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 crossbands are required, then the cross bands must be trimmed to the same size as the face and back.
Balanced Construction - Figure 200-03 from AWI quality Standards, Seventh Edition.
Panel Construction Balance (from AWI Quality Standards, Seventh Edition)
To achieve balanced construction, panels should be an odd number of layers (plies) symmetrical from the center line, i.e, all inner plies, except the innermost ply, should occur in pairs, using materials and adhesives on both sides that contract and expand, or are moisture permeable, at the same rate. A ply may consist of a single veneer, particleboard, medium density fiberboard, or hardboard. Each pair of inner plies should be of the same thickness and direction of grain. Each ply of each pair is placed on opposite sides of the innermost ply or layer, alternating grain directions from the center out.
(Particleboard and MDF do not have a specific grain orientation.)
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.
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:
- 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.
- 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.
- The face is then lain 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 fine or more time in the press than less porous veneers and cores.
- 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.
- 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.
- After the sanding process, each panel is trimmed to the exact size and then stacked and bundled for shipment.
- 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.