Plywood production problems? The countermeasures you want to know are here!
Guide: China’s plywood production is in full swing, but the quality of plywood on the market is mixed. Many small and medium-sized enterprises are lack of production experience and can not effectively control the problems in plywood production. To improve the quality of plywood, we should know the possible defects in plywood production, and prevent and deal with them in time. The rapid growth of China’s economy, the influx of fast-growing and high-yield forest timber, and the constant supplement of high-quality broad leaved timber from abroad have promoted the rapid development of China’s plywood industry. At present, China is not only a big plywood exporter, but also the largest plywood producer in the world. The so-called plywood refers to a multilayer (odd number) board formed by peeling or planing logs into veneer, gluing and gluing according to the direction of adjacent layers of wood grain perpendicular to each other. It is one of the three major man-made boards. Common plywood types include plywood, plywood, etc. China’s plywood production is in full swing, but the quality of plywood on the market is mixed. Many small and medium-sized enterprises are lack of production experience and can not effectively control the problems in plywood production. Before introducing the common defects and countermeasures of plywood, let’s briefly review the production process of plywood. 1、 Plywood production process 1. General process flow At present, most plywood manufacturers in China choose to purchase veneer and adhesive, so the production process mainly includes gluing, forming, cold and hot pressing, plywood, edge trimming, sanding, etc. ▲ Plywood production process flow chart 2. Basic principles to be followed: plywood production follows two basic principles, one is symmetrical, the other is vertical. The principle of symmetry requires the production of plywood to use odd layers of veneer, which is symmetrically arranged on both sides of the central layer or the board core. The two veneers in the symmetrical position should be the same in material, thickness, fiber direction and moisture content. The panel and back panel are allowed to be different from each other. The principle of vertical type refers to that the veneers are arranged in a crisscross manner in the direction of wood grain when assembling, so as to ensure that the fibers of adjacent veneers are perpendicular to each other. The quality of plywood in China is uneven, largely because the manufacturers can not effectively prevent and solve the plywood quality defects in the production process. What defects might plywood have? The minor editor sorted out common defects and countermeasures, including four categories and eleven points, as follows. 2、 Structural defect Structural defect refers to that the content integrity and layout rationality of the internal and external structures of plywood cannot meet the requirements, mainly including plywood warping, core lamination, parting and corner defects. 1. Warpage is a concave surface formed by the upwarping of both ends of plywood or the arching of the middle. Plywood warping is mainly caused by the large internal stress of plywood due to the fact that the plywood structure and processing technology do not conform to the principle of symmetry. Therefore, the measures to deal with plywood warpage are to produce plywood in strict accordance with the principle of symmetry:. (2) To improve the drying quality, the veneer shall be naturally dried to a certain extent before being sent to the dryer to control its distortion during the drying process. (3) During the hot pressing process, the temperature of the upper and lower platens of the hot press shall be the same to ensure that the upper and lower platens are heated uniformly. 2. Core lamination and split core lamination refer to the overlapping of two adjacent core veneers (or two parts of a cracked veneer) 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. The causes of core lamination and gap separation are mainly as follows: (1) When assembling, the core plates are overlapped, misaligned or the reserved gap is inappropriate. (2) The core plate moves or is dislocated when the plate is installed or the slab is moved. (3) The core plate is seriously warped or deformed, or the core plate edge is not straight enough. For the first two reasons, as long as the operator is careful in assembling and moving, the defects can be largely avoided. Or, after the veneer is glued, it can be stored for a period of time, and then the veneer is assembled and pre pressed, so that the veneer is initially adhered to prevent displacement and dislocation during handling. In the third case, the warped core plate can be softened and leveled first, and the edges can be aligned twice to make the surface flat and the edges straight, and then the treated veneer can be used for core arrangement and blank assembly. 3. Corner defect Corner defect refers to the loss or damage of corners or edges of the product caused by improper mechanical or human operation. There are many factors causing corner defects: (1) raw materials have defects: poor quality and insufficient length of core plates; Surface plate edge defect. (2) Non conformance in the operation process: the principle of “one end at one side” was not strictly followed during the assembly of billets; When the slab is loaded into the hot press, the upper and lower slab corners are not aligned, and the slab is loaded askew and pressed unevenly. (3) Equipment failure: poor heat transfer of hot pressing equipment and low corner temperature. To deal with the problem of corner defects, first of all, we should control the quality of the raw materials of the purchased boards. Second, we should strictly follow the operating procedures during the operation. In addition, regular maintenance of the equipment is also essential. 3、 Gluing defect Gluing defect refers to the defect of the adhesive itself or the defect of loose adhesion, blistering and glue opening caused by the operation non-compliance during the sizing process. 1. Low bonding strength refers to the firmness of bonding between veneers in plywood. Bonding strength is one of the important indexes to evaluate the quality of plywood. ▲ Analysis of plywood strength test equipment The causes of low bonding strength or glue layer degumming can also be analyzed from three aspects: raw materials, process and equipment: (1) raw materials have defects: the purchased veneer has quality problems, such as poor rotary cutting quality or deep burr grooves; The moisture content of veneer is not appropriate; The adhesive has poor quality, deterioration or too low concentration. (2) Non conformance in the operation process: the gluing process is not up to standard, the amount of glue is too small or the gluing is uneven; The storage time of glue coated veneer is too long or too short; The hot pressing parameters are not set properly, such as insufficient hot pressing pressure, too low hot pressing temperature or short hot pressing time. (3) Equipment failure: for example, the pressure cannot be increased and the temperature cannot be transmitted in place. The solution is to control the quality of raw materials, strictly control the process quality and regularly repair and maintain the equipment from these three aspects. 2. Bubbling, glue opening and bubbling refer to that when the pressure is reduced, the steam in the slab destroys the structure of the plywood, forming a raised bubble on the board surface, sometimes accompanied by a loud noise. If the slab is completely damaged in the adhesive layer, it can be regarded as glue opening. The generation of blistering is mostly related to the moisture content of veneer. The moisture content of veneer affects the quality of plywood to a large extent. Too high or too low is not conducive to the production of plywood. If the moisture content is too high, it is easy to blister; The nonstandard sizing process and hot pressing process will also lead to blistering. ▲ The vacuum drying equipment shall take the following measures for this defect: (1) The moisture content of the dried veneer shall be strictly controlled within the range of 8%~14%. (2) When applying glue, pay attention to apply it evenly. Neither empty white spots nor glue accumulation is allowed. At the same time, clean up the debris on the veneer surface. (3) Reasonably set the time and temperature of hot pressing, including the speed of pressure reduction and temperature reduction after hot pressing. 3. Corner opening refers to the phenomenon that the edges and corners of plywood are not glued together. The causes of such defects are: (1) material defects: poor adhesive quality; The veneer thickness is uneven. (2) Process defect: lack of glue at corners; The aging time of glue coated veneer is too long, and the edges and corners are dry and lose their activity; The edges and corners of the slab are not aligned, and the slab is skewed and pressed unevenly during loading. (3) Equipment defect: deformation of pressing plate; The temperature of pressing plate surface is uneven. ▲ For the hot press, in order to avoid the defect of glue opening at the corners, it is necessary to use high-quality veneer and adhesive first; Secondly, in order to ensure the quality of gluing, the amount of gluing should be even. The parts not coated with glue should be manually brushed. When gluing, attention should be paid to the corners to prevent the corners from lacking glue, the amount of gluing is small or the glue dries up prematurely; Finally, check the equipment before hot pressing and eliminate the fault. 4、 Appearance defect Appearance defect refers to the defect that can be perceived on the plywood surface through visual observation and hand touch, including glue penetration, unevenness, indentation and discoloration. 1. Glue penetration refers to the defect that the adhesive penetrates into the plywood surface through the surface board and causes the board surface pollution. The causes of glue penetration are: (1) adhesive problem: too thin glue; Excessive sizing. (2) The watch board is too thin; The surface crack is too large. (3) Hot pressing process problem: The hot pressing pressure is too large, extruding the adhesive to the plywood surface. For the above reasons, the glue should be adjusted reasonably to control the thinness and consistency of the glue solution; Check the quality of gauge plate; Select the appropriate hot pressing pressure. 2. Plywood unevenness Plywood unevenness, that is, the thickness of the board is different, is easy to cause the thickness of the plywood to exceed the allowable tolerance range. The reasons for the different thickness of plywood may be: (1) the thickness of veneer is different; (2) Too many boards are pressed at one time; (3) Inclination of pressing plate; (4) The hot pressing pressure calculation is incorrect. The countermeasures are to select high-quality veneer, adopt the hot pressing process of “one piece and one press”, correct the press installation to the level, and correctly calculate and adjust the hot pressing pressure. 3. The indentation indentation refers to the local dent on the plywood surface caused by external factors. The indentation of the plate surface mainly occurs in the hot pressing process. First, the surface of the hot pressing pad is uneven, or there are adhesive blocks and other sundries on the surface. Second, there are glue blocks and other sundries on the back surface of plywood joint. For this defect, check carefully whether there are sundries on the hot pressing pad and the surface plate when assembling the hot pressing plate. If any sundries are found, clean them in time. 4. Discoloration of plywood refers to abnormal color of plywood surface due to contamination. If the plywood is not stored properly, it will change color, which will directly affect the quality and service life of the board. The color pollution of the board has always been one of the problems concerned by manufacturers and technicians. There are many ways of color pollution on the board surface, which can be basically attributed to fungal discoloration and chemical discoloration. Iron ion pollution is the most common chemical discoloration. Fungal discoloration is mainly caused by fungus contamination of the planing logs of plywood during storage. Under the appropriate temperature and humidity environment, fungi grow and develop longitudinally along the vessels of logs, and logs deeply polluted by fungi will appear discoloration and decay. Iron pollution is due to the color reaction of tannin contained in logs when encountering iron ions, forming blue black pollution on the surface of wood. For fungal pollution, as long as the fungal growth environment is damaged, it can be effectively prevented. The suitable environment for fungus growth is: (1) temperature: 22~25 ℃; (2) Humidity: log moisture content 20%~60%; (3) Oxygen: fungus is an aerophilic fungus, which is suitable for growing in the tube hole containing air; (4) Nutrition: lignin, cellulose, monosaccharide and starch in wood are all nutrients of fungi; (5) Acidity and alkalinity: fungi are suitable for weak acid medium, pH 4.5~5.5. Looking at the above five conditions, the humidity condition is the easiest to control and destroy, so controlling the moisture content of veneer is the most economical way to prevent fungal pollution. For iron ion pollution, it is necessary to control the contact of plywood with iron ion during production, storage, transportation and use: (1) softened water is used during production; (2) Avoid long-term contact between plywood and cement, which contains a lot of iron ions; (3) Clean the fine iron filings on the surface of the machining tool in time; (4) Avoid using adhesives contaminated by iron ions. 5、 Formaldehyde emission exceeds the limit. Formaldehyde emission exceeds the limit of the national standard. This is a very troublesome problem in plywood production. Once formaldehyde emission exceeds the standard, it is difficult to remedy it. Plywood produced with “three formaldehyde resin” series adhesives, such as urea formaldehyde resin, phenolic resin and melamine formaldehyde resin, will continuously volatilize free formaldehyde from the outside during manufacturing, stacking and use. Formaldehyde emission exceeds the standard for two reasons: (1) Formaldehyde content of adhesives exceeds the standard; (2) Too much sizing and uneven sizing. For the first reason, the manufacturer should strictly control the purchased adhesives, strictly test each batch of new adhesives, and refuse to use unqualified adhesives; For the second reason, it is necessary to strengthen the training of operators to improve their technical level and sense of responsibility. 6、 In conclusion, we introduced the common defects and countermeasures in plywood production from four aspects: structural defects, bonding defects, appearance defects and formaldehyde emission exceeding the standard. During the production process, possible problems should be prevented in advance, and all measures should follow the principle of “prevention first and prevention combined”.
