Plywood shrinkage??? Laminated soundboards???

[Delwin D Fandrich]

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Re: Plywood shrinkage??? Laminated soundboards???
  -----Original Message-----
  From: pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]On
Behalf Of Sarah Fox
  Sent: February 15, 2004 8:58 AM
  To: Pianotech
  Subject: Re: Plywood shrinkage??? Laminated soundboards???




  I'm just curious about the moisture blocking properties of the SB finish:
How much does the finish alter the rate of MC change in a SB?  I presume it
depends on the type and thickness of finish used.  Has anyone done any
side-by-side measurements?
Yes. Check the archives. Basically, I know of no finish material other than
multiple coats of epoxy that will form a effective vapor barrier when
applied to wood.


  Back to the plywood...

  Regarding quality:  Although the drawer bottoms may  be constructed of
relatively poor quality material, I would strongly suspect only the best
materials are used in boat construction.  In fact knowing some boating
enthusiasts, who pour unbelievable amounts of money into their surrogate
babies, I'd suspect the quality of these materials would rival what you
builders/rebuilders use!  Furthermore, the reference to boat deck plywood
shrinkage is most likely with regard to modern-day materials.
The shrinkage/expansion potential has more to do with the physical
characteristics of the wood used (its compression strength perpendicular to
grain, etc.), the type of adhesive used (its rigidity and/or its cold-flow
characteristics) and the thickness of the veneers used than its "quality."
In other words, a "high quality" plywood using top-grade, relative thick,
hardwood veneers might be less stable than a "cheap" plywood using thinner
doug-fir peeler veneers.

And on the starting moisture content. Wood, regardless of how it is
configured, does not continue to shrink just because it gets older. It must
start its shrink from somewhere and that somewhere depends on how much
moisture is bound in the cells of the wood. Once that moisture is gone the
wood stops shrinking. If moisture gets back in the wood expands -- that is
the opposite of shrinking -- until the cells are saturated, then it stops
expanding. In other words there will be a range within which the wood will
move. There will be a maximum dimension and a minimum dimension, both
dependent on moisture. Regardless of what you do to it, it doesn't just keep
on shrinking. Well, I guess you could burn the stuff ... it shrinks quite a
bit then.

Because of its cross-banded nature plywood is far more stable than so-called
"solid" wood. But there will still be some nominal movement with changes in
moisture content, but these changes will always be subject to the basic laws
that govern wood strength and wood movement.


  I suspect shrinkage may be the fate under extreme conditions.  The boat
deck plywood obviously gets baked in the sun, doused with water, and so
forth.  Drawer bottoms can frequently be wiped out with wet rags -- which
while not as extreme, still creates a rapid and drastic changes in MC.
Perhaps more importantly, another dynamic is going on in both these cases.
Deposition of water is never uniform, whether through leakage past finish
defects (boat) or through differential deposition of water with a wet rag.
This can cause a lot of internal stress and strain in the panel.  For
instance, if a small spot in the center of a panel is soaked with water,and
the surrounding board remains dry, the swollen center spot will be
compressed within the "frame" of the surrounding wood.

All of this is true, but please remember that in most plywood construction
the laminae are quite thin. Thin enough that they are no longer capable of
creating much in the way of compression (to expand the panel) or tension (to
cause it to shrink). That is the whole point of plywood. It is the relative
thinness of the laminae and the crossbanding that gives plywood its relative
stability.

Nor am I convinced by the many tales boat owners tell of their plywood decks
"shrinking." Solid teak overlays certainly do swell and shrink and boat
hulls certainly do twist and warp and I wonder just how any of them are so
certain it is the plywood decking that is "shrinking." I've lived on and
been around too many boats and have listened to way too much pure poppycock
from boat owners (many of whom should know better) about their boats to
believe much of anything they have to say about their boats without
verifying it first. The most memorable tale of shrinking plywood decking --
it couldn't be the fiberglas hull, could it? -- turned out to be the hull
that was literally coming apart. The was spreading across the beam,
ultimately splitting the joint between the hull and the fiberglas deck
molding. The plywood decking turned out to be about the only thing that was
holding the thing together.

Del

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