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 Wall Shear Stress
Meaning
Rheological force parameters define the actual shear stress exerted by a polymer melt at a capillary die wall after correcting for entrance and exit pressure losses. Calculating true wall shear stress requires applying the Bagley correction to raw pressure data gathered during capillary rheometry testing across multiple die length-to-diameter ratios. The resulting value represents true mechanical resistance to flow independent of die entrance geometry.
Application covers capillary rheometry of viscoelastic polymer melts, stopping at non-confined free surface flows.
Mathematical Correction
Capillary dies introduce excess pressure drops at entry constrictions where polymer chains align. Subtracting entrance pressure losses from total measured extrusion pressure isolates wall shear forces along the fully developed capillary flow field. Corrected stress values yield accurate viscosity curves.
Process Modeling
Polymer melt processing in extrusion dies and runner systems requires precise rheological input data for finite element flow simulations. Utilizing true wall shear stress instead of uncorrected apparent values prevents underestimating pressure drops across complex mold gates. Moulders designing thin-wall injection molds rely on true wall shear stress data to predict required clamping forces and prevent incomplete mold cavity filling during high speed injection steps.
Melt Instability
Polymer melt flow instabilities such as sharkskin melt fracture occur when wall shear stress exceeds critical material thresholds. Identifying these stress limits establishes maximum processing throughputs for extrusion dies.