Wood is a natural material. Trees are often affected by various external factors during the growth process, resulting in abnormal wood structure, various defects that are unfavorable to painting, and material changes caused by diseases, insects, etc. (called parasitic) Sexual defects), these are natural defects of wood.
Common natural defects of wood include knots, discoloration, insect holes, cracks, etc. These will bring hazards to the painting, so they must be removed and remedied before painting.
Cracks are the phenomenon that the fibers of wood are separated from each other due to the influence of external force or temperature and humidity changes.
There are several forms of wood cracking: surface cracking, internal cracking, end cracking and wheel cracking. Let's briefly understand the reasons for these types of cracking:
1. Surface cracking:
Refers to surface cracks, and surface cracks refer to cracks on the log body or the surface of finished wood. Cracks are usually confined to the chord plane and develop radially. When the wood is dry, water evaporates from the surface first, and when the moisture content of the surface layer drops below the fiber saturation point, the surface wood begins to shrink, but at this time the moisture content of the adjacent inner wood is still above the fiber saturation point, and no shrinkage occurs. The shrinkage of the surface wood is limited by the inner wood and cannot shrink freely, so internal stress is generated in the wood: the surface wood is in tension, and the inner wood is in compression. 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 transverse grain, the wood tissue is torn. Since the tensile strength of the tissue along the wood ray is smaller than that of the adjacent wood fibers, the cracks are first generated along the wood ray.
2. Inner fissure:
That is, internal cracks. Inner fissure is also often called honeycomb fissure. Inner cracking occurs in the later stage of drying, sometimes during the storage period of dry materials. It is usually not easy to find from the outside of the wood, but in serious cases, it can be judged by the depression on the surface of the wood. Inner cracking is caused by the tensile stress of the inner layer of wood.
3. End fissure:
That is, the end face crack. End-cracking 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 of the wood along the grain direction is much greater than that of the cross grain direction. When the wood is dry, the water evaporates from the end face much faster than from the side. The moisture content of the end is lower than that of the middle, and the shrinkage of the end is limited by the wood in the middle, so tensile (tensile) stress is generated at the end. When the tensile stress exceeds the transverse tensile strength of the wood, the end surface cracks.
4. Wheel cracking:
This kind of crack develops along the direction of the growth ring, and often extends to several adjacent growth rings. Wheel cracks usually occur at the early stage of drying, appearing on the end face of the wood, and the cracks deepen and lengthen as the drying progresses. Sometimes it occurs inside, but it appears in the later stage of drying, which is caused by severe internal tensile stress.
Solutions for wood cracking
1. Using high temperature qualitative treatment:
The method of reducing internal cracking of wood can be treated qualitatively at high temperature. The tensile residual deformation of the wood surface that produces internal cracking can be eliminated by treating the wood with high temperature and high humidity before the drying process ends. During the processing, the residual deformation of the wood surface due to humidification and expansion will be offset by the original residual deformation of the extension, and the internal cracking of the wood will also be eliminated.
2. Mechanical crack prevention:
Bind the ends of the dried wood with wire, use anti-split rings, combined nail boards, etc., and use mechanical methods to force the wood not to expand and shrink, which can also prevent the wood from cracking.
3. Apply waterproof paint:
Apply waterproof paint to the end and surface of the wood to slow down 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. Impregnation treatment with waterproofing agent:
The more effective method is to use waterproofing agent for pressure treatment, so that the waterproofing agent can penetrate deeply into the wood, so as to achieve a long-lasting and good anti-cracking effect.
5. Improve the sawing method when making timber:
Wood is anisotropic. Under the same temperature and humidity changes, the chord direction is the largest coefficient of wet expansion and dry shrinkage, followed by the radial direction, and the longitudinal change is the smallest. Therefore, more quarter-sawn boards are produced when sawing. Can reduce cracking. Especially the board with pith core is prone to severe splitting when it is dry, which is caused by the difference in shrinkage in the radial and chord directions near the pith core. The best way is to avoid producing board with pith core when making lumber.
Dongguan Mumu Woodwork Co., Ltd. is a PRO provider of high-quality interior and exterior decorative materials in Dongguan, China.
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