July 25, I 91 9 



Hardwood Record — Veneer & Panel Section 



23 



Factors Affecting Warping of Wood 



Symmetrical Construction, Moisture Conditions and Direction of Grain Important 



By ARMIN ELMENDORF* 



HE WARPING OF PLYWOOD panels is a mat- If the distance between the edges is 1 inches and the 



ter of considerable concern to manufacturers w^ho recession 1 inch, we would say that the panel has cupped 



make panels for cabinet work where large flat I inch in 1 inches. A panel may, however, also be 



surfaces are desired. Experience and numerous tests twisted as shown in Fig. 1, in which two edges that were 



made at the Forest Products Laboratory of the U. S. parallel originally are now twisted relative to each other. 



Forest Service, Madison, Wis., for the army and navy. The angle that the edge A A- in Fig. 1 makes with the 



have shown that many factors may contribute to the dis- surface upon which the panel rests is the angle of twist. 



tortion. It is the purpose of this discussion to analyze Twisting is measured by placing the panel upon a smooth 



briefly some of 



the causes of 



warping and in- 

 d i c a t e several 

 methods for min- 

 imizing it. 



Methods of 



Measuring 



Warping 



When a piece 

 of plywood 

 bends over as 

 shown in Fig. 2, 

 assuming the 

 form of a cylin- 

 drical surface, it 

 is said to cup. 

 In measuring cup- 



FIG. 1— TWISTIXG RESULTING FROM A CONSTRUCTION WITH GRAIN OF FACES AT 45° 

 WITH GRAIN OF CORE; FIG. 2— CUPPING WHICH RESULTS FROM UNSVMMETRICAL CON- 

 STRUCTION IN PLYWOOD. 



surface so that 

 three corners are 

 resting upon the 

 surface, and then 

 measuring the 

 distance from the 

 fourth corner to 

 the surface. If, 

 for example, an 

 edge that is 1 

 inches long has 

 twisted so that 

 one corner is 2 

 inches above the 

 surface, it would 

 be said that the 

 panel has twisted 

 2 inches in 1 

 inches. 



ping a straight edge is placed across the center of the panel A panel that is twisted is usually also cupped, although 



so that it rests upon the upturned edges and the maximum panels that are cupped need not be twisted. In either case 

 recession of the panel from the straight edge is measured. the panel is said to have warped. 



Relation of Moisture to Warping 



One of the most pronounced results of a change in 



•Engineer in Forest Prodvicts, Forest Products Laboratory. V. S. Forestry 

 Service. Madison, Wis. 



FIG. 6-FAULTV PLYWOOD CONSTRUCTION CAl'SING WARPING 

 Panel No. 1—2 ply, 1/16 maple veneer, grain of 1 ply at 90 to grain of other; Panel No. 2—4 ply, 1/16 maple veneer, grain of successive plies at 90'; Paiiel 

 No. 3—3 ply, 1/16 maple veneer on one face and 1/16 basswood core, and 1/16 hasswood on other face, grain of successive plies at 90'; I anel No. 4— 3 ply, 

 1/16 red gum veneer, angle between grain of faces 10°. between core and faces 85'; Panel No. 5—3 ply, 1/16 red gum veneer, angle between grain of laces 

 20°, between core and faces 8°0; Panel No. 6—3 ply, 1/16 red gum veneer, angle between grain of faces 30°, between core and faces 75 . 



