Plywood
is composed of a minimum of three layers, the back, the face and
the core, and is held together by glue.
Though three layers is the minimum, five ply plywood, seven ply
plywood and multi-ply plywood are also available. Most plywood is
pressed under pressure using heat to speed the setting process.
With veneering, the concept of balanced construction is very important.
In other words everything is centered on the core, and what you
do to the front of the panel must be done to the back of the panel
so that it will remain straight.
Preparation
of Faces
First the face veneer is selected and lay-up drawings are produced.
The veneer is then trimmed to size and stitched, taped or glued
together to form a face. The completed faces are marked to show
which side is the face, direction to the top, sequence and size.
Veneer can be hand taped together, but there are other alternatives.
Usually it is joined by edge gluing each leaf to the next in a
special machine that applies glue, pushes the veneer leaves together
and then sets the glue instantaneously with heat. In other cases,
the veneer is stitched together using a thread of glue that is
zigzagged back and forth across the joint line.
Backs
are usually of a similar veneer or a veneer that has similar density
and expansion and contraction properties. This is another illustration
of the need for balanced construction. If cross-bands are required,
then the cross-bands must be trimmed to the same size as the face
and back.
Preparation
of Cores
Each core is cut to the appropriate size for the panel being pressed.
The core is usually slightly smaller than the veneer faces. Thus,
the face, back and core sizes must be coordinated in the processing
plant.
Pressing
Plywood
Once the face, back and core are cut to size the pressing process
can begin. Large sequences of backs, faces and cores are assembled
so that the pressing process can be continuous in the following
steps:
o
First the back is laid upon the bed of the press, taking care
that no dust, dirt or waxy film is on the surface of the back.
o The core is fed through a brush to clean off any dust particles,
wax or grease, and then through a double-sided glue spreader to
coat the core with glue on both sides. The core is then laid on
top of the back.
o The face is then laid on top of the glue-coated core and the
stacked components are then rolled into the press. The press closes
and under pressure and heat fuses and dries the glue. Normal pressures
are between 20 and 30 pounds per square inch, and normal press
times are between one minute twenty seconds and four minutes.
Timing is highly dependent upon the core type, the glue type,
and the face type. Some species, such as cherry, are more prone
to shrink in the press and thus they are sometimes wetted with
a fine spray of water prior to pressing. Their cycle time and
temperature in the press are kept to a minimum. More porous woods
may require a heavier glue line or more time in the press than
less porous veneers and cores. o After pressing the plywood is
rolled out of the press and stacked horizontally to cool and set.
Stacks of newly pressed plywood can hold their temperature for
hours or even days because of the insulating properties of wood.
In some cases it is recommended that the panel products be stacked
vertically with air between them to permit cooling and drying
more efficiently and more evenly.
Sanding
As it leaves the press, plywood has raised grain and can have
glue or tape on its surface. Thus the panel product must be sanded
thoroughly using manual or automated sanders to smooth the surface
and remove any glue or tape.
Trimming
After the sanding process, each panel is trimmed to the exact
size and then stacked and bundled for shipment.
Storage
Plywood must be stored in absolutely flat racks on pallets so
the sheets do not twist or warp when in storage. The plywood should
not be stacked too high because the excessive weight of plywood
can warp the sheets located at the bottom of the pile. The whole
process from veneer selection through lay-up can take weeks or
even months depending upon the project size.
The
moisture content of the core prior to being cut and pressed, during
and after pressing, and when in storage must be fairly consistent
or the cores may shrink or expand. When the panel products reach
the woodworker's shop, it is a very good idea to allow a week
for them to acclimate and stabilize.
Plywood
is often manufactured in areas of the country that may be very
different in terms of temperature and humidity compared to the
final installation site. It is important that the equilibrium
moisture content (EMC) of the plywood be achieved prior to cutting
to final sizes.
