Plywood
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Plywood was the first type of engineered wood to be invented. It is made from thin sheets of wood veneer, called plies, which are stacked together with the direction of each ply's grain differing from its neighbors by 90° (cross-banding). The plies are bonded under heat and pressure with strong adhesives, usually phenol formaldehyde resin1, making plywood a type of composite material. A vast number of varieties of plywood exist, tailored for all manner of conditions and uses. Plywood can be made of Mahogany, Pine, and other woods. Plywood meant for indoor use generally uses a cheaper, water soluble glue, while outdoor and marine grade plywood are designed to withstand rot and use an insoluble glue to prevent delamination and retain strength in high humidity. Plywood production requires a good log, called a peeler, generally straighter and larger in diameter than that required for processing by a sawmill. The log is peeled into sheets of veneer which are then cut to the desired dimensions, dried, patched and glued together to form the plywood panel. The panel can then be patched, resized, sanded or otherwise refinished, depending on the market it was intended to be sold in.
The most common varieties of plywood comes in three, five or seven plies with dimensions of 1.2 m × 2.4 m (4 feet × 8 feet). Each ply is 1/8 inch. Roofing can use the thinnest 3/8-inch plywood. Floorboards are at least 5/8-inch depending on the distance between floor joists. Plywood is often tongue and grooved for flooring applications. Two of the edges will have "grooves" notched into them to fit with the adjacent "tongue" that protrudes from the next board.
A common reason for using plywood instead of plain wood is because plywood is more stable and because it is less prone to change (shrink, twist or warp).
History
Plywood has been made for thousands of years; the earliest known occurrence of plywood was in ancient Egypt around 3500 BC when wooden articles were made from sawn veneers glued together crosswise. This was originally done due to a shortage of fine wood; thin sheets of high-quality wood were glued over a substrate of lower-quality wood for cosmetic effect, with the structural benefits arising only incidentally. This manner of inventing plywood has occurred repeatedly throughout history; for example, many of the great English furniture makers such as Sheridan used veneer as a raw material.
Modern plywood in which the veneer are cut on a rotary lathe from softwood logs is of relatively recent origin, invented by Emmanuel Nobel (the father of the more-famous Alfred Nobel). The first such lathes were set up in the United States in the mid 19th century. Plywood has been one of the most ubiquitous building products for decades.
Compare to OSB (Oriented strand board)and MDF (Medium-density fibreboard).
References
- Handbook of Finnish Plywood, Finnish Forest Industries Federation, 2002, ISBN 952-9506-63-5