The aim of drying is the extraction of humidity absorbed by hygroscopic materials and the prevention of moisture depo­sits in the plastic granulate. Mainly air driers and in some cases also vacuum or infrared driers are employed.
With air driers, the plastic granulate is dried by air and sometimes also with nitrogen as an inert gas. The air absorbs the humidity in or on the granulate. The higher the temperature of the drying air, the more humidity can be absorbed.
At the same time, however, the plasticising process of the plastic granulate is also promoted by heat during drying. The air temperature cannot therefore be increased above a certain level. In order to enhance its moisture absorption capacity and consequently its drying performance, the humidity level of the air can be reduced prior to heating. In this case, the air is referred to as dry air rather than warm air.
In contrast to working with warm air, dry air also has the advantage that the moisture content of the air can be regulated precisely. The drying time of the plastic granulate can then be determined in a targeted manner. The lower the moisture content of the air, the shorter the drying time.
Although drying of the material can be a complicated process, optimum material preparation still forms the basis for consistently high injection moulding quality. The key to stable, top-quality moulded part production is the overall consideration of all production steps. This also always includes perfect drying, conveying and dosing of the plastics used.