Carey Blue, age 10, of West Linn, Ore., for her question:
HOW IS PLYWOOD MADE?
Plywood is used extensively in building construction on floors, roof linings and wall paneling. Contractors also find it an ideal material in building forms for concrete work. Cabinetmakers couldn't get along without plywood. It is used extensively in furniture building and also in the manufacture of boats, house trailers, sporting goods and many other products. Thin layers of wood glued together form plywood. The thin sheets, called plies or veneers, are arranged in a way so that the direction of the grain of each layer crosses at right angles to the layer next to it.
Most common type of plywood is made with three plies of veneer, although five, seven, nine and even more plies can be used.
To make plywood involves three steps: selection of the log, obtaining the veneer and making the lay up.
Straight, round logs that are free from knots and decay are selected. The bark is removed and the log is cut to the right length before it is put through a steam ¬heating process to soften it.
Veneer can be obtained by slicing, sawing or rotary cutting the log. Sawing is used only for fine finishing woods, such as ebony, which is too brittle for slicing. Using the slicing technique, the log is moved against a heavy, stationary knife.
About 90 percent of plywood veneer is obtained by rotary cutting with a lathe. The log goes from a storage pond and is placed in a lathe. It then revolves against a stationary knife that extends the full length of the log. The veneer is obtained by unwinding it in a long, continuous ribbon.
Lay up is completed after the veneer is dried, trimmed and matched. Thin layers of glue are applied and the plies are put together. Large hydraulic presses then squeeze the plies together with just the right amount of heat and pressure, or pressure alone.
The plywood is then dried again, trimmed, sanded and finished to complete the work order. Some sheets of plywood, especially those used for paneling, are stained and varnished as part of the manufacturing process. Most softwood plywood is made of Douglas Fir. Western hemlock, ponderosa pine, redwood, white fir and other types of wood can also be used.
More than 80 different kinds of wood are used to make hardwood plywood. Oak, poplar, birch, cherry and walnut are favorites. Imported woods, including mahogany, are also cut into thin sheets and used as a top layer in plywood.