Double screw extruders used for PVC pipe extrusion are available in both parallel and conical screw designs. While conical machines historically offered mechanical advantages, parallel extruders are increasingly preferred, particularly for larger sizes.
Design and Processing Implications
The distinction between parallel and conical designs has significant implications for screw design, gearbox technology, and processing capabilities.
Parallel Extruders: For bigger extruders, parallel designs are favoured because they offer greater freedom in screw design for optimal processing. Advances in FEM calculation techniques have improved parallel gearbox designs, allowing for equal torque flexibility and achieving high reliability and long lifetime. Parallel extruders can also be more easily extended (e.g., from 22D to 33-36D) by stretching the pitches of the C-ring chambers to increase output while maintaining similar processing conditions.
Conical Extruders: Conical machines were historically easier to design mechanically due to more space for gears and bearings. However, they are often designed primarily for mechanical reasons, which can limit processing priorities. Optimal processing, especially at higher outputs, is limited by the shorter available screw length for generating pressure in the metering zone and mixing near the screw tip. Conical machines cannot easily be made longer to increase output; their diameter needs to increase instead.
Processing Window
The processing window for conical machines can be relatively narrow, particularly when aiming for a well-gelled melt that passes tests like the DMC test. Achieving sufficient gelation for the DMC test might lead to over-fusion in the vent zone, causing air inclusions. Consequently, the melt might be processed less gelled at the vent to ensure air extraction, resulting in a melt that is borderline for the DMC test but easily passes the ASTM acetone test. Parallel designs can offer a wider processing window.
Technical Specifications (Design Considerations)
Feature
Parallel Extruder
Conical Extruder
Preferred Size Range
Bigger Extruders
Historically Easier for Gears
Screw Design Freedom
Less Limited for Processing
More Limited by Mechanics
Gearbox Design
Improved by FEM, High Reliability
More Space for Gears
Length Extension for Output
Easier (Stretching pitches)
Requires Diameter Increase
Screw Length for Processing
Can be Longer
Limited for Pressure/Mixing
Processing Window
Can be Wider
Can be Narrow
Venting Flexibility
Better (e.g., early vent)
Limited
FAQ Section
Why are parallel extruders often preferred over conical extruders for large PVC pipes?
Parallel extruders are preferred for bigger sizes due to greater flexibility in screw design for optimal processing and improvements in gearbox technology providing high reliability. They can also be extended in length more easily to increase output compared to conicals.
What processing challenges can arise with conical extruders compared to parallel designs?
Conical extruders can have limitations in screw length available for generating pressure and mixing, potentially narrowing the processing window. Achieving optimal gelation while effectively venting air can be challenging, sometimes resulting in a compromise on fusion level to prevent air inclusions.
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Gerrit Jan Wilpshaar
