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Analysis and control of common quality defects of plywood

Analysis and control of common quality defects of plywood

At present, China is a big exporter of plywood and the largest producer of plywood in the world. However, on the one hand, China’s plywood products are frequently squeezed by foreign trade barriers, and on the other hand, they are impacted by international advanced productive forces. They are facing great challenges and threats from outside. It is necessary to adjust and upgrade the plywood industrial structure, improve production technology, control the production process, produce high-quality plywood products continuously and stably, and enhance core competitiveness. Only in this way can there be a stable market. Based on the production practice of plywood in China, this paper comprehensively analyzes and discusses the production technology of plywood, the quality problems that are easy to occur in the production process and the control of the production process, so as to provide reference for the production and process control of plywood in enterprises.

(1) brief description of plywood production process

At present, most plywood production enterprises in China do not produce veneers (veneers and core boards) and adhesives, but directly outsource them. Therefore, the production process and production process discussed in this paper omit the process from log cutting to veneer processing by rotary cutting and glue making process. Fig. 1 is a flow chart of plywood production process.

(2) analysis and control of common quality defects of plywood

2.1 core lamination and seam separation

Laminated core 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. Split joint, also known as split core, refers to the separation between the same layer of inner core plate or two adjacent spliced core plates in plywood. Core lamination and seam separation are often encountered in plywood production, and should be analyzed and controlled from the following aspects: 1) core plate overlap, dislocation or reserved gap is too large or too small when arranging cores and assembling billets. For these problems caused by core arrangement and billet assembly, it is necessary to strengthen the training and responsibility of core arrangement operators, improve the technical level of operators in billet assembly, and ensure that the combination between veneers and veneers is just right during core arrangement. 2) The core plate moves or dislocates during loading or slab moving. In this case, it is necessary to put the core plate coated with glue for a period of time before assembling the slab. After assembling the slab, pre press it first to make the slab initially bonded together, and keep it as stable as possible when moving the slab. 3) The core plate is seriously warped or deformed or the edge of the core plate is not straight enough. In this case, the warped and deformed core plate needs to be softened and leveled before core arrangement and blank assembly, and the secondary edge alignment needs to be carried out to make the surface flat and the edge straight.

2.2 low bonding strength or degumming of rubber layer

Gluing strength refers to the firmness of gluing between veneers in plywood. The gluing strength is one of the important indexes to measure the quality of plywood. In case of quality problems such as low bonding strength or degumming of the adhesive layer during production, the following aspects should be analyzed and controlled: 1) there are quality problems with the adhesive used, such as poor quality, deterioration or low concentration of the adhesive. During the production process, the quality of the adhesive should be checked at any time, and the adhesive with poor quality or deterioration must 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 veneers, and pay attention to the appropriate and even 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, and after drying, the moisture content of the core plate shall be controlled within the range required by the production process. 4) The aging time of glued veneer does not meet the requirements. For example, if the aging time is too long, the glue liquid is dry when the blank is hot pressed; The aging time is too short to form a continuous adhesive film before hot pressing. In order to effectively infiltrate the glue into the plate and increase the viscosity, it is beneficial to form a continuous glue layer and improve the bonding strength during hot pressing. After the blank is assembled, it should be aged for 15-30 minutes before being sent to the next process. The specific aging time after gluing should depend on the weather conditions. 5) The veneer has quality problems, such as too deep burr and groove marks, and poor veneer rotary cutting quality. The veneer quality is an important factor affecting the bonding strength, so 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, too low hot pressing temperature or short hot pressing time. The hot pressing process affects the quality of plywood, so the appropriate hot pressing process parameters should be selected according to different products. On the premise of meeting the production requirements, the hot pressing temperature and pressure should be appropriately increased or the hot pressing time should be extended. 7) There are problems in the hot pressing equipment, such as the pressure cannot be increased and the temperature cannot be transferred in place. It is necessary to frequently check whether the hot press equipment is in good condition. If there is any problem, it should be repaired in time to ensure that the hot press is in good working condition.

2.3 bubbling and local glue opening

Bubbling refers to the bulge caused by the gas contained on the product surface. Local glue opening refers to the cracking of a part of the board due to the lack of firm bonding. Bubbling and local glue opening should be analyzed and controlled from the following aspects: 1) the moisture content of veneer is too high or uneven. The moisture content of veneer affects the quality of plywood to a great 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) During gluing, there are local blank spots or sundries or stains on the veneer. In order to ensure the gluing quality, the gluing amount shall be uniform. The parts that have not been glued shall be brushed manually. No blank points are allowed. The excess glue shall be removed from the places where the glue is accumulated. The sundries and sticky dirt on the veneer must be cleaned up. 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 depressurization speed is too fast. In the plywood production process, the speed of pressure reduction from equilibrium pressure to zero during hot pressing should be controlled. The pressure reduction speed at this stage should be slow, so that the hot pressing temperature can be properly reduced. If the pressure reduction is too fast, the water vapor in the plate is discharged too fast, which is easy to produce bubbles.

2.4 glue penetration

Glue penetration refers to the defect that the adhesive penetrates into the surface of plywood through the surface plate and causes the surface pollution. The problem of glue penetration should be analyzed and controlled from the following aspects: 1) check the surface plate quality, such as the surface plate crack is too large and the surface plate is too thin. In plywood production, the surface plate should not be too thin, and it should be guaranteed to have a certain covering capacity; If the surface crack is too large, it needs to be repaired. 2) The viscosity of the adhesive should be appropriate. 3) The hot pressing process is unreasonable, such as excessive hot pressing pressure. In production, if the pressure is too high, the adhesive will be extruded to the plate surface. Therefore, the appropriate pressure should be selected in production to make the glue flow reasonably between slabs.

2.5 warpage

Warping is a concave phenomenon formed by the upwarping of both ends or the arching of the middle of plywood. Plywood warpage is caused by the large internal stress of plywood. The warpage problem should be analyzed and controlled from the following aspects: 1) the plywood structure is asymmetric. In the production process, the principle of odd layers shall be followed, and the tree species shall be reasonably matched. The veneer species, thickness and moisture content of the two symmetrical layers from the middle layer shall be the same. 2) The moisture content of veneer is uneven. When the veneer is dried, it should be dried naturally first, and then sent to the dryer for drying after drying to a certain extent, so as to control the veneer from twisting and deformation during drying. Monitor the temperature of each point in the dryer at any time during the drying process to ensure that the moisture content of each part of the dried veneer is uniform, so as to improve the drying quality. 3) The hot pressing process should 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 itself is uneven, and the surface of the hot pressing pad is stuck with rubber blocks or sundries. 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) Rubber blocks or sundries are stuck on the surface plate after surface bonding. Check whether there are rubber blocks or sundries on the surface of plain slab after it is finished and hot pressed, and clean them in time.

2.7 corner defect or glue opening

Corner defect refers to the loss or damage of four corners or edges of products caused by improper mechanical or human operation. Corner opening is the cracking of the product because the corners are not firmly bonded. This problem should be analyzed and controlled from the following aspects: 1) quality problems of core plate, such as insufficient length of core plate and corner defects of core plate. The length of the purchased core plate shall be strictly controlled to ensure that the length of the core plate meets the production process requirements; For the core plate with edge and corner defects, it shall be handled in time when the veneer is sheared. 2) During blank assembly, a right angle reference edge shall be used for blank assembly according to the principle of “one side, one end flush”. 3) There are problems in hot pressing, such as the misalignment of the upper and lower slab edges and corners when the slab is loaded into the hot press, skew loading of the slab, uneven compression, etc. Strengthen production management, improve operators’ skills and sense of responsibility. When hot pressing plates, the position of slabs in each laminate should be kept as flat as possible (i.e., up, down, left and right alignment), so as to ensure that the laminates are evenly pressed. 4) Hot pressing equipment problems, such as insufficient pressure caused by wear of hot pressing plate edges and corners, and low temperature caused by poor heat transfer at edges and corners. Check the hot pressing pad frequently, and replace it in time in case of worn edges and corners; Frequently check whether the heat transfer oil passage of the hot pressing plate is smooth. If the corner temperature of the hot pressing plate is found to be low, unblock the heat transfer oil passage. 5) Gluing quality problems, such as lack of glue at the corners and premature drying due to insufficient gluing.

2.8 formaldehyde emission exceeds the standard

Formaldehyde release refers to the wood-based panel pressed with formaldehyde series adhesives such as urea formaldehyde resin, which is used in the manufacturing The phenomenon that formaldehyde gas is constantly emitted to the outside during stacking and use [. This problem should be analyzed and controlled from the following aspects: 1) the formaldehyde content of the adhesive used exceeds the standard. The manufacturer should inspect every batch of adhesive purchased. Once the formaldehyde content exceeds the standard is found, the adhesive should not be used. 2) the amount of glue applied is too large and uneven.

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