Rabinowitsch Shear Rate Corrections Integrated with Bagley Wall Stress Regressions
Integrating Rabinowitsch shear rate corrections with Bagley wall stress regressions converts raw capillary metrics into true viscosity, protecting yield margins.
Non-Newtonian Melt
Meaning
Polymer processing deals with liquid plastics whose viscosity is not constant but varies depending on the rate at which they are sheared or deformed. This behavior is typical of long-chain polymer molecules that disentangle and align themselves along the flow direction when subjected to high stresses. Characterizing a non-Newtonian melt is essential for designing extrusion dies and injection molds, as the pressure required to move the material decreases as the processing speed increases.
Failing to account for this shear-thinning behavior leads to severe over-design of heating and pumping systems.
Shear Dependency
Viscosity drops by several orders of magnitude as the shear rate increases within the die channel. This shear-thinning response of a non-Newtonian melt allows high-speed processing machinery to operate at manageable pressures. Tooling must be designed to generate sufficient shear to ease flow without causing thermal degradation of the polymer.
Extrusion Behavior
Elastic recovery occurs as the melt exits the constrained environment of the die. Because the polymer chains were stretched during their passage, they tend to contract axially and expand radially upon release. This phenomenon causes the extruded profile to swell, requiring careful calibration of the takeoff equipment to maintain the correct product dimensions.
Testing Method
Capillary rheometers measure the viscosity over a wide range of shear rates. If the test is conducted too slowly, the melt behaves like a Newtonian fluid, producing an incomplete profile that can mislead tool designers.