3 core board processing and the influence of the blank
The core board has two forms of side glue and no glue. The blockboard made by sanding or planing the core board after side glue has a high surface flatness. At present, most domestic manufacturers still use the lateral non-glue process, which has the advantage that the stress generated by the core strip during dry shrinkage and swelling will disappear in the gap between the slats, and the bending deformation of the sheet is small. However, since the core plate which is not side-spliced ​​has no reworking of the side core plate, the dimensional tolerance of the core strip and the arrangement at the time of the blanking have a great influence on the flatness of the surface of the blockboard.
3.1 The effect of lateral rubber core board processing
The core side is a good process, which is very advantageous for reducing the unevenness of the surface of the blockboard, because the plane reworking after the side stitching ensures the flatness of the core board. However, it should be noted that the core board must be placed for 24 to 48 hours before being glued. Because the core strip absorbs the water in the adhesive, the local expansion will occur, the time is too short, the water content does not tend to be uniform, such as sanding or planing immediately, after the water content tends to be uniform, it will appear in the original expansion. The depression brings ripples to the pressed blockboard. In addition, after the surface processing of the core board, such as surface cracking, cracks, joints, blunt edges and other defects, necessary repairs must be made, otherwise it will cause unevenness of the board surface.
3.2 Influence of the core strip blank when the side is not glued
There are many ways to combine the sideless rubber core strips into a core sheet, but it is critical that the lateral gaps must be as small as possible. When the gap is large and the middle plate is thinner, the surface will have concave stripes. In addition, the gap between the end faces of the core strips should not be too large, otherwise the same situation will occur.
3.3 The situation of the blank
The attention should be paid to the placement of the middle plate in the blank of the back sheet, the middle plate and the core strip. Most domestic manufacturers use multiple pieces of matching medium plates. As with the plywood blank, if the laying of the middle plate produces a so-called core and separation phenomenon, the unevenness of the surface of the blockboard is generated.
4 The impact of hot pressing process
According to the production experience, the flatness of the blockboard produced by the cold pressing process is generally better than that of the blockboard produced by the hot pressing process. This is due to the large internal stress generated by the sheet under high temperature and high pressure. For modern industrial production, cold pressing certainly cannot meet the production requirements. Therefore, how to control the hot pressing process and produce blockboard with high surface flatness has become one of the important links in the production of blockboard.
4.1 Impact of pressure
In the production of wood-based panels, the high compression ratio not only reduces the utilization of wood, but also increases the internal stress of the sheet, making the sheet more susceptible to deformation. Under excessive pressure, the blockboard core board is over-compressed. During the placement or use after hot pressing, the core strip is deformed by rebound or hygroscopic expansion. This deformation may vary in size due to various reasons. The difference is that the result is uneven on the surface of the board. Therefore, under the pressure conditions required to ensure the bonding, the compression ratio should be as small as possible, that is, as small as possible. Structurally speaking, blockboard is a thick-core plywood with a small number of layers and easy to flatten without the need for higher pressure. According to the production experience, when the processing of the middle plate and the core strip are all as required, the pressure can be 0.4-0.6 MPa.
4.2 Influence of temperature
At high temperatures, the plasticity of the wood increases, and the layers of the sheet are easily flattened and compacted. The higher the temperature, the better the plasticity of the wood and the greater the compression ratio. At the same time, however, the problem of increased stress has arisen, and as a result, the same problems as those caused by excessive pressure will occur. The main purpose of the temperature in the hot pressing is to cure the glue layer, and the glue layer of the blockboard is close to the hot platen, and it is easy to reach the temperature required for curing. Therefore, the hot pressing does not have to use an excessively high temperature, and it is generally about 100 ° C.
4.3 Influence of time
During hot pressing, temperature, pressure, and time are related to each other. Under the same temperature and pressure conditions, the longer the time, the greater the compression ratio of the sheet. Excessive time will obviously not benefit the unevenness of the board. In actual hot pressing, the time control should be based on the temperature and pressure. According to research, if high flatness is required, the hot pressing of blockboard should adopt low-pressure delay process, because this process is beneficial to reduce the compressibility and internal stress of the board. When the temperature is 95 ° C and the pressure is 0.5 MPa, the time can be 8 to 10 min.
5 Conclusion
When it is necessary to have a higher flatness of the blockboard, the following measures are recommended.
(1) The material of the middle plate should be consistent, the surface is smooth and flat without large cracks and ruffles, and the thickness is between 1.5 and 2.2 mm. When the back sheet of the watch is thick, take a lower limit, and vice versa.
(2) The ratio of the width to the thickness of the core strip is 1:3 to 6, and the height of the wood with a small deformation is taken as a high limit, and vice versa.
(3) The processing precision of the core strip is ±0.3mm～±0.6mm, and the soft material takes the high limit, and vice versa.
(4) The material of the core strip should be made of wood with a density of 0.4 to 0.6 g/cm3 and a hard or slightly soft material. The core strip has a moisture content of no more than 12%. The processing of the core board is preferably a whole board process in which the side is spliced ​​and then sanded or planed.
(5) The hot pressing of the blockboard is preferably carried out by a low temperature and low pressure delay process. When the temperature is 95 ° C, the pressure is 0.5 MPa, when hot pressing
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