Venting in PVC Extrusion

Effective venting is a critical process step in PVC pipe extrusion using double screw extruders. It serves to remove entrapped air from the dry blend and volatiles generated during heating.

Venting Zones

Double screw extruders for PVC typically incorporate venting zones. The intake section can serve as an atmospheric vent, allowing a significant amount of entrapped air in the dry blend to escape backwards. This atmospheric venting assists in heating up the dry blend more quickly as air acts as an insulator. Downstream of the powder lock, a second vent zone is typically under vacuum.

Importance of Vacuum Venting

Vacuum venting is necessary to extract remaining air and volatiles from the melt, aiming for a void-free pipe wall. However, maximizing the vacuum level can introduce risks. High vacuum might lead to the dry blend becoming too gelled in the venting zone to prevent powdering. If the dry blend fuses too much, it becomes impossible to extract the air trapped within the already fused material, leading to air inclusions in the pipe wall.

Optimizing Venting

To mitigate the risk of air inclusions while ensuring effective venting, it is safer to have a screw design with inherent venting-safe characteristics. This design allows for a void-free pipe wall even at vacuum levels lower than the maximum possible. Such a design means an older vacuum pump or a leaking seal does not immediately result in reject material. An early vent design is also beneficial, particularly for processing quick-gelling materials like MPVC and CPVC. It makes foaming easier, as a later vent extracts more decomposed blowing agent gas. An early vent also provides more screw length downstream for proper mixing at the screw tip.

Technical Specifications (Venting Design Aspects)

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