High Shear Rheology Calibration Thresholds for Recovered Polyolefin Industrial Blends
Calibrating high-shear capillary rheology via Bagley and Weissenberg-Rabinowitsch corrections prevents off-spec polyolefin lot processing failures.
Gross Melt Fracture
Meaning
Severe surface and structural defects occur when polymer melt flows exceed a critical shear stress. In extrusion and injection moulding, gross melt fracture produces highly distorted, rough and irregular surfaces across the extruded shape or moulded part. The defect indicates that the flow of the polymer has transitioned from a stable state to an unstable, chaotic state at the die or gate exit.
Rheological Cause
Elastic rupture of the polymer melt at the entrance of the die zone initiates the chaotic flow. When the shear rate exceeds the ability of the polymer chains to relax, the melt fractures internally, rather than sliding smoothly. This instability is typical for materials with high molecular weight or narrow molecular weight distribution, which have long relaxation times.
Defect Profile
Distortions from this instability range from deep ridges to a completely ruptured, corkscrew structure in the extruded profile. In injection moulding, gross melt fracture appears as localized tearing, surface dullness or weak knit lines where the fractured melt fronts join. These issues compromise both the structural integrity and the cosmetic appearance of the product, leading to rejected batches.
Process Adjustment
Raising the die temperature or reducing the extrusion speed lowers the shear stress below the critical threshold. Adding processing aids such as fluoropolymers can also coat the metal surfaces, allowing the melt to slip and reducing the stress at the wall. This change restores a smooth flow and eliminates the surface distortions.
Alternatively, redesigning the die with a more gradual approach angle reduces the elongation acceleration at the entry point, allowing the polymer chains to adjust without fracturing even at higher output speeds.