Causes of drying defects in the drying process using a Rotary Dryer

There are two types of cracks in the early dry period,Rotary Dryer namely end-face cracking and surface cracking. The cracks on the end surface are mostly the cracks of the growth stress and dry shrinkage of the logs before the timber is made. A new end crack will occur when the dry conditions are severe,Rotary Dryer and the original crack will be further extended. The appearance of cracks will extend from the end of the wood to the inside. The appearance of cracks at the initial stage of the dryness of the board less than 2 cm thick is fierce,Rotary Dryer and the board less than 1 cm thick is almost absent. The reason for the appearance of the crack is that the surface layer shrinks after being dry, but it is subject to internal constraints.

The factors related to it are the dry condition of the wood, the shrinkage rate,Rotary Dryer the difficulty level of water movement and the ability of the material to resist deformation. Under the same dry conditions,Rotary Dryer the greater the density of the wood, the easier it is to crack;Rotary Dryer the shrinkage of the width of the string material is about twice that of the radial material,Rotary Dryer so the string material is prone to cracking on the surface. The relationship between dry temperature and appearance cracking is intimate. Cracking is prone to occur at low temperature of 0-5℃. The main reasons are low temperature,Rotary Dryer small water diffusion coefficient and large water content gradient. Therefore, when winter is naturally dry,Rotary Dryer try to prevent the easy-cracking wood from being exposed to the intense north wind. When the temperature is above 50 ℃ dry wood, Rotary Dryer it is also prone to cracking on the surface.

For trees that are prone to cell collapse, use 60-75℃ to stop the slow and dry,Rotary Dryer and the temperature difference of some wet bulbs increases sharply, which is easy to crack on the surface. Under certain temperature conditions,Rotary Dryer when the wood that collapses is small, but the wood that is easy to crack is dry,Rotary Dryer the temperature is high and it is easy to crack. Therefore, it is necessary to choose appropriate dry conditions to avoid the initial cracking of the wood.Rotary Dryer The so-called collapse mainly refers to the abnormal deformation of the wood due to the extreme deformation of the cell. It is because the cell cavity produces the tensile force and compressive stress caused by the change of water.Rotary Dryer Ordinary wood with high water content is prone to collapse when the temperature is too high in the early dry period. Depending on the tree species,Rotary Dryer the areas where the collapse is concentrated will show unevenness on the surface of the board. In order to prevent such defects, Rotary Dryerthe tree species with large collapse can be air-dried after a period of time or stopped by low temperature.

Internal cracking: Dry thin plates with a thickness of less than 1 cm or air-drying methods will not cause internal cracking. Internal cracking is caused by the appearance of cracks after the appearance of cracks inward,Rotary Dryer and there are also cases where there are no cracks on the surface and only cracks occur inside. The internal cracking of the string-oriented material is at the end of the dry period,Rotary Dryer due to the reason that the inner layer shrinks in the width direction than the appearance. The relationship between internal cracking and dry temperature is great. Generally,Rotary Dryer the initial dry temperature is low (about 50℃), and it is difficult for the surface cells to collapse. However,Rotary Dryer the inner layer of wood is heated for a long time under the condition of high water content. As the dryness stops, the dryness temperature gradually increases,Rotary Dryer and the cell collapse increases. Therefore,Rotary Dryer most thick plates are prone to internal cracking due to the longer internal heating time. In addition, if the temperature difference between the wet and dry bulbs is large at the beginning of the dry period,Rotary Dryer the surface tensile stress is large, and if the internal cells collapse, internal cracks are also likely to occur.