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.
Flow Curve Analysis
Meaning
Evaluation of a polymer melt’s viscosity across a wide range of shear rates characterizes its non-Newtonian behavior. This method, termed flow curve analysis, uses data from capillary or rotational rheometers to plot shear stress or viscosity against shear rate. It allows process engineers to determine the shear-thinning behavior and processing limits of a polymer resin.
Rheological Graph
Polymer viscosity decreases as the shear rate increases due to the alignment of polymer chains in the direction of flow. Conducting flow curve analysis involves generating a curve that spans several orders of magnitude of shear rate to represent different processing methods. This graph helps distinguish between resins that behave similarly at low shear but diverge at high shear.
Viscosity Modeling
Engineers use mathematical equations like the power law or Cross models to fit the experimental data points from the rheometer. In flow curve analysis, these models provide the parameters needed for computer simulations of extrusion dies and injection molds. The resulting equations predict the pressure drop and flow behavior under actual manufacturing conditions, allowing for accurate mold and die designs.
This modeling approach enables the virtual testing of multiple die geometries before investing in expensive steel tooling, saving time and resources during new product introductions.
Processing Prediction
Selecting the right polymer grade requires understanding how the material flows under specific shear conditions. In flow curve analysis, these results show if a resin has a low enough viscosity to fill thin-walled mold sections.