Plywood is a type of board made from thin sheets of wood. The layers are glued together, each with its grain at right angles to adjacent layers for greater strength. Plywood is made from logs that are cut into blocks between 8 and 10 feet long. A lathe is used to peel-off long, thin, continuous sheets called veneer. Clippers cut the veneer to the required width, and dryers reduce the wood’s moisture content. Next, the veneer is graded and conveyed to glue spreaders that apply adhesive or glue. The plywood is then exposed to heat and pressure in a large hydraulic press with multiple openings. After the glue cures, plywood panels and other plywood products are removed from the press and cut to size. Some grades of industrial plywood are sanded.
Plywood is classified by exposure durability based upon moisture resistance and other factors. There are four classes of exposure durability: exterior, exposure 1, exposure 2, and interior. Exterior plywood uses fully waterproof glue and is designed for permanent exposure to the elements. Exposure 1 plywood may also use waterproof glue, but can develop some localized glue delamination under high-moisture conditions. Exposure 2 plywood or intermediate glue (IMG) plywood is designed for protected applications in environments with low to moderate levels of humidity. Interior plywood uses moderately moisture-resistant glues or adhesives. This type of industrial plywood delaminates under high-humidity conditions and is designed for indoor use only.
Plywood products are grouped into five numbered species. Group 1 plywood is made of woods such as American beech, birch, Douglas fir, western larch, sugar maple, pine and southern pine. Group 2 plywood is made from cedar, cypress, fir, hemlock, black maple, spruce, sweetgum, and certain types of pine. Groups 3 plywood includes plywood panels and other plywood products made from red alder, paper birch, big leaf maple, ponderosa pine, redwood, and white spruce. Group 4 plywood is made from aspen, western cedar (red cedar), eastern cottonwood, and sugar pine. There are two types or species of Group 5 plywood: basswood and poplar. Plywood may also be produced from trees that are not indigenous to North America. Examples include Apitong from the Philippines and Keruing from Malaysia.