What causes cracking during wood drying process？
Wood will crack if it is not careful in the process of drying wood, mainly due to internal cracking and surface cracking in the process of drying. What is the reason for these phenomena?
1. Internal cracking
for thin plates with a thickness of less than one centimeter or by air drying, internal cracking will hardly occur. Internal cracking is formed after the surface cracking develops inward and the surface cracking is closed. There are also cases where there is no surface crack and only cracking occurs inside.
the internal crack of chord-wise wood occurs at the end of drying because the shrinkage of the inner layer along the width direction is greater than that of the surface. Internal cracking is closely related to drying temperature. Generally, the temperature of wood drying equipment at the initial stage of drying is low, that is, about 50 ℃, and it is difficult for surface cells to collapse. However, the inner layer of wood is heated for a long time under the condition of high moisture content. With the progress of drying, the drying temperature of wood drying equipment increases gradually, and the cell collapse increases. Therefore, most thick plates are prone to internal cracking due to the extension of internal heating time. In addition, if the dry wet bulb temperature difference is large at the initial stage of drying, the surface tensile stress will be large. In addition, if the internal cells collapse, it is also easy to produce internal cracking.
2.Surface cracking
The reason for surface cracking is that the inner and outer layers of wood are unevenly dried, and the radial and chord directions are different the difference in contraction is an important additional factor. When wood is dry, it is first steamed from the surface when the moisture content of the surface layer decreases below the fiber saturation point, the surface wood opens at this time, the moisture content of the adjacent inner wood is still above the fiber saturation point, no shrinkage occurs.
The shrinkage of surface wood is limited by the inner wood and cannot be harvested freely shrinkage, resulting in internal stress in the wood: the surface wood is pulled and the inner wood is compressed. The more intense the drying conditions, the greater the difference in moisture content between the inner and outer layers of wood, and the greater the internal stress the bigger.
If the tensile stress of the surface layer exceeds the transverse tensile strength of the wood, the wood structure torn, because the tensile strength of the tissue along the wood ray is lower than that of the adjacent wood fibers,therefore, cracks first occur along the wood ray.
Treatment measures to prevent wood cracking
1. Mechanical crack prevention
Tie the end of the dry wood with iron wire, and force the wood not to expand or shrink by mechanical means such as anti crack ring and combined nail plate, so as to avoid wood cracking.
2. Improve the sawing method when making wood
Wood anisotropy. Under the same temperature and humidity change, the largest coefficient of wet expansion and dry shrinkage is in the chord direction, followed by the radial direction, and the longitudinal change is the smallest. Cracking should be reduced when sawing. In particular, the board with pith core is prone to serious splitting during drying, which is caused by the difference between radial contraction and chord contraction near the pith core. The best way is to avoid making the board with pith core when making wood.
3.High frequency vacuum wood drying
High frequency vacuum wood dryer kiln adopts high frequency medium heat-ing and vacuum dehydration technology.
High frequency is also called radio frequency, it is a kind of direct heating. The principle is using the wood as a medium, capacitor is a group between two pieces of metal plate.Then give them voltage, the electric field produced between the two plates. The water molecules between electrodes will be polarized by the alternating magnetic field and move in very high speed rapidly. The impact and friction (billions of times per second) among water molecules creates heating in very short time. It is kind of directly heating,this continuous effect let whole wood heated surface to core wholly and uniformly.
So the wood will be shrink at the same time. That will avoid the wood surface cracking. That is also one of the reason why the high frequency vacuum machine is 10 times faster than conventional wood drying kiln our kiln also adopts vacuum dehydration technology. The stainless steel vacuum pump can make the vacuum oven reach -0.093mpa condition.
The vacuum condition is to make water vaporized in low temperature below 55 ℃,at the same time, vacuum can also suck the wood moisture out. The machine cooling system will separate the hot air and hot water out, then cool them and release them out. Wood is drying under a vacuum and wet condition at low temperature. That will ensure wood won’t internal cracks during wood drying process.
