Plywood and laminated wood are both made of layers (laminae) of wood glued together. The basic difference is that in plywood the grain of alternate layers is crossed, in general at right angles, whereas in laminated wood it is parallel. The development of these products (as well as particleboard, described in the next section) was made possible by the production of improved adhesives—especially synthetic resins—in the 1930s and ’40s.
Plywood is a panel product manufactured by gluing one or more veneers to both sides of a central veneer layer or a lumber-strip core. Most plywood is all-veneer; lumber-core plywood is produced only in small quantities. Lumber cores are made by the lateral gluing of strips of wood. In both plywood products, the species, thickness, and grain direction of each layer are matched with those of its counterpart on the other side of the central layer. Consequently, the total number of layers is usually odd (three, five, or more), the exception being when the central veneer layer consists of two sheets glued together with their grains parallel. After the glue is spread, the panels are assembled and brought for pressing, usually in large, multistoried hot presses, where loading is automatic. Adhesives are thermosetting synthetic resins—phenol-formaldehyde for exterior-use plywood and urea-formaldehyde for interior-use plywood. Phenol-formaldehyde resin can produce joints more durable than the natural wood itself—highly resistant to weather, microorganisms, cold water, hot water, boiling water, seawater (“marine” plywood), steam, and dry heat. After pressing, the panels are stacked to cool and then are sanded, graded, and stored. Plywood ranges in thickness from 3 mm (about 0.12 inch) for all-veneer to 30 mm (1.2 inches) for lumber-core.
