Plastic Pipes Conference Association # 2023 Lake Buena Vista
Dipl. Ing. Rainer Vießmann
The production of PVC pipes is mainly produced on counter rotating twin screw extruders as the PVC Dryblend material is readily available in powder form. Whereas PVC granules, mainly processed on single screw extruders, play a minor role in pipe production for economic reasons. The design of the extruder screws has a major influence on the quality of plastic pipes produced. Various processing problems may occur by using unsuitable screws e.g., impact failure, colour deviations, lumps. The formulation of the PVC Dryblend and the screw design has to be well-matched otherwise a high product quality, stable production process and high outputs cannot be achieved.
General market conditions have been changing rapidly and so the economic pressure for PVC pipe production has increased. Environmental issues, sustainability, uncertain supply chains and / or rising raw material cost are only a few factors to mention. As a consequence, the pipe manufacturer modifies formulations by using more recycled material, raises CaCO3 content or changes stabilizers and additives. To the same extent as the PVC formulation changes the screw design has to change with it. To meet these challenges a new, patented screw design the so called IF-screw or wave screw has been developed.
The principle of conventional counter-rotating twin-screw extruders for PVC processing is as following.
In counter-rotating, intermeshing twin-screw extruders, the material is plasticized mainly by introducing mechanical energy (from the drive) into the pre-heating zone and the compression zone. A large part of the energy is supplied by the calendar effect between the two screws and the compression of the plastic Dryblend between the feed and compression zone. Only a small part of the energy supplied comes from the electric heater bands clamped to the outside of the barrel. Till now, this basic principle has applied to all conventional counter-rotating twin screws in use.
The mixing effect with counter-rotating twin-screw extruders is only moderate due to the design and is significantly worse compared to compounders. The chamber volume filled with material remains almost unmixed in the C-chamber from intake to the metering zone.
The mixing effect of a conventional screw can only be improved by additional mixing tips in the metering zone and or grooves in the pre-compression area. Without mixing elements, streaks, lumps or other discoloration on the pipe or profile might occur.
The challenge of every extruder is to deliver high output with homogeneous melt as good as possible across the total outlet section. Constant, even melt flow ensure accurate product sizing and optical good- looking products. To improve melt quality in terms of gelation, homogenization and dispersion the use of the IF-screw is one way to achieve it.
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