Wood moisture content in the plywood
Plywood construction that consists several layers of
thin wood is more
stable than solid wood. The plywood surface is relatively closed to make the wood
surface becomes more stable and less affected by changes in the
outside humidity. However the plywood is highly
depend to the conditions of moisture
content of the wood pieces which
form it. The change of moisture content of the constituent
elements will continue
to make changes in the shape and volume of the plywood
and if the change is too large will result
for the occurrence of surface defects such as
checks, sunken joints,
wavy surfaces and
other defects.
When the plywood used laminated wood as its
cores, then the variation at the wood pieces of the cores is potential to make
the wood defects since the size changes of the wood. That is way the moisture
content of the wood pieces in the construction of the plywood must be controlled
to be as uniform as possible.
The moisture content of the core stock at
the time gluing is very important. The best authorities seem to agree that
moisture content about 5 % is advisable and then the gluing process will add
the moisture content. The moisture will distribute itself by conditioning
treatment after the veneer has been laid off. The thinner the core stock the
drier it should be at the time of laying cross banding and veneer.
Since the cross banding usually rather thin
as compared to the core, it should be dried and treated as the sane ways as
face veneer. As General rule, the moisture content should be about 5 % at the time of laying.
The cross banding is usually spread with glue on both side. One side is being
place against the core and the other against the face veneer. The wood will
pick up considerable moisture from the gluing process, but this can be
redistributed by mean of condition treatment after the panels has been built up.
Just before laying the veneer, it should be
re dried to a moisture content of 4 to 6 percent. Some prefer higher
M.C., this lessen to the liability of fancy, cross grained veneer to break in
handling. Consequently such veneer is sometimes laid with moisture content of 8
to 10 %. The amount of shrinkage in the face veneer when it dries out will be greater
under this condition than if the veneer were laid at moisture content of 5% – 6 %. If the veneer was
too fragile to be handled at lower moisture content it is almost certain not
too be strong enough to resist the greater tendency to shrink, and checking of
the face veneer will probably result causing check and cracks in the finish.
The use of moist veneer in building the
panels also results in much more warping than is obtained when the dry veneer
is used. Where there is a manufacturing advantage in using moist veneer, this
increased warp and liability to checking must be taken into consideration as disadvantage which will
partially or entirely offset the gain from the from using moist veneer. The moisture
content of the back veneer should be as the same with the face veneer.
Otherwise one will shrink more than the other upon drying the glue up work and
may result in warping.
The pressure should be applied to the
panels as quickly as the veneer is laid to prevent the thin veneer to expanding
too much. The incorrect position of the veneer when held by glue will result in
the formation of check as the panel dry out.
The panels should be left in the retainers
until the glue is fully dry. As the panels are removed from the retainer, they
should be piled on special trucks with stick of exactly uniform width and
thickness between the panels. Each stick should
be placed directly above the one below to ensure the plywood drying is straight.
It is evident that the built up panels must
also be given a conditioning treatment after gluing to bring the moisture
content to the right point
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