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.
True Shear Stress
Meaning
Actual frictional force per unit area exerted by flowing polymer melt along a capillary channel wall defines true shear stress. In polymer capillary rheometry, true shear stress represents the corrected shear force calculated after subtracting the entrance and exit pressure drops from the total measured barrel pressure. Molten plastics dissipate measurable energy simply converging into a constricted die entrance, which inflates apparent pressure readings above what wall friction alone causes.
The metric applies to laboratory capillary rheometer measurements and numerical injection flow simulations. It stops applying to rotational rheometers where uniform shear fields eliminate converging entrance pressure losses.
Bagley Correction
Calculation of accurate wall stress requires the Bagley correction method using capillary dies of identical diameter but varying length-to-diameter ratios. By plotting total driving pressure against die length-to-diameter ratios at a given shear rate, technicians extrapolate the linear trend back to zero length to identify the excess entrance pressure loss. Subtracting this entrance loss from the total pressure drop yields the net driving force responsible solely for steady shear along the capillary bore.
Data uncorrected for entrance loss overstates the true shear stress by twenty to fifty percent, especially in short capillary dies. Accurate stress characterization is essential for establishing true material viscosity curves.
Viscosity Derivation
Dividing true shear stress by the true wall shear rate, obtained via the Weissenberg-Rabinowitsch correction, produces the absolute steady-state shear viscosity of the resin. Corrected viscosity profiles provide the baseline data needed for injection moulding simulation software to predict fill pressures accurately. Inaccurate stress values skew simulated cavity pressures, causing toolmakers to misjudge required clamp tonnages and runner sizing.
Virgin resins produce smooth linear Bagley plots, whereas highly filled compounds or degraded regrinds exhibit scatter due to wall slip and elasticity variations.
Tooling Calibration
Designers use true stress values to predict pressure drops through extrusion dies and runner manifolds under actual manufacturing conditions. Die swell and melt fracture onset correlate directly with critical shear stress limits at the die exit. Uncorrected shear stress metrics fail to predict the onset of sharkskin defects on extruded profiles.