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.
Die Swell Ratio
Meaning
Polymer elasticity defines the characteristic behavior where a material recovers its shape after passing through an extrusion orifice. The die swell ratio represents the cross-sectional area of the extrudate compared to the area of the exit opening. This enlargement occurs because long molecular chains stored in a state of compression during flow regain their original configuration upon release of stress.
Melt elasticity dictates the magnitude of this recovery, which varies significantly between polymers based on molecular weight distribution and shear history.
Elasticity Performance
Extrusion lines manage this property to maintain wall thickness and dimensional accuracy during production. A higher ratio indicates increased storage of energy within the melt, forcing the material to expand beyond the constraints of the tooling. Moulders verify these dimensions during start-up to compensate for the anticipated expansion of the profile or parison.
Consistent results rely on maintaining uniform melt temperature and steady flow rates through the head, since thermal fluctuations alter the viscous response of the resin. Variations between virgin pellets and regrind material often appear as shifts in this ratio, as recycled chains undergo scission that decreases the total elastic recovery.
Tooling Geometry
Engineers design flow channels with specific draw-down ratios to account for the physical expansion inherent to the resin grade. Correcting the die geometry allows for the production of parts with precise tolerances even when the material exhibits high internal memory. This calculation rests on comparing the exit cross section to the measured product dimension after cooling.
Precise control prevents excessive scrap rates during high-speed manufacturing runs.
Process Stability
Thermal history and shear rate conditions govern the repeatability of the observed swell throughout a production run. Stable outputs depend on the interaction between residence time in the die and the relaxation time of the polymer chains. An inconsistent melt stream leads to surging and thickness variations that compromise the integrity of the finished article.
Monitoring the ratio provides a constant validation of the processing state.