In the production process of pulp molded products, about 3.5-4 kg of moisture should be removed per kilogram of finished products through the drying process, so that the moisture content of the paper molded products is reduced to 10%-12%. Therefore, the production cost of the drying process occupies a large proportion in the production of paper molded products, and improving the drying efficiency is a key measure to increase the production efficiency of paper molded products. At present, the domestic production of paper moldings mostly adopts the method of hot air convection drying.
Three methods of hot air convection drying
(1) Hot air drying. Hot air is used as the drying medium, and the heating source can be coal, fuel oil, gas, etc.; the drying equipment used mainly includes drying box, tunnel dryer, chain dryer, etc. The drying process can be divided into three stages: heating, high temperature dehydration, and cooling. The main purpose of heating and cooling is to avoid severe deformation of paper moldings due to sudden changes in temperature: the high-temperature dehydration zone is the main stage of drying, and the temperature can be controlled between 130 and 200 °C according to different paper moldings. The drying efficiency of hot air drying is generally around 63%.
(2) Drying by far infrared radiation. Because the radiated energy can penetrate deeply into the inside of the paper molded product, the inside and outside of the paper molded product can be dried evenly, and the distortion is small.
(3) Microwave heating and drying. Microwave has automatic balance, uniform heating, avoids local over-drying or over-heating of paper molded products, small distortion and fast drying speed, and plays the role of sterilization and detoxification. The drying process is also easy to realize automatic control, and the drying efficiency can reach 80%, but the equipment cost is high and the power consumption is large. If the manufacturing and assembly are improper, there is a danger of leakage wave radiation hurting people.
Most domestic manufacturers use the hot air convection drying method to produce paper molded products, which generally has the problem of low drying efficiency. To improve the drying efficiency, it should mainly start from two aspects: improving the condition of the drying equipment and controlling the reasonable drying process conditions. The drying equipment itself must have a good moisturizing layer to reduce heat loss and ensure that the temperature of all parts of the drying equipment is uniform. The temperature, humidity and flow rate of the hot air are the three major process parameters in the drying process, which must be reasonably controlled. Usually, the temperature of the hot air is generally controlled between 130 and 200 °C. The drying efficiency is improved, but the plant fiber will be thermally degraded, and the paper molding will produce excessive shrinkage deformation.
The humidity in the drying equipment has a great influence on the drying process. The hot air acts as a moisture carrier to absorb and discharge the moisture that is continuously evaporated from the wet paper mold. The driving force is the difference in moisture concentration between the hot air and the wet paper mold. The hot air in the drying equipment maintains a low humidity, which is very beneficial to improve the drying efficiency. Generally, through the continuous replenishment of fresh hot air and the timely removal of waste hot air, the hot air in the drying equipment maintains a substantially constant humidity. Sometimes in order to improve the utilization rate of thermal energy, the exhausted hot air needs to be partially circulated. The amount of circulation of the exhaust hot air should be strictly controlled according to the humidity requirements in the drying equipment, which must be paid attention to. In the drying process, it is required to have enough hot air to blow evenly on the surface of the paper molding, and the flow rate of the hot air entering the drying equipment should be reasonably controlled. The high flow rate of the hot air will cause the power consumption of the drying system Increase: If the flow rate is too low, the drying efficiency will be reduced. Usually, the flow rate of hot air should be kept at about 5m/s.