Plywood is a sheet material manufactured from thin layers or "plies" of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. It is an engineered wood from the family of manufactured boards which includes medium-density fibreboard (MDF) and particle board (chipboard).
All plywoods bind resin and wood fiber sheets (cellulose cells are long, strong and thin) to form a composite material. 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 expansion and shrinkage, providing improved dimensional stability; and it makes the strength of the panel consistent across all directions. There are usually an odd number of plies, so that the sheet is balanced-this reduces warping. Because plywood is bonded with grains running against one another and with an odd number of composite parts, it is very hard to bend it perpendicular to the grain direction of the surface ply.
Smaller thinner plywoods and lower quality plywoods (see Average-quality plywood photo below and right) may only have their plies (layers) arranged at right angles to each other, though many better quality plywood products will by design have five plies in steps of 45 degrees (0, 45, 90, 135, and 180 degrees), giving strength in multiple axis. The highest quality specialty plywoods often have plies at 30 degrees (0, 30, 60, 90, 120, 150, and 180 degrees) in seven layers, or have nine layers with two layers of 45 and 135 degrees in the sandwich. The smaller the step rotations the harder it is to manufacture, increasing manufacturing costs and consequently retail price.
Types of plywood
Different varieties of plywood exist for different applications:
a) Softwood plywood Hardwood plywood
b) Tropical plywood
c) Aircraft plywood
d) Decorative plywood (overlaid plywood)
e) Flexible plywood
f) .Marine plywood
g) Other plywoods
Grading rules differ according to the country of origin. Most popular standard is the British Standard (BS) and American Standard (ASTM). Joyce (1970), however, list some general indication of grading rules:
A Face and back veneers practically free from all defects.
Face veneers practically free from all defects. Reverse veneers with only a few
small knots or discolorations.
A/BB Face as A but reverse side permitting jointed veneers, large knots, plugs, etc.
B Both side veneers with only a few small knots or discolorations.
B/BB Face veneers with only a few small knots or discolorations. Reverse side permitting jointed veneers, large knots, plugs, etc.
BB Both sides permitting jointed veneers, large knots, plugs, etc.
WG Guaranteed well glued only. All broken knots plugged.
X Knots, knotholes, cracks, and all other defects permitted.
Face as BB, back as CC. BB as very little knots of less than 1/4 inches, slight BB/CC discoloration, no decay, split and wormholes mended skillfully, matched colors,
no blister, no wrinkle. Most popular choice for most applications.
Plywood is used in many applications that need high-quality, high-strength sheet material. Quality in this context means resistance to cracking, breaking, shrinkage, twisting and warping.
Exterior glued plywood is suitable for outdoor use, but because moisture affects the strength of wood, optimal performance is achieved in end uses where the wood's moisture content remains relatively low. On the other hand, subzero conditions don't affect plywood's dimensional or strength properties, which makes 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, which was primarily made using a moulded sandwich of two layers of birch plywood around a balsa core. 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. Plywood is currently successfully used in stressed-skin applications. The American designers Charles and Ray
Eames are famous for their plywood-based furniture, as is Finnish Architect Alvar Aalto and his firm Artek, while Phil Bolger is famous for designing a wide range of boats built primarily of plywood.
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.
Softwood plywood applications
Typical end uses of spruce plywood are:
· Floors, walls and roofs in home constructions
· Wind bracing panels
· Vehicle internal body work
· Packages and boxes
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.:
· Concrete shuttering panels
· Ready-to-paint surfaces for constructions
Hardwood plywood applications
Phenolic resin film coated (Film Faced) plywood is typically used as a ready-to-install component e.g.:
· Panels in concrete form work systems
· Floors, walls and roofs in transport vehicles
· Container floors
· Floors subjected to heavy wear in various buildings and factories
· Scaffolding materials
("Wire" or other styles of imprinting available for better traction)
Birch plywood is used as a structural material in special applications e.g.:
· Wind turbine blades
· Insulation boxes for liquefied natural gas (LNG) carriers
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.:
· High-end loud speakers
· Die-cutting boards
· Supporting structure for parquet
· Playground equipment
· Signs and fences for demanding outdoor advertising
· Musical instruments
· Sports equipment
Tropical plywood applications
Tropical plywood is widely available from the South-East Asia region, mainly from Malaysia and Indonesia. Tropical plywood boasts premium quality, and strength. Depending on machinery, tropical plywood can be made with high accuracy in thickness, and is a highly preferable choice in America, Japan, Middle East, Korea, and other regions around the world.
· Common plywood
· Concrete panel
· Floor base
· Structure panel
· Container flooring
· Lamin board
· Laminated veneer lumber (LVL)
In woodworking, veneer refers to thin slices of wood, usually thinner than 3 mm (1/8 inch), that typically are glued onto core panels (typically, wood, particle board or medium-density fiberboard) to produce flat panels such as doors, tops and panels for cabinets, parquet floors and parts of furniture. They are also used in marquetry. Plywood consists of three or more layers of veneer, each glued with its grain at right angles to adjacent layers for strength. Veneer beading is a thin layer of decorative edging placed around objects, such as jewelry boxes. Veneer is also used to replace decorative papers in Wood Veneer HPL. Veneer is also a type of manufactured board.