Views:0 Author:Site Editor Publish Time: 2020-11-24 Origin:Site
Plywood is composed of veneers. When the veneer is peeled, the thickness of the veneer must be determined according to the thickness of the finished plywood. However, in actual production, there is a vague understanding of the determination of the thickness of the veneer, and even a misunderstanding. This issue is explained below.
The thickness of the veneer can be divided into the nominal thickness of the veneer, the actual peeling thickness of the veneer, the thickness of the remaining veneer, and the thickness of the veneer in the finished product.
When measuring the thickness of the veneer at each stage, in order to obtain accurate values, a dedicated person must use the same screw micrometer, and under the same operation method, after the veneer making machine, before veneer drying, after veneer drying, and gluing After that, select the same measuring point for measurement in order to measure the slight change 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 rotary cutting. This is due to the partial rebound of the rotary cut veneer, that is, the veneer is compressed after being pressed by the pressure ruler during the rotary cutting of the veneer making machine. Part of the compressed amount is compressed into elastic deformation, and rebounds after being placed for a period of time. After veneer drying, the size of the veneer is smaller than before drying due to the shrinkage of the veneer in the dryer (the moisture content of the veneer is controlled at 8%-12% after drying, and the size is basically unchanged during the placement process). After the veneer is glued, due to the adhesive carrying some moisture, the veneer will swell and increase in thickness. Compression during the hot pressing process reduces the thickness of the veneer. It can be seen from the above that after peeling, the veneer has undergone several changes in thickness.
Therefore, when determining the thickness of the peeling veneer, the veneer making machine must consider the processing allowance in the thickness of the plywood, the amount of compression during the hot pressing process, and the shrinkage of the thickness when the veneer is dried. The thickness deviation of the veneer during cutting, and the rebound and expansion of the veneer thickness should not be ignored.
When estimating the thickness of the veneer from the finished product forward, it is necessary to be clear here: the calculation of the compression is the thickness of the veneer after the glue is pre-pressed and glued, not the thickness of the veneer after drying. The calculation of shrinkage ratio should be careful not to use the moisture content to reach the shrinkage ratio under absolute dry conditions. One more thing to be clear about the drying of veneer is that the veneer is compressed by the pressure ruler when the veneer is peeled. The shrinkage rate given in wood science is the shrinkage rate of the wood without compression. The shrinkage of compressed wood is different from that of uncompressed wood. The determined shrinkage rate should be the shrinkage rate of the veneer after drying under the current process conditions of our dryer.
Because the drying process is different, the shrinkage rate is also different, it is best to verify the value of the shrinkage rate by field measurement. The factors contributing to the thickening of the veneer after peeling are the rebound after peeling and the expansion after gluing. The amount of rebound and expansion must be measured and mastered.
When determining the nominal thickness of the veneer (specified veneer peeling thickness), in addition to the thickness of the finished product, the amount of compression, and the amount of shrinkage, the rebound of the veneer should be considered in the process after the veneer making machine. And the expansion amount is taken into account, otherwise the determined nominal thickness of the veneer is thicker than required, which is easy to overlook.
After the above analysis, the nominal thickness of the veneer can be expressed as: Nominal veneer thickness = (finished thickness + surface processing allowance) / plywood layers + compression of each layer of veneer-expansion of each layer of glued veneer thickness +The thickness shrinkage of the veneer in the dryer-the rebound of the veneer thickness after peeling.