When drying the veneer, if the veneer thickness is not uniform, which will cause cracking.
In the process of drying wood veneer with a continuous veneer dryer, the anisotropy of wood veneer shrinkage is not considered, and it is assumed that water movement occurs only in the thickness of the wood veneer, and the changes in water content distribution, stress and thickness deformation can be analyzed:
1. The stage where no stress is generated at the beginning of drying. At this stage, although the surface moisture content is low and the moisture content thickness is not uniform, above the fiber saturation point, no dry shrinkage occurs and no stress occurs.
2. In the initial stage of drying, the stress is outside the tensile pressure stage. After the drying process begins, the free water on the wood surface first evaporates. After a short time, depending on the temperature and relative humidity of the dry medium, the surface water content drops below the fiber saturation point, and the water content gradient on the cross section increases. There are "wet lines", the areas outside the "wet line" are lower than the fiber saturation point, and the inner area is still higher than the fiber saturation point. As the drying progressed, the "wet line" continued to move inward.
Because the moisture content of the wood veneer surface is lower than the fiber saturation point, dry shrinkage is required, but the moisture content of the inner layer is higher than the fiber saturation point and the size is unchanged. Therefore, the surface layer shrinks and dry, so the surface layer is subject to tensile stress and internal effects.
3、Because the cross-section of the wood veneer in the early stage of drying is thin, the area with a moisture content below the fiber saturation point is thinner, and the area corresponding to the tensile stress is smaller and the area is smaller. The compressive stress is large, the total tensile force and the total pressure are balanced, the compressive stress per unit area is small, and the tensile stress per unit area of the surface layer is large, which develops rapidly. And reach great tensile stress. When the stress is greater than the tensile strength limit of the surface layer, cracks occur. This is also the main cause of cracks in the early stages of drying.
