Drying Of Pulp Molding

The raw materials for pulp molding and papermaking are pulp containing only about 1% plant fiber. After the wet paper sheet is formed, the moisture content of the wet paper sheet must be reduced to about 55% by mechanical pressing. However, the moisture content after pulp molding is as high as 65%~75%, and it is difficult to reduce the moisture content by mechanical methods. In the production process of pulp molded products, about 3.5~4kg of moisture must be removed through the drying process for each kilogram of paper mold product. Therefore, drying is an important process in papermaking and pulp molding production. The cost of the drying process accounts for a large proportion in the production of pulp molded products. Improving drying efficiency is a key measure to increase the production efficiency of pulp molded products.

1. Drying OF Pulp Molding Mechanism

The wet paper mold with plant fiber as the main component is a porous object with a capillary structure inside. Water exists on the surface of the fiber and between the pores of the fiber capillaries. Paper molded products used for product inner packaging are generally required to have good dynamic buffering performance. The moisture in them cannot be completely removed by mechanical pressing, but needs to be removed by drying. Therefore, compared with the conventional paper sheet drying process, the drying intensity of pulp molded products is very large. On the one hand, hot air acts as a heat source to provide energy for removing moisture; on the other hand, it acts as a moisture carrier to take away the continuously evaporated moisture. The removal of moisture from the wet paper mold depends on the internal diffusion and external diffusion of moisture. The so-called internal diffusion refers to the migration process of moisture from the inside to the outer surface in the wet paper mold; external diffusion refers to the process of moisture being taken away after evaporation from its surface. The drying rate of pulp molded products depends on the speed of internal and external diffusion of moisture and the balance and coordination between the two. Reasonable drying process conditions should be formulated around the above two aspects and the drying properties of pulp molded products.

After vacuum filtration, the wet paper mold has three forms of water in the wet plant fiber: bound water, adsorbed water and free water. The purpose of drying paper mold products is to remove the free water (adsorbed water and free water) in them so that the paper mold products reach the equilibrium water content. The principle of pulp molding drying is that when the water vapor partial pressure on the surface of the wet paper mold is greater than the water vapor partial pressure in the drying medium, the water on the surface of the wet paper mold will continue to vaporize and enter the air, and the water inside the wet paper mold will continue to diffuse to its surface. The speed of vaporization and diffusion varies with the medium temperature, the moisture content of the wet paper mold, the thickness and size of the product, etc. Therefore, the medium temperature, medium speed, drying speed, etc. in the process of drying the wet paper mold change according to a certain rule.

2. Drying Process Conditions

(1) Drying Temperature

Increasing the temperature can speed up the drying speed. However, too high a temperature will reduce the overall strength and other performance indicators of the paper mold product. The various parts of the wet paper mold will form a large temperature gradient, which will produce large thermal stress and cause defects in the paper mold product. In addition, too high a hot air temperature will also increase energy consumption and seriously reduce the thermal efficiency of drying. Therefore, when the wet paper blank is dried, the temperature of the hot air in contact with the wet paper blank is generally controlled at around 110°C, and the temperature of each part of the drying equipment should be kept consistent.

(2) Control of Hot Air Humidity During Drying

The control of hot air humidity is very important. If the water evaporated in the early stage of drying the wet paper blank is not removed in time, the humidity in the drying equipment will continue to increase, thereby prolonging the drying time of the paper mold product and reducing the drying efficiency. For drying equipment that requires hot air circulation, the ratio of circulating hot air and fresh hot air should be adjusted at any time to avoid affecting the drying efficiency of the drying equipment due to excessive humidity.

(3) Speed and Flow of Hot Air

Adjusting the speed and flow of hot air entering the drying equipment is also an effective way to increase the drying speed. The speed of moisture diffusion of the wet paper blank depends largely on the speed and flow of hot air in the drying equipment, and requires high-speed hot air to blow evenly to the surface of the wet paper blank and ensure sufficient hot air flow so that the wet paper blank can be dried evenly. Generally, when the hot air speed reaches 5m/s or more, the paper mold product can be dried faster.

The drying process of paper mold products directly affects the quality and production cost of paper mold products. Therefore, it is very important to master the drying mechanism and characteristics of paper mold products and determine reasonable drying process conditions. At present, the drying process of paper mold products generally has the problems of high energy consumption and low drying efficiency. Therefore, only by reasonably determining and controlling the three major process elements of temperature, humidity and flow rate (flow) of hot air in the drying process can a better drying effect be obtained.

3. Drying Process

The moisture content of wet paper mold blanks when they leave the molding machine is generally 65%~75%. The moisture content of paper mold products that do not require shaping is 10%~12% after drying, while the moisture content of products that require shaping is generally 30%~35%. The process conditions during drying in the drying tunnel have a greater impact on the quality of paper mold products. Although the high-temperature strong drying method has little effect on the strength of paper mold products, it has a greater impact on their shrinkage rate; the use of a gentle drying method, although the shrinkage rate of paper mold products is reduced and the quality is improved, but the drying time is too long, which affects the production efficiency of the entire paper mold product production line. Therefore, reasonable temperature-raising drying process conditions should be selected, that is, a reasonable drying curve should be formulated. The maximum drying temperature can be determined by controlling the shrinkage rate of the finished product. Under normal conditions, the drying of paper mold products should be a dehydration process. In order to understand the dehydration mechanism, we must first know what types of water there are in the wet paper mold and how they are combined. Because the energy required to remove water in different combinations is different, and the influence of external conditions is also different.

(1) Free water, also known as mechanically bound water, is water added to facilitate paper mold forming. It is distributed between solid particles and is combined with the material by cohesion. It is water absorbed by the material when it comes into direct contact with water. Free water is loosely combined with the material and is easy to remove. Drying is to remove free water.

(2) Adsorbed water. When an absolutely dry material is placed in the atmosphere, the material absorbs a certain amount of water from the atmosphere as the temperature and humidity in the atmosphere change. This water adsorbed on the surface of the particles is called adsorbed water. When the water adsorbed by the blank reaches a certain level and is in balance with the external conditions, the water is called equilibrium water.

(3) Chemically bound water refers to the water contained in the molecular structure of the raw material. This form of water is the most solid and requires a lot of energy to remove.

In summary, when drying, free water must be removed first until the water is balanced. If the surrounding medium conditions (temperature and humidity) change during drying, the balance water content of the wet paper mold will also change accordingly. From a process point of view, the drying process only needs to remove free water and adsorbed water.

Draw three curves with time as the horizontal axis and temperature, drying speed and moisture content of the wet paper mold as the vertical axis. The drying process of wet paper molds can be divided into four stages, namely, the rising speed drying stage, the constant speed drying stage, the falling speed drying stage and the balance drying stage.

(1) Rising speed drying stage. The wet paper mold is placed in air with a high temperature and a relative humidity of less than 100%. In a very short time, the surface of the wet paper mold is heated to the wet bulb temperature of the drying medium, and the evaporation rate of water also increases rapidly. After increasing to point A, the heat absorbed by the wet paper mold is equal to the heat consumed by evaporating water, and equilibrium is achieved. Since the speed-up stage is very short, not much water is removed in this stage, and the isotropic drying stage is reached after this.

(2) Isotropic drying stage. In this stage, the water evaporated from the surface of the wet paper mold is replenished from its interior to the surface of the wet paper mold. The surface of the wet paper mold always remains moist, just like the speed at which water evaporates on a free surface. The drying speed during this period remains unchanged. The surface temperature of the wet paper mold remains unchanged, approximately equal to the wet bulb temperature of the drying medium, and the vapor pressure of the wet paper mold surface is equal to the vapor pressure of the pure water surface. The drying speed (evaporation speed) is related to the water vapor concentration difference, partial pressure difference or temperature difference between the wet paper mold surface and the surrounding medium. The greater the difference, the greater the drying speed. In addition, the drying speed is also related to the air speed on the surface of the wet paper mold, because there is always a layer of air film on the surface of the wet paper mold that is not easy to flow. The steam diffusion and heat exchange between the wet paper mold and the medium must pass through this air film. The reduction of the air film thickness is beneficial to evaporation and heat exchange, and increasing the air flow speed on the surface of the wet paper mold will make the air film thinner. Therefore, increasing the air flow speed on the surface of the wet paper mold can increase the drying speed. When the drying reaches the K point, the diffusion rate of moisture inside the wet paper mold begins to be lower than the surface evaporation rate, and the moisture in the wet paper mold cannot completely wet the surface to maintain surface evaporation. At this time, the deceleration drying stage begins. The K point is the critical moisture point. The critical moisture point is the turning point from the constant speed drying stage to the deceleration drying stage. It is different from adsorbed water. The critical moisture is expressed as the average moisture content of the entire wet paper mold body. After reaching the K point, the surface layer stops shrinking. When drying continues, the pores in the wet paper mold increase. Therefore, the constant rate drying stage is an important stage, because in this stage, the wet paper mold shrinks and often produces shrinkage stress that can make the wet paper mold become waste.

(3) Decreasing rate drying stage. When the moisture content of the wet paper mold reaches the K point, the moisture diffusion rate inside the wet paper mold is lower than the surface evaporation rate, and the moisture in the wet paper mold cannot completely wet the surface. Therefore, the wet surface of the wet paper mold gradually decreases, and the drying rate also gradually decreases. This stage is called the decreasing rate drying stage. In this stage, the evaporation rate and heat energy consumption are greatly reduced, the surface temperature of the wet paper mold gradually increases, the temperature difference between the wet paper mold surface and the carrier decreases, and the water vapor partial pressure on the wet paper mold surface decreases and becomes lower than the saturated water vapor partial pressure at the wet paper mold surface temperature.

(4) Balanced drying stage. When the wet paper mold is dried to the equilibrium moisture content on the surface, the drying rate drops to zero. At this time, the moisture in the paper mold reaches a state of equilibrium with the surrounding medium. The amount of equilibrium moisture varies according to the properties of the paper mold and the humidity and temperature of the surrounding medium. The moisture in the paper mold at this time is also called the final moisture of drying. Generally speaking, the final moisture of the paper mold should not be lower than the equilibrium moisture during storage, otherwise it will absorb moisture again after drying to reach the equilibrium moisture. During the drying process, if the temperature and humidity of the carrier change, the above drying stages will also change, but not significantly.

The setting of the heating and cooling zones is mainly to prevent the paper mold products from being severely deformed due to sudden changes in temperature. The high-temperature dehydration section generally accounts for more than 60% of the entire drying line and is the main area for drying and dehydrating the wet paper mold. Depending on the product, the temperature of this section can be controlled within the range of 130~200℃. During the drying process, the moisture on the surface of the wet paper mold is evaporated first, while the internal moisture is difficult to evaporate. If the drying temperature is too high, the surface of the wet paper mold will dry quickly, but the inside is still very wet, making it difficult for the internal moisture to dissipate. If the temperature curve of the drying process is not set properly, the difference in the internal and external diffusion speed of the moisture of the wet paper mold body will be too large, causing the surface of the paper mold product to shrink violently and warp, and the internal moisture content is large and the waterproof and oil-proof are uneven. Therefore, for paper mold products of different sizes, the running speed of the transmission chain plate in the drying tunnel, the temperature during the drying process, the hot air speed and the discharge of water vapor should be reasonably adjusted to form a reasonable drying curve, so that the deformation of the paper mold product is as small as possible to ensure the quality of the product. The humidity in each stage should also be regulated to ensure high efficiency during drying. Since the wet paper mold is in a free state in the drying tunnel, the product has a large deformation space, which is not conducive to maintaining the accuracy of the product. No matter how reasonable the drying process curve is, the product coming out of the drying tunnel will be more or less deformed. After drying in the drying tunnel, for packaging products with general geometric shapes and dimensions that are not very strict, it is not necessary to hot press and shape them. The moisture content of the dried product is not too strict. Generally, the moisture content of the product coming out of the drying tunnel can be 10%~12%. However, for food tableware and fine industrial packaging products, they must also go through the hot pressing shaping process to ensure the flatness and beauty of the products. In order to ensure the smooth completion of the hot pressing shaping process, there should be strict requirements on the moisture content of the product after drying in the drying tunnel. If the moisture content is too low, the product has been shaped and the shaping process has lost its effect; if the moisture content is too high, it will affect the efficiency and quality of the shaping process. Generally, depending on the product, the moisture content before hot pressing shaping should be controlled at 30%~35%.

4. Ways to Improve the Drying Efficiency of Pulp Molded Products

During the drying process of pulp molded products, in order to improve the drying efficiency, it is necessary to reduce the humidity of the drying medium (hot air), increase the temperature and air flow rate, but it is necessary to increase the internal and external diffusion rate of moisture at the same time. In production practice, this can be achieved through the following ways.

(1) Appropriately reduce the thickness of pulp molded products. The thicker the pulp molded product, the more difficult it is to dry. Therefore, when designing the thickness of pulp molded products, the thickness should be reduced as much as possible while ensuring its overall strength and meeting the packaging performance requirements. This is conducive to improving the drying efficiency and reducing the consumption of fiber raw materials. The thickness difference of each part of the paper mold product should not be too large to avoid affecting the overall drying speed due to the difficulty of local moisture diffusion outward.

(2) Appropriately increase the moisture gradient between the surface and the interior of the wet paper mold. During the drying process, the difference in moisture concentration between the surface and the interior of the wet paper mold is the main driving force for the internal diffusion of moisture. Increasing the moisture gradient by adjusting the temperature, humidity, wind speed and air volume of the hot air is beneficial to improving the internal diffusion of moisture. However, if the external diffusion rate of moisture is too high, it is easy to cause the surface of the wet paper mold to shrink too quickly and too much, affecting the internal diffusion rate of moisture, which will have a negative effect on the drying of pulp molded products.

(3) Appropriately increase the temperature during drying. Increasing the drying temperature is conducive to the migration of moisture from the inside of the wet paper mold to the surface. However, in production practice, the increase in temperature is subject to many factors, so it can only be appropriately increased.