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.
History
Plywood 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.
[citation needed]
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 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 is consistent throughout the depth of the panel.
[citation needed]
Types
Average-quality plywood with 'show veneer'
High-quality concrete pouring plate in plywood
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 (collectively known as
spruce-pine-fir or SPF) or
redwood and is typically used for construction and industrial purposes.
[1]
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 plywood
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.
[2]
Tropical plywood
Tropical
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 plywood
Certain plywoods do not have alternating plies. These are designed for specific purposes.
Aircraft plywood
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
[citation needed]
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 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
[citation needed].
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 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. 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 plywoods
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.
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 (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.
[3]
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.
[4]
Sizes
The 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.
[5]
Sizes on specialised plywood for concrete forming range from 6 to
21mm, and a multitude of formats exist, though 15x750x1500mm is very
commonly used.
Grades
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:
[6]
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 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. |
JPIC Standards
| Grade |
Description |
| 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. |
Applications
Plywood being used to cover the many broken windows in a building in Miami after
Hurricane Wilma.
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 is currently successfully used in
stressed-skin applications.
[citation needed]. 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 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
Birch plywood applications
Phenolic-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)
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
- 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 (LVL)
