The plywood is composed of single veneers. When the rotary cutting machine rotates the veneers, the thickness of the veneer veneers is determined according to the thickness of the finished plywood. However, in the actual production, the determination of the thickness of the veneer has a fuzzy understanding and even a misunderstanding. The following is a description of this problem.
The thickness of the veneer can be divided into the nominal thickness of the veneer, the actual thickness of the veneer, the thickness of the veneer, and the thickness of the veneer in the finished product.
In the measurement of the thickness of the veneer at each stage, in order to obtain accurate values, a person should use the same spiral micrometer, under the same operation method, after the rotary cutting, before drying, after drying, after coating, select Measurements are taken at the same measuring point to measure small changes in the thickness of the veneer. It can be seen from the actual measurement results that the value measured before drying is slightly larger than that after the rotary cutting, which is due to the partial rebound of the rotary-cut veneer, that is, the compression of the veneer after being pressed by the pressure gauge in the rotary cutting, the amount of compression Some of them are compressed into elastic deformation, and will rebound after being placed for a while. The size after drying, because the veneer is shrinking in the dryer, the thickness is made smaller before drying (the moisture content of the veneer is controlled to 8% to 12% after drying, and the size is basically unchanged during the placement process). After the veneer is glued, the veneer expands and the thickness increases due to the partial moisture of the adhesive. Compressed during hot pressing, which reduces the thickness of the veneer. It can be seen from the above that after the rotary cutting, the veneer has undergone several changes in thickness.
Therefore, when determining the thickness of the rotary cutting veneer, the rotary cutting machine should consider the machining allowance on the thickness of the plywood, the compression amount during the hot pressing process, the dry shrinkage in the thickness of the veneer when drying, and also consider the time of the rotary cutting. Veneer thickness deviation, and do not ignore the amount of rebound and expansion on the thickness of the veneer.
When calculating the thickness of the veneer from the finished product, it is necessary to clarify here that the calculation of the compression amount is the thickness after the veneer is pre-pressed and coated, and the thickness of the veneer is not the thickness of the veneer after drying. The calculation of the shrinkage rate should be careful not to use the moisture content to achieve the dry shrinkage rate under dry conditions. It is also clear that the drying of the veneer is that the veneer is compressed by the pressure bar during the rotary cutting, and the shrinkage rate given in the wood science is the shrinkage rate of the wood without compression, and is compressed. The shrinkage rate of wood and uncompressed wood is different. The determined shrinkage rate should be the shrinkage rate of the veneer after drying under the current process conditions of the dryer.
Because the drying process is different, the shrinkage rate is also different. It is best to verify the dry shrinkage value by field measurement. The factors for the thickening of the veneer after the rotary cutting are the rebound after the rotary cutting and the expansion after the rubber coating. The amount of rebound and the amount of expansion should be measured to grasp the value.
When determining the nominal thickness of the veneer (the specified veneer thickness of the veneer), in addition to considering the thickness of the finished product, the amount of compression, and the amount of dry shrinkage, the process of rewinding and expansion of the veneer Considering it, otherwise the nominal thickness of the board is determined to be thicker than required, which is easy to ignore.
After the above analysis, the nominal thickness of the veneer can be expressed. To: nominal thickness of veneer = (finished product thickness + surface processing allowance) / number of plywood layers + compression of each veneer - the amount of expansion on the thickness of each veneer veneer + the thickness of the veneer in the dryer Shrinkage - The amount of springback on the thickness of the veneer after spin cutting.
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