In order to improve the anisotropic properties of natural wood as much as possible, and to make the properties of plywood uniform and stable in shape, the structure of plywood generally follows two basic principles: firstly, symmetry; The second is that adjacent layers of veneer fibers are perpendicular to each other. The principle of symmetry requires that the veneer on both sides of the symmetrical central plane of the plywood, regardless of wood properties, veneer thickness, number of layers, fiber direction, moisture content, etc., should be symmetrical with each other. In the same plywood, single tree species and thicknesses of veneer can be used, as well as different tree species and thicknesses of veneer; But any two layers of symmetrical veneer trees on both sides of the symmetrical central plane should have the same thickness. The top and back panels are allowed to be of different tree species.
To ensure that the structure of plywood conforms to both of the above basic principles, its number of layers should be odd. So plywood is usually made into three layers, five layers, seven layers, and other odd layers. The names of each layer of plywood are: the surface layer of veneer is called surface board, and the inner layer of veneer is called core board; The front panel is called the panel, and the back panel is called the back panel; In the core board, the fiber direction parallel to the surface board is called a long core board or medium board. When forming the cavity table slab, the panel and back panel must face outward tightly.