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. Softwood plywood is usually made either of Douglas fir or spruce, pine, and fir. Decorative plywood is usually faced with hardwood, including red oak, birch, maple, lauan (Philippine mahogany) and a large number of other hardwoods.
Plywood meant for indoor use generally uses the less expensive phenol-formaldehyde glue (which has limited water resistance), while outdoor and marine grade plywood are designed to withstand rot and use a water resistant phenol-resorcinol 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 softwood 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 boardA 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.
Sources
Plywood is made by gluing together a number of thin veneers or plies of softwood or hardwood.
Advantages
There is always an odd number of veneers and each ply is at a right angle to the one below, this gives the material it's strength. The more veneers used, the stronger the plywood becomes. Both the type of glue and veneers determine the suitability of a sheet for a particular application. The finish quality of plywood varies considerably, some plywood have attractive grains while others can contain knots. Plywood may be used inside and outside. Plywood is graded for exterior or interior use depending upon the water resistance of the glue used to stick the plies together. Code letters shows this grading on each sheet.Exterior grade plywood (WBP - Weather and Boil Proof). This type of plywood can be used outside. Water-resistant adhesives are used and can resist a certain amount of moisture.
Uses
Sheds and cladding are made from this material.
- Weather boiled proof plywood requires paint or varnish to protect the outer veneer from the elements.
- Internal plywood does not contain water-resistant adhesive. The code used to represent this plywood is (Int.interior use only).
- It may be used for wall panelling, flooring and furniture.
- Shuttering boxes are made from shuttering plywood. Shuttering boxes are used in the construction industry to contain concrete while it sets. The material is water resistant to a certain extent. The surface of this material does not have a decorative veneer and is generally not suitable for use where an attractive quality finish is required.
- Marine Plywood is made with waterproof adhesive so that it can be used under water. The material should still be protected with paint or varnish.
- Other codes used on the surface of plywood are: BR: -Boil Resistant, MR: -Moisture Resistant.
- The smoothness of the surface and the number of defects in it grade plywood. Plywood can be nailed and screwed. Thin plywood is flexible and can be formed into curved shapes.