Softwood Plywood: Balanced Construction
In producing a plywood panel (or any panel for that matter) it is essential that the principles of balanced construction be observed. What this means, in essence, is that the potential for dimensional change must be equal on the two sides of the panel at equal distances from the center of the panel. In other words, in moving from the center of a panel outward toward the surfaces, the panel makeup should be a mirror image on both sides of the panel.
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Consider, for example, a three-ply panel which has the grain direction of
veneers arranged as shown. Here, the top and bottom veneers will tend to
shrink in one direction (across the grain),
as they lose moisture, while the middle veneer will tend to shrink in
the other direction (also across the grain).
The result is a panel that exhibits little shrinkage. Recall that wood shrinks very little along the grain (or in the grain direction), but a considerable amount across the grain. Wood is also strongest when stressed parallel to the grain. Thus, when veneers are glued together such that the grain direction in adjacent plies are at right angles to one another, the tendency to shrink and swell with moisture change is largely eliminated in the flat dimensions of the panel. (Adapted from drawing by Karen Lilley as published in Haygreen, J.G. and Bowyer, J.L. 1996. Forest Products - An Introduction. Ames: Iowa State University Press.) |
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Two veneers are depicted here, with the red arrows indicating the
grain direction of each.
Alone, veneer A will shrink and swell with moisture content change both in thickness and across the grain as indicated by the green arrow. Veneer B, will not change dimensions in the same direction. Veneer B will shrink and swell in thickness and across the grain as indicated by the green arrow that appears on it. If glued together, each veneer will restrain the other. |
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| There is still a problem here, however. Although we have succeeded in creating an assembly that will not shrink and swell in length and width, we do not yet have a product that will remain flat as the moisture content changes. Should the two veneers shown above lose moisture after gluing, the panel would tend to look like this: |
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This problem can be solved by adding another veneer to the assembly:
In this situation forces are balanced, (again, the red arrows indicate grain direction and the green arrows indicate the direction of dimensional change) and the glued-up panel will remain flat. To convince yourself of this consider the glued-up veneers two at a time, giving consideration as to how they would react with gain or loss of moisture. |
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| The assembly shown above may still warp if veneers A and C are of different densities, thickness, or moisture content. A higher density veneer on one face, for example, will shrink and swell more than a lower density veneer, resulting in an unbalanced panel. |
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