Plywood is made from an odd number of constructional veneers bonded face to face with the grain running in alternate directions
(cross bonding) it then goes in between the shelves of a large hydraulic press, these shelves are heated and squeezed together tightly putting great pressure on the plies, the heat dries the glue and sticks the plies together. Then the plywood is trimmed and can be sanded to give a smoother finish.
The reason for using an odd number of veneers is that the sheet of plywood must be balanced as near as possible to be stable. Plywood can still twist even when it is balanced because no two veneers are completely identical and the tensions are never perfectly balanced. Another factor that can cause warping is the wetting or heating of one face of the ply; this will cause the veneer to expand or contract, both of which can pull the board out of true.
It shrinks, warps, twists and swells less than ordinary wood and has great resistance to splitting at the ends, this permits carpenters to fasten plywood sheets with nails or screws close to the edges. The outside veneer is called face and back and the centre ply or veneer is called core. The quality of the plywood is defined according to its face and back veneer grade. The simplest plywood is made of three layers of veneer, with the grain direction of the two outer sheets being parallel. The greater the number of veneers the stronger the plywood.
Plywood's main advantage is that by gluing together an odd number of layers of veneer, a material that is lightweight and workable, yet rigid and strong can be obtained. Plywood can also be cut to exact sizes worked into curved shapes and produced in large panels with a smooth surface. Expensive woods can be used for the face because only thin sheets are needed, plastic or metal faces are sometimes used to provide surfaces that resist starching.
Uses of Plywood
Plywood is used chiefly for floors, to line roofs and walls, and for wall paneling. It is particularly suited for the forms used for shaping concrete for homes, buildings, bridges and dam foundation. Carpenters and cabinetmakers find wide use for plywood in furniture, cabinets and counters. Manufactures use it in boats, lorry trailers, office equipment, packing cases, trains, cars, road signs, sporting goods and other products.