Designing Balanced Multi Cavity Runner Systems for Polymer Injection Tooling
Balanced multi cavity runners match flow length, shear history, and pressure drop across all branches to achieve simultaneous mold cavity filling.
Viscous Dissipation Heating
Meaning
Internal friction within flowing polymer melt converts mechanical work from the injection ram into thermal energy during high-velocity cavity filling. Experiencing viscous dissipation heating elevates local resin melt temperature above set barrel temperatures as fluid passes through restrictive runners and narrow gates. The effect vanishes when flow stops, making it a dynamic process variable tied strictly to injection velocity and shear rate.
Shear Heating
Melt friction across molecular chains generates heat within high-shear gate regions. Controlling viscous dissipation heating prevents melt stream temperature spikes during rapid cavity filling.
Thermal Degradation
Excessive temperature rise within localized flow zones breaks polymer backbone bonds, reducing molecular weight and degrading mechanical properties. When viscous dissipation heating raises local temperatures beyond resin processing windows, degradation products cause splay marks, material discoloration or gas burning in molded components. Heat-sensitive materials such as polyvinyl chloride or polycarbonate suffer severe impact strength loss when gate shear rates drive local melt temperatures beyond datasheet maximums.
Reclaimed regrind resin contains pre-degraded chains that break down at lower thermal thresholds, compounding the risk of material degradation under high shear. Mold designers lower gate shear rates by enlarging gate dimensions or widening runner transitions to manage thermal rises.
Process Optimization
Process engineers adjust barrel heater band settings downward to compensate for internal frictional heat generation. Balancing nozzle temperatures with viscous dissipation heating reduces overall energy consumption while maintaining required melt flow rates inside cavity channels. Lowering barrel temperatures also speeds up cooling times, shortening total cycle time.