Meaning
Viscous forces developing across a polymer melt stream during injection dictate how polymer chains orient under pressure. Shear stress gradients govern molecular orientation and localized viscosity changes within the runner system and the cavity. Bounded strictly by the gate entry and the final flow front, this fluid dynamic parameter ceases to apply once solidification locks the frozen layer in place.
Flow Distortion
Differential velocity profiles inside a runner generate uneven molecular stretch that persists into the molded part. Polymeric chains near the cold wall experience high drag while core material travels faster, creating persistent orientation layers. Virgin feedstock tolerates these velocity disparities better than regrind because degraded polymer chains break prematurely under extreme velocity shifts.
Processing temperatures set during barrel heating directly alter melt viscosity, which subsequently amplifies or dampens the resulting internal friction profile.
Thermal Variance
Localized frictional heating generated by velocity extremes alters the actual temperature of the flowing resin. Viscous dissipation adds thermal energy disproportionately where velocity changes peak, creating hot zones inside narrow mold sections. Extrusion pressures must compensate for these thermal fluctuations to prevent localized scorching or incomplete fill patterns.
Virgin polymer maintains predictable thermal thresholds, whereas mixed regrind batches exhibit unpredictable viscosity drops that disrupt calculated cooling rates.
Structural Defect
Uneven molecular alignment causes differential shrinkage across the cross section of a finished component. Material specifications for structural parts demand tight control over flow kinetics to eliminate warp and internal void formation. Moulders struggle to hold datasheet values across a production run when melt turbulence introduces hidden stress concentrations.
Component specifications fail inspection whenever internal warp exceeds dimensional tolerances established for the final assembly.
