The simplest plywood is made of three plies of veneer. However, five, seven, nine, or more plies may be used. In some cases, plywood may have an even number of plies, with the grain direction of the two center plies being parallel. The term plywood is also used for door panels that have a solid lumber core up to 3 inches (7.6 centimeters) thick.
Kinds of plywood.
Plywood is classified in two ways--by material and by use. Most softwood plywood is made of Douglas-fir or southern pine. Western hemlock, white fir, ponderosa pine, redwood, and many other trees are also used. Hardwood plywood is available in over 80 kinds of wood. They include domestic woods such as oak, red gum, poplar, birch, cherry, and walnut. Imported woods used in plywood include mahogany and other attractive tropical woods.
Interior plywood is usually made with glues that are moisture-resistant. Exterior plywood is designed to withstand severe conditions resulting from moisture and humidity. It is always made with waterproof glues.
The most commonly available types of plywood panels are 4 feet (1.2 meters) wide, 8 feet (2.4 meters) long, and from 1/4 to 3/4 inch (6 to 19 millimeters) thick. Dimensions of plywood panels usually range from 3 to 5 feet (0.9 to 1.5 meters) wide, 5 to 12 feet (1.5 to 3.7 meters) long, and 3/16 to 13/16 inches (5 to 30 millimeters) thick. Three, five, or seven plies are normally used.
Use of plywood. Plywood's main advantages over ordinary lumber are that it is lightweight and workable, yet rigid and strong. Plywood can also be cut to exact sizes and produced in large panels for ease of application, strength, and smooth surfaces. It shrinks and swells less than ordinary wood and has greater resistance to splitting at the ends. This permits carpenters to fasten plywood sheets with nails or screws close to the edges. Plywood also has little tendency to warp or twist. Decorative hardwood veneers can provide the look of expensive woods without the cost because only thin sheets are needed. Plastic or metal faces are sometimes used to provide surfaces that resist scratching. Plywood can also be made in curved shapes.
Softwood plywood is used chiefly as a structure upon which finished walls, flooring, and roofing are laid. It is also particularly suited for the forms used in shaping concrete for building, bridge, and dam foundations. Carpenters and cabinetmakers find wide use for hardwood plywood in furniture, cabinets, counters, and decorative wall paneling. Manufacturers use both hardwood and softwood in boats, recreational vehicles, office equipment, railroad cars, road signs, sporting goods, and other products.
Making plywood is done in three steps. They involve (1) the log, (2) the veneer, and (3) the lay-up.
Logs used for plywood are selected for straightness, roundness, and freedom from knots and decay. After the bark is removed and the logs cut to the desired lengths, they are often steam-heated. This softens their surfaces, and they are placed into the lathe or slicer to be converted to veneer.
Veneer is made in one of three ways. These are (1) sawing, (2) slicing, or (3) rotary cutting. Sawing is used only for fine finishing woods, such as ebony or knotty pine, which are too brittle or unsuitable for slicing. Slicing is used chiefly for fine-figured woods for furniture or wall-panel faces. Slicing is done by moving the log, called a flitch, against a heavy, stationary knife.
About nine-tenths of veneer is rotary cut with a lathe. The log is placed in a lathe and then revolved against a stationary knife extending across its length. The veneer is then unwound in a long, continuous ribbon.
The lay-up takes place after the plies are dried, trimmed, and matched. A thin layer of glue is applied to each ply. Workers then lay-up, or place, the plies with the grain in each ply opposite to that in the adjacent ply. Hydraulic presses squeeze the plies together with heat and pressure, or pressure only. Then the finished plywood is again dried, trimmed, sanded, or otherwise finished into sheets.
History. Beginning about 1500 B.C., the people of ancient Egypt made furniture that included parts made of two wood veneers glued together. In 1830, the first three-veneer plywood was created by German furniture maker Michael Thonet. The first official patent for plywood was issued to John K. Mayo of Maine in 1865.