Meaning
Differential velocity across a melt flow front defines the variation in molecular orientation that occurs when resin travels through a tool cavity. Shear imbalance describes the condition where plastic experiences uneven strain rates due to geometric features like abrupt wall thickness changes or non-uniform runners. High-speed injection cycles amplify these differences in viscosity because the material does not occupy the space symmetrically.
Residual stress remains locked in the molded part after cooling if the velocity profile across the width of a flow path departs from a singular equilibrium.
Flow Mechanism
Variations in runner layout force the polymer to reach the gate at different times and pressures. Melt enters the cavity from primary channels that supply secondary paths with unequal flow resistance. Discrepancies in cooling rates across these paths prevent uniform solidification of the amorphous chains.
Part dimensions suffer from inconsistent shrinkage when the molded object exhibits localized variations in density caused by these uncontrolled velocity gradients.
Tooling Consideration
Cavity geometry determines the propensity for non-uniform material deposition during the filling phase. Mold designers prioritize balanced runner systems to ensure that each gate experiences an identical pressure drop from the sprue. Sharp corners inside the manifold accelerate the degradation of the viscosity profile by forcing fluid molecules into sudden redirection.
Proper venting at the end of fill positions alleviates air entrapment that would otherwise worsen the instability of the incoming melt front.
Economic Consequence
Off-specification dimensions emerge when shear imbalance prevents the attainment of consistent part tolerances across large production runs. Scrap rates increase when parts exhibit warpage or internal voids resulting from the asymmetric cooling of the polymer matrix. Virgin resin costs mount as rejects accumulate from processes that fail to replicate the intended density distribution.
Consistent control over injection speed and barrel temperature minimizes the cost of secondary sorting or manual finishing.
