Meaning
Polymer processing failures frequently result from the mechanical breakdown of polymer chains caused by high shear stresses in restricted flow channels. This shear degradation occurs when the polymer melt experiences extreme velocity gradients, such as those found in narrow gates, hot runner nozzles, or tight extruder screw clearances. The resulting reduction in molecular weight alters both the processing behaviour and the performance of the moulded part.
Rheological Impact
High shear forces cleave the longest polymer chains and permanently reduce the melt viscosity of the resin. This change makes the material flow more easily, which can lead to flashing at the parting line of the mould. Because the viscosity has been permanently altered, the material’s behavior during subsequent processing or recycling becomes unpredictable.
Quality Consequences
Molded parts made from shear-degraded material suffer from reduced mechanical strength, lower impact resistance, and increased brittleness. This defect often appears as brittle failure in areas of the part that experienced the highest flow velocity, such as near the gates. In optical parts, the broken chains can cause haze and discoloration, ruining the cosmetic appearance of the product.
Equipment Design
Optimizing the geometry of the runner system and the gate design minimizes the risk of mechanical degradation. Large gates and generous runner diameters reduce the shear rates experienced by the melt during injection. Screw configurations in extruders should avoid aggressive mixing elements that generate excessive shear heating and degrade the resin.
In addition, using streamlined melt channels without sharp corners prevents high-stress concentration zones where the polymer can break down. This design approach is particularly important when processing shear-sensitive polymers like polycarbonate or polyvinyl chloride.
