Dry-air Dryers

Digicolor dry-air dryers work according to the dry-air drying principle. This process is used for the energy-efficient dehumidification of hygroscopic plastic granules without compressed air in order to achieve the required residual moisture tolerances in the granules. In plastics processing, granulate dryers with different drying hopper sizes are used for this purpose, in order to meet the application-specific granulate throughputs in kg/h. Dry-air dryers from Digicolor are available in the modular Drywell system as top-mounted dryers, next-to-the-machine-dryers or central dryers.

Dry-air drying works in such a way that the process air of the dryer is specifically passed through the adsorption medium, thereby achieving a desired degree of dryness.

In the further course of the drying process, dry air is heated up to a material-specific drying temperature. Next this heated dry air flows through the plastic granulate in order to dehumidify it. Consequently the dry-air dryer guides the process air in a closed circuit to enable a continuous, industrial drying process.

The dew point is the measure of the water absorption capacity of the air. A dew point of -20°C is absolutely sufficient for drying most plastics, as only insignificantly more moisture can be absorbed below this limit. The relationship between dew point temperature and water absorption of the air is illustrated by the Mollier diagram.

Residual moisture tolerances in dried granulate

Material-specific residual moisture tolerances are determined on the one hand by the manufacturers of the plastic granulates. On the other hand, there are in-house specifications that are set for the production of the respective plastic parts. Adherence to prescribed residual moisture tolerances plays a significant role in ensuring that the desired physical and optical properties of the plastics are achieved in the end product.

Monitoring or control of residual moisture values is ideally carried out directly before or above the feeding zone of the injection moulding machine or the extruder. Moisture measuring devices are used to determine the residual moisture value. These devices must be able to measure moisture values that are within the range of the required residual moisture tolerances of the plastic granulates. A suitable moisture measuring device should measure moisture values of at least 0,01% H₂O respectively 100 ppm.

The German standard DIN EN ISO 15512 Plastics – Determination of water content describes the procedures used for the determination of the water content of plastics in form of granulate or finished product.