Basic knowledge of wood drying
Wood drying refers to the natural and artificial method of forcing the evaporation of water in the wood to reduce the moisture content of the wood to the moisture content of the use requirements, to avoid the erosion of the wet materials during the storage, transportation and use. The wood splits and deforms, improving the quality of wood products and prolonging the service life.
The natural drying method of wood is a drying method using gas convection. Among the artificial drying methods, convection heating and dielectric heating are important: the former has kiln drying, solar drying, suspicion of water drying and centrifugal drying; the latter has microwave drying and high frequency drying. There are also infrared drying methods for radiant heating and contact drying methods for contact heating. At present, convection heating air drying and kiln drying are the main methods at home and abroad, especially kiln drying is the most important method.
When the wood is dry, the moisture on the surface of the wood first evaporates. The moisture content of the surface layer is lower than that inside the wood, and the internal moisture moves to the surface layer under the influence of the water content gradient. The speed of the initial stage of drying depends on the rate of evaporation of the surface water of the wood. In the middle and late stages of drying, the drying speed depends on the speed of movement of the moisture inside the wood, which is inversely proportional to the thickness of the board and the specific gravity of the wood.
In addition, the sapwood drying speed is faster than the heart material. The chord panel of broad-leaved trees is faster than the diameter panel; however, the difference in the string surface of coniferous trees is not obvious. The tree species have a great influence on the drying: the easy-to-dry wood has red pine, fish scale spruce, fir fir, red spruce, masson pine, Chinese fir, scotch pine, elm, paulownia, elm, etc. The wood has poplar, color wood, maple, yellow pineapple, walnut, eucalyptus, camphor, eucalyptus, eucalyptus, persimmon, maple, etc.; hard-to-dry wood has larch, ash, Cyclobalanopsis, Cyclobalanopsis, palsy, lotus, alfalfa, Dalbergia, etc.
Why does wood have to be cooked, what is the role?
Chinese entry name: (wood) cooking treatment
Why the wood should be cooked, mainly because the wood can change the color of the wood after cooking; reduce the color difference of the wood heart and sapwood; maintain the natural luster of the wood; ease the initial moisture content gradient of the wood; reduce the probability of wood drying defects, at the same time, It is not easy to crack after cooking, and it also helps to prevent corrosion and moth.
Wood cooking process
The purpose of cooking is to soften the wood, increase the plasticity and moisture content of the wood, to reduce the cutting resistance during cutting, and to remove some of the oil from the wood and extracts such as tannin.
Generally, the cooking of the prescription is boiled. The cooking temperature and time should be controlled according to the hardness of the tree species and the wood, and the thickness of the sliced ​​wood. The temperature is higher when the hardness is high, and the cooking time is longer when the thickness is thicker.
Pay attention to the following points when cooking wood:
1 When the square is placed in the cooking pool, the ice temperature should preferably be kept at room temperature, so as to prevent the surgeon from suddenly cracking due to thermal expansion.
2 The opening temperature is 10-15 degrees Celsius/hour. If the temperature rises too fast, it will cause thermal stress in the wood, which will cause the wood to crack. When the temperature rises above 40 °C, the temperature should be slowed down.
3 Wood cooking should be carried out according to the tree species and diameter.
4 Excessive cooking will reduce the quality of the thin wood.
5 It is necessary to remove the oil, bark and muddy water in the cooking pool in time to avoid polluting the wood.
3 The cooked wood should be transferred to the warm water storage tank in front of the planer for cutting. The optimum temperature for cutting is 50-60 degrees. Excessive temperature will deform the planer, resulting in uneven thickness of the thin wood. , or make the surface of the thin wood fluff.
The following is an example of the cooking process of a wood section of a factory:
Ash: The water temperature rises from 15 °C to 50 °C, heats for 4 h, then heats up from 50 °C to 90 °C, the heating rate is guaranteed to be 2 ~ 3 degrees / hour, and then kept for 10 h.
Yellow Polo: The water temperature rises from 15 ° C to 70 ° C, and the heating rate is 4 to 5 ° C / h. Then, the water is naturally cooled and soaked for 24 hours.
Elm: The water temperature rises from 15 °C to 80 °C, and the heating rate is 4-5 °C/h. Then, the water is naturally cooled and soaked for 24 hours.
Elm: The water temperature rises from 15 degrees to 50 degrees, and the temperature is kept for 4 hours. The water temperature is further increased to 100C. The heating rate is 2 to 3 degrees/h, and the temperature is maintained for 15 to 20 hours. Then, the water is naturally cooled.
The following is an example of the three-stage cooking process of the ash wood section of a factory in the north:
winter:
1 After the wood is thrown into the pool, it is covered, drained, soaked for 6-8 hours, and the water temperature is maintained at 38-39 °C.
2 The first temperature is raised to 50-55 ° C, this time is 16 hours, and the heating rate is 0.5 ° C / hour.
3 Closed steam insulation for 16 to 18 hours to balance the temperature inside and outside the wood section.
4 The wood section is heated to 75-80 degrees for the second time. This period is 18 hours and the heating rate is 1 degree/hour.
5 Closed air insulation for 2 to 4 hours to balance the temperature inside and outside the wood section.
summer:
(The wood section is soaked for 4 to 6 hours after the pool is thrown. If it is rainy season, the time can be shortened.
2 The first temperature is raised to 50 to 55 degrees, and the heating rate is controlled at 1 ° C / hour.
3 The following steps are the same as above.
After the actual measurement, according to the above procedures, the temperature of the wood core can reach more than 45 degrees.
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