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Hot air dryers are designed to remove surface moisture from non-hygroscopic (non-moisture absorbing) polymer, and also to preheat material before processing. They comprise an insulated stainless-steel hopper with an attached blower and heater. Hot air is blown through the polymer granules, and the wet air is then expelled to the atmosphere. These units are not suitable for use with materials that absorb moisture (hygroscopic).
Dehumidifying dryers are designed to remove any moisture present in the polymer before being processed (moisture held within the plastic granules due to absorption from the atmosphere). Air is forced through a desiccant bed to make it extremely dry. This air is then heated to a specified temperature and fed into a drying hopper containing the material to be dried. This hot, dry air draws the moisture out of the material. This removal of internal moisture is critical for hygroscopic material grades such as Nylon and ABS.
Compressed air dryers are designed to be fitted directly to the throat of smaller throughput machines. These compressed air dryers are compact and light in weight, but still use desiccant bed technology, making them ideally suited for the technical moulding sector. The units use factory compressed air rather than using the electric blowers that are typically fitted to larger floor standing dehumidifying units.
Vacuum dryers do not require the use of desiccants. After the material is heated with hot air, a vacuum is applied to it, which reduces pressure to a level below the vapour pressure of water, causing the water to boil off at a relatively low temperature. This reduces energy consumption and is less likely to cause material degradation.