Drying defects in the wood drying equipment process I. […]
Drying defects in the wood drying equipment process
I. Collapse: The so-called collapse mainly refers to the abnormal deformation of the wood due to the extreme deformation of the cell. It is due to the tension and compressive stress caused by the change of water in the cell cavity. Generally, wood with a high water content tends to collapse if the temperature is too high at the beginning of drying. Depending on the species of the tree, the unevenness of the plate surface will occur. In order to avoid such defects, for a large collapsed tree species, it may be air dried or dried at a low temperature for a period of time.
Second, the internal cracking: drying the thickness of the sheet below 1 cm or using air drying method, almost no internal cracking. Internal cracking is formed by the surface cracking and inward after the surface cracking develops inward, and there is also a case where the surface is cracked only inside the crack.
The internal crack of the chord occurs at the end of the drying because the inner layer shrinks in the width direction more than the surface. Internal cracking has a great relationship with drying temperature. Generally, the initial temperature of the drying is low (about 50 ° C), and the surface cells are difficult to collapse. However, the inner layer of wood is heated for a long time in a state of high water content, and as the drying progresses, the drying temperature gradually rises and the cell collapse increases. Therefore, most of the thick plates are prone to internal cracking due to the lengthening of the internal heating time. In addition, if the temperature difference between the dry and wet bulbs is large at the initial stage of drying, the surface tensile stress is large, and if the internal cells collapse, it is easy to generate internal cracking.