Capillary Rheology

Melt Behavior

Pressure drops across circular dies dictate how polymer melts behave during high speed mould filling cycles. Extruders generate high shear environments inside barrel zones, changing molecular chain alignment before material enters the runner system. Viscosity drops non-linearly as shear rates increase inside small gating orifices, allowing dense engineering resins to fill thin walled cavities.

Resin batches with identical melt flow index numbers often exhibit completely different pressure drops during capillary testing due to hidden variations in branching. Virgin polymer grades maintain predictable viscosity curves across repeated thermal cycles, whereas regrind fractions suffer chain scission that lowers resistance and causes flash.

Pressure Drop

Pressure losses across the die entrance reflect extensional viscosity components that standard rotational instruments fail to capture properly. Entrance losses accumulate rapidly when processing high molecular weight high density polyethylene through narrow barrel geometries. Moulders adjust barrel temperatures upward to counteract sudden viscosity spikes, risking thermal degradation if residence times exceed safe limits.

High shear regions near channel walls generate significant frictional heat, altering the local viscosity profile and causing part dimensions to drift out of tolerance.

Processing Limit

Operating windows narrow severely when shear sensitive polycarbonates pass through restricted gate geometries on multi cavity tools. Parts develop internal residual stresses when injection speeds exceed the critical shear rate defined by the material characterization curve. High internal stress levels trigger premature environmental stress cracking in finished housings subjected to secondary chemical exposure.

Dimensional distortion on moulded enclosures often traces back to uncompensated batch to batch viscosity shifts that alter post gate volumetric shrinkage.