Establishing Rheological Dispersity Limits to Prevent Strain Hardening Failure in Regrind Blends
Establishing Trouton ratio floors via capillary extensional rheology prevents catastrophic web tear and parison sag in regrind polyolefin blends.
High-Pressure Capillary Rheology
Meaning
Polymer viscosity characterization under shear rates exceeding one thousand inverse seconds defines this analysis method. High-pressure capillary rheology forces molten resin through a narrow die of known dimensions while measuring the required force to maintain constant flow. Precise calculation of the shear stress and shear rate provides the viscosity curve necessary to model filling behavior in injection moulding tools.
Laboratories employ this data to predict pressure drops and shear heating effects during rapid cavity filling.
Processing Dynamics
Consistent control of melt flow remains critical when manufacturers transition from pelletized resin to final geometry. High-pressure capillary rheology simulates the extreme shear environments found inside runners and gates where material degradation or surface defects commonly originate. Datasheet values often represent low-shear conditions that fail to predict performance during the injection phase.
Resin producers provide these high-shear curves so moulders can adjust machine settings to prevent burn marks or short shots.
Material Economics
Regrind integration into production streams requires verification of molecular weight stability through comparative flow testing. High-pressure capillary rheology detects subtle shifts in viscosity that signal chain scission or additive depletion in recycled polymers. Virgin resins possess a stable flow profile that allows for predictable cycle times and consistent part weight.
Economical operations depend on these measurements to define the ratio of regrind acceptable without compromising the mechanical integrity of the end component.
Viscosity Measurement
Absolute measurement of polymer resistance to deformation at elevated flow rates forms the technical basis for this test. High-pressure capillary rheology captures the non-Newtonian behavior of thermoplastics by varying the plunger speed and recording the resulting back pressure. Correction factors for entrance pressure and wall slip improve the accuracy of the final viscosity report.
Accurate curves ensure that computer simulation software yields results matching actual performance on the shop floor.