Plywood
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Plywood is a type of engineered wood made from thin sheets of wood veneer, called plies or veneers. The layers are glued together, each with its grain at right angles to adjacent layers for greater strength.There are usually an odd number of plies, as the symmetry makes the board less prone to warping [1], and the grain on the outside surfaces runs in the same direction. The plies are bonded under heat and pressure with strong adhesives, usually phenol formaldehyde resin,[2] making plywood a type of composite material. Plywood is sometimes called the original engineered wood.[3]
A common reason for using plywood instead of plain wood is its resistance to cracking, shrinkage, twisting/warping, and its general high degree of strength. It has replaced many dimensional lumbers on construction applications for these reasons.
Types of plywood
A vast number of varieties of plywood exist, with many conditions and uses. Softwood plywood is usually made either of Douglas fir or spruce, pine, and fir, and is typically used for construction and industrial purposes.[4] Decorative plywood is usually faced with hardwood, including red oak, birch, maple, lauan (Philippine mahogany) and a large number of other hardwoods.
Plywood for indoor use generally uses the less expensive urea-formaldehyde glue which has limited water resistance, while outdoor and marine grade plywood are designed to withstand rot, and use a water resistant phenol-formaldehyde glue to prevent delamination and to retain strength in high humidity.
The most common varieties of softwood plywood come in three, five or seven plies with a metric dimension of 1.2 m × 2.4 m or the slightly larger imperial dimension of 4 feet × 8 feet. Plies vary in thickness from 1/10" through 1/6" depending on the panel thickness. Roofing can use the thinner 3/8-inch plywood. Floorboards are at least 5/8-inch depending on the distance between floor joists. Plywood for flooring applications is often tongue and grooved. Two of the edges will have "grooves" notched into them to fit with the adjacent "tongue" that protrudes from the next board.
High-strength plywood, known as aircraft plywood, is made from mahogany and/or birch, and uses adhesives with increased resistance to heat and humidity. It was used for several World War II fighter aircraft, including the British-built Mosquito bomber.
Plywood production
Plywood production requires a good log, called a peeler, which is generally straighter and larger in diameter than one required for processing into dimensioned lumber by a sawmill. The log is peeled into sheets of veneer which are then cut to the desired dimensions, dried, patched, glued together and then baked in a press at 140 °C (280 °F) and 19 MPa (2800 psi) to form the plywood panel. The panel can then be patched, resized, sanded or otherwise refinished, depending on the market for which it is intended.
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US plywood grades
Plywood grades are determined by a veneer quality on the face and back of each panel. The first letter designates quality of face veneer (best side), while the second letter denotes the surface quality of the back of the panel.[5] The letter "X" indicates the panel was manufactured with scrap wood as the center plies, not "exterior" as is commonly thought. The A-D rating is only good for construction plywood, not for hardwood plywoods such as oak or maple.
"A": Highest grade quality available. Can be defect free or contain small knots, providing they are replaced with wooden plugs (the fillers having a "boat" or a "football" shape) or repaired with synthetic patch. This grade may contain occasional surface splits that are repaired with synthetic filler. The surface is always sanded and provides for smooth paintable face quality.
"B": Second highest quality veneer grade. Normally a by-product of downgraded "A" quality veneer. Solid surface, but may contain small diameter knots and narrow surface splits. Normally repaired with wooden plugs or synthetic filler. The surface is normally sanded smooth.
"C": Considered to be a lower end face quality, but a reasonable choice for general construction purposes. May contain tight knots up to 1½ inches diameter, some open knot holes, some face splits, and discoloration. Some manufactures may repair the defects with synthetic filler. Panels are typically not sanded.
"D": Considered to be the lowest quality veneer and often used for the back surface for construction grade panels. Allows for several knots, large and small, as well as open knots up to 2½ inches diameter. Open knots, splits, and discoloration are acceptable. "D" grade veneers are neither repaired nor sanded. This grade is not recommended for permanent exposure to weather elements.
Plywood applications
Plywood is used in any application that needs high quality wooden sheet material. High quality in this context means resistance to cracking, breaking, shrinkage, twisting and warping. Plywood is also used as an engineering material for stressed-skin applications.Also, Plywood has been used in this fashion for marine and aviation application since the WWII era. Most notable is the British De Havilland Mosquito bomber, (which was primarily made out of wood.) Plywood is currently used in stressed-skin applications quite successfully.[citation needed]. The American designers Charles and Ray Eames and Phil Bolger are world-famous for their plywood-based furniture.
See also
- Engineered wood
- Fiberboard
- Glued laminated timber
- Hardboard
- Masonite
- Marine plywood
- Medium-density fiberboard
- Oriented strand board
- Particle board
- Pressed wood
Further reading
- APA - The Engineered Wood Association (formerly known as the American Plywood Association)
- Canadian Plywood Association - Plywood Manufacturing Process, Grades and Specifications, Engineering Design Values, Technical Literature, Mill Contact Info, Environmental Practices, International Certification, Hobby Ply-Plans
- Sellers, Terry. 1985. Plywood and Adhesive Technology. New York: Dekker. ISBN 0-8247-7407-8
- Yahoo about Article On Plywood Grades
- ^ http://www.westsystem.com/webpages/epoxyworks/18/plywood.html
- ^ Handbook of Finnish Plywood, Finnish Forest Industries Federation, 2002, ISBN 952-9506-63-5 [1]
- ^ "Milestones in the History of Plywood", APA - The Engineered Wood Association. Accessed October 22, 2007. "Plywood is often called the original engineered wood product because it was one of the first to be made by bonding together cut or refashioned pieces of wood to form a larger and integral composite unit stronger and stiffer than the sum of its parts."
- ^ O'Halloran, p.221.
- ^ http://www.timber.org.au/NTEP/menu.asp?id=104#Veneer_quality