Plywood

Plywood is a combination of hardwood or softwood thin sheets that are bonded together with an adhesive. The sheets are laid together so that their grains run perpendicular to one opposite of it. For the weight of a board, it has an inverse relation to its strength. It is strong, resist splitting, and can be molded into curves. Plywood is ideal for both structure and decoration. “Plywood” is a term founded in 1919, trying to reduce the several other names that had come before, such as: built up wood, past wood, or scale boards.

History
John K. Mayo patented the first “scale boards” in 1865. It was recognized for being ideal for covering and lining structures. The use of thin sheets of wood attached together with several different scales or thicknesses crossed was meant to prevent cracking, splitting, shrinking, swelling, or expanding. While Mayo held his patent, plywood never received much recognition. Panels made of plywood were made with hardwood facings, or veneers in the late 19th century for custom orders from furniture makers looking for drawer bottoms and other unseen parts. Pin planks in pianos, seating, desktops, and sewing machine covers also employed the use of plywood. In the early 1890s, plywood was being used for panels in doors instead of the traditional raised lumber panels. In 1905 Portland Manufacturing Company introduced softwood plywood using Douglas fir plywood panels at an exposition in Portland, Oregon. Soon, the softwood was being used in airplanes and automobiles. The Pacific Coast Manufacturers Association (later to be named the Douglas Fir Plywood Association) created a grading system and tried to make a new market for plywood use in house construction. The quality of the glue used to attach the wood pieces was used to define the quality of the plywood. Hide glue was the first glue used, replaced by Vegetable glue. Vegetable glue was inexpensive for plywood and was easy to store and manipulate. Casein glue was the first water-resistant glue for plywood introduced in the United States. Soybean glue became the glue of choice for plywood until 1931 when Germany introduced synthetic resin in sheet form, which offered greater water resistance. Phenolformaldehyde began being reproduced domestically in 1935 and became the standard glue in the plywood business. Custom-ordered veneer panels became more popular in the early 1900s and by 1919, U.S Plywood was established. This company was the most pioneering plywood company. After World War II, the number of manufacturers of plywood grew to nearly 150 companies.

Manufacturing Process
While some technology has improved, the manufacturing process still uses many historical techniques. Logs are brought to the mill, where they are stripped of bark, and rotated against a knife to create veneers. The veneers are flattened, clipped, taped, patched, and dried. After the plies have dried, they are glued together using either a cold or hot press. In the cold process, liquid glue is placed by a mechanical spreader. Plies are then pressed together with a hydraulic press. With hot press, panels are processed immediately, saving time. Hot pressing uses synthetic resin sheets or liquid adhesives. The panels are trimmed to size afterward, then sanded. Electronic heating devices emerged after World War II and took the place of convection systems. This allowed adhesives to cure at lower temperatures without changing the moisture content of th wood. Molded plywood emerged after the war as well; curves could be created using a process called bag molding.

Uses and Installation
Plywood is capable of being used for a vast variety of purposes, both structural and nonstructural. Nonstructural began with doors, paneling, and formwork for concrete slabs. Houses were being built with a majority of plywood products after the 1930s. In 1934, the first waterproof plywood was introduced and soon the Forest Products Laboratory had constructed a plywood house, using exterior plywood and stressed-skin panels to help carry some of the wall’s load. Plywood was experimented with heavily in the 1930s, and Foster Gunnison soon developed the first prefabricated house with a plywood exterior. The Douglas Fir Plywood Association bought the rights to the Dri-Bilt housing system, which had no plaster, only plywood walls, ceiling, and partitions. Even though the country had been presented with ideas like the Dri-Bilt housing style, plywood remained a favorite for sheathing and subfloors. When selling plywood, it was beneficial to sell it in sheets; on the job site, it was easy enough to cut to size. In construction, plywood was sometimes used as a road to cover muddy holes that may prove difficult for a weighted truck to cross. These sheets were laid down side by side, and were sturdy enough that they could support a full truck load. Plywood doors were common in public buildings and gave way to the hollow core plywood doors in the 1930s. Plywood also gave way to parquet floors in the same time frame. Stock plywood panels were originally 3 feet by 6 feet but found today’s standard of 4 feet by 8 feet in the early 1930s. The grading for plywood are as follows: 1 Side, sound 2 sides, sound 1 side, Wallboard, and concrete form plywood. Installation of plywood included nailing panels to boards every 6 inches. Paneling could be glued and then nailed to furring and studs. Flush, concealed with battens, V-joint, tongue and groove, and lap are all varieties of joints that have developed over the years.