China’s plywood production and export volume is very large. The value of plywood is inseparable in practice. Therefore, it is necessary to further develop and study these issues. Unqualified materials exist in plywood due to material, manufacturing and other problems. Proactive measures need to be taken to solve this problem. Common defects such as lamination, parting, debonding and 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 productivity, and are facing huge 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 process, quality problems and process control of plywood in the production of solid wood in China, providing a reference for the production and process control of plywood.
1、 Brief Description of Plywood Production Process
Most plywood manufacturers do not produce veneer and adhesives, but directly purchase them. Therefore, the process and manufacturing process introduced in this paper do not include the process from cutting to peeling veneer. Figure 1 shows the manufacturing process of plywood.
2、 Current situation of glue mixing process management
- Enterprise leaders did not understand the importance of glue mixing process, nor did they realize that the quality of glue mixing process would affect the production and product quality of plywood.
- 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 raw rubber are taken as the basis for the type and use of the curing agent, the free formaldehyde content is the basis for adding the formaldehyde catcher.
- Improper addition of curing agent. The function of the curing agent is to reduce the pH value of the gelling agent under neutral or alkaline conditions. Accelerate the coupling speed of urea formaldehyde resin molecular chain, shorten the setting time of gel, and strictly control the use of curing agent, such as excessive use, short service life of adhesive, and even curing before hot pressing; If it is not used enough, it cannot act as a catalyst.
- The mixing quality does not meet the technical requirements. Apply – 4 cups of adhesive for testing before mixing to meet the viscosity required by the enterprise’s production process. However, some plywood manufacturers do not use equipment to test the viscosity agent. However, only visual inspection and manual contact test can determine whether the process requirements are met.
3、 Analysis and Control of Common Quality Defects of Plywood
- Quality control methods for core lamination and parting. Plywood lamination is an overlapping problem between two adjacent boards. Seam separation and core separation are the same, which refers to the separation of plywood core materials. The control process of this defect should start from the core arrangement stage and the assembly stage of the workpiece to avoid the core lamination from leaving the joint, and check whether the reserved joint of the core plate meets the requirements. The core arrangement and blank assembly shall have professional technical knowledge and a high sense of responsibility. The blank assembly shall be carried out after the core plate is coated for a period of time to avoid dislocation and movement. During the core plate, the core arrangement and blank assembly shall be treated to ensure that the flatness of the core plate meets the requirements.
- 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 amount of glue should be controlled scientifically to ensure uniform curing. For threshing caused by veneer moisture content, the best threshing method should be selected from the core moisture content. In practical application, it is necessary to prevent long-term deterioration in storage, control veneer quality, optimize hot pressing process and improve the quality of hot pressing equipment.
- Control the bubbling and opening method. Plywood blistering is due to the presence of gas on the surface of the core board, which is usually related to the problem of local gluing of the core board. 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 veneer. Manual operation shall be carried out to avoid gluing. Bubbling and opening during hot pressing shall start from the process flow diagram. Refer to the process drawing for specific operations to ensure that all relevant parameters meet the requirements. After the hot pressing is completed, maintain a slow decompression rate to reduce the possibility of bubbling.
- Quality control of glue penetration method. Defects are related to adhesives. Improper use of adhesives may cause plywood surface penetration and glue contamination. The inspection of quality defects shall start from the inspection of single panel to avoid excessive cracking and thinning of the panel. In addition, the consistency of the adhesive shall be monitored, and the adhesive concentration can be adjusted if necessary, starting from the hot pressing process to avoid cross problems caused by excessive pressure. Provide sufficient scientific basis for pressure control.
- Quality control warping. It is one of the main problems of plywood quality. It refers to the phenomenon that the left side at both ends of the plywood or the notch surface in the middle of the plywood is warped. The cause of warping is related to the internal stress of plywood. Deviation will occur when the internal stress is too high. To check this defect, you must first check the internal stress of the plywood. The plywood shall be manufactured in accordance with the principle of odd number of layers, the correct tree combination shall be selected, and the thickness and water content of plywood shall be controlled. If the moisture content of veneer is uneven, it may also cause leakage. The veneer shall be dry dry to ensure its durability 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 so that the upper and lower clamping plates can be heated evenly.
- Quality control method of plate surface indentation. Indentation is a quality defect visible to the naked eye. Plywood surface dominated by local dents. There are many methods to solve the dent problem, and only two representative methods are selected here. During operation, the operator should check the hot pad. If the surface is uneven, it can be judged that the hot pressing pad is affected by various external factors, such as rubber blocks caused by poor operation. Therefore, it is necessary to arrange professionals to inspect the hot pressing pad before opening it. If abnormal conditions are found, rubber blocks and fragments shall be removed in time. If it cannot be removed, the pressure pad must be replaced in time. In addition, other fragments of the bonding surface behind the rubber block must be handled in a correct way in time, otherwise it must be replaced.
- Quality assurance methods for corner defects. It is often directly caused 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 broken 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 correctly plan the size of the core plate, the length of the plate must match 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, the corresponding guidelines shall be followed, usually one end is flush. In the case of edge and corner defects, check whether the hot pressing 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.
- Plywood plays an important role in China’s manufacturing industry. With the development of social economy, the quality of plywood is constantly improving. With the increasing demand for plywood, formaldehyde in plywood production often becomes a problem that affects human health and the environment. Excessive formaldehyde emission will have adverse effects on people and the environment. Formaldehyde content of plywood should be reduced from material selection, use and storage. If the formaldehyde content of the adhesive exceeds the standard, it is prohibited to use the adhesive with excessive formaldehyde to avoid excessive formaldehyde in the production process. When purchasing adhesives, they should be tested according to relevant standards to ensure that the adhesive parameters are accurate before being 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 amount, so as 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 powerful country.
Plywood has been used in China for nearly 100 years. After decades of development, production conditions, technologies and capabilities have been greatly improved and developed. However, to become the first plywood producer in the world, there is still much work to be done in terms of product quality and standardization, which requires concerted efforts by the industry.
