In order to improve the anisotropy of natural wood as much as possible, so that the characteristics of plywood are uniform and the shape is stable, the general plywood must abide by two basic principles in structure: one is symmetry; Second, the fibers of the adjacent layers of veneer are perpendicular to each other. The principle of symmetry is to require that the veneer on both sides of the symmetrical center plane of the plywood should be symmetrical with each other regardless of the nature of the wood, the thickness of the veneer, the number of layers, the direction of the fiber, the moisture content, etc. In the same plywood, veneers of a single species and thickness can be used, as well as veneers of different species and thicknesses; However, any two layers of veneer trees on either side of the symmetrical center plane should be of the same kind and thickness. The backing of the face is allowed not to be of the same species.
For the structure of plywood to conform to both of the above basic principles, its number of layers should be odd. Therefore, 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 veneer is called the surface board, and the inner layer of the veneer is called the 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 is parallel to the surface plate, which is called the long core board or the middle plate. When composing the cavity table slab, the panel and backplate must be facing outward.
