Various defects occur during the wood drying equipment, […]
Various defects occur during the wood drying equipment, and most of these defects can be prevented or mitigated. The deformation is generally determined by factors such as tree species and material grade; collapse is likely to occur on the radial material of some tree species close to the core; cracking occurs in the early stage of drying, which is end cracking and surface cracking, and internal drying occurs later. Cracking.
Initial cracking: There are two types of cracking at the initial stage of drying, that is, end cracking and surface cracking. Most of the end face cracking is the growth stress of the logs before the wood and the cracks caused by the shrinkage. A new end crack occurs when the drying conditions are bad, and the original crack is further expanded. Surface cracking extends from the wood end to the inside. When the plate having a thickness of 2 cm or less has a surface cracking at the initial stage of drying, the plate having a thickness of 1 cm or less is hardly present. The reason for the surface cracking is because the surface layer shrinks after drying, but it is internally restrained. The factors associated with it are the drying conditions of the wood, the shrinkage rate, the ease with which the moisture moves, and the ability of the material to resist deformation. Under the same drying conditions, the greater the density of wood, the more likely it is to crack; the amount of dry shrinkage in the width direction of the string is about twice that of the radial material, so the surface of the string is prone to surface cracking.
The drying temperature is closely related to surface cracking. Cracking easily occurs at a low temperature of 0 to 5 ° C, which is mainly due to low temperature, small water diffusion coefficient, and large water content gradient. Therefore, when drying naturally in winter, the crack-prone wood should be avoided to be exposed to strong north wind. Surface cracking is also prone to occur when the temperature is dried above 50 °C. For tree species prone to cell collapse, slowly drying at 60-75 ° C, the temperature difference between several wet bulbs increases sharply, and surface cracking is prone to occur. Under certain temperature conditions, when the wood collapses, but the wood which is easy to crack is dried, the temperature is high and cracking is easy. Therefore, it is necessary to select suitable drying conditions to prevent initial cracking of the wood.