Plywood is a manufactured wood panel from the family of manufactured boards (such as MDF,Chipboard etc.) made from thin sheets wood veneer. Plywood layers (called veneers or plywoods) are glued together, with adjacent plies having their wood grain rotated relative to adjacent layers up to 90 degrees.
All plywoods bind resin and wood fiber sheets(cellulose cells are long, strong and thin) to form a composite material. This alternation of the grain is called graining and has several important benefits: it reduces the tendency of wood to split when nailed at the edges; it reduces expansion and shrinkage, providing improved dimensional stability; and it makes the strength of the panel consistent across all directions. There is usually an odd number of plies, so that the sheet is balanced -this reduces warping. Because plywood is bonded with grains running against one another and with an odd number of composite parts, it is very hard to bend it perpendicular to the grain direction of the surface plywood.
Smaller thinner plywoods and lower quality plywoods may only have their plywoods (layers) arranged at right angles to each other, though many better quality plywood products will by design have five plywoods in steps of 45 degrees (0, 45, 90, 135, and 180 degrees), giving strength in multiple axes. The highest quality specialty plywoods often have plys at 30 degrees (0, 30, 60, 90, 120, 150, and 180 degrees) in seven layers, or have nine layers with two layers of 45 and 135 degrees in the sandwich. The smaller the step rotations the harder it is to manufacture, increasing manufacturing costs and consequently retail price.
A typical plywood panel has face veneers of a higher grade than the core veneers. The principal function of the core layers is to increase the separation between the outer layers where the bending stresses are highest, thus increasing the panel's resistance bending. As a result, thicker panels can span greater distances under the same loads. In bending, the maximum stress occurs in the outermost layers, one in tension, the other in compression. Bending stress decreases from the maximum at the face layers to nearly zero at the central layer stress by contrast, is higher in the center of the panel, and zero at the outer fibers.
Plywood is an ideal product for wide application in architectural formwork, interior decoration, all kinds of furniture, office supplies, combination speakers, packaging and exhibition. Plywood production line is divided according to its process flow: veneer peeling machine, veneer dryer, glue spreader, cold press machine, hot press machine, plywood cutting machine, sanding machine, etc.