Abstract: Because the mechanical properties of MDF are very good, and its surface is very smooth and stable size, MDF is widely used in furniture manufacturing, packaging, and construction industry. MDF can be produced with too rich raw materials, and its production cost is relatively low, which also makes it replace natural boards in the production industry such as ships and vehicles. These industries must use the production of raw materials. Because MDF such as a good market, its sales are very good, so many domestic manufacturers have joined the production of MDF, such a result is the product quality is uneven, this article from how to improve the quality of MDF to discuss and analyze.
Keywords: medium density fiberboard; Quality control; Probe into
Introduction
The production process of MDF is very important and complex, and most of the reasons for product quality problems are due to the production process is not up to standard. Through the analysis of the factory’s product problems in these years, the problems are mainly found to be insufficient strength, cracks, deformation, stratification, etc, resulting in these reasons and other factors, such as the thickness of each part of a board is inconsistent, the density is not the same, the cooling time of the board is short, the amount of uneven sanding on the upper and lower surfaces, the pavement accuracy is not enough, etc. However, the most important reasons are the density of the board, the moisture content of the fiber after drying, the quality of the adhesive and the amount of sizing, and the hot pressing process. In order to improve product quality, increase output, reduce production costs to the maximum extent, and obtain the highest economic benefits, we need to do the following aspects of work.
Improve the mechanical strength and performance of the board
Improve the properties of raw materials to reduce the content of bark or decay; Improve the specification quality of wood chips, improve the qualified rate of wood chips, reduce the amount of debris, reduce the bark content; The ratio of fiber thickness should be appropriate, too thick will affect the interweaving and binding force, too fine specific surface area is too large, the amount of glue used will increase and its own strength will be weakened; Control the amount of sizing and uniform sizing to ensure the stability of the performance of the glue; Ensure uniform pavement thickness and stable thickness to reduce density differences; Make reasonable hot pressing curve, control suitable hot pressing, etc.
Control the board density
The density of the board is mainly related to the type and material of raw materials. The raw materials of our factory are mainly hard leaf materials based on tussah branches, and the ratio of raw materials themselves is significant (0.76g/om3). According to the experience of our factory over the years, the board density is lower than 0.70g/om3, the finished products are stratified, cracked, and the corners are more soft, and the plane tensile strength and static bending strength are not up to the standard. When the density is controlled at about 0.76g/om3, the surface of the board is smooth, and the cracking delamination phenomenon rarely occurs. When the density of the board exceeds 0.80g/om3, the warping deformation of the board is very serious. This phenomenon indicates that the board density should be controlled at 0.74 ~ 0.78g/om3 for hardwood and raw materials with high density. For soft broadleaf wood and raw materials with low density, the board density should be controlled at 0.65 ~ 0.70g/om3. In the case that the performance indicators of the product meet the standard, the lower the density, the better, the lower the density so the lower the weight, the use of less raw materials.
Grasp the moisture content of dried embryo
The moisture content of the fiber after drying has great influence on the delamination, cracking and warping deformation of the board. We have also done some experiments to solve this problem. From the production practice of our factory for many years, different specifications (referring to thickness) of products, in different seasons have different moisture content. Taking 15mm thick products as an example, the temperature difference between day and night in spring is large, the moisture content is 10.8% ~ 12% when 8:00 ~ 19:00 dryer import temperature (our factory uses hot oil furnace) is 85 ° C ~ 105 ° C, the export temperature is 66 ° C ~ 71 ° C; At 19:00-8:00, the moisture content is 9.8% ~ 10.8%, the inlet temperature is 96℃ ~ 112℃, and the outlet temperature is 67℃ ~ 72℃. After drying, the fiber enters the bin through the Φ430mm air feed pipe, and then enters the laying machine through two Φ330mm air feed pipes. Although the air feed pipe is insulated, the feeding medium inhaled by the fan belongs to the gas at normal temperature. When the fiber passes through the air feed pipe, cyclone separator and feeding valve, part of the water in the fiber is evaporated. The water content should be reduced by 2% to 3%, so the water content of the board before entering the press should also have a control range, for example, the same quarter, the water content before entering the press should be controlled between 8.7% and 10.1%, less than 8.5%, there will be insufficient sizing of the board side longitudinal corner soft, board hair and other phenomena; Higher than 10.3%, the board will appear stratification, bubbling phenomenon; In general, the moisture content of the fiber entering the press should be a relatively stable value to ensure the quality of the fiberboard. In addition, the change in the temperature of the dry inlet and outlet is directly affected by the external temperature and humidity, for example, the minimum temperature of the weather is -15℃, while the maximum temperature is 38℃, and the temperature difference is 50℃, so the dry inlet temperature of the coldest day should be increased to 130℃, while the inlet temperature of the hottest weather can be about 70℃. In addition, the size of the dryer when entering the air and the length of time the fiber stays in the drying pipe have a direct impact on the drying effect.
Master the performance indicators of the adhesive and control the sizing amount
Medium density fiberboard dry production, this process leads to a particularly low water content of the fiber, the plasticity of the fiberboard is poor, and due to the low density of the product, resulting in a relatively large gap between the fibers, only relying on the control of temperature and pressure control to make the fiber closely combined with each other is very difficult, so that the strength of the produced fiberboard and other performance indicators are also unqualified. Therefore, it is particularly important to apply adhesives in order to achieve product performance standards. The selection of urea-formaldehyde resin as an adhesive is often used in the production of fiberboard, and it is necessary to pay attention to his pH value when using this adhesive, and be sure to choose the appropriate pH value of the adhesive. In addition, if the selected adhesive concentration is not enough, its viscosity is low, it will increase the moisture in the fiber, which not only increases the difficulty of the hot pressing process, but also prone to product quality problems. On the contrary, if the concentration is too large and the viscosity is increased, then the adhesive is difficult to penetrate in the fiber, which will make the fiber clumped together, which will lead to uneven distribution of the adhesive, and product quality problems such as glue spots, so we must choose the concentration of the adhesive.
Master the hot pressing process
There is a particularly important process in the production process of MDF, that is, hot pressing process, which directly affects the output and quality of the product, according to the performance of MDF, the type of adhesive used in production and the production efficiency of the hot press to choose the right hot pressing temperature. The improvement of the strength of MDF and the improvement of water resistance is produced with the improvement of temperature, for example, the hot pressing temperature is increased from 140℃ to 170℃, the static bending strength is increased by 9.8%, the plane tensile strength is increased by 33.6%, the water absorption rate is reduced by 38.2%, and the thickness expansion rate is reduced by 15.2%. However, if the temperature continues to rise, reaching 185 ° C, there will be a decrease in the strength and water resistance of the plate, which is caused by resin degradation, excessive fixed-line, embrittlement. The hot pressing pressure of medium density fiberboard is generally 2.5-3.5mpa.
When carrying out the hot pressing process steps, it must be strictly in accordance with the designed hot pressing curve line, and control the size of the pressure on each process and the length of the stress time. It is also necessary to control the temperature difference between the hot press plate under the hot press, if the temperature difference is large, the heat uneven situation will appear, the heat transfer is not equal, the internal stress of the board is different, easy to cause the warping deformation of the board and the phenomenon of stratification. In addition, the hot pressing process of the plate opening time is too short, the unloading speed is too fast, the board is also prone to delamination, cracking, bubbling and other defects. Under normal circumstances, the temperature difference between the laminates should be ≤3 ° C, the temperature difference on the same hot press plate is ≤2 ° C, and the pressure relief time is about 40s.
The conclusion
The main quality problems existing in the MDF board industry are hoped to be highly valued by all relevant parties, and supervision of weak links in quality should be strengthened to promote the improvement of quality levels; Further guide enterprises to integrate resources, eliminate backward production capacity, and promote industrial transformation and upgrading; Innovation is the vitality of industrial development, encourage enterprises to actively innovate in product production, sales, application and other fields, and promote the healthy, stable and sustainable development of China’s MDF industry.