Plywood is a material manufactured from thin layers or “plies” of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. It is an engineered wood from the family of manufactured boards which includes medium-density fibreboard (MDF) and particle board (chipboard).
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 cross-graining and has several important benefits: it reduces the tendency of wood to split when nailed in 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.
Phenolic plywood is somewhat of a misnomer as it should more realistically be called “phenolic faced plywood.” The plywood itself isn’t made of phenolic, it’s generally made of birch. However, both surface veneers are soaked in phenolic resin before attaching them to the plywood; creating an extremely stable, smooth, highly water resistant and attractive surface. Like other high grade hardwood, phenolic plywood is made with a large number of layers. Depending on the manufacturer, a 3/4” sheet of phenolic could have from 13 to 15 layers. It is a void-free product, as with most cabinet grade plywood.