Plastic pipe production utilizes specific feeding systems based on the polymer type and extruder design. PVC resin pipe grades are primarily processed in dry blend form on double screw extruders. Polyolefins (PO), such as HDPE and PP, are typically processed in granular form using single screw extruders. The fundamental difference lies in material transport: double screw extruders use intermeshing screws to move material in C-ring shaped chambers independent of barrel friction, offering high pump efficiency. Single screw extruders rely on friction with the barrel. Double screw extruders are well-suited for PVC due to its higher wall slip compared to PO.
Dosing Feeders and Their Impact
The extruder intake serves to transport material and acts as an atmospheric vent, allowing entrapped air to escape backward, which aids dry blend heating. Without a dosing feeder (flood feeding), the maximum intake capacity is restricted to prevent overfilling downstream sections at lower RPM, potentially limiting maximum output and risking wear or breakage.
There are two primary dosing feeder types: the horizontal gravity dosing feeder (starve feeder), which doses material free falling onto the screws, and the vertical crammer feeder, which packs the screws fuller. Vertical crammer feeders can hinder air escape backwards and may stall with dry blend having a high CaCO3 load under pressure. Starve feeders are suitable for direct dosing using an Intermixer.
Technical Considerations
The design of the intake depends on the presence of a dosing feeder and the pourability of the dry blend. Powder lock design is also crucial upstream of the vent to avoid powdering under vacuum and prevent air/volatiles inclusions.
Technical Points:
PVC Dry Blend: Double Screw Extruders
PO Granules: Single Screw Extruders
Double Screw Transport: Positive Displacement
Single Screw Transport: Friction Dependent
Feeder Types: Starve (Horizontal Gravity) vs. Crammer (Vertical)
Crammer Risk: Stalling with high CaCO3
FAQ Section
What type of extruder is typically used for PVC dry blend and what feeding system is preferred?
PVC resin pipe grades are typically processed on double screw extruders. For dry blend, a dosing feeder is important, with starve feeders offering advantages like easier air escape compared to crammer feeders.
Can a vertical crammer feeder be used for dry blend with high filler content like CaCO3?
A vertical crammer feeder packs the material more densely, and this setup might stall if the dry blend has a high CaCO3 load under pressure.
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Navdeep Singh Chawla
