Forced moisture balance is to quickly balance the moisture content of wood and reduce internal stress. Forced moisture balance treatment is usually carried out when processing into solid wood panels and laminated materials. If the wood is not dried well, it must be carried out in the raw materials before being made into finished products. However, which method to use should be determined according to the actual production situation of the enterprise.
4.1 Forced moisture balance of solid wood narrow boards or small square materials
If the wood is not well treated in the later stage of drying, that is, the baseline conditions in the moisture balance stage are not well controlled, the wood must also be forced to balance the moisture content of solid wood narrow boards and finger-jointed small square materials in the drying kiln, which is also called secondary drying treatment. At this time, the dry bulb temperature in the kiln should be controlled at 50-60℃, and the dry-bulb temperature difference corresponds to the equilibrium moisture content in the kiln (Table 1, Table 2). The corresponding relationship between the equilibrium moisture content in the kiln can greatly shorten the storage time of the wood in the later stage of drying and reduce the warping and cracking of the finished product to a certain extent. However, there are also some problems. First, after the forced moisture content treatment of the wood, although the moisture content deviation of the entire wood can be controlled to the lowest range (ideally, the moisture content deviation of the entire pile of wood can be controlled at about 1%), after the forced treatment, it needs to be stored for a certain period of time before subsequent processing. A large amount of stored wood will also be affected by the humidity of the environment, causing the moisture content of the wood to change and reducing the amount of finished products after splicing; second, there is an extra production process, which increases the cost. At the same time, the post-treatment of wood drying in production is less taken seriously, which increases the moisture content deviation. Sometimes, even after forced moisture balance treatment, the final moisture content of the wood cannot be achieved.
4.2 Forced moisture balance of solid wood panels or integrated materials
The forced moisture balance of the solid wood panels or integrated materials requires stricter process control. The forced moisture balance of solid wood panels or integrated materials should be stacked according to the requirements of wood drying. When multiple piles are balanced together, the distance between each pile should not be less than 100mm. Each pile should be square and "three straight" and "one side pressed", that is, the top and two sides of the pile are straight, the pressure strips are straight and pressed on the top of the pile. The process conditions for forced moisture balance of solid wood panels or integrated materials are as follows: The dry bulb temperature in the kiln should be controlled at about 50℃, and the temperature should not be too high, otherwise it is easy to crack. The wet bulb temperature should be determined according to the final moisture content of the wood. At this time, the process conditions in the kiln can refer to Table 1.
During the entire forced moisture balance stage, appropriate steam spraying is carried out according to the requirements of the dry-wet bulb temperature difference, and the dry-wet bulb temperature difference is maintained. At the same time, the style is turned on and the temperature is determined every hour according to the thickness of the wood. For 30mm thick hardwood, the processing time is generally 72 hours. For example, when processing Northeast Elm, when the wood thickness is 25mm, the dry bulb temperature and wet bulb temperature are 50-52℃ and 45-47℃ respectively, the processing time is 72 hours, and the final moisture content of the wood is 12%. After forced moisture balance, the moisture content of the solid wood plywood or integrated wood is uniform. After reaching the processing time, open the dehumidification hole, start the fan, and start cooling. The cooling speed should not be too fast. Generally, the cooling speed is 3-6℃ per hour to prevent the wood from cracking. When it is reduced to the room temperature, it can be taken out of the kiln and further processed after aging for 24 hours. The forced moisture balance treatment at this stage can effectively prevent the warping and cracking of solid wood panels and integrated materials. It can also detect quality problems caused by incorrect control of process conditions before panelization and avoid them flowing into the next production process.
In short, there are many reasons for the warping and cracking of solid wood panels or integrated materials. In addition to the above, it also involves the selection of adhesives, but the main reason is the moisture content of the wood during splicing. Therefore, strictly controlling the moisture content of the wood and taking effective process measures can completely avoid the warping and cracking of solid wood panels and integrated materials.
