24 



Hardwood Record — Veneer & Panel Section 



July 25. 191 y 



moisture content of an ordinary board is change in di- 

 mension, either as shrinkage or as exp^.nsion. Changes of 

 moisture conditions are responsible for almost all the un- 

 desirable distortions that occur in boards. For simplicity, 

 only changes of dimension which result from a reduction 

 in moisture will be considered in this article. 



The shrinkage of wood parallel to the grain is negligible 

 compared to the shrinkage across the grain. On account 

 of the great difference in shrinkage of wood in these two 

 directions, a change in moisture content of plywood will 

 inevitably either introduce or relieve internal stresses. 

 Plywood consists of layers of wood glued together so that 

 the grain of successive layers or plies cross, consequently 

 the shrinkage of one ply across the grain takes place at 

 right angles to the across-the-grain shrinkage of the adja- 

 cent ply or plies. We will consider a three-ply construc- 

 tion for an example and subject it to low humidity con- 

 ditions so that the moisture content of the plywood is 

 lowered. Since the grain of the core is at right angles 

 to the grain of the faces, the core will tend to shrink a 

 great deal more than the faces, in the direction of the grain 

 of the faces. This shrinkage subjects the faces to com- 

 pression (pressure) and the core to tension (pull). The 

 magnitude of these internal forces depends upon changes 

 in moisture content, density, species, and the relative thick- 

 ness of the core and face veneer. 



The shrinkage of ordinary flat-sawn boards in drying 

 from the soaked or green to the oven-dry condition ranges 

 from about 4 to 1 5 per cent, being greater for the heavy 

 or high density woods than for the low density woods. 

 A maple board I inches wide when changing moisture 

 content from about 1 5 to 5 per cent may shrink 0.4 inch. 

 When in the form of a lamination between other lamin- 

 ations of wood whose grain crosses that of the first, it is 

 obvious that maple wood in the center will be restrained 

 from shrinking, or in other words will be stretched across 

 the grain almost to full 0.4 inch because there is little 

 give" in the outer laminations or plies parallel to the 

 grain. The condition is analogous to that which would 

 exist if an elastic sheet of strong rubber were stretched and 

 then glued between two plies of wood. 



In order to minimize the distortions of plywood panels 

 that are caused by changes in moisture content combined 

 with inaccurate or faulty construction, it is imperative that 

 all plies be at the same moisture content before gluing and 

 that the moisture content of the finished panel on leaving 

 the conditioning room be about the same as it will average 

 when in use. The limits of from 1 to 15 per cent moisture 

 in the finished panel will usually give satisfactory results 

 when the panel is in service in the open air. 



Symmetrical Construction of Plywood 



Fig. 5 shows the distribution of the internal stresses in 

 panels made of two, three, four and five plies. 



An equal reduction in moisture content of all plies is 

 assumed to have taken place, causing the plies showing 

 end grain in Fig. 5 to shrink in the direction indicated by 

 the arrows. It is obvious that the 2 -ply construction is 

 going to cup so that the side b will be concave. In the 



3-ply construction the ply c prevents the bending of the 

 other two plies. When the plies a and c are of the same 

 thickness, density, and species, the pressures set up in each 

 due to the pull of the center b will be approximately equal 

 and the panel should not warp. On adding another ply 

 d an additional pull is brought into play and the panel 

 again bends, although not so much as in the two-ply con- 

 struction. By adding one more ply e we have a 5-ply 

 panel and it will be seen that the pressure in e should equal 

 that in a and the pull in d should equal the pull in b. The 

 stresses in the 3-ply and 5-ply panels are seen to be bal- 

 anced and the panel should not warp if other conditions 

 are satisfactory. Panels constructed in this way are said 

 to be symmetrically built. A continuation of this discus- 

 sion for larger numbers of plies leads to the generalization 

 that in order to be symmetrical a panel must consist of an 

 odd number of plies. 



In order to obtain a balanced construction it is also 

 necessary that symmetrical plies (a and e in the 5-ply 

 panel. Fig. 5 or a and c in the 3-ply panels of the same 

 figure) be of the same thickness. If they are not of the 

 same thickness the pressure or push exerted by the thinner 

 ply will be less than the pressure of the opposite or sym- 

 metrical ply and the panel will tend to bow. Care in 

 sanding a panel is therefore necessary to prevent non- 

 uniform thicknesses. Cases of machine sanding are 

 frequently seen in which thin face veneer is sanded through 

 in spots. Panels having one-half the thickness of the face 

 veneer ground away are quite common and can of course 

 not be expected to remain very flat when subjected to 

 moisture variations. 



a b 



a be 



abcde 



". 



¥^ 



2 PLY 



3 PLY 4 PLY 



FIG. 5 

 ^Continued on piujc 26) 



.S PLY 



