Proper lubrication is a fundamental aspect of achieving reliable and high-quality PVC pipe extrusion. Lubricants, such as stearates and waxes, are key components of the PVC dry blend formulation.
Sufficient lubrication is critical to create adequate wall slip between the PVC melt (rubber-like solid during extrusion) and the metal surfaces of the extruder screws and die. This slip prevents excessive local friction and heat build-up, which can lead to burning of the polymer. Burning causes defects like burned particles that negatively impact pipe performance in tests such as burst pressure and impact.
Lubricants include waxes with different melting points. For reliable production at higher outputs and to prevent issues like beard grow (die drool), practice shows it is beneficial to shift the balance of lubricants towards higher melting waxes. Double compression dies, for instance, allow the use of formulations with external lubricants at higher melting points, further reducing the risk of burning.
Beyond preventing degradation, adequate lubrication contributes to a smoother inner bore surface of the pipe and helps maintain consistent wall distribution, especially when combined with advanced die designs like double compression tooling. A well-lubricated formulation processes more consistently, contributing to lower reject rates and the ability to achieve higher output levels. Insufficient wall slip, regardless of heat stabilizer level, can lead to burning.
|
Lubricant Type |
Primary Function during Extrusion |
Impact on Process/Product |
|
Stearates |
Part of lubricant package |
Contributes to wall slip |
|
Low Melting Waxes |
Part of lubricant package |
Contributes to wall slip |
|
Higher Melting Waxes |
Part of lubricant package |
Preferred for high output/prevents burning |
|
Overall Lubrication |
Prevents friction & burning |
Improves pipe quality (smooth bore), allows higher output |