ever, when these laminated pieces are sent to a humid environment, a lot more moisture increase will occur, which means that
some swelling will occur. In fact, with composite wood, we often
see that the first time the wood is exposed to high moisture, the
swelling is much larger than expected due to springback of the
wood. (The wood was pushed tightly together in manufacturing
and now the pressure is relieved with excessive swelling.) Now,
if the veneer or other laminated material on the surface does not
swell as much, this will create tension, which will try to pull the
veneer apart. When a thin piece of veneer fights against a large,
solid core, the core wins.
For your special humid environment, you need to make sure
that the manufacturer of the legs allows the core material to
reach 10 percent MC before the legs are veneered. Is this time
consuming and expensive? Yes, but it is the cure. You might be
tempted to switch to a lumber core. You will have some of the
same issues if the lumber is too dry at the time of laminating, but
the MC difference is less and there is no springback phenomena
with lumber.
Q:We are in a discus- sion about the role
that water and moisture
play in bending wood.
I know that how well a
piece bends is dependent
on the moisture content
of the wood but what role
does the moisture play
in the plasticity of the
sample? If you were to
take a piece of wood that
is oven dry and we’re able
to cool it in a chamber
with 0 percent RH, would application of heat alone be
enough to bend the piece and maintain its shape?
Wood pushed
together in
manufacturing will
relieve the pressure
by swelling.
A:Heat does indeed make the wood plastic, which is what we need in order to bend wood and have it maintain its
bent shape. (If wood were only elastic, it would bend, but then
when the bending force was released, the wood would return to
its original shape.) Moisture extends the plastic range of wood.
I cannot say which factor, heat or moisture, is more critical; both
are important, but perhaps heat is a bit more important than
moisture. Note that at 0 percent MC, the wood is so brittle that
the plastic range is very small. Failure can occur as soon as the
wood bends a little bit even when using heat to plasticize the
wood. ❮
Gene Wengert, “The Wood Doctor,” has been training
people in efficient use of wood for 35 years. He is extension
specialist emeritus at the University of Wisconsin-Madison.
A GUARD
SYSTEM YOU’LL
ACTUALLY USE.
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