Quantifying Wall Shear Stress Thresholds Struck from Capillary Rheometry Data
Quantifying true wall shear stress requires Bagley end-loss and Rabinowitsch corrections on capillary data to prevent premature extrudate fracture.
True Wall Shear Rate
Meaning
Actual rate of shear deformation experienced by a non-Newtonian polymer melt at the boundary of a flow channel provides the basis for accurate viscosity calculations. This metric, known as the true wall shear rate, corrects for the velocity profile distortion that occurs in shear-thinning fluids. It is calculated by applying the Rabinowitsch correction to the apparent shear rate obtained from capillary rheometry data.
Flow Correction
Newtonian fluids exhibit a parabolic velocity profile during pipe flow, but polymer melts undergo shear thinning, which flattens the velocity profile in the center and steepens it at the wall. The true wall shear rate reflects this steepened velocity gradient, which is higher than the apparent shear rate calculated for a Newtonian fluid. This difference is critical because polymer viscosity is highly dependent on the local shear rate, and underestimating it can lead to severe errors in flow predictions.
Calculation Methodology
Obtaining this value requires collecting flow data at multiple flow rates and calculating the slope of the shear stress versus apparent shear rate curve. Through true wall shear rate calculations, engineers apply the Rabinowitsch equation, which adjusts the apparent value based on the non-Newtonian index of the polymer. This correction is essential for converting raw capillary rheometer measurements into fundamental rheological properties that are independent of the testing geometry.
It ensures that the viscosity data used in mold flow simulations accurately reflects the material’s behavior under high-stress production conditions.
Extrusion Design
Accurate shear rates are necessary for designing extrusion dies that prevent polymer degradation and melt fracture. If the true wall shear rate is underestimated, the designed die channels may be too narrow, causing excessive pressure drops during production.