Capillary Rheology Qualification Protocols for High Strain Extrusion Regrind
Twin-bore capillary testing at 1,000 to 10,000 s⁻¹ qualifies high-strain regrind by isolating entrance losses, true wall slip, and elastic die swell shifts.
Twin Bore Rheometer
Meaning
Laboratory testing equipment known as a twin bore rheometer measures melt volume rate and melt flow rate under standardized load and temperature conditions. Two independent test channels operate simultaneously within a single heated block to double throughput during polymer quality control routines. Thermoplastic resins undergo forced extrusion through capillaries of specified dimensions while pistons apply constant downward force.
Moulders rely on these empirical values to verify incoming raw material consistency before initiating large production runs. Melt viscosity behavior observed during the procedure correlates directly with molecular weight distribution and thermal degradation history.
Flow Divergence
Material verification distinguishes virgin pellets from recycled regrind streams by quantifying viscosity shifts caused by previous thermal cycles. Extrusion parameters specified on resin datasheets reflect laboratory conditions that differ significantly from shear rates encountered inside commercial injection moulding machines. Operators establish processing windows by comparing melt flow rates measured at two distinct piston loads within the dual channels.
Excessive viscosity variation between identical resin lots leads to part dimensional distortion, sink marks, and localized structural failure in finished components.
Shear Sensitivity
Capillary die geometry generates high shear rates that reveal non Newtonian flow characteristics typical of engineering thermoplastics. Viscosity drops as shear stress increases during the transit of molten polymer through the narrow orifice. Dual barrel arrangements capture this rheological response efficiently by executing parallel tests at contrasting load configurations without thermal equilibrium delays.
Processors adjust barrel temperature profiles on moulding equipment when incoming feedstock exhibits unexpected shear sensitivity deviations.
Thermal Stability
Residence time inside the heated test chamber determines whether polymer chains degrade during extended processing runs. Degradation manifests as molecular chain scission or crosslinking, which alters melt flow behavior and compromises mechanical performance in moulded parts. Technicians monitor viscosity changes over timed intervals to establish safe residence limits for heat sensitive materials.
Accurate melt index control prevents flash, short shots, and excessive clamping force requirements during high speed production cycles.