Plastic Extrusion Screw
There are five possible zones in a thermoplastic screw. Since terminology is not standardized in the industry, different names may refer to these zones. Different types of polymer will have differing screw designs, some not incorporating all of the possible zones.
Most screws have these three zones:
Feed zone. Also called solids conveying. This zone feeds the resin into the extruder, and the channel depth is usually the same throughout the zone.
Melting zone. Also called the transition or compression zone. Most of the resin is melted in this section, and the channel depth gets progressively smaller.
Metering zone. Also called melt conveying. This zone, in which channel depth is again the same throughout the zone, melts the last particles and mixes to a uniform temperature and composition.
In addition, a vented (two-stage) screw will have:
Decompression zone. In this zone, about two-thirds down the screw, the channel suddenly gets deeper, which relieves the pressure and allows any trapped gases (usually moisture or air) to be drawn out by vacuum.
Second metering zone. This zone is like the first metering zone, but with greater channel depth, and repressurizes the melt to get it through the resistance of the screens and the die.
Wear occurs due to addition of non metallic fillers in plastic. Coatings of NiCrBSi with or without addition of tungsten carbide and tungsten carbide Ã¢â‚¬â€œcobalt protect against wear caused by abrasion & corrosion. Coatings are fused with base metal. Coatings are applied by TIG, PTA or HVOF using Robot or servo driven X-Y-Z Manipulator.
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