Dry-air drying is a process for de-moisturising hygroscopic plastic granulates in order to meet the required residual moisture tolerances in the granulate. Special dry-air dryers known as desiccant dryers are used in plastic processing for this purpose. The desiccant media used are mostly molecular sieves or adsorption rotors. The process uses a focused transport of air through the desiccant medium until the desired degree of dryness is achieved.
The dew point is the measurement for the air’s capacity to absorb water. Subsequently, the dry air heated to a material-specific drying temperature flows through the plastic granulate to remove the moisture contained therein. A dry-air dryer carries the process air into a closed circuit to enable a continuous, industrial drying process.
In almost all cases, material-specific residual moisture tolerances are specified by the plastic granulate manufacturer. If this specified residual moisture is achieved, flawless processing without moisture issues will be the result. A dew point of -20°C is perfectly sufficient for drying most plastic materials as, below this limit, insignificantly more moisture can be absorbed.
The residual moisture value is monitored and verified in the production process, ideally just immediately upstream or above the feed zone of the plasticising screw of the processing machine such as an injection-moulding or extrusion machine. So-called residual moisture sensors are used to determine the residual moisture value.
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.
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