Plywood is made of layers (plies) of wood veneers. A layer may be 1/16 inch to 5/8 inch thick.
The center layer is known as the core. As layers are added on either side of the core, they are placed alternately at a right angle, which increases the strength of the sheet.
Layers with the grain at right angles to the core are called crossbands.
The outer exposed layer is known as the face veneer.
Plywood: How it's Made
First, logs are cut to a specific length; they are then softened in a hot water or steam bath. The bark is then removed and the log locked into its center on a lathe. As the lathe turns the log, a long knife slices off a thin veneer. A steel roller at the rear of the knife assists in keeping the veneer intact and maintains a uniform thickness of approximately 1/16 inch to 5/16 inch.
This continuous veneer is trimmed into smaller sheets that are fed into a hot oven or dryer to reduce the moisture content. The moisture content is reduced to a range of 3 to 8 percent. At this point, the veneers are separated into core and crossband materials.
The veneer that will be used for the face now goes through a patching machine to remove and correct defects in the veneer. The patching machine will also match face grains.
After all this, the materials go through one final step called splicing, prior to becoming a sheet of plywood.
In the final manufacturing process, hot glue is applied by the machine to the core, crossbands, faces, and plies.
The rough plywood sheet will now go into a hot press. This hot press will apply a great amount of pressure to the pieces to squeeze out the excess glue and will compress the rough plywood to its approximate final thickness.
This process takes from two to twenty minutes. Once the process is completed, the sheets are cut and sanded to their final thickness.
Plywood Sheet Sizes
The average or standard size of plywood is 4’ X 8’. There are three types of edges on plywood:
• Butt joints (two standard pieces joined)
• A shiplap cut or edge
• Interlocking tongue-and-groove
Opposite edges or all four edges may be cut to match.
Grading for Softwood Construction Plywood
After the plywood has been manufactured, the finished product is graded. Plywood is rated for interior or exterior use.
Exterior-rated plywood is used for sheathing, siding, and other applications where there may be exposure to moisture or wet weather conditions.
Exterior plywood panels are made of high-grade veneers bonded together with waterproof glue that is as strong as the wood itself.
Interior plywood uses lower grades of veneer for the back and inner plies.
Although the plies can be bonded with water-resistant glue, waterproof glue is normally used. However, the lower-grade veneers reduce the bonding strength, which means that interior-rated panels are not suitable for exterior use.
Plywood Glues, Cores, and Face Veneers
A. Plywood Glues:
Most plywood today is rated for interior use; most of the glue used on plywood is exterior glue.
B. Plywood Cores:
Plywood for industrial use is often called soft plywood. This product has a veneer core. This core is manufactured by the same procedure used to make other plies. It may be of single ply or two plies, having the grain running in the same direction and glued together to form a single core or layer.
The plywood used in the manufacture of cabinets, doors, furniture, and finished components is known as hardwood plywood. This type of plywood may have any of four different types of core:
a. A lumber core
b. A particleboard core
c. A veneer core (common in construction plywood)
d. A fiberboard core
C. Face Veneers:
Plywood that is designated for sheathing, subflooring, concrete forms, and sheets used for special structural purposes is called engineered plywood.
The veneers are either unsanded or lightly sanded.
Remember, when stacking plywood only two methods may be used.
Sheets may be stacked flat on a solid surface and off the floor or ground, or the sheets may be stacked in a full vertical position between two posts.
Keep the plywood off the floor or ground. Storing plywood at an angle will cause the material to warp or twist.