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Increasing the water content will gradually reduce the shear viscosity of the material. During the processing, due to the change of the melt flow properties, the quality of the product and a series of processing parameters will also change accordingly. For example, too long a dwell time will result in too low a residual moisture content resulting in an increase in viscosity, which will result in insufficient mold filling and also yellowing of the material. In addition, some changes in properties cannot be observed directly with the naked eye, but only through relevant testing of the material, such as changes in mechanical properties and dielectric strength.
When selecting the drying process, it is of crucial importance to identify the drying properties of the material. Materials can be divided into two kinds of hygroscopicity and non-hygroscopicity. Hygroscopic materials absorb moisture from the surrounding environment and non-hygroscopic materials do not absorb moisture from the environment. For non-hygroscopic materials, any moisture present in the environment remains on the surface, becomes "surface moisture" and is easily removed. However, colloidal particles made from non-hygroscopic materials may also become hygroscopic due to the action of additives or fillers. In addition, the calculation of the energy consumption of a drying process may be related to the complexity of the processing operations and other factors, so the values ​​presented here are for reference only.
Convection drying
For non-hygroscopic materials, hot air dryers can be used for drying. Because moisture is only loosely constrained by the interfacial tension between the material and water, it is easy to remove. The principle of this type of machine is to use a fan to absorb the air in the environment and heat it to the temperature required to dry a specific material. The heated air passes through a drying hopper and heats the material by convection to remove the moisture.
The drying of hygroscopic material is generally divided into three drying stages: the first drying stage evaporates water from the surface of the material; the second drying stage focuses evaporation on the interior of the material, and the drying speed slowly decreases. The temperature of the dried material begins to rise; in the final stage, the material reaches a moisture-absorbing balance with the dry gas. At this stage, the temperature difference between the interior and the exterior will be eliminated. At the end of the third stage, if the dried material no longer releases moisture, this does not mean that it does not contain moisture, but merely indicates that a balance has been established between the colloidal particles and the surrounding environment.
In the drying technology, the dew point temperature of air is a very important parameter. The so-called dew point temperature is the temperature corresponding to when the relative humidity reaches 100% while keeping the moisture content of the moist air constant. It represents the temperature at which the air reaches the condensation of moisture. In general, the lower the dew point of the air used for drying, the lower the amount of residual water obtained and the lower the drying speed.
At present, the most common method for producing dry air is to use a dry gas generator. The device is based on an adsorptive dryer consisting of two molecular sieves, where the moisture in the air is absorbed. In the dry state, the air flows through the molecular sieve, and the molecular sieve absorbs the moisture in the gas to provide a dehumidified gas for drying. In the regeneration state, the molecular sieve is heated by hot air to the regeneration temperature. The gas flowing through the molecular sieve collects the removed moisture and brings it to the surrounding environment. Another method to generate dry gas is to reduce the pressure of the compressed gas. The advantage of this method is that the compressed gas in the supply network has a lower pressure dew point. After the pressure is reduced, the dew point reaches about 0°C. If a lower dew point is desired, membrane or adsorption dryers can be used to further reduce the dew point of the air before the compressed air pressure is reduced.
In dehumidified air drying, the energy required to produce dry gas must be extra calculated. In adsorption drying, the molecular sieve in the regenerated state must be heated from the temperature in the dry state (about 60°C) to the regeneration temperature (about 200°C). For this reason, it is common practice to continuously heat the heated gas through a molecular sieve to a regeneration temperature until it reaches a certain temperature when it leaves the molecular sieve. Theoretically, the necessary energy for regeneration consists of the energy required to heat the molecular sieve and its internal water, the energy required to overcome the molecular sieve's adhesion to water, and the energy necessary for the evaporation of moisture and the heating of water vapor.
In general, the dew point resulting from adsorption is related to the temperature of the molecular sieve and the amount of water carried. In general, a dew point of less than or equal to 30° C. allows the molecular sieve to achieve a moisture carrying capacity of 10%. For the preparation of dry gas, the theoretical energy demand calculated from the energy is 0.004 kWh/m3. However, this value must be slightly higher in practice because the calculation does not take into account fan or heat loss. By contrast, the specific energy consumption of different types of dry gas generators can be determined. In general, the energy consumption for dehumidified gas drying is between 0.04 kWh/kg and 0.12 kWh/kg, depending on the material and initial moisture content. In practice, it is also possible to reach 0.25 kWh/kg or more.
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Plastic drying is inevitable for every plastics processor. At the same time, this process is also very important in order to produce high quality products. Choosing a reasonable drying technology can help to save costs and reduce energy consumption. The right assessment of drying technology and cost is of great importance for the selection of a suitable drying technology.