Drying of materials 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.
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.