Analysis and control of common quality defects of plywood ,sourcing,importing,export,quality control,buying guide ,quality control

Analysis and control of common quality defects of plywood

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At present, China is a big plywood export country and the largest plywood production country 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 under the impact of international advanced productivity, facing huge challenges and threats from abroad. Only by adjusting and upgrading the plywood industrial structure, improving production technology, controlling the production process, effectively and continuously producing high-quality plywood products, and improving the core competitiveness, can we There is a stable market. Based on the production practice of plywood in China, this paper makes a comprehensive analysis and Discussion on plywood production technology, quality problems that are easy to appear in the production process and production process control, in order to provide a reference for plywood production and process control of enterprises.

Brief introduction of plywood production process

At present, most plywood manufacturers in our country do not manufacture veneers (surface and core boards) and adhesives, but directly purchase them. Therefore, the production process and production process discussed in this paper omit the process from log cutting to rotary cutting to veneer and glue making process. Figure 1 shows the plywood production process flow chart.

Analysis and control of common quality defects of plywood

2.1 core stacking and seam separation

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. Also known as core separation, it refers to the separation phenomenon between the inner core plate of the same layer or the adjacent two splicing core plates in plywood. Core lamination and seam separation are often encountered in plywood production meeting, which should be analyzed and controlled from the following aspects: 1) core plate overlap, dislocation or reserved gap is too large or too small in core plate assembly. To solve these problems, it is necessary to strengthen the training and responsibility of core arranging operators, improve the technical level of core arranging operators, and ensure the proper combination of single board and single board. 2) The core plate moves or dislocates when the plate is loaded or the slab is moved. In this case, it is necessary to lay the core plate coated with glue for a period of time before forming the blank. After forming the blank, pre press it first to make the slab initially bond together and keep it as stable as possible when moving the slab. 3) The core plate is warped seriously or the edge of the core plate is not straight enough. In this case, the warped core plate needs to be softened and leveled before core arrangement, and the edge should be leveled twice to make the surface flat and the edge flat.

2.2 low adhesive strength or degumming of adhesive layer

Gluing strength refers to the firm degree of gluing between veneers in plywood. Gluing strength is one of the important indexes to evaluate the quality of plywood. In the production process, the quality problems of low adhesive strength or degumming of adhesive layer should be analyzed and controlled from the following aspects: 1) there are quality problems in the adhesive used, such as poor quality, deterioration or too low concentration of the adhesive. In the production process, the quality of the adhesive should be checked at any time, and the bad or deteriorated adhesive should not be used; if the concentration is too low, flour can be added for thickening adjustment. 2) The coating quality does not meet the process requirements, such as too little or uneven coating. In the production process, it is necessary to select the appropriate amount of glue according to the different species and thickness of the veneer, and pay attention to the proper and even amount of glue. 3) The moisture content of single board is too high or too low. The moisture content of the core plate shall be checked before coating, and the moisture content of the core plate after drying shall be controlled within the range required by the production process. 4) The aging time of glue coated veneer does not meet the requirements. If the aging time is too long, the glue solution will be dry during hot pressing; if the aging time is too short, the hot pressing will be carried out without forming a continuous glue film. In order to make the glue liquid penetrate into the board effectively, increase the viscosity, and facilitate the formation of continuous glue layer and improve the bonding strength during hot pressing, the green body should be aged for 15-30 minutes before being sent to the next process, and the specific aging time after gluing should be determined according to the climatic conditions. 5) There are quality problems in veneer, such as too deep burr and groove mark, poor veneer rotary cutting quality, etc. The quality of veneer is an important factor affecting the bonding strength. Therefore, the separation and finishing process of veneer should be strictly controlled to improve the shearing quality of veneer to ensure that the veneer quality meets the production process requirements. 6) The process parameters of hot pressing 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. It is necessary to select the appropriate hot pressing process parameters according to different products, and increase the hot pressing temperature and pressure or extend the hot pressing time properly on the premise of meeting the production requirements. 7) There are problems in the hot pressing equipment, such as the pressure can not be increased, the temperature transmission is not in place, etc. Always check whether the hot pressing equipment is in good condition. If there is any problem, timely repair it to ensure that the hot pressing machine is in good condition.

2.3 bubbling and local opening

Bubbling refers to the bulge caused by the gas on the product surface. Partial gluing means that some part of the board is not bonded firmly and cracks. The problems of bubbling and local opening should be analyzed and controlled from the following aspects: 1) the moisture content of single plate is too high or uneven. The moisture content of veneer affects the quality of plywood to a great extent, too high and too low are not conducive to plywood production. Therefore, the moisture content of veneer after drying should be strictly controlled within the range of 8% – 14%. If the moisture content is too high, it is easy to open glue and bubble. 2) When applying glue, there are some blank spots or sundries on the veneer or they are stuck. In order to ensure the quality of glue application, the amount of glue application shall be even, and the parts not applied to the glue shall be manually brushed, no blank points are allowed, and redundant glue shall be removed at the place where the glue is piled up. For the sundries and dirt on the veneer, it must be cleaned. 3) Unreasonable hot pressing process, such as insufficient hot pressing time, low local hot pressing temperature, resulting in insufficient resin condensation. In the whole plywood production process, the hot pressing process always needs to be strictly controlled, and the hot pressing process parameters that meet the production needs must be selected according to different products. 4) After hot pressing, the pressure reduction speed is too fast. In the plywood production process, the speed of decreasing from equilibrium pressure to zero should be well controlled. In this stage, the speed of reducing pressure should be slow, so that the temperature of hot pressing can be properly reduced. If the pressure is too fast, the water vapor in the board will be discharged too fast, which is easy to generate bubbles.

2.4 penetration

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

2.5 warpage

Warpage is a concave phenomenon formed by warping of both ends of plywood or arching in the middle. The warpage of plywood is caused by the large internal stress of plywood. Warpage should be analyzed and controlled from the following aspects: 1) plywood structure asymmetry. In the production process, the principle of odd number of layers should be followed, the tree species collocation should be reasonable, and the tree species, thickness and moisture content of the two symmetrical layers from the middle layer should be the same. 2) The moisture content of single board is not uniform. 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 distortion and deformation in the drying process. 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 and consistent, so as to improve the drying quality. 3) The setting of hot pressing process should be reasonable. During normal production, the temperature of the upper and lower platens of the hot press shall be close to each other to ensure the same heating of the upper and lower platens.

2.6 plate surface indentation

Indentation refers to the local dent on the surface of plywood caused by external factors. The problem of plate surface indentation should be analyzed and controlled from the following aspects:

1) The surface of the hot pressing pad is uneven, and there are rubber blocks or sundries on the surface of the hot pressing pad. Before the hot pressing machine is started, the hot pressing pad shall be checked and cleaned in time; if the surface of the hot pressing pad is uneven, the hot pressing pad shall be replaced.

2) There are rubber blocks or sundries on the rear surface plate of the joint. Check whether there are rubber blocks or sundries on the surface of plain slab after it is closed and when it is hot pressed, and clean them in time.

2.7 corner defect or gluing

Corner defect refers to the phenomenon that the corner or edge of the product is missing or damaged due to improper mechanical or human operation. Corner opening is the product corner is not bonded firmly and cracks. This problem should be analyzed and controlled from the following aspects: 1) quality problems of core board, such as insufficient length of core board, edge and corner defects of core board. 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 corner defect, it shall be handled in time when the single plate is cut. 2) During the assembly operation, a right angle datum edge shall be used for the assembly according to the principle of “one side and one end are aligned”. 3) There are some problems in the hot pressing, such as the misalignment of the upper and lower slab corners when the slab is loaded into the hot press, the skewness and uneven pressure of the slab loading, etc. Strengthen the production management, improve the operator’s skills and sense of responsibility. During hot pressing, the position of slab in each laminate shall be kept as flush as possible (i.e. up and down, left and right alignment) to ensure that the plywood is pressed evenly. 4) Problems of hot pressing equipment, such as insufficient pressure caused by corner wear of hot pressing plate and low corner temperature caused by poor corner heat transfer. Check the hot pressing base plate frequently, and replace it in time in case of corner wear; check whether the heat transfer oil channel of the hot pressing plate is unobstructed frequently; if it is found that the corner temperature of the hot pressing plate is low, dredge the heat transfer oil channel. 5) Glue quality problems, such as lack of glue on corners, little glue application and premature drying.

2.8 excessive formaldehyde emission

Formaldehyde emission refers to the phenomenon that formaldehyde gas is continuously emitted to the outside world during the manufacturing, stacking and use of wood-based panels made of formaldehyde series adhesives such as urea formaldehyde resin. This problem should be analyzed and controlled from the following aspects: 1) the formaldehyde content of the adhesive used exceeds the standard. Every time the manufacturer purchases a batch of adhesives, it shall conduct inspection. Once it is found that the formaldehyde content exceeds the standard, the adhesives shall not be used. 2) Too much glue, uneven glue.

3 conclusion

Plywood has a history of nearly a hundred years in China. After decades of development, it has been greatly improved and developed in terms of production conditions, production technology, production capacity, etc. However, to become the world’s largest plywood production power, there is still a long way to go in terms of product quality and standardization, which requires the joint efforts of peers to create.

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