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Analysis and quality control of common quality defects of plywood

Analysis and quality control of common quality defects of plywood

Abstract: the production and export volume of plywood in China is very large. The value of plywood is inseparable in practice. Therefore, it is necessary to further develop and study these issues. Plywood is unqualified due to material, manufacturing and other problems. We need to take proactive measures to solve this problem. Common defects such as core lamination, seam separation, degumming and glue penetration are analyzed, and corresponding control measures are provided.

China is the largest producer of plywood. However, our plywood products are often affected by external trade barriers, which in turn are affected by international advanced productive forces and face enormous external challenges and threats. Only by adjusting and improving the industrial structure of plywood, improving the manufacturing technology, controlling the production process, upgrading and improving the production of high-quality plywood products, and improving the basic competitiveness can we have a stable market. This paper analyzes the production technology, quality problems and process control of plywood in the production of solid wood in China, providing reference for the production and process control of plywood.

1、 Brief introduction of plywood production process

Most plywood manufacturers do not produce veneers and adhesives, but directly purchase them. Therefore, the process and manufacturing process introduced in this paper are not included in the cutting to rotary cutting veneer process. Figure 1 shows the manufacturing process of plywood.

2、 Current situation of glue mixing process management

1. Enterprise leaders do not understand the importance of glue mixing process, nor do they realize that the quality of glue mixing process will affect the production and product quality of plywood.

2. Adjust the glue according to the quality of auxiliary materials, semi-finished products and finished products. If the pH value and curing time of the original glue are used as the basis for the type and use of curing agent, the content of free formaldehyde is the basis for adding formaldehyde capturing agent.

3. Improper addition of curing agent. The function of curing agent is to reduce the pH value of gelling agent under neutral or alkaline conditions. Accelerate the linking speed of urea formaldehyde resin molecular chain, shorten the setting time of gel, and strictly control the use of curing agent. If it is used excessively, the service life of the adhesive will be short, and it will even cure before hot pressing; If it is not used enough, it cannot act as a catalyst.

4. The glue mixing quality fails to meet the technical requirements. Apply – 4 cups of testing before dispensing the adhesive to meet the viscosity required by the production process of the enterprise. However, some plywood manufacturers do not use equipment to test viscosity agents. However, only visual inspection and manual contact test can determine whether it meets the process requirements.

3、 Analysis and control of common quality defects of plywood

1. Quality control methods for core lamination and seam separation. Plywood core lamination is an overlapping problem between two adjacent panels. Seam separation is the same as core separation, which refers to the separation of plywood core materials. The control process of this defect shall start from the core arrangement stage and assembly stage of the workpiece, so as to avoid core lamination and seam separation, and check whether the reserved seam of the core plate meets the requirements. The assembly of cores and billets shall have professional technical knowledge and a high sense of responsibility. The assembly of billets shall be carried out after the core plate is coated for a period of time to avoid dislocation and movement. The core plate shall be processed during the core plate assembly to ensure that the flatness of the core plate meets the requirements.

2. Degumming and quality control methods. Degumming is the firm adhesion of the core board. Only when the bonding strength reaches the standard can it enter the market. If the problem of plywood degumming is encountered, it may be caused by insufficient adhesion of plywood. In order to ensure the quality of plywood, it is necessary to control these problems. High quality adhesives can be selected from the beginning according to the requirements of adhesive concentration. The use of flour with adjustable glue concentration will cause degumming because the curing process does not meet the requirements. The glue amount should be controlled scientifically to ensure uniform curing. For the threshing caused by the moisture content of the veneer, the best threshing method should be selected from the moisture content of the core plate. In practical application, it is necessary to prevent long-term deterioration, control veneer quality, optimize hot pressing process and improve the quality of hot pressing equipment.

3. Control bubbling and glue opening methods. Plywood bubbling is due to the presence of gas on the surface of the core plate, which is usually related to the local glue opening problem of the core plate. In order to control the quality of these problems, distinguish the specific causes, control the water content of the single layer and keep it within a reasonable range, it is necessary to improve the process level and eliminate the dirt and impurities in the single plate. Manual operation shall be carried out to avoid gluing. Bubbling and opening during hot pressing shall start from the process flow chart. Refer to the process drawing for specific operation to ensure that all relevant parameters meet the requirements. After hot pressing, maintain a slow decompression rate to reduce the possibility of bubbles.

4. Quality control: glue penetration method. Defects are related to adhesive. Improper use of adhesives may cause plywood surface penetration and glue pollution. The inspection of quality defects shall start from the inspection of single panel to avoid excessive cracking and thinning of panel. In addition, the consistency of the adhesive should be monitored. If necessary, the adhesive concentration can be adjusted, and the hot pressing process should be started to avoid cross problems caused by excessive pressure. Provide sufficient scientific basis for pressure control.

5. Quality control warpage. It is one of the main problems of plywood quality. It refers to the upward warping of the left side of both ends of plywood or the notch surface in the middle of plywood. The cause of warpage is related to the internal stress of plywood. Deviation occurs when the internal stress is too high. To check this defect, the internal stress of the plywood must be checked first. The principle of odd layers shall be followed in plywood manufacturing, correct tree combination shall be selected, and the thickness and moisture content of plywood shall be controlled. If the moisture content of the veneer is not uniform, it may also lead to leakage. Veneer shall be dry dry to ensure the durability of veneer and avoid deformation during processing. It is also very effective to control the offset from the hot pressing process stage. The operator must control the temperature of the upper and lower clamping plates to uniformly heat them.

6. Quality control method of plate surface indentation. Indentation is a visible quality defect. Plywood surface mainly with local dents. There are many ways to solve the dent problem, and only two representative methods are selected here. During operation, the operator should check the hot press pad. If the surface is uneven, it can be judged that the hot press pad is affected by various external factors, such as rubber blocks caused by poor operation. Therefore, it is necessary to arrange professionals to check the hot press pad before opening it. If abnormal conditions are found, rubber blocks and debris shall be removed in time. If it can’t be removed, replace the pressure pad in time. In addition, other debris on the bonding surface behind the rubber block must be handled in time with correct methods, otherwise it must be replaced.

7. Quality assurance methods for corner defects. It is often caused directly by external forces and may be caused by improper work and machine movement. Of course, in practice, targeted measures should be taken for the reason of corner opening. If the adhesive is opened at the corners, the problem is whether the adhesive does not meet the requirements or whether the adhesive is used improperly. When the core plate has quality problems, there will also be corner defects. To properly plan the size of the core plate, the length of the plate must be consistent with the technical solution. In case of serious failure of the core plate, the core plate shall be cut in time to avoid affecting the next operation. When assembling billets, corresponding guidelines shall be followed, usually one side and one end. In case of corner defects, check whether the hot press mounting plate causes these errors. In this case, production must be properly managed to improve the sense of responsibility and quality of operators. The position of the hot press needs to be neat.

8. Plywood plays an important role in China’s manufacturing industry. With the development of society and economy, the quality of plywood is constantly improving. With the increasing demand for plywood, formaldehyde often becomes a problem affecting human health and the environment in plywood production. Excessive formaldehyde release will have adverse effects on people and the environment. The formaldehyde content of plywood should be reduced from the aspects of material selection, material use and storage. If the formaldehyde content of the adhesive exceeds the standard, it is forbidden to use the adhesive with formaldehyde exceeding the standard to avoid excessive formaldehyde in the production process. When purchasing adhesives, tests shall be carried out according to relevant standards to ensure that the parameters of adhesives are accurate before they can be put into use. In the production process, formaldehyde exceeded the standard due to unscientific materials. In order to optimize the production process and ensure the high glue content in the plywood production process, it is necessary to organize personnel with rich knowledge and experience to control the glue application amount to reduce the occurrence of quality problems. In order to greatly improve the uniformity of glue and fundamentally improve the production capacity of plywood in China. Reduce production costs, improve social and economic benefits, promote the development of China’s socialist construction, and achieve the goal of becoming a strong country.

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