Plywood

Plywood is a sheet material manufactured from thin layers or "plies" of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. It is an engineered wood from the family of manufactured boards which includes medium-density fibreboard (MDF) and particle board (chipboard).

All plywoods bind resin and wood fibre sheets (cellulose cells are long, strong and thin) to form a composite material. This alternation of the grain is called cross-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 ply.

Smaller thinner plywoods and lower quality plywoods (see Average-quality plywood photo below and right) may only have their plies (layers) arranged at right angles to each other, though some better quality plywood products will by design have five plies in steps of 45 degrees (0, 45, 90, 135, and 180 degrees), giving strength in multiple axes.

In 1797 Samuel Bentham applied for patents covering several machines to produce veneers. In his patent applications, he described the concept of laminating several layers of veneer with glue to form a thicker piece – the first description of what we now call plywood. Samuel Bentham was a British naval engineer with many shipbuilding inventions to his credit. Veneers at the time of Bentham were flat sawn, rift sawn or quarter sawn; i.e. cut along or across the log manually in different angles to the grain and thus limited in width and length.

About fifty years later Immanuel Nobel, father of Alfred Nobel, realized that several thinner layers of wood bonded together would be stronger than one single thick layer of wood ; understanding the industrial potential of laminated wood he invented the rotary lathe.

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Wikipedia