44 



Hardwood Record — Veneer & Panel Section 



August 25, 1922 



Some Fundamentals of Plywood Construction 



{< 'out i nurd /. 



Here use formula (2) : 



p' 



PX A 



A' 



P' ^= pressure actually secured on panel 

 P = 1 100 

 A =78.34 

 A' = 8 X 36 or 288 

 1100 X 78.54 



Then P' = or 300 pounds per square 



288 inch which is four times that 



used on the larger panels. 



The necessity for calculating the required gauge read- 

 ing for each size panel is thus obvious. A table, showing 

 gauge readings to be used for all sized panels manu- 

 factured and for the different pressures used, can be com- 

 puted and placed near the press where the operator may 

 see at a glance the amount of pressure required on the 

 gauge reading for each run of panels. 



Drying Panels Made With Wet Glue 



Panels made with wet glues take up a good deal of 

 moisture and after coming from the press are usually 

 placed on stickers and run into a kiln or left at room 

 conditions for final drying. Drying under room condi- 

 tions is slow and, because of the space required, is ex- 

 pensive. The use of panel kilns has become more com- 

 mon of late years. The necessity for quick drying of 

 panels in connection with aircraft work has aided con- 

 siderably in this development. 



Results of experiments in kiln drying aircraft panels 

 have indicated that the essential requirements of minimum 

 injury to the material, a sufficiently rapid drying rate, and 

 convenience and economy of operation can best be met 

 by maintaining a constant temperature of about 120 de- 

 grees Fahrenheit and a constant maximum relative humid- 

 ity wrhich will permit the stock to dry down to the final 

 degree of dryness desired, but which will not allow^ an ap- 

 preciable amount of further drying below the required 

 moisture content. The use of constant temperature and 

 humidity conditions which w^ill dry the panels to a definite 

 moisture content makes the drying simple, safe, and easy. 

 For panels of three- and five-ply veneer or of veneer 

 faces and crossbanding and a thick core and glued at a low 

 moisture content, drying at 120 degrees Fahrenheit and 

 the necessary maximum humidity may be accomplished 

 in from a few hours to over night. Temperatures above 

 1 20 degrees Fahrenheit have the advantage of decreasing 

 the drying time but are more liable to lower the quality 

 of the panel by inducing checking, warping, and open 

 joints. Panels dried from a high to an excessively low 

 moisture content are very liable to warp and should be 

 dried relatively slowly. 



Gluing by a Dry Glue Process 

 In connection with aircraft work a process of laying 



'Dllt {Hint' -i-) 



veneer with dry glue was vvrorked out at the Forest Prod- 

 ucts Laboratory.* It consists of making an adhesive by 

 coating a tissue paper with blood albumin glue, allowing 

 it to dry, and using the coated paper as the glue layer for 

 plywood. The sheets of glue are alternated w^ith sheets 

 of veneer until the required number of plies is obtained 

 and the w^hole is pressed in a hot press. The use of the 

 dry glue enables the construction of plywood with the 

 addition of little or no moisture. Veneers as thin as 

 I /150-inch can be glued successfully into thin sheets of 

 pljrwood or panels. The process can be used w^ith thicker 

 stock but appears to be adapted especially to fancy, cross- 

 grained, and very thin veneer. The method eliminates a 

 number of troubles resulting from the use of wet glues, 

 such as checking, warping, open joints, and overlaps, and 

 makes the handling of thin material much easier. The 

 preparation of the glue tissue is discussed more fully in 

 a mimeograph, entitled "Blood Albumin Glues — Their 

 Manufacture, Preparation and Application." 



Factors Affecting the Warping of Plywood : Symmetrical 

 Construction 



On account of the great difference in shrinkage of wood 

 in the direction parallel to the grain and perpendicular 

 to it, a change in moisture content of plywood will in- 

 evitably either introduce or relieve internal stresses. Take 

 for example, a three-ply construction and subject it to low^ 

 humidity conditions so that the moisture content of the 

 plywood is lowered. Because 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 compression stresses and the core to tensile 

 stresses. If the faces are of exactly the same thickness 

 and of like density, the stresses are symmetrically dis- 

 tributed and no cupping should ensue. 



Now let it be assumed that one face of a three-ply panel 

 has been glued with the grain in the seune direction as the 

 core, and that the moisture content of the panel is re- 

 duced. It is obvious that the internal stresses are now no 

 longer symmetrically distributed, inasmuch as the com- 

 pressive stresses in one face have been removed. This 

 face now shrinks a great deal more than the other face 

 in the direction of the grain of the latter. The result is 

 that cupping takes place. 



The necessity for exercising care in sanding the faces 

 of a panel is obvious, inasmuch as different thicknesses 

 on the faces would introduce unequal forces w^ith chang- 

 ing moisture content and produce distortion. 



In order to obtain symmetry it is also necessary that 



'At least two patents on similar process hare reeently heen iliseorered, 

 one a British patent Ko. 77,327 issued in 1902 and the other U. S. patent 

 No. 1,299,11,1 issued in 1919. The value of these patents is not definitely 

 known, Init it is suggested that anyone interested in producing this material 

 shonltl faniili<iri::e hitnself with them. 



(Continned on ttage ."iO) 



