Exploring how to control the production quality of melamine impregnated paper veneer panels

Melamine resin impregnated paper veneer artificial board (referred to as melamine veneer board) is rapidly expanding into the material industries of office and civil furniture, vehicle manufacturing, construction, shipbuilding, interior decoration, etc. due to its advantages of low production materials, low energy consumption, few processes, and low cost, as well as beautiful appearance, surface wear resistance, heat resistance, pollution resistance, and easy cleaning. It has already occupied a considerable market share. According to statistics, the market share of melamine impregnated paper veneer panels in Europe was 8% in 1980, rising to 18% in 1995. It is expected that there will still be great growth potential in the next 10 years, so melamine impregnated paper veneer panels are receiving increasing attention. Based on several years of practice, the author has conducted a preliminary exploration of quality control in the production process of melamine impregnated paper veneer. 1. Quality control of production process 1.1 Paper quality and impregnation process To ensure the quality of impregnated film paper, high-quality decorative base paper must be used. Regardless of the type of paper, it should be used after passing the inspection at the time of entry/exit. Any paper that does not meet the standards must not be sent to the production line. During the impregnation process, strictly follow the process requirements and take samples for inspection every half hour to ensure that the adhesive content is between 130% and 150%, the volatile content is between 6% and 7%, and the pre solidification degree is ≤ 65%; The impregnated and dried adhesive film paper should be immediately packaged with plastic film and sealed with adhesive tape; Stored in a warehouse with air conditioning installed at a temperature of 20-25 ℃ and humidity of 55% -65%, the storage time should not exceed 3 doors; When using, the principle of opening as needed should be adopted. Paper that cannot be used up at once should be immediately sealed and sent to the warehouse for storage to prevent moisture absorption from causing the adhesive film paper to stick together and become unusable, resulting in unnecessary losses. 1.2 Substrate Quality: The substrates commonly used for veneer artificial boards are medium density fiberboard and particleboard. For particleboard, it should be purchased according to different customer requirements, but particleboard products must meet the requirements of GB/T4897-92 first-class products. Quality requirements for the substrate: (1) The surface of the board should be flat and clean, without any water or oil stains, and there should be no slipping, gnawing or other phenomena within the effective size; (2) The thickness deviation of the board should not exceed ± 0.2mm, and the sanding should be uniform without any sand leakage; (3) The moisture content of the board should be controlled within the range of 6% -10%. Due to the fact that particleboard is mostly purchased externally, changes in the environment can cause variations in the moisture content of the board. If the moisture content is too high, it may cause phenomena such as blasting and delamination during the veneer process; If the moisture content is low or the volatile content of impregnated paper is low, it may cause the surface to not wet well during hot pressing, resulting in delamination of the cardboard. Therefore, it is necessary to observe and promptly remove unqualified substrates during the production process. The three elements of hot pressing process are interdependent and mutually restrictive, and need to be analyzed and adjusted uniformly when making process adjustments. The three factors have different effects on the adhesive panel, and the hot pressing temperature mainly catalyzes the chemical reaction of the impregnated resin, that is, accelerates the curing process. Based on the actual production requirements and the author's experience, a temperature range of 145-165 ℃ is more suitable for hot pressing plates. High temperature is beneficial for demolding after pressing, and can shorten the hot pressing period and increase production. However, excessive temperature prevents the resin from flowing evenly and solidifying, resulting in small pores on the surface of the board. According to the quality inspection data of impregnated film paper veneer artificial board in 1998, a considerable number of enterprises have shortened the hot pressing time by increasing the hot pressing temperature in order to increase production, resulting in the board surface being prone to rebound and not smooth. Seemingly increasing production, it is actually at the cost of sacrificing product quality, while also causing an impact on legitimate product manufacturers. Appropriate pressure can ensure a good bond between the substrate and the impregnated paper. Under appropriate temperature and pressure, the resin in the impregnated paper melts and solidifies, forming a closed and dense surface. It can also fill the unevenness and small pores on the substrate surface. Usually, the pressure is between 2.0 and 3.0 MPa. Under the premise of not affecting product quality, it is advisable to use a lower pressure as much as possible, which is beneficial for the service life of equipment, hydraulic oil, and the internal structure of the substrate; However, excessively low pressure affects the bonding strength and resin flow ability between the substrate and impregnated paper. Therefore, in actual production, the unit pressure should be adjusted appropriately according to the age of different substrates and buffer pads. The length of hot pressing time depends on the curing speed of the impregnating resin and the hot pressing temperature, and is generally suitable between 40-50 seconds. Excessive curing time can cause resin to lose its elasticity, which can easily cause cracks or internal stress in the product, leading to cracking and warping during subsequent processing; If the time is too short and the resin curing is not sufficient, it is easy to cause sticking and affect the physical and chemical properties of the product surface, which in turn affects the durability of the product. The required time for different materials varies, and a curing degree test should be conducted on the surface of each new product, and then the hot pressing parameters should be adjusted based on the test results. In addition, during the operation, the movement of the impregnated paper should be careful to avoid damage; Do not wear dirty gloves during operation; If the board surface is found to be unclean during the assembly process, it should be cleaned up in a timely manner; Careful inspection should be carried out when feeding the substrate. If defects such as large shavings, missing corners, damage, sand marks, etc. are found on the surface of the board, they should be promptly removed and rejected for use; The pressure rollers of the paving machine, the large suction cups for discharging, the small suction cups for stacking, and the various conveying rollers should be cleaned regularly to prevent suction cup marks and dust on the board surface from causing product degradation. 2 Common problem analysis and solutions 2.1 Adhesive pad phenomenon Adhesive pad refers to the partial or complete adhesion of the board to the pad during the hot pressing process. There are several reasons for this: (1) Short hot pressing time, or low or locally low temperature of the hot pressing plate, which leads to incomplete resin curing; (2) The moisture content of the substrate is too high; (3) Excessive adhesive content and high volatile matter content on impregnated paper; (4) The surface of the steel plate is not clean, the type of release agent is inappropriate or the coating is uneven; (5) Inaccurate positioning of the slab during hot pressing. The phenomenon of sticking plates often occurs around the edges of the plate, which is due to the low temperature at the edges, or it may be caused by the deviation of the slab from the position inside the press. It is necessary to inspect each item to identify the true cause of the sticking of the board, solve it in a timely manner, and reduce losses. The occurrence of adhesive sticking is often due to incomplete curing of the adhesive, therefore, increasing the temperature or extending the hot pressing time appropriately is an effective method to solve such problems. 2.2 Board warpage (1) From the perspective of hot pressing, board warpage is caused by the large temperature difference between the upper and lower pressure plates, which results in the asynchronous curing speed of the adhesive film on both sides of the substrate during the pressing process, causing the board to bend towards the side with the fastest curing speed. The author believes that the appropriate temperature difference between the upper and lower pressure plates should be 3-5 degrees higher than that of the lower pressure plate. (2) From the perspective of the material itself, the uneven performance of the two-layer impregnated paper used in the village material can also cause the board to warp. If a single-sided sticker is used, warping is an inevitable result. From a production perspective, it is possible to ensure that the board does not warp by using the same impregnated paper or adjusting the appropriate temperature difference between the upper and lower pressure plates. (3) The incorrect stacking method of finished products after hot pressing is another major reason for the warping of the board. The solution is to use uniform and straight wooden pads or trays to store the hot pressed melamine impregnated paper for panel pasting. In summary, to produce high-quality products, it is necessary to use excellent impregnated paper and substrate, as well as reasonable processing techniques; In addition, it is necessary for operators and inspectors to cooperate fully and provide timely feedback when problems arise. Only in this way can we ensure stable product quality and occupy a larger market share.