Wood is a natural substance. Trees are often affected by various external factors during the growth process, resulting in abnormal wood structure. There are such unfavorable defects in painting, and the material changes caused by diseases such as diseases and insects (called parasitic Sexual defects), these are all natural defects in wood.
Common wood natural defects include knots, discoloration, insect eyes, cracks, and the like. These can be harmful to the painting, so they must be removed and remedied before painting.
Cracks are caused by changes in the external force or temperature and humidity of the wood, causing the fibers to separate from each other.
Wood cracking has the following forms: crack, internal crack, end crack and wheel crack. Below we briefly understand the causes of these types of cracking:
First, the table split:
Refers to surface cracks, which are cracks on the surface of the original wood or material. Cracks are usually limited to the chord plane and develop radially. When the wood is dry, the water is first evaporated from the surface. When the moisture content of the surface layer decreases below the fiber saturation point, the surface wood begins to shrink, but at this time, the moisture content of the adjacent inner layer wood is above the fiber saturation point, and no shrinkage occurs. The shrinkage of the surface wood is limited by the inner layer of wood and cannot be freely contracted, thus causing internal stress in the wood: the surface wood is pulled and the inner layer is pressed. The more severe the drying conditions, the greater the difference in moisture content between the inner and outer layers of wood, and the greater the internal stress generated. If the tensile stress of the surface layer exceeds the tensile strength of the wood grain, the wood structure is torn, and since the tensile strength along the wood ray tissue is smaller than that of the adjacent wood fiber, the crack is first generated along the wood ray.
Second, the internal crack:
That is, internal cracks. Internal cracks are also often referred to as honeycomb cracks. Internal cracking occurs in the late stages of drying and sometimes occurs during the storage of dry materials. It is usually not easy to find from the outside of the wood, but in severe cases, it can be judged by the depression of the surface. The internal crack is caused by the tensile stress of the inner layer of the wood.
Third, the end crack:
That is, the end face crack. The end crack is either limited to the end face of the wood or extends to one or both sides of the end, the latter being commonly referred to as splitting. The main reason is that the water conductivity in the direction of the grain is much larger than that in the grain direction. When the wood is dry, the evaporation of water from the end face is much faster than evaporation from the side. The end water content is lower than the middle portion, and the end portion shrinkage is restricted by the middle wood, so that tensile stress is generated at the end portion, and when the tensile stress exceeds the transverse tensile strength of the wood, the end surface is cracked.
Fourth, the wheel crack:
Such cracks develop in the direction of the growth wheel and often extend to adjacent growth wheels. The crack usually occurs at the beginning of the drying and appears on the end face of the wood. As the drying progresses, the crack deepens and lengthens. Sometimes occurs inside, but occurs in the late stage of drying due to severe internal tensile stress.
Solution to deal with wood cracking 1. Qualitative treatment with high temperature:
The method for reducing the internal crack of the wood can be qualitatively treated by high temperature, and the residual deformation of the surface layer of the wood which produces the internal crack can be eliminated by high temperature and high humidity treatment of the wood before the end of the drying process. During processing, the surface layer of the wood is compressed and deformed due to humidification expansion, which is offset by the original tensile residual deformation, and the internal crack of the wood is thus eliminated.
2, mechanical method crack prevention:
Bundle the end of the wire on the dried wood, use a crack-proof ring, a combination nail plate, etc., and mechanically force the wood not to expand and contract, so as to avoid cracking of the wood.
3, paint waterproof coating:
Apply a waterproof coating to the ends and surfaces of the wood to reduce the evaporation intensity of the wood surface, which can reduce the moisture content gradient inside and outside the wood, and also reduce the cracking of the wood.
4. Infusion treatment with water repellent:
A more effective method is to use a water repellent for pressure treatment, so that the waterproofing agent penetrates deeply into the wood to achieve a long-lasting good anti-cracking effect.
5. Ways to improve the sawing of the material:
Wood anisotropy, in the case of the same temperature and humidity changes, the maximum wet and dry shrinkage coefficient is the chord direction, followed by the radial direction, the longitudinal change is the smallest, and some cutting boards are produced during the sawing. Can reduce cracking. In particular, the cored sheet is prone to severe splitting when dried due to the difference in radial and chordwise shrinkage near the core. The best method is to avoid the production of cored sheets during the production process.
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