Wood is anisotropic and its shrinkage is complex. The shrinkage situation is not only different with the tree species, but also the same piece of wood. The longitudinal, chordwise, and radial shrinkage are also different. The longitudinal dry shrinkage is extremely small, the string shrinkage is the largest, and the radial shrinkage is about the string shrinkage. 1 / 3-1 / 2. Wood is made up of many long cells. Below the saturation point of the fiber, when the sorption water is reduced, the shrinkage of the length of the wood cell is not as large as the thinning of the cross section, so the longitudinal shrinkage is extremely small.
The reason why the chordwise dry shrinkage is greater than the radial shrinkage is:
( 1 ) The wood ray cell is its length in the radial direction, its end face in the chord direction, and the lateral shrinkage of the wood ray is larger than the longitudinal shrinkage;
( 2 ) The wood rays are arranged in the radial direction, which restricts the contraction of other longitudinal cells, and the string upwards are not used for such traction;
( 3 ) When some cells are dry, the chord direction is under pressure and the radial direction is slightly elongated;
( 4 ) The diameter of the wood is large and the number of pores on the cell wall is small, and the content of the cell wall is small, and the dryness is reduced; while the cell wall of the wood chord is small and small, and the content of the cell wall is large, and the shrinkage is large.
Due to the different shrinkage of wood in all directions, attention should be paid to the use of wood, longitudinal shrinkage (drying along the fiber direction) is extremely small, and no dry allowance is left in industrial production; They are all string panels, and there must be enough dry shrinkage when mixing.
In the drying process, due to the difference in the shrinkage ratio of the wood in the radial direction and the chord direction, the greater the difference between the two in the same moisture content stage, the greater the possibility of wood cracking. If the difference between the radial and chordwise shrinkage ratios is large at the beginning of drying, the wood will be cracked. When the difference between the two is relatively large in the middle and late stages of drying, the wood will be prone to internal cracking. In order to avoid the above phenomenon, the spraying process (preheating treatment, intermediate treatment, post treatment) must be taken in the drying process.
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