Wood deformation due to shrinkage and swelling and wood shrinkage stress can be overcome during the storage and drying of logs and sawn timber by controlling the wood mixing rate and developing a suitable drying process.
2. l Dry shrinkage and swelling control method
Control the uniformity of the moisture content of the wood, so that it can be reduced to the local or the use of the atmospheric equilibrium water combination rate, which can overcome the dry shrinkage and wet expansion deformation of the sawn timber. In the process of transportation and storage of logs, the water mixing rate is kept uniform, which can avoid cracking of logs, and during the sawing process, the bending deformation caused by the stress caused by uneven drying can be reduced. Therefore, when transporting and storing logs, the surface should be prevented from drying too fast. When conditions permit, the water transport, water storage method, and wet storage method can be used. The use of artificial drying, using the principle of moisture hygroscopic hysteresis (average reduction of about 2.5%), can stabilize the size of sawn timber or wood products.
2.2 Control of dry shrinkage stress
The shrinkage stress of wood is caused by the unsynchronization of drying and shrinkage of the inner and outer layers during the drying process. The occurrence and development of dry shrinkage stress is related to the drying process and directly affects the deformation of the sawn timber. Therefore, in the kiln drying process, it is necessary to understand the changes in the shrinkage stress and take corresponding measures. The method of eliminating the dry shrinkage stress is to carry out the humidity conditioning treatment at different stages of artificial drying. According to its time and effect, the humidity control treatment is divided into the following four stages:
The pre-heat treatment is carried out in the early stage of wood drying, which can speed up the core temperature, eliminate the surface tension of the gas material, and make the water content more uniform. In addition, the conductivity of moisture can be improved. Treatment method: first raise the temperature of the medium to 45-55 C, and maintain 0.5~lh; then combine the steaming and heating to raise the temperature and humidity to the required medium state for a certain period of time. Let the wood heat through and then dry.
The intermediate treatment refers to the humidity control treatment for eliminating the surface tension and surface hardening of the sawn material during the kiln drying process, usually in the middle of the drying period, using high temperature and high humidity treatment to eliminate the dry shrinkage stress and the surface hardening. Treatment method: increase the dry bulb temperature by 8 to 10 °C, but the maximum is not more than 100 °C. The humidity is determined by the specified value of the equilibrium water combination rate of 5% to 6%. The temperature difference between the control dry and wet bulbs is about 1.5. ~2 C. The treatment time depends on the tree species, the thickness of the sawn timber and the degree of stress. Generally, hard broad-leaved trees or larch below 50 mm are treated for about 1 h every 10 mm; for thick and difficult materials, it can be extended; coniferous trees, Soft broad-leaved trees and sheets can be left without intermediate treatment. The number of intermediate treatments is 1 to 3 times, which is determined according to the tree species, thickness, initial moisture content and dryness of the sawing material.
Balance treatment A process in which the moisture content of a portion of the sawn timber is not uniform when the moisture content of the sawn timber reaches the desired final moisture content. The method is to increase the temperature of the medium by 5 to 8 C, so that the equilibrium moisture content of the medium is 2% lower than the final moisture content of the sawn timber, and the treatment time is maintained for 2 to 6 hours per 10 mm thickness. The coniferous tree, the soft broad-leaved tree and the thin plate Can not be balanced.
The final treatment is a wetting treatment for eliminating residual dry stress existing inside the material. The temperature of the finished medium can be 5-8 C higher than the baseline; the humidity is about 4% higher than the equilibrium moisture content of the medium state, or the dry-wet bulb temperature difference is about 3 °C. The treatment time depends on the tree species, the thickness, the hardness of the reference, and the presence or absence of intermediate and equilibrium treatment. It is usually treated for 2 to 5 hours per 10 mm thickness. At the end of the treatment, care should be taken not to deal with excessive, otherwise it will produce "reverse surface hardening", which is the reverse dry residual stress.
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