Rheological Verification of Recycled High Density Polyethylene in Multicavity Tooling
Verifying recycled HDPE in multicavity tools requires capillary rheometry beyond standard melt index to map shear thinning and prevent cavity filling imbalance.
Cross-Yasuda Model
Meaning
Rheological equations describe the flow behaviour of non-Newtonian polymer melts across a wide range of shear rates. The cross-yasuda model expresses this viscosity by combining a zero-shear plateau with a power-law transition region. This mathematical framework applies to materials showing stable viscosity at low shear rates that diminish under high stress conditions.
Shear Sensitivity
Polymer processing requires an accurate prediction of viscosity to size screws and define cavity fill pressures. The cross-yasuda model captures the transition between the Newtonian region and the shear-thinning regime through three specific parameters. These variables represent the zero-shear viscosity, the critical shear rate, and the transition index that governs the curvature of the transition.
Practitioners adjust these inputs to match capillary or rotational rheometer data obtained during the material characterization phase.
Moulding Performance
Injection moulding simulation software relies upon this equation to calculate pressure drops within runners and gates. An incorrect transition index causes deviations in predicted clamp force and short shot analysis for high-speed filling scenarios. Resin suppliers provide these coefficients on technical datasheets to ensure that processors can model the injection gate correctly.
Moulders verify these values against actual process data to confirm that the theoretical flow balance matches the physical injection run.
Material Economics
Quality control protocols distinguish between virgin feedstock performance and the altered flow profile of regrind. Processing degradation changes the molecular weight distribution, which shifts the critical shear rate identified by the cross-yasuda model. Recycled materials typically show a narrower Newtonian plateau or a shift in the transition index compared to base resins.
Consistent product dimensions depend upon the ability of the moulder to compensate for these rheological variations through adjusted temperature or profile settings. The utility of this numerical model lies in the predictable link between molecular architecture and machine pressure requirements.