על ידי moshik » 19 יולי 2009, 23:13
Plywood Construction:
Plywood's construction addresses both of these concerns. By gluing numerous thin strips of wood in alternating directions layer by layer, plywood is much less likely to expand or shrink based on moisture in the environment, and the alternating construction creates a board that is much stronger in each direction than a similarly sized board cut from a single tree.
Most plywood is made from an odd number of layers, called plies (typically 3, 5 or 7), with an equal number of plies on each side of the center ply. In this manner, the surface plies are always parallel, and the grain of the surface plies usually follows the longest side of the sheet (although not always). The sheet is strongest parallel with (as opposed to against) the two surface plies.