Achieving a homogeneous melt is a crucial factor for producing high-quality plastic pipes. Non-uniformities in the melt can lead to defects in the finished pipe, affecting mechanical properties, appearance, and dimensional consistency.
For PVC, which is processed as a rubber-like solid that forms a network of tie molecules, melt homogeneity is essential for optimal properties. The screw length downstream of the vacuum venting zone should be sufficient to gently treat the visco-elastic melt, which has a memory. Mixing at the screw tip is necessary because the material in the center of the C-chambers and the material near the steel surfaces have different processing histories. Elongation flow through gaps or holes in the screw flights helps to homogenize the melt and improve dispersion. Examples of mixing principles include Starke mixers. Proper mixing also reduces the need for cooling by generating less viscous heat dissipation compared to shear flow.
For Polyolefins, which exit the extruder as a real melt, melt homogeneity is also critical, especially for the dispersion of masterbatch. High specific outputs, often enabled by grooved intakes in single screw extruders, can challenge melt homogeneity. Mixers at the screw tip, such as the Rapra cavity transfer mixer, Twente mixing ring, or CRD mixer, are used to improve masterbatch dispersion. While intensive mixers like Rapra and Twente generate significant heat, the CRD mixer generates less. Devices like the Maddock/Egan element at the screw tip can help ensure un-molten particles are flattened for quick melting.
For foam core pipes, especially PVC, achieving an even melt temperature is paramount for homogeneity. An uneven temperature leads to inconsistent expansion of the blowing agent gas mixed into the core melt, resulting in thickness variations (thicker or thinner sectors) in the pipe wall. A homogeneous melt temperature without hot spots reduces the need for high levels of heat stabilizer.
|
Aspect |
Importance / Method |
|
Screw Length (PVC) |
Sufficient length downstream of vent for gentle treatment |
|
Mixing at Screw Tip |
Necessary to blend material from different flow histories |
|
Elongation Flow |
Through gaps/holes in flights, aids homogenization/dispersion (PVC) |
|
Mixer Types |
Starke (PVC), Rapra, Twente, CRD, Staromix (PO) |
|
Masterbatch Dispersion |
Key challenge in PO, improved by mixers |
|
Maddock/Egan Element |
Forces melt over narrow gap, melts un-molten particles (PO) |
|
Melt Temperature |
Must be even for foam core pipes for consistent expansion |