Plywood is a three- or multi-layer board material made from wood segments that are rotary cut into veneer or from wood squares that are planed into thin wood and then glued together with an adhesive.
Principles of plywood composition
Wood is anisotropic. This anisotropy is mainly reflected in the drying and expansion of the wood. The various physical and mechanical properties vary considerably in all respects. The shrinkage and swelling is 0.1% to 0.3% in the down-grain direction, 3% to 6% in the radial direction and 6% to 12% in the chord direction for freshly harvested timber until it is completely dry. The mechanical properties are only in terms of tensile strength. It is about 20 times higher in the sinew direction than in the cross-grain direction. In order for plywood to have good properties, this inherent disadvantage of the wood should no longer be reflected in the overall composition of the plywood. The following principles should be followed when forming plywood from veneer.
Principle of symmetry
The principle of symmetry means that veneers on both sides of the symmetrical central plane of the plywood are required. All species, veneer thicknesses, ply numbers, manufacturing methods, fibre orientation and moisture content should correspond to each other.
Plywood has a homogeneous composition (each layer of veneer has the same thickness) and a non-homogeneous composition (each layer of veneer has a different thickness). The corresponding layers and veneer thicknesses must be the same for non-homogeneous layers. For plywood with mixed species, the corresponding layers should generally be of the same species.
As wood is characterised by hygroscopic expansion and hygroscopic contraction, each layer of veneer is subject to deformation when the moisture content changes. This deformation varies with the direction of the fibre in the wood and therefore causes a different amount of stress.
Principle of odd numbered layers
The number of layers in plywood must be an odd number. The structure of plywood is such that the fibre of adjacent veneers are perpendicular to each other and must conform to the principle of symmetry. Therefore, the total number of layers must be odd.
Plywood is often subjected to forces in the form of bending. The distribution of horizontal shear stress in its interior during bending shows that the maximum horizontal shear stress is distributed in the central ply plane. The principle of symmetry is also fulfilled in the case of an even number of layers of plywood. However, the central plane of symmetry for even layers is on the glue layer. This is because the plies are generally more brittle and less elastic than wood. The even numbered layers are subject to the greatest horizontal shear stress, which tends to make the glue layer flake off and reduce the bonding strength of the plywood. In addition, even layers of plywood should be symmetrical. The fibre direction of the two adjacent veneers must be the same. This structure is equivalent to a three-ply plywood with a thick core. This structure does not lead to an improvement in board properties, but only to more consumption of wood, man-hours and adhesives. If the principle of an odd number of layers is followed, the maximum horizontal shear stress is distributed over the middle layer of the core. This avoids the drawbacks associated with using an even number of ply structures.
The principle of ply thickness has also been proposed in the past, meaning that for the same thickness of plywood. The thinner the veneer and the more plies it has, the less the difference in the longitudinal and transverse strength of the plywood and the more balanced the physical and mechanical properties. In practice, however, the thickness of the veneer is limited. Furthermore, the manufacture of plywood from very thin veneers leads to an excessively complex production process. As a result productivity decreases, raw material consumption increases and costs rise. The principle of ply thickness is therefore no longer advocated. In actual production should be based on the product use to choose the appropriate veneer thickness.