PLYWOOD. Board made of several plies or veneers (q.v.) of wood, glued together with the object of obtaining comparatively large sheets free from some of the natural defects and limitations of timber. Rotary cut (peeled) or sliced veneers are used, if neces sary joined edgewise, to form each ply, and thus large sheets with an unbroken surface are obtained; the plies are sorted as to their appearance, and the inferior (knotty, shaky or sappy) material is generally utilized for the interior of the board (core), leaving the best plies for the outer surfaces (face and back).
The natural tendency of wood to shrink, swell and warp is effec tively neutralized, and the comparatively low tensile strength of wood along its grain is greatly improved upon, by carefully cross graining the plies. In the result, a large flat board of wood is pro duced, practically unaffected by climatic influences, and possessing considerably higher shearing, braking and bending properties than ordinary wood of similar thickness.
The plies are glued (cemented) together under considerable pressure, different adhesives being used, dependent upon whether or not the gluing process is aided by heat, and also varying according to the kind of timber used. The veneers are wet when produced from the log and may be glued together either in their wet state or after a pre-drying process. Plywood is, therefore, grouped into two classes, wet and dry produce. The drying of veneers before being cemented and pressed into a board prevents their shrinkage during the press ing process and, therefore, helps to make a better plywood board. If the veneers be glued wet the drying process is combined with the glueing operation in the press, and this often produces face checks (cracks) on the surfaces of the finished board.
In the construction of plywood the following different types are known: The outside plies of the lami nated type and of battenboards may be occasionally superimposed on so-called crossbandings (ve neers lying crosswise to the outer plies) to ensure still greater rigidity of the boards.
Although there is evidence that the principle of "plying" thin strips of wood into panels to ensure permanent flatness was known and practised in very early days of civilization and was continued by the cabinet-maker and builder all over the world, mechanical production of plywood originated comparatively recently, i.e., in the eighteen-eighties, when the first factories specializing in the production of plywood were erected in Russia. The industry has since spread to all parts of the world wherever large timber resources are available and where widespread employment of the material makes local manufacture profitable.