1 Drying method and drying mechanism of pulp moldings Most domestic pulp moldings adopt hot air drying method. In a specific drying equipment, hot air is used as a drying medium to dry pulp moldings. Hot air is mainly obtained by two methods. One is to directly generate hot air by burning in a hot air oven; the other is to use hot air to exchange hot air through heat exchanger and air. The equipment used for the drying of pulp mouldings includes drying ovens, tunnel dryers and chain dryers. No matter which type of drying equipment is used, the drying mechanism is basically the same.
The wet paper mold blank consisting mainly of pulp is a porous body having a capillary structure inside. Moisture exists between the surface of the fiber and the pores of the fiber capillary. Pulp mouldings used for internal packaging of products generally require good dynamic buffering properties, in which the moisture cannot be removed by means of mechanical squeezing, and all of them need to be removed in a dry manner first. Therefore, the pulp molded article has a great drying strength compared to the conventional sheet drying process. Hot air, on the one hand, acts as a heat source to provide energy for removing moisture; on the other hand, it acts as a water carrier and takes away the water that has been continuously evaporated. The moisture in the wet paper mold blank depends on the internal diffusion and the external diffusion of water. The so-called internal diffusion refers to the migration process of water from the interior to the surface in the wet paper mold blank; the external diffusion refers to the moisture being evaporated after evaporating from the surface of the wet paper mold blank. The process of walking. The degree of drying efficiency of pulp moulding products depends on the speed of internal and external diffusion of moisture and the balance and coordination between the two, and reasonable process conditions are formulated around these two aspects and the drying properties of pulp mouldings.
2 Ways to Improve the Drying Efficiency of Pulp Molded Products In the drying process of pulp molded products, it is necessary to increase the drying efficiency, reduce the humidity of the drying medium (hot air), and increase the temperature and air flow rate. However, it is necessary to increase the moisture content at the same time. Outward diffusion rate. In production practice, it can be achieved through the following ways.
2.1 Appropriately reduce the thickness of pulp mouldings The greater the thickness of pulp mouldings, the more difficult it is to dry. Therefore, when designing the thickness dimension of pulp mouldings, the thickness of the pulp mouldings should be reduced as far as possible, on the premise of ensuring their overall strength and meeting the packaging performance, which is not only beneficial to the improvement of the drying efficiency, but also reduces the consumption of the fibre raw materials. The difference in thickness of each part should not be too large, so as to avoid the difficulty of out-diffusion of local moisture and affect the overall drying speed.
2.2 Appropriately increase the moisture gradient on the surface and inside of the wet paper mold blank. During the drying process, the moisture concentration on the surface and inside of the wet paper mold blank is the main driving force for water diffusion. Increasing the moisture gradient by adjusting the temperature, humidity, wind speed, and air volume of the hot air is advantageous for increasing the internal diffusion of moisture. However, if the out-diffusion rate of water is too high, the shrinkage of the surface of the wet paper mold is too fast and the internal diffusion rate of moisture is affected, which will have a negative effect on the drying of the pulp molded product.
2.3 Appropriately increase the drying temperature The increase of the drying temperature facilitates the migration of moisture from the interior of the wet paper mold blank to the surface. However, in the production practice, the increase in temperature has to be limited in many aspects, so it can only be properly improved.
3 Drying process conditions
3.1 Drying temperature Increasing the temperature can speed up drying. However, the high temperature will reduce the overall strength and other performance indicators of the pulp molded products. Each part of the wet paper mold blank will form a large temperature gradient, which will cause greater thermal stress and cause defects in the pulp molded products. In addition, an excessively high hot air temperature also increases the energy consumption, and the thermal efficiency of drying is severely reduced. Therefore, when the wet paper mold blank is dried, the temperature of the hot air is generally controlled at about 110° C. It is absolutely not allowed to exceed 120° C., and it is required that the temperature in various parts of the drying equipment should be consistent.
3.2 Control of hot air humidity during drying Hot air humidity control is very important. The moisture that evaporates out of the wet paper mold blank at the initial stage of drying, if not eliminated, will cause the humidity in the drying equipment to increase continuously, thereby prolonging the drying time of the pulp molding product and reducing the drying efficiency. For some hot air circulation drying equipment, the proportion of circulating hot air and fresh hot air must be adjusted at any time, so as not to make the drying efficiency of drying equipment colorful due to excessive humidity.
3.3 Speed ​​and Flow of Hot Air Adjusting the speed and flow of hot air into the dryer is also an effective way to increase the drying speed. The speed of out-diffusion of moisture in the wet paper mold blank depends to a large extent on the speed and flow rate of hot air in the drying equipment, and requires that high-speed hot air can be evenly blown onto the surface of the wet paper mold blank and ensure sufficient hot air flow. , It is made to be evenly dried, and generally the pulp molded product can be dried faster when the hot air speed reaches 5 m/s or more.
4 Conclusions The drying process of pulp moulding products directly affects product quality and production costs. Therefore, it is very important to understand its drying mechanism, characteristics, and determine a reasonable drying condition. At present, in the domestic production enterprises, the drying process of pulp molding products generally has the problems of high energy consumption and low drying efficiency. Some of them are defects in the drying equipment itself, but most of them are not controlled in the production of hot air temperature. The three major process elements, humidity and flow rate (flow rate), reasonably determine these three major process elements in order to obtain a better drying effect of pulp molded products.
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