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Hardwood Record — Veneer & Panel Section 



August 25, 1922 



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i;> * uuili'sy American Walnut Manufacturers' Association 

 A Two-Piece Matched Walnut Panel of Figured Rotary Veneer 



i Cnntiniifd from (iOffc 44 I 



both faces of symmetrical plies be of the same species, or 

 species of approximately the same properties. 



To summarize: A veneer panel to retain its form ^vith 

 changes of moisture must be symmetrically constructed. 

 Symmetry is obtained by using an odd number of plies. 

 The plies should be so arranged that for any ply of a 

 particular thickness there is a parallel ply of the same 

 thickness and of the same species on the opposite side of 

 the core and equally removed from the core. 

 Direction of the Grain of the Plies 



In the discussion of symmetry of construction it was un- 

 derstood that the successive plies were always glued with 

 the grain either parallel or exactly at right angles to the 

 core. In careless construction this may not always be the 

 case. Tests have shown that deviations as small as five 

 degrees from the standard 90-degree construction may 

 introduce considerable twisting. 



In building up a three-ply veneer panel the core should 

 be glued with the grain at 90 degrees with the faces, or as 

 close as this is feasible. 



The previous discussion brought out the fact that a 

 change in moisture content of a panel may introduce 

 cupping and twristing in the panel if it is not carefully con- 

 structed. Hence it is highly desirable that all plies be at 

 about the same moisture content before gluing, and that 

 the moisture content of the panel when it leaves the dry- 

 ing room should be about the same as it will average 

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

 ture in the finished panel will usually give satisfactory re- 

 sults when the panel is in service in the open air. 

 Moisture Content 



The moisture content of the plies should be varied for 

 the purpose for which the plywood is to be used. Veneer 

 for furniture panels should generally be low in moisture, so 

 that when removed from the press the moisture content, 

 increased by the moisture from the glue, will be as near 



as possible to that required for factory use. For fancy, 

 cross-grained veneer the gluing at low^ moisture content 

 is of particular importance, since drying of the panel fre- 

 quently results in checking of the face. Non-water-re- 

 sistant glues also require the use of dry veneer, but with 

 casein and blood glues it is possible to use wet veneer. 



Good strength and water resistance can be obtained 

 with these water-resistant glues with veneer at a moisture 

 content as high as 50 per cent. For maximum wet 

 strength of plywood the veneer should be glued at a mois- 

 ture content which would bring the panels to about 30 

 per cent or more when removed from the press. The 

 high moisture content of the panels made in this way 

 necessitates considerable care in drying if a good quality 

 panel is to be secured, and it is improbable that much use 

 can be made of this method of gluing in furniture manu- 

 facture. It offers possibilities, however, for cutting costs 

 in the manufacture of articles where glue strength, water 

 resistance, or speed of manufacture is more important than 

 appearance. 



Relation of Density of Veneer to Warping 



Numerous tests have shown that the warping of ply- 

 wood panels when subjected to varying moisture contents 

 is least for the panels made of low density veneer, such 

 as basswood, poplar, and cedar, and that, in general, 

 v/arping increases with increasing density. 

 Effect on Warping of Increasing the Ratio of the Core to 

 the Total Plywood Thickness 



A high proportion of core to total plywood thickness 

 helps to maintain a flat unwarped surface. In general, 

 the core should comprise 5 I to 7,'' 10 of the total thick- 

 ness of the panel where flatness is an important considera- 

 tion. Of three-ply panels having cores of the same weight 

 the panels having cores of low density, such as poplar 

 and basswood, will, in general, show less warping than the 

 panels having high density cores, such as maple and 

 birch. 



