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Plywood
was the first type of engineered wood to be invented. It
is made from thin sheets of wood veneer, called plies or
veneers. These are stacked together with the direction of
each ply's grain differing from its neighbors' by 90°
(cross-banding). [1] The plies are bonded under heat and
pressure with strong adhesives, usually phenol formaldehyde
resin, [2] making plywood a type of composite material.
A common reason for using plywood instead of plain wood
is its resistance to cracking, shrinkage, twisting/warping,
and its general high degree of strength.
Types
of Plywood
A
vast number of varieties of plywood exist, with many conditions
and uses. Softwood plywood is usually made either of Douglas
fir or spruce, pine, and fir, and is typically used for
construction and industrial purposes. [3] Decorative plywood
is usually faced with hardwood, including red oak, birch,
maple, lauan (Philippine mahogany) and a large number of
other hardwoods.
Plywood meant 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 most common varieties of softwood plywood comes in three,
five or seven plies with dimensions of 1.2 m × 2.4
m (4 feet × 8 feet). Plies vary in thickness from
1/10" through 1/6" depending on the panel thickness.
Roofing can use the thinner 3/8-inch plywood. Floorboards
are at least 5/8-inch depending on the distance between
floor joists. Plywood for flooring applications is often
tongue and grooved. Two of the edges will have "grooves"
notched into them to fit with the adjacent "tongue"
that protrudes from the next board.
High-strength plywood, known as aircraft plywood, is made
from mahogany and/or birch. It was used for several World
War II fighter aircraft, including the British-built Mosquito
bomber. Airplane plywood was adapted for furniture by Alvar
Aalto.
Plywood 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 peeled into sheets of veneer which are then cut to
the desired dimensions, dried, patched, glued together and
then baked in a press at 280 degrees Fahrenheit and 2800
psi to form the plywood panel. The panel can then be patched,
resized, sanded or otherwise refinished, depending on the
market it was intended for.
History
of Plywood
Plywood has been made for thousands of years; the earliest
known occurrence of plywood was in ancient Egypt around
3500 BC when wooden articles were made from sawn veneers
glued together crosswise. This was originally done due to
a shortage of fine wood. Thin sheets of high quality wood
were glued over a substrate of lower quality wood for cosmetic
effect, with incidental structural benefits. This manner
of inventing plywood has occurred repeatedly throughout
history. Many of the great English furniture makers such
as Sheridan used veneer as a raw material.
Modern plywood in which the veneer are cut on a rotary lathe
from softwood logs is of relatively recent origin, invented
by Immanuel Nobel. The first such lathes were set up in
the United States in the mid 19th century. Plywood has been
one of the most ubiquitous building products for decades.
Compare to OSB (Oriented strand board) and MDF (Medium-density
fibreboard).
Plywood
Applications
Plywood
is used in any application that needs high quality wooden
sheet material. High quality in this context means resistance
to cracking, breaking, shrinkage, twisting and warping.
Plywood is also used as an engineering material for stressed
skin applications. Plywood has been used in this fashion
for marine and aviation application since the WWII era.
Most notable is the British De Havilland Mosquito bomber
which was primarily made out of wood. Plywood is currently
used in stressed skin applications quite successfully[citation
needed]. The American designers Charles and Ray Eames and
Phil Bolger are world famous for their work with plywood.
Thanks to http://en.wikipedia.org/wiki/Plywood
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