Meaning
Shear-induced velocity gradients created as polymer melt flows between stationary mold cavity walls drive molecular alignment and thermal heating. Injection molding shear controls apparent melt viscosity through shear-thinning behavior inherent to long-chain macromolecular fluids. The parameter governs cavity fill pressures, gate design, and runner dimensions across all thermoplastic processing operations.
Analysis stops at zero-shear relaxation zones or post-gate hold phases where flow velocity drops to zero.
Viscous Dissipation
Friction generated between rapidly sliding polymer chains converts mechanical screw work into thermal energy inside cold mould channels. High injection molding shear near gate locations elevates localized melt temperatures by ten to thirty degrees Celsius without barrel heater input. Non-Newtonian pseudoplastic flow lowers apparent viscosity, allowing complex thin-wall geometry fill at reduced peak injection pressures.
High shear exposure degrades molecular weight if melt residence time and shear rates exceed polymer thermal limits.
Molecular Orientation
Macromolecules align along flow lines near cavity walls under high velocity gradients, creating anisotropic mechanical properties in finished components. Rapid freezing locks orientation in place, boosting tensile strength along the flow direction while reducing transverse impact resistance. Core regions cool more slowly, allowing polymer chains to relax into random coiled configurations.
Defect Threshold
Excessive wall shear stresses cause jetting or gate blush that ruins component surface aesthetic standards.
