Product Code Database
Plywood is a composite material manufactured from thin layers, or "plies", of wood veneer that have been stacked and glued together. It is an engineered wood from the family of manufactured boards, which include plywood, medium-density fibreboard (MDF), oriented strand board (OSB), and particle board (or chipboard).
All plywoods bind resin and wood fibre sheets (cellulose cells are long, strong and thin) to form a composite material. The sheets of wood are stacked such that each layer has its grain set typically (see below) perpendicular to its adjacent layers. This alternation of the grain is called cross-graining and has several important benefits: it reduces the tendency of wood to split when nailed at the edges; it reduces thickness swelling and shrinkage, providing improved dimensional stability; and it makes the strength of the panel consistent across all directions. There is usually an odd number of plies, so that the sheet is balanced, that is, the surface layers have their grains set parallel to one another. This balance reduces warping. Because plywood is bonded with grains running against one another and with an odd number of composite parts, it has high stiffness perpendicular to the grain direction of the surface ply.
Smaller, thinner, and lower-quality plywoods may only have their plies (layers) arranged at right angles to each other. Some better-quality plywood products by design have five plies in steps of 45 degrees (0, 45, 90, 135, and 180 degrees), giving strength in multiple axes.
The word ply derives from the French verb plier,Collins Dictionary of the English Language, 2nd edition, London, 1986, p. 1181. "to fold", from the Latin verb plico, from the ancient Greek verb πλέκω.Cassell's Latin Dictionary, Marchant, J. R. V, & Charles, Joseph F. (eds.), Revised Edition, 1928, p. 421.
The most common dimension is or the slightly larger imperial dimension of 4 feet × 8 feet. Plies vary in thickness from 1.4 mm to 4.3 mm. The number of plies—which is always odd—depends on the thickness and grade of the sheet. Roofing material can use the thinner plywood. Subfloors are at least thick, the thickness depending on the distance between floor . Plywood for flooring applications is often tongue and groove (T&G); This prevents one board from moving up or down relative to its neighbor, providing a solid-feeling floor when the joints do not lie over joists. T&G plywood is usually found in the range.
Structural aircraft-grade plywood is most commonly manufactured from African mahogany, spruce or birch veneers that are bonded together in a hot press over hardwood cores of basswood or poplar or from European Birch veneers throughout. Basswood is another type of aviation-grade plywood that is lighter and more flexible than mahogany and birch plywood but has slightly less structural strength. Aviation-grade plywood is manufactured to a number of specifications including those outlined since 1931 in the Germanischer Lloyd Rules for Surveying and Testing of Plywood for Aircraft and MIL-P-607, the latter of which calls for shear testing after immersion in boiling water for three hours to verify the adhesive qualities between the plies meets specifications. Aircraft grade plywood is made from three or more plies of birch, as thin as thick in total, and is extremely strong and light.
Howard Hughes' H-4 Hercules was constructed of plywood. The plane was built by the Hughes Aircraft Company employing a plywood-and-resin Duramold process.Winchester, Jim. "Hughes H-4 'Spruce Goose'." Concept Aircraft: Prototypes, X-Planes and Experimental Aircraft. Kent, UK: Grange Books plc., 2005. p. 113. The specialized wood veneer was made by Roddis Manufacturing in Marshfield, Wisconsin. Marshfield women recall building engineering marvels of the skies , Marshfield News Herald
In the United Kingdom single-ply sheets of veneer were used to make Top hat in Victorian times, so flexible modern plywood is sometimes known there as "hatters ply", although the original material was not strictly plywood, but a single sheet of veneer.
Marine plywood can be graded as being compliant with BS 1088, which is a British Standard for marine plywood and IS:710 is Bureau of Indian Standards (BIS) for marine grade plywood. There are few international standards for grading marine plywood and most of the standards are voluntary. Some marine plywood has a Lloyd's of London stamp that certifies it to be BS 1088 compliant. Some plywood is also labeled based on the wood used to manufacture it. Examples of this are Okoumé or Shorea.
In the UK, one can find builders' merchants advertising a grade of ply as "marine ply" that does not conform to BS 1088 - generally online adverts for these products will include a caveat along the lines of "not suitable for boat".
In India, marine ply is popularly called "Kitply" after the brand which popularised the concept of water proof marine plywood in the 1980"s.
Baltic Birch plywood is a product of an area around the Baltic Sea. Originally manufactured for European cabinet makers but now popular in the United States as well. It is very stable composed of an inner void-free core of cross-banded birch plys with an exterior grade adhesive. The face veneers are thicker than traditional cabinet grade.
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 moisture, and use a water-resistant resorcinol-formaldehyde or phenol-formaldehyde glue to prevent delamination and to retain strength in high humidity.
The adhesives used in plywood have become a point of concern. Both urea formaldehyde and phenol formaldehyde are carcinogenic in very high concentrations. As a result, many manufacturers are turning to low formaldehyde-emitting glue systems, denoted by an "E" rating. Plywood produced to "E0" has effectively zero formaldehyde emissions. Engineered Wood Products Association of Australasia . (PDF). Retrieved on 2012-02-10.
In addition to the glues being brought to the forefront, the wood resources themselves are becoming the focus of manufacturers, due in part to energy conservation, as well as concern for natural resources. There are several certifications available to manufacturers who participate in these programs. Programme for the Endorsement of Forest Certification (PEFC) Forest Stewardship Council (FSC), Leadership in Energy and Environmental Design (LEED), Sustainable Forestry Initiative (SFI), and Greenguard are all certification programs that ensure that production and construction practices are sustainable. Many of these programs offer tax benefits to both the manufacturer and the end user. Pro Woodworking Tips.com . Pro Woodworking Tips.com. Retrieved on 2012-02-10.
Sizes on specialised plywood for concrete-forming can range from , and a multitude of formats exist, though 15 × (19/32in × 2 ft-6in × 4 ft-11in) is very commonly used.
Aircraft plywood is available in thicknesses of (3-ply construction) and upwards; typically aircraft plywood uses veneers of 0.5 mm (approx 1/64 in) thickness although much thinner veneers such as 0.1 mm are also used in construction of some of the thinner panels.
| + Typical Western grades, after Joyce |
| + Grades according to Japan Plywood Inspection Corporation (JPIC) Standards |
Exterior glued plywood is suitable for outdoor use, but because moisture affects the strength of wood, optimal performance is achieved where the moisture content remains relatively low. Subzero conditions do not affect the dimensional or strength properties of plywood, making some special applications possible.
Plywood is also used as an engineering material for stressed-skin applications. It has been used for marine and aviation applications since WWII. Most notable is the British de Havilland Mosquito bomber, with a fuselage made of birch plywood sandwiching a balsa core, and using plywood extensively for the wings. Plywood was also used for the hulls in the hard-chine Motor Torpedo Boats (MTB) and Motor Gun Boats (MGB) built by the British Power Boat Company and Vosper's, American , and the Higgins boat critical to the D-Day landings. The American designers Charles Eames and Ray Eames are known for their plywood-based furniture, as is Finnish Architect Alvar Aalto and his firm Artek, while Phil Bolger has designed a wide range of boats built primarily of plywood. Jack Köper of Cape Town designed the plywood Dabchick sailing dinghy, which is still sailed by large numbers of teenagers. Plywood is often used to create curved surfaces because it can easily bend with the grain. Skateboard ramps often utilize plywood as the top smooth surface over bent curves to create transition that can simulate the shapes of ocean waves.
There are coating solutions available that mask the prominent grain structure of spruce plywood. For these coated plywoods there are some end uses where reasonable strength is needed but the lightness of spruce is a benefit, e.g.:
Birch plywood is used as a structural material in special applications e.g.:
Smooth surface and accurate thickness combined with the durability of the material makes birch plywood a favorable material for many special end uses e.g.:
|
|
