Plywood is a manufactured wood panel made from thin sheets of wood veneer. It is one of the most widely used wood products. It is flexible, inexpensive, workable, re-usable, and can usually be locally manufactured. Plywood is used instead of plain wood because of its resistance to cracking, shrinkage, splitting, and twisting/warping, and its general high degree of strength.
Plywood layers (called veneers) are glued together with adjacent plies having their grain at right angles to each other. Cross-graining has several important benefits: it reduces the tendency of wood to split when nailed at the edges, it reduces expansion and shrinkage equating to improved dimensional stability, and it makes the strength of the panel consistent across both directions. There are usually an odd number of plies so that the sheet is balanced—this reduces warping. Because of the way 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.
HistoryPlywood was invented around 3500 B.C. by the Egyptians, who attached several thinner layers of wood together to make one thick layer. They originally did this during a shortage of quality wood, gluing very thin layers of quality wood over lesser-quality wood.
Modern plywood was invented by Immanuel Nobel, father of Alfred Nobel. Nobel realized that several thinner layers of wood bonded together would be stronger than one single thick layer of wood.
Structural characteristicsA typical plywood panel has face veneers of a higher grade than the core veneers. The principal function of the core layers is to increase the separation between the outer layers where the bending stresses are highest, thus increasing the panel's resistance to bending. As a result, thicker panels can span greater distances under the same loads. In bending, the maximum stress occurs in the outermost layers, one in tension, the other in compression. Bending stress decreases from the maximum at the face layers to nearly zero at the central layer. Shear stress is consistent throughout the depth of the panel.
Softwood plywoodSoftwood panel is usually made either of cedar, Douglas fir or spruce, pine, and fir (collectively known as spruce-pine-fir or SPF) or redwood and is typically used for construction and industrial purposes.
The most common dimension is 1.2 m × 2.4 m or the slightly larger imperial dimension of 4 feet × 8 feet. Plies vary in thickness from 1.4mm to 4.3mm. The amount of plies depends on the thickness and grade of the sheet but at least 3. Roofing can use the thinner 5/8" (15mm) plywood. Subfloors are at least 3/4" (18mm) thick, the thickness depending on the distance between floor joists. Plywood for flooring applications is often tongue and groove; This prevents one board from moving up or down relative to its neighbor, so providing a solid feeling floor when the joints do not lie over joists. T&G plywood is usually found in the 1/2" to 1" (12-21mm) range
Hardwood plywoodUsed for demanding end uses. Birch plywood is characterized by its excellent strength, stiffness and resistance to creep. It has a high planar shear strength and impact resistance, which make it especially suitable for heavy-duty floor and wall structures. Oriented plywood construction has a high wheel-carrying capacity. Birch plywood has excellent surface hardness, and damage- and wear-resistance.
Tropical plywoodTropical plywood is always made of mixed species of tropical wood in the Asian region. Tropical plywood is superior to softwood plywood due to its density, strength, evenness of layers, and high quality. It is usually sold at a premium in many markets if manufactured with high standards. Tropical plywood is widely used in the UK, Japan, Taiwan, Korea, Dubai, and other countries worldwide. It is the most preferred choice for construction purposes in many regions.
Special-purpose plywoodCertain plywoods do not have alternating plies. These are designed for specific purposes.
Aircraft plywoodHigh-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, which was nicknamed "The Wooden Wonder".
Structural aircraft-grade plywood is more commonly manufactured from African mahogany or American birch veneers that are bonded together in a hot press over hardwood cores of basswood or poplar. 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. All aviation-grade plywood is manufactured to specifications outlined in MIL-P-607, which calls for shear testing after immersion in boiling water for three hours to verify the adhesive qualities between the plies and meets specifications.
Decorative plywood (overlaid plywood)Usually faced with hardwood, including ash, oak, red oak, birch, maple, mahogany, Philippine mahogany(often called lauan), rose wood, teak and a large number of other hardwoods. However, Formica, metal and resin-impregnated paper or fabric bonded are also added on top of plywood at both side as a kind of ready for use in the decoration field.
Flexible plywoodFlexible plywood is very flexible and is designed for making curved parts. In the UK this is sometimes known as "Hatters Ply" as it was used to make stovepipe hats in Victorian times. It is also often referred to as "Bendy Ply" due to its flexibility. However these may not be termed plywood in some countries because the basic description of plywood is layers of veneered wood laid on top of each other with the grain of each layer perpendicular to the grain of the next. In the US, the terms "Bender Board" and "Wiggle Board" are commonly used.
Marine plywoodMarine plywood is manufactured from durable face and core veneers, with few defects so it performs longer in humid and wet conditions and resists delaminating and fungal attack. Its construction is such that it can be used in environments where it is exposed to moisture for long periods. Each wood veneer will be from durable tropical hardwoods, have negligible core gap, limiting the chance of trapping water in the plywood and hence providing a solid and stable glue bond. It uses an exterior Water and Boil Proof (WBP) glue similar to most exterior plywoods.
Marine plywood can be graded as being compliant with BS 1088, which is a British Standard for marine 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 Okoume or Meranti
Marine plywood is frequently used in the construction of docks and boats. It is much more expensive than standard plywood: the cost for a typical 4-foot by 8-foot 1/2-inch thick board is roughly $75 to $100 US or around $2.5 per square foot, which is about three times as expensive as standard plywood.
Other plywoodsOther types of plywoods include fire-retardant, moisture-resistant, sign-grade and pressure-treated. However, the plywood may be treated with various chemicals to improve the plywood's fireproofing. Each of these products is designed to fill a need in industry.
ProductionPlywood 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 laid horizontally and rotated about its long axis while a long blade is pressed into it (rather like turning a Swiss Roll against the edge of a ruler[clarification needed]), causing a thin layer of wood to peel off. In this way 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 (284 °F) and 1.9 MPa (280 psi) to form the plywood panel. The panel can then be patched, re-sized, sanded or otherwise refinished, depending on the market for which it is intended.
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 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 ("E0" possessing the lowest formaldehyde emissions). Plywood produced to "E0" has effectively zero formaldehyde emissions.
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. 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.
SizesThe most commonly used thickness range is from 0.14 to 3.0 in (0.36 to 7.6 cm). The sizes of the most commonly used plywood sheets are 4 by 8 ft (1.2 by 2.4 m) or 3 by 6 ft (0.91 by 1.8 m). Width and length may vary in 1 ft (0.30 m) increments.
In the United States, the most commonly used size is: 4 ft by 8 ft or 5 ft by 5 ft.
Sizes on specialised plywood for concrete forming range from 6 to 21mm, and a multitude of formats exist, though 15x750x1500mm is very commonly used.
GradesGrading 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.|
|A/B||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, knot-holes, cracks, and all other defects permitted.|
|BB/CC||Face as BB, back as CC. BB as very little knots of less than 1/4 inches, slight discoloration, no decay, split and wormholes mended skillfully, matched colors, no blister, no wrinkle. Most popular choice for most applications.|
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 is currently successfully used in stressed-skin applications.. The American designers Charles and Ray Eames are famous for their plywood-based furniture, 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 applicationsTypical end uses of spruce plywood are:
- Floors, walls and roofs in house constructions
- Wind bracing panels
- Vehicle internal body work
- Packages and boxes
- Concrete shuttering panels
- Ready-to-paint surfaces for constructions
Birch plywood applicationsPhenolic-film Coated birch 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)
- Wind turbine blades
- Insulation boxes for Liquefied Natural Gas (LNG) carriers
- 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 applicationsTropical 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)