All plywoods bind resin and wood fibre sheets (cellulose cells are
long, strong and thin) to form a composite material. This
alternation of the grain is called crossgraining 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 is 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 has high
stiffness perpendicular to the grain direction of the surface ply.
Smaller, thinner, and lower quality plywoods may only have their
plies (layers) arranged at right angles to each other. Some
better-quality plywood products will by design have five plies in
steps of 45 degrees (0, 45, 90, 135, and 180 degrees), giving
strength in multiple axes.
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, causing a thin layer of wood to peel off (much as
a continuous sheet of paper from a roll). 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
(depending on wood species) when dried. The sheets are then
patched, graded, glued together and then baked in a press at a
temperature of at least 140 °C (284 °F), and at a pressure of up to
1.9 MPa (280 psi) (but more commonly 200 psi) 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
moisture, 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. Programme for the Endorsement of Forest Certification
(PEFC) 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.
