Determine The Thickness for Peeling Veneer

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. The following is an explanation of this problem.

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 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 under the same operating essentials to select after peeling, before drying, after drying, and after gluing. Measure at the same measuring point in order 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 rotary cutting. This is due to the partial rebound of the rotary cut veneer, that is, the amount of compression that the veneer is compressed after being pressed by the pressure ruler during rotary cutting Some of them are compressed into elastic deformation, and will rebound after being placed for a period of time. After 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 expand 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 peeling machine must consider the processing allowance in the thickness of the plywood, the amount of compression during the hot pressing process, the shrinkage of the thickness during the drying of the veneer, and the thickness during the peeling. The thickness deviation of the veneer, and do not ignore the rebound and expansion of the thickness of the veneer.

When calculating the thickness of the veneer from the finished product, 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 during peeling. The shrinkage rate given in wood science is the shrinkage rate of the wood without compression, after compression. The shrinkage rate of wood and uncompressed wood is different. The determined shrinkage rate should be the dry shrinkage rate of the veneer after drying under the current process conditions of the dryer in our factory. 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 for the thickening of the veneer after peeling are the springback after peeling and the expansion after gluing. The amount of springback and expansion must be measured to master their values.

When determining the nominal veneer thickness (specified veneer peeling thickness), in addition to the finished product thickness machining allowance, compression, and dry shrinkage, the rebound and expansion of the veneer should be considered for the process after the peeling Take it into consideration, 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. for:

Nominal thickness of veneer = (finished thickness + surface processing allowance) / plywood layers + compression of each veneer-expansion of each layer of glued veneer thickness + veneer thickness in the dryer shrinkage -Rebound on the thickness of the veneer after peeling

When peeling a veneer, determine the feed amount according to the nominal thickness of the veneer (the feed amount refers to the distance that the rotary knife advances when the rotary cutter shaft rotates once). At the same time, consider the amount of compression by the pressure ruler during the peeling process. In other words, the nominal thickness of the veneer includes the amount compressed by the pressing ruler, and this pressing rate must meet the technological requirements. How to make the crush rate meet the requirements? In addition to strictly operating in accordance with the operating rules when installing the rotary knife pressure ruler, the pressing rate can also be corrected during rotary cutting. Correction method: ①When the knife door is opened, measure the unscrewed veneer to obtain the actual thickness of the unscrewed veneer without pressure rule compression. ②Measure the thickness of the peeled veneer when compressed by the pressing ruler, and then calculate the pressing rate. ③Compare the calculated pressing rate with the technological requirements, and then adjust the gap width of the knife. The amount of compression here can be understood as the difference between the amount of knife feed and the width of the knife door gap.

The rotary cutting machine determines the feed amount according to the nominal thickness of the veneer and the compression of the pressure ruler. However, due to various factors such as machine tool accuracy, equipment maintenance status and processing technology, the actual veneer thickness and the required nominal thickness of the veneer ( Excluding the compression of the veneer by the normal pressure ruler, there is always a deviation. In other words, the nominal feed amount and the actual board thickness (measured by opening the knife door) are inconsistent. When determining the feed amount, it should be adjusted according to the actual situation. That is to say, if the thickness of the unscrewed veneer is greater than the amount of the adjusted knife, the amount of the adjusted knife is smaller than the specified thickness of the veneer to meet the requirements. For example, if the nominal thickness of the veneer is required to be 1.20mm, it may be adjusted to 1.10mm when determining the feed amount to meet the requirement. The specific thickness is more or less than the nominal thickness. Different peeling machines have different rules. Generally, it takes many repeated trials to adjust. The adjustment of the actual feed amount should be done at the outer circle of the rotary wood section, that is, when the waste veneer is discharged, it should be adjusted to reduce the loss of wood.