Analysis and control of common quality defects of plywood in March after the Spring Festival in 2024
Since the end of February and early March, most plywood factories have resumed production. Some plywood factories have begun to adjust and upgrade the structure of the plywood industry, improve production technology, control the production process, and effectively and continuously produce high-quality plywood products. Only by improving core competitiveness can we have a stable market. Our QC also analyzes and discusses the production technology, quality problems that are prone to occur during the production process, and production process control based on the current production practice of plywood, aiming to provide reference for importers and buyers in plywood procurement, production and process control.
1. Production process of plywood
At present, most of China’s plywood manufacturing enterprises do not produce veneers (surface and core panels) and adhesives, but directly purchase them. Therefore, the production process and production steps discussed in this article omit the process of cutting logs and rotating them into veneers, as well as the process of making adhesives. Figure 1 shows the production process flowchart of plywood.
2. Common quality defects analysis and control of plywood
2.1 Stacked core, gap
Stacking core refers to the phenomenon of adjacent two single-core veneers (or two parts of a cracked single-core veneer) overlapping each other within the same layer of plywood. Separation, also known as core separation, refers to the phenomenon of separation between the core veneer or adjacent two spliced core veneers within the same layer of plywood. Stacking core and separation are often encountered in the production of plywood, and should be analyzed and controlled from the following aspects: 1) When arranging the core veneers for assembly, there may be overlapping, misalignment, or excessive or insufficient gap reservation. These problems caused during core arrangement require strengthening the training and responsibility of the core arrangement operators, improving the technical level of the operators’ assembly skills, and ensuring that the combination of veneers during core arrangement is appropriate. 2) The core veneer may move or misalign during loading or moving of the board blank. In this case, it is necessary to place the glued core veneer for a period of time before assembly, and conduct pre-pressing after assembly to initially bond the board blank together. When moving the board blank, try to keep it stable. 3) The core veneer may be severely warped or deformed, or the edge of the core veneer may not be straight. In this case, before arranging the core veneers for assembly, it is necessary to perform softening and leveling treatment on the warped and deformed core veneer, and conduct secondary edge trimming to make the surface smooth and the edge straight.
2.2 Low bonding strength or adhesive layer delamination
Glue bonding strength refers to the firmness of the bond between individual veneers in a plywood. Glue bonding strength is one of the important indicators to measure the quality of plywood. In the production process, if there are quality problems such as low glue bonding strength or delamination of the glue layer, it should be analyzed and controlled from the following aspects: 1) The adhesive used has quality problems, such as poor quality, deterioration, or too low concentration. During the production process, the quality of the adhesive should be checked at any time, and the adhesive with poor quality or deterioration should not be used; if the concentration is too low, it can be adjusted by adding flour. 2) The glue application quality does not meet the process requirements, such as too little glue or uneven application. During the production process, it is necessary to choose the appropriate amount of glue according to the different species and thickness of the veneers, and pay attention to applying glue evenly and appropriately. 3) The moisture content of the veneers is too high or too low. Before applying glue, it is necessary to check the moisture content of the core veneer. After drying, the moisture content of the core veneer should be controlled within the requirements of the production process. 4) The storage time of the veneers after applying glue does not meet the requirements, such as too long storage time, resulting in dried glue during assembly and hot pressing; too short storage time, resulting in continuous glue film formation during assembly and hot pressing. In order to effectively penetrate the glue into the board and increase viscosity, it is beneficial to form a continuous glue layer during hot pressing, which can improve the bond strength. After assembly, it is necessary to store for 15-30 minutes before sending it to the next process, and the specific storage time after applying glue should be determined according to climate conditions. 5) There are quality problems in the veneers, such as deep burrs and grooves in the veneers, poor quality of the veneer rotation, etc. Veneer quality is an important factor affecting bond strength. Therefore, strict control should be exercised over the sorting and finishing processes of veneers to improve their shearing quality, so as to ensure that their quality meets the requirements of production processes. 6) The parameters of hot pressing process are unreasonable, such as insufficient hot pressing pressure, too low hot pressing temperature or short hot pressing time. Hot pressing process affects the quality of plywood. Therefore, appropriate parameters should be selected according to different products, and under the premise of meeting production requirements, hot pressing temperature and pressure should be appropriately increased or hot pressing time should be extended. 7) There are problems with the hot pressing equipment, such as failure to increase pressure or failure to transfer temperature properly. It is necessary to regularly inspect whether the hot pressing equipment is in good working condition, and any problems should be promptly repaired to ensure that the hot pressing machine is in good working condition.
2.3 Bubbles and local glue separation
Blistering refers to the bulge caused by gas inclusion on the surface of the product. Local delamination refers to the cracking of a part of the board due to poor bonding. For blistering and local delamination problems, the following aspects should be analyzed and controlled: 1) The moisture content of the veneer is too high or uneven. The moisture content of the veneer greatly affects the quality of the plywood. Too high or too low moisture content is not conducive to plywood production. Therefore, the moisture content of the dried veneer should be strictly controlled within the range of 8% to 14%. If the moisture content is too high, it is easy to debond and blister. 2) During gluing, there are blank spots or debris attached to the veneer or it is stained. In order to ensure the quality of gluing, the amount of glue should be uniform, and areas not glued should be manually brushed. Blank spots are not allowed, and areas with excess glue should be removed. For debris and contamination on the veneer, they must be cleaned up. 3) The hot pressing process is not reasonable, such as insufficient hot pressing time and too low local hot pressing temperature, resulting in insufficient resin condensation. The hot pressing process should be strictly controlled throughout the production process of plywood, and the hot pressing process parameters that meet production needs should be selected according to different products. 4) After hot pressing, the pressure reduction speed is too fast. During the production process of plywood, the speed of reducing pressure from equilibrium pressure to zero should be controlled during hot pressing and decompression. The pressure reduction speed during this stage should be slow, allowing the temperature of hot pressing to be appropriately reduced. If the pressure reduction is too fast, the water vapor in the board will be discharged too quickly, which is prone to blistering.
2.4 Glue penetration
Gel penetration refers to the defect of adhesive penetrating through the surface of the plywood to contaminate the board surface. The problem of gel penetration should be analyzed and controlled from the following aspects: 1) check the quality of the surface plate, such as excessive cracks and thin plate. In the production of plywood, the surface plate should not be too thin and should ensure a certain covering ability; if the cracks on the surface plate are too large, they need to be repaired. 2) The viscosity of the adhesive should be appropriate. 3) The hot pressing process is not reasonable, such as excessive hot pressing pressure. In production, if the pressure is too large, it will squeeze out the adhesive and penetrate into the board surface, so it is necessary to choose a suitable pressure in production to ensure that the glue flows reasonably between the slabs.
2.5 Warping
Warping is a concave phenomenon caused by the upwarping of the ends or the arching of the middle of the plywood. Warping of plywood is caused by high internal stress in the plywood. The warping problem should be analyzed and controlled from the following aspects: 1) asymmetry of the plywood structure. During production, the principle of odd-layer should be followed, and the tree species should be reasonably matched. The species, thickness, and moisture content of the two symmetrical layers from the middle layer should be consistent. 2) uneven moisture content of veneers. When drying veneers, natural drying should be carried out first, and after a certain degree of drying, they should be sent to a dryer for drying to control the distortion and deformation of the veneers during drying. During the drying process, the temperature at various points in the dryer should be monitored at any time to ensure that the moisture content of each part of the dried veneer is uniform, so as to improve the drying quality. 3) The setting of the hot pressing process should be reasonable. During normal production, the temperature of the upper and lower plates of the hot press should be close to ensure that the heating of the upper and lower plates is consistent.
2.6 Indentation on the board surface
Impression refers to the local indentation of the plywood surface caused by external factors. The problem of plate surface indentation should be analyzed and controlled from the following aspects:
The surface of the hot pressing pad itself is uneven, and the surface of the hot pressing pad is covered with glue blocks or debris. Before starting the hot pressing, the hot pressing pad should be inspected and cleaned in time; if the surface of the hot pressing pad is uneven, the hot pressing pad needs to be replaced.
2) There are glue blocks or debris on the surface after the surface is combined. After the surface is combined and during the hot pressing of the board, the board surface should be inspected for glue blocks or debris, and cleaned up in a timely manner.
2.7 Corner defects or glue failure
Corner defects refer to the phenomenon of missing or damaged corners or edges of products caused by mechanical or human misoperations. Corner delamination refers to the cracking of corners or edges of products due to weak bonding. This issue should be analyzed and controlled from the following aspects: 1) quality problems of core panels, such as insufficient length of core panels and missing corners or edges of core panels. The length of outsourced core panels should be strictly controlled to ensure that the length of core panels meets the production process requirements; for core panels with missing corners or edges, they should be handled in time during single-board shearing. 2) During the assembly operation, a right-angle reference edge should be used to assemble the blank according to the principle of “one side and one head in alignment”. 3) There are problems in hot pressing and loading, such as the misalignment of upper and lower board corners when loading the blank into the hot press, uneven pressure on the unevenly loaded blank, etc. Strengthen production management, improve operator skills and responsibility, and try to keep the position of the blank in each layer of the press plate as level as possible (i.e., aligned up and down and left and right) during hot pressing and loading to ensure uniform pressure on the composite plate. 4) Problems with hot pressing equipment, such as insufficient pressure caused by wear of the corners of the hot pressing plate, and low temperature caused by poor heat transfer at the corners due to poor heat transfer at the corners. Regular inspection of hot pressing pads is necessary, and if there is any wear at the corners, it should be replaced in time; regular inspection of the heat transfer oil path of the hot pressing plate is necessary, and if there is low temperature at the corners of the hot pressing plate, it is necessary to dredge the heat transfer oil path. 5) Quality problems of glue application, such as missing glue at corners, insufficient glue application, and premature drying.
2.8 Excessive formaldehyde emission
Formaldehyde emission refers to the phenomenon of continuous emission of formaldehyde gas to the outside during the manufacturing, stacking and use of wood-based panels made from urea-formaldehyde resin and other formaldehyde-based adhesives. This problem should be analyzed and controlled from the following aspects: 1) The formaldehyde content of the adhesive used exceeds the standard. When purchasing a batch of adhesives, manufacturers must conduct inspections and resolutely refuse to use adhesives with excessive formaldehyde content. 2) The amount of glue applied is too large and the application is uneven.
