Capillary Rheometry Limits for Post Consumer Resin in Multicavity Tools
Capillary rheometry establishes binding high-shear viscosity limits that prevent multicavity tool imbalance and part weight variance in post-consumer resins.
Non Newtonian Index
Meaning
Mathematical quantification of the degree to which a fluid’s viscosity changes in response to shear rate defines the non Newtonian index. Within rheological analysis, the non Newtonian index, often expressed as the flow behavior index n in the Ostwald-de Waele power law model, measures how intensely a polymer melt deviates from Newtonian fluid mechanics. Values equal to unity characterize Newtonian fluids whose viscosity remains constant regardless of shear, while values below one indicate pseudoplastic, shear-thinning behavior typical of thermoplastic melts.
The index applies to capillary rheometer data and injection fill simulations. It stops applying at extremely low or high shear rate plateaus where polymer melts reach zero-shear or infinite-shear Newtonian limits.
Shear Thinning
Thermoplastic polymers display pseudoplastic flow behavior because entangled macromolecular chains uncoil and align parallel to the direction of flow under mechanical shear. A lower non Newtonian index signifies stronger shear-thinning response, meaning viscosity plunges steeply as injection speed increases. Polypropylene and high-density polyethylene often exhibit index values between zero point three and zero point four, making them highly responsive to injection fill speed adjustments.
Conversely, polycarbonate has an index closer to zero point seven, reflecting stiffer molecular chains that resist orientation and demand higher processing temperatures. Tool designers rely on this value to predict flow resistance through narrow gates and thin part walls.
Processing Influence
Injection moulders utilize shear-thinning characteristics to fill complex, thin-walled cavities without exceeding machine pressure limits. Increasing injection speed sharply reduces effective viscosity at the gate, allowing resin to fill distant cavity regions before freezing begins. However, high-shear processing generates frictional dissipation that raises melt temperatures, increasing the risk of thermal degradation in temperature-sensitive resins.
Extrusion operations with low index values exhibit steep velocity profiles across die channels, requiring targeted die lip geometry adjustments to ensure uniform extrudate thickness.
Formulation Sensitivity
Blending virgin resin with post-consumer regrind or mineral fillers shifts the flow behavior index upward toward Newtonian response. Mineral fillers and glass fibers hinder polymer chain orientation, resulting in higher flow resistance at elevated shear rates. Melt flow index datasheets fail to capture this dynamic, requiring multi-point capillary rheometry to confirm the active power law index across processing shear regimes.