Plywood is a manufactured wood panel from the family of manufactured boards, particle
board, etc.) made from thin sheets of wood veneer. Plywood layers are glued together,
with adjacent plies having their wood grain rotated relative to adjacent layers up to
90 degrees. All plywoods bind resin and wood fiber sheets
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 may only have their plies arranged
at right angles to each other, though many better quality plywood products will by design
have five plies in steps of 45 degrees, giving strength in multiple axis. The highest quality
specialty plywoods often have plies at 30 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.
History In 1797 Samuel Bentham applied for patents
covering several machines to produce veneers. In his patent applications, he described the
concept of laminating several layers of veneer with glue to form a thicker piece – the
first description of what we now call plywood. Samuel Bentham was a British naval engineer
with many shipbuilding inventions to his credit. Veneers at the time of Bentham were flat sawn,
rift sawn or quarter sawn; i.e. cut along or across the log manually in different angles
to the grain and thus limited in width and length.
About fifty years later Immanuel Nobel, father of Alfred Nobel, realized that several thinner
layers of wood bonded together would be stronger than one single thick layer of wood ; understanding
the industrial potential of laminated wood he invented the rotary lathe.
There is little record of the early implementation of the rotary lathe and the subsequent commercialization
of plywood as we know it today, but in its 1870 edition, the French dictionary Robert
describes the process of rotary lathe veneer manufacturing in its entry Déroulage. One
can thus presume that rotary lathe plywood manufacture was an established process in
France in the 1860s. Plywood was introduced into the United States in 1865 and industrial
production started shortly after. In 1928, the first standard-sized 4 ft by 8 ft plywood
sheets were introduced in the United States for use as a general building material.
As for artists’ use of plywood as support for easel paintings, replacing traditional
canvas or cardboard; a recent JSTOR article has brought to light that ready-made artist
boards for oil painting in three-layered plywood were produced and sold in New York as early
as 1880. Structural characteristics
A 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, by contrast, is higher in the center of the panel, and
zero at the outer fibers. Types
Different varieties of plywood exist for different applications:
Softwood plywood Softwood panel is usually made either of cedar,
Douglas fir or spruce, pine, and fir or redwood and is typically used for construction and
industrial purposes. The most common dimension is 1.2m × 2.4m
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 depends on the thickness and grade of the
sheet but at least three are required as the minimum odd number of plies. Roofing can use
the thinner 5/8" plywood. Subfloors are at least 3/4" 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" range Hardwood plywood
Hardwood plywood is made out of wood from angiosperm trees and used for demanding end
uses. Hardwood 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. Hardwood plywood has excellent surface hardness, and damage- and wear-resistance.
Tropical plywood Tropical plywood is made of mixed species
of tropical wood. Originally from the Asian region, it is now also manufactured in African
and South American countries. 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, United States, Taiwan, Korea, Dubai, and other countries worldwide. It is
the preferred choice for construction purposes in many regions due to its low cost. However,
many countries’ forests have been over-harvested, including the Philippines, Malaysia and Indonesia,
largely due to the demand for plywood production and export.
Aircraft plywood High-strength plywood also 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. Although the
British-built Mosquito bomber, nicknamed "The Wooden Wonder", was constructed of a plywood
monocoque, this was formed in moulds from individual veneers of birch, balsa and birch,
rather than machined from pre-laminated plywood sheets.
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 Usually faced with hardwood, including ash,
oak, red oak, birch, maple, mahogany, Philippine mahogany, 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. This plywood is a lot easier to dye and draw on than any other plywoods.
Flexible plywood Flexible 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 U.S.A, the
terms "Bender Board" and "Wiggle Board" are commonly used.
Marine plywood Marine 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. More recently, tropical
producers have become dominant in the marine plywood market. Okoumé from Gabon is now
the accepted standard for marine plywood, even though the wood is not very resistant
to rot and decay. Each wood veneer will be from 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 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
Okoumé 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 U.S. or around $2.50 per square
foot, which is about three times as expensive as standard plywood.
Other plywoods Other types of plywoods include fire-retardant,
moisture-resistant, wire mesh, 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. 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 laid horizontally and rotated about its long axis while a long blade is pressed
into it, causing a thin layer of wood to peel off. An adjustable nosebar, which may be solid
or a roller, is pressed against the log during rotation, to create a "gap" for veneer to
pass through between the knife and the nosebar. The nosebar partly compresses the wood as
it is peeled; it controls vibration of the peeling knife; and assists in keeping the
veneer being peeled to an accurate thickness. In this way the log is peeled into sheets
of veneer, which are then cut to the desired oversize dimensions, to allow it to shrink
when dried. The sheets are then patched, graded, glued together and then baked in a press at
a temperature of at least 140 °C, and at a pressure of up to 1.9 MPa to form the plywood
panel. The panel can then be patched, have minor surface defects such as splits or small
knot holes filled, 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. 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, Leadership in Energy
and Environmental Design, Sustainable Forestry Initiative, 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.
Sizes The most commonly used thickness range is
from 0.14 to 3.0 inches. The sizes of the most commonly used plywood sheets are 4 by
8 feet. Width and length may vary in 1-foot increments.
In the United States, the most commonly used size is 4 × 8 feet or 5 × 5 feet. A
common metric size for a sheet of plywood is 1,220 × 2,440 millimetres.
Sizes on specialised plywood for concrete-forming can range from 6 to 21 mm, and a multitude
of formats exist, though 15×750×1,500 mm is very commonly used.
Grades Grading rules differ according to the country
of origin. Most popular standard is the British Standard and American Standard. Joyce, however,
list some general indication of grading rules: JPIC Standards
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 and Motor Gun Boats 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 house constructions
Wind bracing panels Vehicle internal body work
Packages and boxes Fencing
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 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
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 Furniture
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 See also
BS 1088 Engineered wood
Fiberboard Glued laminated timber
Hardboard Masonite
Medium-density fiberboard Oriented strand board
Particle board Pressed wood
References
External links APA – The Engineered Wood Association
Material Uses Pro Woodworking Tips.com Canadian Plywood Association
Plywood