Meaning
This procedure determines the melt viscosity of thermoplastic resins by forcing material through a small orifice of known dimensions under controlled pressure. Capillary rheometry defines the resistance to flow as a function of shear rate across a range of temperature settings that mimic injection moulding environments. It provides the data necessary to predict how a polymer fills a mould cavity before the tool exists.
Moulders use the output to calculate the pressure drop required for filling thin walled components or complex geometries. Any resin manufacturer provides these curves to verify that a production batch stays within the established flow windows required for specific machine settings and tooling parameters.
Viscosity Profile
Testing occurs by loading pellets into a heated barrel where a piston drives the molten plastic through a calibrated die. The equipment records the force needed to maintain a constant speed of the piston while sensors track the pressure at the die entrance. Capillary rheometry links the measured stress to the shear rate through mathematical corrections for pressure losses.
These calculations translate raw force data into a viscosity value that accounts for the non-Newtonian behavior of polymer chains. A material specification sheet presents this data as a curve of viscosity versus shear rate. Moulders hold these values as a reference during the setup of filling velocities on the injection unit.
When the measured value deviates from the baseline, the processing window narrows and forces an adjustment to barrel temperatures.
Flow Consistency
Stable production relies on the consistency of the melt flow behavior throughout the duration of a run. Capillary rheometry identifies batch variations that impact part weight or dimension stability when using high percentages of regrind. Virgin resins possess a stable molecular weight distribution that results in predictable shear thinning, whereas contaminated or thermally degraded plastics exhibit erratic flow patterns.
Such deviations show up as short shots or excessive flash on the finished parts. Cost increases when machine operators compensate for poor flow by adjusting pressures rather than rejecting the faulty resin. This measurement technique detects subtle differences in material rheology before the resin enters the hopper, which avoids the accumulation of scrap during long production cycles.
Shear Sensitivity
Polymeric materials demonstrate different flow characteristics when subject to high speed deformation inside the nozzle or gate. Capillary rheometry isolates the shear sensitivity of the resin by comparing viscosity at varied piston speeds. This information identifies the risk of mechanical degradation or aesthetic defects that appear as streaks on the surface of moulded parts.
Processing variables set during the validation phase depend on the understanding of how the resin responds to these extreme rates of shear. Resin manufacturers report these limits to define the maximum injection velocity that a part design tolerates without structural failure. Excessive shear causes chain scission that weakens the molded article permanently.
Accurate data from this test remains the primary defense against catastrophic processing failures in high pressure applications.
