Shear is a fundamental aspect of polymer processing. It is the relative motion of adjacent layers of the material under stress. In extrusion, shear is generated by the movement of the screw relative to the barrel and the material itself.
Impact of Shear on PVC and PO Quality
PVC Processing
PVC extrusion requires carefully controlled shear. While shear is effective at heating up the dry blend, excessive or over-shear, especially after venting, can destroy the tie-molecule network essential for optimal PVC properties like long-term strength, ductility, and impact resistance. A well-gelled melt with an entangled network is the desired outcome, achieved through balanced shear and heating. The screw design plays a vital role in realizing this. Sufficient motor load at normal barrel temperatures indicates appropriate shear levels downstream of the vent for optimal elongation properties. Elongation flow through gaps or holes in screw flights contributes to homogenizing the melt with lower viscous heat dissipation than shear flow.
PO Processing
Over-shear in Polyolefins (PO) can destroy tie-molecules, similar to PVC. This can lead to degradation and unmolten particles. High shear sensitivity of some modern PO grades, like LS (Low Sagging) types, can increase issues like die drool. Techniques that promote forced transport and low shear, short residence time in the die head are becoming popular for high-grade PO. Modern spider dies, compared to spiral mandrel dies, offer lower shear for PO.
FAQ Section
How does excessive shear negatively affect plastic pipe quality?
Excessive or over-shear can destroy the tie-molecule network within the polymer structure, particularly for PVC and high-grade PO. This degradation weakens the material, leading to reduced long-term strength, ductility, impact resistance, and potentially causing unmolten particles or die drool.
How do extruder and die designs manage shear to optimize quality?
Extruder screw design controls shear and heating profiles to achieve a well-gelled melt or a melt with an optimal tie-molecule network. For instance, counter-rotating double screw extruders for PVC provide controlled shear. Die design, like spider dies with sufficient compression or short residence times, minimizes detrimental shear, especially for sensitive PO grades.
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Gerrit Jan Wilpshaar
