Several factors causing drying defects
2019-09-24 15:59:36 Hebei Shuowei Metal Manufactuering Co.,Ltd
—— Edit by Carlos Sheng
In the process of wood drying, there will be various defects, most of which can be prevented or alleviated.
Deformation is mainly determined by tree species, material grade and other factors;
Collapse orcorrugation easily occurs on the radial wood near the pith center of some tree species;
Cracking occurs at the end surface and surface cracking at the early stage of drying, and internal cracking occurs at the later stage of drying.
1) Initial cracking: There are two kinds of cracking in the early drying stage,
end cracking and surface cracking. Most of the cracks on the end face are caused by the growth stress and shrinkage of logs before lumbering. When the drying conditions are bad, new end cracks will occur, and the original cracks will further expand. Surface cracking extends from the end face of the wood to the interior. At the beginning of drying, the surface cracking of the plate with thickness less than 2 centimeters is severe, and the plate with thickness less than 1 centimeter has almost no cracking. The reason for surface cracking is that the surface shrinks after drying, but is subject to internal constraints. The related factors are drying conditions, shrinkage rate, difficulty of moisture movement and material resistance to deformation.Under the same drying condition, the greater the density of wood, the more likely it is to crack; the shrinkage in the width direction of tangential timber is about twice that of radial timber, so the tangential timber is prone to surface cracking.Drying temperature is closely related to surface cracking. It is easy to crack at low temperature of 0 - 5 C. The main reasons are low temperature, small water diffusion coefficient and large water content gradient. Therefore, when natural drying in winter, we should try our best to avoid exposing crackable wood to strong northerly wind.When wood is dried at temperatures above 50 ℃, surface cracking is also prone to occur. For the species prone to cell collapse, the surface cracking is easy to occur when the temperature difference between some wet bulbs increases sharply after slow drying at 60-75 C. Under certain temperature conditions, wood with small cell collapse but easy cracking tends to crack when it is dried at high temperature. Therefore, suitable drying conditions should be selected to prevent wood from cracking at the initial stage.
2) Collapse: The so-called collapse mainly refers to the abnormal deformation of wood caused by the extreme deformation of cells, which is the cause of the tension and compressive stress caused by the change of water content in the cell cavity. Generally, wood with high moisture content is prone to collapse at the early stage of drying if the temperature is too high. According to the different tree species, the bump and unevenness of the plate surface will appear in the part of collapse concentration. In order to avoid this defect, the trees with large collapse can be dried by air drying for a period of time or by low temperature.
3) Internal cracking: Drying thin plates with thickness less than 1 cm or using air-drying method will hardly cause internal cracking. Internal cracking is formed by the closure of surface cracking after the surface cracking develops inward. There are also cases where there are no surface cracks and only internal cracks occur.
The internal cracking of chordal timber occurs at the end of drying because the inner layer shrinks more than the surface along the width direction. Internal cracking is closely related to drying temperature. Generally, the initial drying temperature is low (about 50 C), and it is difficult for surface cells to collapse. However, the wood inner layer is heated for a long time under the condition of high moisture content. With the drying process, the drying temperature gradually rises and the cell collapse increases. Therefore, most thick plates are prone to internal cracking because of the prolonged internal heating time. In addition, if the temperature difference between wet and dry bulbs is large at the early stage of drying, the surface tension stress will be large, and if the internal cells collapse, internal cracking will easily occur.
4) Deformation: The main deformations of dried wood are transverse bending, longitudinal bending, distortion and warping. The main reasons are different shrinkage of different parts, difference of shrinkage between different tissues (such as wood ray, cellulose, heartwood) and local collapse.
(5) discoloration: wood discoloration occurs to varying degrees after drying, some of which are more serious. There are two kinds of discoloration:
One is caused by the reproduction of discolored fungi and decaying fungi; the other is caused by the acidification of the components in wood in hot and humid state.
Drying wood with high moisture content at high temperature often deepens or darkens the color of the wood, and sometimes darkens the surface of the wood due to excessive humidity during spray treatment or long-term non-cleaning of the drying chamber.
Comparaed with steam drying, our high frequecny vacuum dry kiln is more suitable for Large Section Wood and Hard Broad-leaved Wood
high-frequency heating is dielectric heating by increasing the temperature to evaporate water, which means that the molecule in wood rotates at a high speed and generates heat rapidly.
The wood is heated inside and outside uniformly, and then dries water quickly through vacuum negative pressure.the vacuum condition make the wood dried at low temperature.
If you are severe wood drying defects, welcome contact us, we are rich experience with wood drying, especially for high frequecny vacuum wood drying.
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