Symmetry principle: veneer on both sides of the symmetry center plane, regardless of the thickness of the tree veneer, the number of layers, the manufacturing method, the fiber direction and the moisture content of the veneer should correspond to each other, that is, the symmetry principle plywood center plane on both sides of the corresponding layer in different directions The stress is equal. Therefore, when the moisture content of the plywood changes, its structure is stable, does not produce deformation, cracking and other defects; the other hand, if the symmetry on both sides of the plane corresponding to some differences, will be symmetrical center plane on both sides of the board stress Equal to the plywood to produce deformation, cracking.
Odd-level principle: Since the structure of the plywood is the fiber direction of the adjacent veneer, it must meet the principle of symmetry, so its total number of layers must be odd. Such as: three boards, five board, seven board and so on. The maximum horizontal shear stress on the odd-level plywood is bent on the center veneer to give it greater strength. Even the highest horizontal shear stress on the adhesive layer acts on the adhesive layer rather than on the veneer, making it easy to break the adhesive layer and reduce the strength of the plywood.