Plywood is a type of manufactured timber 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, 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 for greater strength. 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.
Plywood was invented around 3500 B.C. by the Egyptians, who first thought of sticking 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 qualitative wood over lesser-quality wood. What we know as plywood was invented by Immanuel Nobel, father of Alfred Nobel, the inventor of dynamite and founder of the Nobel Prize. Nobel realized that several thinner layers of wood bonded together would be stronger than one single thick layer of wood. Plywood is now a staple of the construction industry, used in floors and roofing. Forssmanholz or Holzblechär is an extra-resilient plywood patented in Germany in 1921-1922 by Swedish engineer Villehad Forssman.[1]
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It is quite usual for the outer layer on each face to be of a higher-grade timber, and sometimes thinner than the other layers, and perhaps more decorative. The principal reason for the central layers is to increase the separation of the outer layers, where the stresses are highest - in bending, the maximum stress is in the outer layers, one in tension, the other in compression, changing lineally with distance from the surface, i.e. zero bending stress in the middle, where only a very weak layer is needed. Shear stress is constant throughout the depth.
Different varieties of plywood exist for different applications:-
Softwood 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.[2]
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"(2.54mm) through 1/6"(4.23mm) depending on the panel thickness. Roofing can use the thinner 5/8-inch(15.875mm) plywood. Subfloors are at least 3/4-inch(19.05mm) thick, the thickness depending on the distance between floor joists. Plywood for flooring applications is often tongue and groove; the mating edge will have a "groove" notched into it to fit with the adjacent "tongue" that protrudes from the next board. 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. Tongue & groove flooring plywood is typically 1"(25.4mm) in thickness.
Used 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.[3]
Tropical plywood is always made of mixed species of tropical wood in the Asian region. Tropical plywood boasts its superiority over 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.
Certain plywoods do not have alternating plies. These are designed for a specific purpose.
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 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 piles and meets specifications.
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 plywood is very flexible and is designed for making curved parts. In the UK this is known as "Hatters Ply" as it was used to make stovepipe hats in Victorian times. 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 plywood is specially treated to resist rotting in a high-moisture environment. Its construction is such that it can be used in environments where it is exposed to moisture for long periods. Each wood veneer will 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 Boiled Proof (WBP) glue similar to most exterior plywoods. 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. 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
Other 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.
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 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), 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 (280 °F) and 1.9 MPa (2800 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.[4]
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.[5]
The most commonly used thickness range is from 0.6 in.(1.6 cm.) to 3.0 in. (76 mm.). The sizes of the most commonly used plywood sheets are 4 ft.(1.2 m.) wide by 8 ft.(2.4 m.) or 3 ft. (0.9 m.) by 6 ft. (1.8 m.). Width and length may vary in 1 ft.(0.3 m.) in increments.
In US, the most commly used size is: 4 ft by 8 ft or 5 ft by 5 ft. [6]
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:[7]
| Grade | Description |
|---|---|
| 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 as reverse of A/B |
| B/BB | Face as reverse of A/B. Reverse side as reverse of A/BB |
| BB | Both sides as reverse of B/BB |
| WG | Guaranteed well glued only. All broken knots plugged. |
| X | Knots, knot-holes, cracks, and all other defects permitted. |
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 out of wood. 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.
Typical end uses of spruce plywood are:
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.:
Coated special birch plywood is typically used as a ready-to-install component 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.:
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.