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.
Mooney Wall Slip
Meaning
Mooney wall slip defines a condition where a polymer melt loses adhesion at the interface of a metal surface, causing the material to move as a solid plug rather than through internal shear. This phenomenon occurs when the shear stress at the boundary exceeds the critical adhesion strength of the melt. The effect fundamentally alters the velocity profile of the fluid inside a flow channel, often resulting in lower pressure drops than calculations based on no-slip boundary conditions predict.
Rheological Deviation
Analytical models of extrusion rely on the assumption that polymer velocity is zero at the die wall. Once mooney wall slip occurs, the actual output rate exceeds the predicted volumetric flow because the material translates along the surface. Moulders identify this state when a sudden drop in die pressure happens without a change in screw speed or melt temperature.
High molecular weight resins and formulations containing heavy lubricant loadings exhibit this behavior at lower shear rates than standard grades.
Polymer Economics
Processing additives often induce this slip intentionally to reduce torque or prevent surface fracture in extruded profiles. Manufacturers select specific processing aids that bloom to the surface, creating a lubricated layer that lowers the required extrusion force. Excessive reliance on this mechanism risks degradation of the mechanical properties in the final part because the polymer chains do not undergo sufficient shear to homogenize within the barrel.
Virgin resin batches may react differently than regrind material due to the depletion of these additives, necessitating periodic recalibration of the dosing equipment.
Tooling Geometry
Die design plays a central role in managing the transition from steady state flow to interfacial sliding. Channels with abrupt contractions force local shear stresses to levels that exceed the adhesion threshold, causing flow instabilities that mar the appearance of the product. Engineers mitigate these effects by polishing internal surfaces or adjusting the land length to maintain consistent flow attachment throughout the cooling phase.
Flow control at the interface determines the surface finish and dimensional stability of high-precision molded parts.