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The material leaving the extruder is very hot and must be cooled immediately, either by means of compressed air or with water. In the latter case, blowers are used to blow dry the plastics.
Plastic granules are subjected to varying temperatures in storage and when being transported. Through the condensation of the atmospheric humidity on the surface of the granules, moisture may be absorbed by capillary action, depending on the time of year, the weather conditions and the type of plastic involved. For high quality end-products, the maximum limits for moisture in the granules must not be exceeded. In basic systems, drying is performed by hot air dryers, while more efficient plants employ dry air dryers. In the latter case (and by contrast with hot air drying), the processing air is carried in a closed circuit and is directed through the material (granules or powder) for the purpose of drying it. A dehumidifier (molecular sieve), positioned in the return airflow, removes the moisture in the atmosphere. In this way, a constant flow of pre-dried air is always available in the approach to the dry silo. The air is conveyed by a blower. Most modern dryers have two drying agent containers so that the processor need not interrupt the drying process in order to regenerate the moisture-laden drying agent. Regeneration is likewise performed by a blower.
In the case of recycled material, vacuum pumps remove air inclusions, moisture and adhesives from the molten plastic. They extract all the low-molecular substances in order to obtain the optimum quality, i.e. a smooth surface, high strength, better insulation properties and a lower out gassing rate, resulting in recyclable material which achieves the same level as new material.
Plastic presses and extruders are automatically fed with the aid of blower or vacuum pumps. A distinction is made between conveying by means of a vacuum and by means of pressure. In the former case, a blower generates a partial vacuum in the material container and the granules are carried along with the air flowing through the hose line. A filter prevents the granules from penetrating into the rotary lobe blower or vacuum pump. More elaborate pressurized conveying systems consist of a compressor blower, a material storage tank, a pressurized conveyor tank (or cellular wheel sluice) with a push-type material feeder and, at the end of the conveying line, a container with dust-collecting filter.
Production units for rubber parts used in the automobile industry require vacuum to evacuate the die casting moulds, to avoid air bubbles and to accelerate the rubber mass