Drying of thin wood
The cooked wood or logs are still sliced ​​or cut into thin wood and still have a high water content, in order to ensure the thin wood and
The bonding strength of the substrate and the decorative effect of the thin wood surface require drying of the thin wood.
(1) Basic principle of thin drying
Wood consists of various cells, of which the coniferous material is mainly tracheid, and the hardwood is mainly wood fiber. These various cells that make up the wood cross-link the cell to form a large capillary system, and the water in the large capillary. Called free water. In the cell wall, there are numerous capillary tubes that cross each other to form a microcapillary system. The water in the microcapillary system is called sorption water. The maximum moisture content of sorbed water in wood is about 30%. The water content at this time is called fiber saturation. Point, free water may only be present in the wood when the moisture content in the wood is greater than the fiber saturation point. Moreover, when the thin wood is dried, firstly, the free water is evaporated from the outer surface of the thin wood, the free water has a small adsorption force on the wood, the evaporation speed is fast, and after the free water is evaporated, the adsorbed water is dried, because the capillary diameter is small ( One hundred does not exceed 0.05um), so the wood has a greater binding force on the absorbing water, and evaporating the absorbing water consumes more energy than the same volume of free water.
(2) Shrinkage and deformation of thin books
When the moisture content of the thin wood is higher than the fiber saturation point, the evaporation of the moisture in the thin wood does not change the size of the thin wood. When the thin wood is dried until the water content is lower than the fiber saturation point, as the thin wood continues to dry, the size of the thin wood shrinks. The phenomenon is called thin wood shrinkage. Since the wood chord has a large dry shrinkage and a small longitudinal shrinkage, the thin wood shrinks less in the direction parallel to the fiber and has a larger performance in the direction perpendicular to the fiber.
The surface of the thin wood has the difference between the early material and the late wood, which makes the density of the board surface different. The wood itself is a kind of heterogeneous material, and in order to improve the decorative effect, the surface of the thin wood board often has twisted lines, swirls, knots and other textures, which will cause unevenness of the structure on the thin wood, which makes the shrinkage rate of each part different. , causing deformation and cracking.
When the thin wood is dried in the dryer, the edge portion evaporates faster than the middle portion, and the contraction of the edge portion is restricted by the intermediate portion, so that the edge portion is liable to cause wave-shaped warping deformation and sometimes even cracking.
The following measures can be taken to reduce the warpage of the thin wood:
Water spray is applied on both sides of the thin wood to increase the moisture content of the edge of the thin wood to slow the evaporation of the edge water to reduce the warpage.
After the thinning is dried, the warp-deformed thin wood is flattened with a pressure plate.
3. The use of new drying equipment, such as mesh belt flattening dryer, roller pressure dryer, etc., allows the thin wood to dry under a certain pressure, so that the veneer shrinks and ripples.
(3) Drying conditions and final moisture content of thin wood
An important factor in the development of thin wood drying process conditions is the thickness of the veneer, which varies with the thickness of the veneer.
The drying process is very different. Generally, the thickness of the thin wood (0.7~0.8mm) is similar to that of the veneer. The standard of the single board drying can be used. The final moisture content is controlled at 8-12%, but the thin wood (0 The drying temperature of .2~0.3mm) should not exceed 100°C. After drying, the moisture content of the thin wood should be kept at about 20%, otherwise the thin wood is easy to be broken and warped. The sticking process of the thin wood also requires that the falling wood has a high water content, and the thin wood has a certain elasticity when the water content is high. It is relatively flat, and it is not easy to be misplaced when it is attached to the substrate. The moisture content of the thin wood is too low, and it is easy to absorb the moisture in the glue, causing the splicing place to overlap. However, if the moisture content of the thin wood is too high, the thin wood shrinks due to the rapid evaporation of water during the hot pressing process, which is easy to cause cracking of the thin wood surface, so it is guaranteed The operation is convenient, and the moisture content should be reduced as much as possible without breaking the thin wood. At the same time, the viscosity of the adhesive is increased, the thin wood is not easily displaced, and the dimensional change caused by the thin wood is absorbed by the moisture in the glue.
In addition, when using the particleboard substrate, it is generally decorated with thick wood, and the moisture content should not exceed 10%. Otherwise, the surface of the particle board will absorb the moisture and cause the surface roughness. However, in some countries, such as Japan, in order to simplify the production process, the micro-thin wood is not dried, and is adhered to the substrate at a moisture content of 40 to 50%.
Undried micro-thick wood should be careful not to let the water escape during storage, and pay attention to mold prevention, so the winter thin book should be wrapped with polyvinyl chloride film, and can be stored in cold storage in summer. Generally, such thin wood should not be stored for a long time. It should be used with the planer during production. It should not be stored. When it is exported, it should be properly dried to a moisture content of 20-30%. After anti-virus treatment, it is sealed and bundled with polyvinyl chloride film and splint.
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