Analysis and Control of Common Quality Defects of Plywood in China
At present, China is a big plywood exporter and the largest plywood producer in the world. However, on the one hand, China’s plywood products are frequently squeezed by foreign trade barriers, on the other hand, they are impacted by international advanced productive forces, facing huge challenges and threats from outside. It is necessary to adjust and upgrade the plywood industrial structure, improve production technology, control the production process, effectively and continuously produce high-quality plywood products, enhance core competitiveness, and then there can be a stable market. Based on the actual production of plywood in China, this paper makes a comprehensive analysis and discussion on the production technology of plywood, the quality problems that are easy to occur in the production process and the production process control, in order to provide a reference for the plywood production and process control of enterprises.
- Brief introduction of plywood production process and plywood production process
At present, most plywood manufacturers in China do not manufacture veneer (surface board and core board) and adhesives, but directly purchase them. Therefore, the production process and production procedures discussed in this paper omit the process from log cutting to veneer processing and glue making. Figure 1 shows the production process flow chart of plywood.
2 Analysis and Control of Common Quality Defects of Plywood
2.1 Core lamination and parting
Core lamination refers to the phenomenon that two adjacent core veneers (or two parts of a cracked veneer) overlap each other in the same layer of plywood. The gap, also known as core separation, refers to the separation between the same layer of inner core board or two adjacent splicing core boards in plywood. Core lamination and parting are often encountered in the plywood production meeting, which should be analyzed and controlled from the following aspects: 1) When arranging cores and assembling blanks, the core boards overlap, misplace or reserve gaps are too large or too small. For these problems caused by core arrangement and billet assembly, it is necessary to strengthen the training and sense of responsibility of the core arrangement operators, improve the technical level of the operators’ billet assembly, and ensure that the combination between veneer and veneer is just right during core arrangement. 2) The core plate moves or is dislocated when the plate is installed or the slab is moved. In this case, it is necessary to place the core plates coated with glue for a period of time before assembling. After assembling, pre pressing is carried out to initially bond the slabs together, and the slabs shall be moved as stably as possible. 3) The core plate is seriously warped or deformed, or the core plate edge is not straight enough. In this case, the warped and deformed core plates need to be softened and leveled before the core arrangement and billet assembly, and the secondary edge alignment should be carried out to make the surface flat and the edges straight.
2.2 Low bonding strength or degumming of adhesive layer
Bonding strength refers to the firmness of plywood between veneers. Bonding strength is one of the important indexes to evaluate the quality of plywood. In case of low bonding strength or degumming of adhesive layer during production, the following aspects should be analyzed and controlled: 1) The adhesive used has quality problems, such as poor quality, deterioration or too low concentration. During the production process, the adhesive quality shall be checked at any time, and the adhesive with poor quality or deterioration shall not be used; If the concentration is too low, consider adding flour for thickening adjustment. 2) The gluing quality does not meet the process requirements, such as too little gluing or uneven gluing. In the production process, it is necessary to select the appropriate amount of glue according to different tree species and different thicknesses of veneer, and pay attention to the appropriate and uniform amount of glue. 3) The moisture content of veneer is too high or too low. Before gluing, the moisture content of the core plate shall be checked. After drying, the moisture content of the core plate shall be controlled within the range required by the production process. 4) The storage time of the glued veneer does not meet the requirements, such as the storage time is too long, and the glue solution has dried during hot pressing of the preform; The aging time is too short to form a continuous adhesive film before hot pressing. In order to make the glue liquid penetrate into the board effectively and increase the viscosity, hot pressing is conducive to forming a continuous adhesive layer and improving the bonding strength. After assembling, the blank shall be stored for 15~30 minutes before being sent to the next process, and the specific storage time after gluing shall be determined according to the weather conditions. 5) The veneer has quality problems, such as too deep burr and groove marks, poor veneer peeling quality, etc. The veneer quality is an important factor affecting the bonding strength. Therefore, the veneer sorting and finishing process should be strictly controlled to improve the veneer shearing quality to ensure that the veneer quality meets the production process requirements. 6) The hot pressing process parameters are unreasonable, such as insufficient hot pressing pressure, low hot pressing temperature or short hot pressing time. The hot pressing process affects the quality of plywood. Appropriate hot pressing process parameters should be selected according to different products. On the premise of meeting production requirements, the hot pressing temperature and pressure should be appropriately increased or the hot pressing time should be extended. 7) There are problems with the hot pressing equipment, such as the pressure can not be increased, and the temperature transmission is not in place. Check whether the hot pressing equipment is in good condition. If there is any problem, repair it in time to ensure that the hot press is in good condition.
2.3 Bubbling and partial opening
Bubble refers to the bulge caused by gas on the product surface. Local glue opening means that a part of the board is not firmly bonded and cracks. The following aspects should be analyzed and controlled for blistering and local glue opening: 1) The moisture content of veneer is too high or uneven. The moisture content of veneer affects the quality of plywood to a large extent. Too high or too low is not conducive to plywood production. Therefore, the moisture content of dried veneer should be strictly controlled within the range of 8%~14%. If the moisture content is too high, it is easy to open the glue and bubble. 2) When gluing, there are some blank spots or the veneer is attached with sundries or stained. To ensure the quality of glue application, the amount of glue should be even. The parts not coated with glue should be manually brushed. No blank spots are allowed. Excess glue should be removed from the places where glue is stacked. The debris and dirt on the veneer must be cleaned. 3) The hot pressing process is unreasonable, such as insufficient hot pressing time and low local hot pressing temperature, resulting in insufficient resin condensation. The hot pressing process needs to be strictly controlled throughout the plywood production process. The hot pressing process parameters that meet the production needs must be selected according to different products. 4) After hot pressing, the pressure drop is too fast. In the plywood production process, the speed at which the balance pressure drops to zero during hot pressing and depressurization should be controlled. The depressurization speed at this stage should be slow, so that the hot pressing temperature can be properly reduced. If the depressurization is too fast, the water vapor in the board will be discharged too fast, which is easy to produce bubbles.
2.4 Gel penetration
Glue penetration refers to the defect that the adhesive penetrates into the plywood surface through the surface board and causes the board surface pollution. For the problem of glue penetration, the following aspects should be analyzed and controlled: 1) Check the surface plate quality, such as the surface plate crack is too large and the surface plate is too thin. In the production of plywood, the surface board shall not be too thin and shall have a certain covering capacity; If the surface plate crack is too large, it needs to be repaired. 2) The viscosity of adhesive shall be appropriate. 3) The hot pressing process is unreasonable, such as excessive hot pressing pressure. During production, if the pressure is too high, the adhesive will be squeezed out to the slab surface. Therefore, appropriate pressure should be selected to make the glue flow reasonably between slabs.
2.5 Warpage
Warpage is a concave phenomenon formed by the upwarping of both ends or the arching of the middle of plywood. Plywood warping is caused by high internal stress of plywood. The warping problem should be analyzed and controlled from the following aspects: 1) Asymmetric plywood structure. The production process should follow the principle of odd number of layers, the tree species should be reasonable, and the veneer species, thickness, and moisture content of the two symmetrical layers should be consistent from the middle layer. 2) The moisture content of veneer is uneven. When veneer is dried, it should be naturally dried first, and then sent to the dryer for drying after drying to a certain extent, so as to control that veneer will not be distorted during drying. During the drying process, monitor the temperature of each point in the dryer at any time to ensure that the moisture content of each part of the veneer after drying is uniform, so as to improve the drying quality. 3) The hot pressing process shall be set reasonably. During normal production, the temperature of the upper and lower platens of the hot press should be close to each other to ensure that the upper and lower platens are heated uniformly.
2.6 Plate surface indentation
Indentation refers to the local dent on the plywood surface caused by external factors. The plate surface indentation problem should be analyzed and controlled from the following aspects:
1) The surface of the hot pressing pad is uneven, and the surface of the hot pressing pad is stuck with rubber blocks or debris. Before hot pressing, check the hot pressing pad and clean it in time; If the surface of the hot pressing pad is uneven, replace the hot pressing pad.
2) Glue blocks or sundries are stuck on the rear watch panel. Check whether there are rubber blocks or sundries on the surface of the slab after the surface of the plain slab is closed and when the slab is hot pressed, and clean them in time.
2.7 Corner defect or glue opening
Corner defect refers to the loss or damage of corners or edges of products caused by improper mechanical or human operation. Glue opening at corners means that the corners of the product are not firmly bonded and crack. This problem should be analyzed and controlled from the following aspects: 1) Quality problems of core plate, such as insufficient length of core plate, edge and corner defects of core plate. The length of purchased core plates shall be strictly controlled to ensure that the length of core plates meets the production process requirements; The core board with edge and corner defects shall be timely treated during veneer shearing. 2) During the billet assembly operation, a right angle datum edge shall be used for the billet assembly according to the principle of “one end is flush”. 3) There are problems with the hot pressing plate, such as the upper and lower slab corners are not aligned when the slab is loaded into the hot press, and the slab is loaded askew and pressed unevenly. Strengthen production management, improve the skills and sense of responsibility of operators, and keep the position of slabs in each laminate as flush as possible (that is, align up and down, left and right) during hot pressing to ensure that the plywood is pressed evenly. 4) Hot pressing equipment problems, such as insufficient pressure caused by wear of corners of hot pressing plate, and low temperature caused by poor heat transfer of corners. Check the hot pressing pad frequently, and replace it in time in case of corner wear; Check whether the heat transfer oil path of the hot pressing plate is smooth. If the corner temperature of the hot pressing plate is low, clear the heat transfer oil path. 5) Glue coating quality problems, such as lack of glue at corners, little glue coating and premature drying.
2.8 Excess formaldehyde emission
Formaldehyde release refers to the wood-based panels made of formaldehyde series adhesives such as urea formaldehyde resin The phenomenon that formaldehyde gas is constantly emitted to the outside world during stacking and use [. This problem should be analyzed and controlled from the following aspects: 1) The formaldehyde content of the adhesives used exceeds the standard. The manufacturer should inspect each batch of adhesives when purchasing them, and once it is found that the adhesives with excessive formaldehyde content are not used. 2) The amount of glue is too large and the amount of glue is uneven.
