Surface cracking refers to a longitudinal crack along t […]
Surface cracking refers to a longitudinal crack along the wood ray on the outer surface of the sawn timber (usually the outer cut surface of the string cutting board). It is caused by excessive surface stress in the early stage of drying. When the surface tensile stress is gradually decreased from the maximum value, the surface cracks also begin to shrink. If the crack is not too serious, it can be completely closed in the middle and late stages of drying, and it is not easy to see by the naked eye. Mild cracks have little effect on quality. However, when the paint is processed into finished products, the crack will penetrate into the paint and leave traces, which will affect the appearance. For members subjected to shear forces, the shear strength of the grain may be reduced. If the surface crack occurs too early in the kiln drying, it will affect the correct implementation of the kiln drying process and may develop into obvious drying defects. Therefore, the surface crack should be avoided as much as possible in the early stage of drying to maintain the integrity of the wood. The way to prevent cracking is that the pre-reference should not be too hard and pay attention to controlling the development of dry stress. The intermediate humidity control should be carried out when the initial moisture content is reduced by about 1/3 because the surface tensile stress may reach a maximum at this time. For thick or hard hardwood, the intermediate treatment should be carried out several times, and the moisture content can be reduced by about 10% or every 2 to 3 days to eliminate the surface tensile stress.