermediate points . Then, during subsequent drying and con - = :' r~=
?NG:
ditioning, greater shrinkage occurs at the joints than elsewhere,
permanent depressions are formed . Such depressions along the glue line may ,_ .,,'-.;x
show very conspicuously in the finished panel when viewed under a side light .' - :_ .,rl .
To avoid sunken joints in edge-glued lumber 1 inch thick, the following con.
"r r
ditioning treatments are recommended :
-
7 days at 8o° F . and 30 percent relative humidity
4 days at 12 00 F . and 35 percent relative humidit y
• 24 hours at 160° F . and 44 percent relative humidit y
16 hours at 200° F . and 55 percent relative humidit y
The conditioning schedules given above are suggested for edge-glued panel s
that will be used with a high-gloss finish and without veneering . If ther e
is an appreciable layover period between the surfacing and the sanding an d
finishing operations, the conditioning period can probably be shortened some what, since surface irregularities of certain magnitudes are usually remove d
by the sanding .
For edge-glued furniture panels that are subsequently covered with .crossbands
(usually 116 or 1/20 inch) and face veneers (usually 1/28 inch), the con ditioning times shown above can be shortened at the different temperatures ,
perhaps as much as one-half .
Report No . 475
Drying Plywoo d
It is necessary to dry a part of the glue moisture from plywood and venee r
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, th e
panels when removed from the clamps or press would contain about 17 percent o f
moisture . Such percentages are common in many types of plywood immediatel y
after gluing, especially if made with casein or soybean glue . For use in
cabinets, in furniture, or ride buildings, more than one-half of thi s
moisture should be removed before the panels are ready to be put into th e
finished article . For use outdoors or in unheated buildings, plywood con taining about 12 percent moisture will generally prove satisfactory . Wher e
veneer is glued over a lumber core, the increase in moisture content of th e
whole panel at thetime of removing the panels from the press is not so larg e
as with thin plywood . In thick-core panels, however, the moisture from th e
glue is largely confined to the outside of the core and to the veneer . There fore, the excess moisture of these parts is as great as in thin plywood an d
must be dried out or allowed to equalize through the core .
The moisture content of panels made with lumber core is also affected by th e
type of glue used, but to a less extent than those made of thin veneer s
throughout . Table 2 shows the amount of moisture added by the glue to tw o
particular lumber core panel constructions . This table also shows that the
moisture added by the resin glues is considerably less than the amount adde d
with casein, animal, or vegetable glue, and this fact must be taken int o
consideration both in drying the core lumber and in conditioning the glue d
panels . If, for instance, the core in the first panel listed in table 2 i s
dried to 5 or 6 percent moisture content and a urea or resorcinol glue i s
used, final drying will not be required if an opportunity is afforded for th e
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 bring the whol e
panel above 7 or 8 percent moisture content, even when casein, animal, o r
vegetable glues are used . Under such a circumstance, the panels are sometime s
stacked solid in piles and allowed to condition . This practice requires a
long conditioning period, and the absorption of moisture by the core afte r
the crossbands have been glued to it subjects the whole panel unnecessaril y
to severe stresses .
A better practice for conditioning thick-core panels that contain excessiv e
moisture, is to place the panels on stickers and allow them to dry in pane l
kilns or in factory workrooms . This practice allows the excess moisture to b e
dried from the panel faces, where it is largely concentrated, and does no t
necessitate drying the thick-core stock to an extremely low moisture conten t
before gluing . Panel kilns permit more rapid drying than factory workrooms ,
give a better means for controlling the conditions during drying, and sav e
factory space .
Report No . 475
-3-
In panel kilns it . is- very easy to dry most three-ply and five-ply panel s
satisfactorily in 24 hours . .Thtck stock and low drying temperatures increas e
the required drying time . Results of tests at the Fores t . Products Laboratory
in panel kilns show that under normal conditions the moisture added in .,glaing
three-ply panels 3/16 of an inch thick, can be . dried out satisfactorily i n
from 8 1t a16-hours : These tests also indicate that the desired essentials in .
drying car be met b y - maintaining a constant temperature and relative, humidit y
throughout the drying-. To save time in such kiln operations, . it i s
_'advantageous to maintain conditions that correspond to a-moisture conten t
slightly below that to which the panels are to be'dried ..
Table 3 shows, several combinationa of temperatures and relative humiditie s
that will bring-the stock to . approximately the desired moisture content, bu t
that will not allow an appreciable amount of drying beyond this point .
Panels_should be-open-piled on strips called stickers . The stickers shthild be
made from dry, straight-grained wood, entirely free from stain or decay .
Moreover, the stickers should be dressed to a uniform thickness . Seven-eighths
by 1-1/4-inch stickers should be used in drying the usual run•of .panels .
In loading a kiln t--ruck, stickers should be placed at the extreme ends of th e
panel and the intervening space so divided that the distance between sticker s
will .not exceed 18 inches : Where there is danger of warping, the sticker s
may be spaced a That apart . It is important that the stickers in each tie r
be placed in verticca1 alinement on solid foundations'to prevent the_panel s
from sagging . .The possibility of Carping in the upper panels may be furthe r
reduced by placing a cover board On stickers on top of the'pile Sometime s
the piles are weighted, but experiments indicat e-,that the application o f .-- .
pressure to panels during drying 'does not reduce warp to an important,degi'ee : '
Whenever practicable, plywood should be so piled as to provide flues from .the
top to the bottom of the load in order that-air may readil y- move in a vertica l
plane-through it .
Drying .'panels to an'excessively low moisture content materially increase s
warping,' checking, opening 'of 'joints, and other defects . Tests show that the ;-amount of warping'on three-ply veneer panels is approximately preTortionalto, .
the percentage of moisture removed from the panel in drying :
In a few instances, plywood has been dried on mechanical veneer driers and o n
hot-plate presses . _- These methods, however, have been confined to plywoo d
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 o
but involves more expensive equipment than the other methods .
Plywood and other members glued on hot presses at around 300° F . commonly contain some 2 or 3 percent moisture when removed from the press . Such
- material should be conditioned to about 6 to 8 percent if intelded for,
.
This
may
service
interior use and to about 12 percent if made for exterior
be done in conditioningtrooms in-which a relative humidity is maintained tha t
. is approximately equal to or slightly in excess of that corresponding to th e
Report No . 475
-4-
desired moisture content . Another method is•to apply sufficient water to the .
hot-pressed panels to bring th'ilt to the required moisture content and then . '
to stack the. solidly, allowing the "moisture to equalize : throughout . .
should be used to apply only :-sufficient water to bring the panels . to-th e
desired moisture content . The correct amount of water can be readily '
calculated after determining . the moisture content and weight of thedry ga elp :.
The moisture is conveniently applied by passing the panels between. rater'.: '
:
covered rolls, .such as in a glue spreader ; or by spraying
By weigiing a
number of panels before and after the application of . the . water, the amount and
uniformity of-theapplication can be checked . The_time required fo r
equilization it the solid piles again varies with . the .thickmessof the
individual . panels . While. the panels " are usually warm .whe.n. the`.waater ts ' applied ,
a circumstance that .aids equilization, the glue lines/especially o f
synthetic-resin glues," . retard diffusion . Conditioning periods for plywdo l .Of •.1 . different thickness , and number of plies should- be based . on actual .moistu
content determinations of_ both the interior and ekterior plies .
Report No . 475
Table 1 .--Calcu.latedpercdntagesl of moisture added to wood ingluin g
five-plyconstructions with roam-temperature-setting glue s
Species
Thickness :
Moisture added in gluing• with
• of each :
: ply or : Casein, :Room-temperature- :Room-temperature :lamination :animal,or : setting urea - :setting resorcinol resin glue s
resin glue s
:
:vegetable :
: glues
Inch
Percent
: Percent
Percent
23 .6
17 .4
11 .8
8. 7
15 .1
11 . 1
:
7 .6
5.6
15 .5
:
5 .9
4.3
3/8
5 .2
:
2..0
1.5
Sitka spruce :
3/4
2 .6
:
1 .0
.7
Yellow birch
1/8
10 .0
:
3 .8
2.8
Yellow birch :
3/8
3 .3
:
1 .3
.9
Yellow birch :
3/4
1 .7
Yellow-poplar :
1/32
62 .0
Yellow-poplar :
1/16
31 .0
Yellow birch
1/32
40 .0
Yellow birch :
1/16
20 .0
Sitka spruce :
1/8
Sitka spruce :
.
.6
-
.5
-Calculated percentages are based on oven-dry weight of wood and volum e
at 12 percent . In the calculations it is assumed that all the sur plus solvent added by the glue is absorbed by the wood . This assump tion is known to be somewhat in error, but it nevertheless affords a
satisfactory basis for comparison .
Spreads of 75 pounds of wet casein, animal, or vegetable glue and 4 7
pounds of wet'urea- and resorcinol-resin glues per 1,000 square fee t
of single glue line are assumed in these calculations . It is assumed '
that the casein, animal, or vegetable glue is mixed one part dry glu e
to two parts of water (solids content•33 percent), the cold-setting _
urea resin one part dry glue to 0 .65 parts of water (solids conten t
60 percent), and that the mixed resorcinol-resin glue has a solid s
content of 70 percent .
Report No . 475
ti
Table 2 .--Calculated percentages) of moisture added to wood in gluing five ply lumber core panels with room-temperature-setting glue s
Face
and
back
: Cross
: bands
:
Core
:
:
Moisture added in gluing- with
: Casein, :
RoomRoom animal, : temperature- :temperature or
:setting urea- : settin g
vegetable resin glues : resorcinol glues
: resin glue s
Percent
1/28-inch :1/20-inch :15/16-inch :
black
: sweetgum : sweetgum :
walnut :
1/20-inch :1/20-inch :5/8-inch
mahogany : yellow- : chestnu
: poplar .
tor
sweetgum :
:
7 .0
10 .7
:
Percent
Percen t
2 .6
2 .0
4 .0
.
3 .0
-Calculated percentages are based on oven-dry weight of wood and volum e
at 12 percent . In the calculations it is assumed that all the sur plus solvent added by the glue is absorbed b y. the wood . This assump tion is known to be somewhat in error, but it nevertheless affords a
satisfactory basis for comparison .
2
"Spreads of 75 pounds of wet casein, animal, or Vegetable glue and 4 7
pounds of wet urea and resorcinol-resin glues per 1,000 square fee t
of single glue line are assumed in these calculations . It is assume d
that the casein, animal, or vegetable glue ismixed one part dry glu e
to two parts of water (solids content 33 percent), the cold settin g
urea resin one part dry glue to 0 .65 parts o f 1 water (solids conten t
60 percent), and that the mixed resorcinol-resin glue has a solid s
content of 70 percent .
Report No . 475
Table 3 .--Combinations of temperatures and relative humiditie s
suitable for dryingplywoodpanels to moisture contentvaluesof4to 12 percent, inclusiv e
Moisture
content
desired
(percent)
6
7
: Percentage of relative humidity for use with stated temperatureZ
••
:
•
°
.
°
.
100° F . :110° F .
120° F .
140° F .
90° F :.
: 70 F . s 80°
•
:
19
:
24
rr ;rrrr .r .rr- ;-r-r-rrr=-rr rr---r~-rrrrr :
:
•
•
:
:
20
26
:
-27
:
-28
:
29
:
32
:
33
:
35
45
:
46
:
48
57
:
58
.30
s
31
10
43
:
44
12
55
:
56
:
rrr-r-ri ;rrrrrr--'r
22
19
8
•
1
4
21
59
24
26
:
31
34
:
37
41
•5o
53
61
..b5.
•
:
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 .
Report No . 475