Capillary Rheometry Shear Rate Correction Protocols for Bimodal High Density Polyethylene
Correcting capillary rheometry data via Bagley and Rabinowitsch protocols is mandatory to prevent up to 50 percent viscosity errors in bimodal HDPE die design.
True Shear Rate
Meaning
Velocity gradients within a polymer melt represent the true shear rate, a metric describing the rate at which adjacent fluid layers slide past one another during flow. The calculation accounts for the non-Newtonian behavior of resins by incorporating a correction factor that adjusts the apparent rate for the parabolic velocity profile found in circular geometries. Viscosity measurements rely on this value to determine the resistance to flow under specific thermal conditions.
Proper quantification of the true shear rate allows engineers to model mold filling performance accurately.
Flow Correction
Capillary rheometers output apparent values that assume Newtonian fluid behavior, which fails to represent reality for long-chain polymer structures. Application of the Weissenberg-Rabinowitsch correction modifies these raw numbers to reflect the actual shear conditions at the wall of the nozzle or runner. Discrepancies between the calculated and observed flow patterns stem from these simplified assumptions about fluid deformation.
Tooling Impact
Processing variables at the injection gate depend heavily on high flow rates during the short interval of mold cavity filling. Excessive shear rates cause molecular degradation or surface defects like jetting and sharkskin, which directly increase scrap rates in high-precision part production. Maintaining values within the material datasheet limit preserves the mechanical integrity of the final molded component.
Cost Efficiency
Resin selection dictates the range of viable shear rates for a given tool geometry. Virgin materials typically exhibit consistent viscosity profiles under known shear, while batches containing regrind often show shifted values that require frequent machine adjustments to maintain quality. Precise control over this variable prevents the waste associated with thermal degradation and inconsistent part geometry across production runs.