Managing Hot Runner Shear Induced Thermal Imbalance across High Density Cavity Layouts
Managing shear imbalance across high-density cavities depends on rotating channel boundary layers at runner splits to equalize melt viscosity before gating.
Boundary Layer Heating
Meaning
Thermal energy generation occurring in the outer margins of a polymer flow channel results from high shear rates between the moving melt and the stationary steel wall. This boundary layer heating arises as the polymer molecules slide past each other under intense friction, transforming mechanical work from the extruder screw into thermal energy. The temperature rise remains concentrated in a thin zone adjacent to the cold metal surface rather than spreading evenly through the molten core.
Shear Mechanism
Friction within the flowing polymer increases when the viscosity of the resin is high or the injection speed is elevated. This local temperature rise reduces melt viscosity in the shear zone, causing the material near the wall to flow much faster than the colder core. Because plastic acts as a thermal insulator, the generated heat cannot escape quickly into the mold steel, causing a localized temperature spike that can exceed the set point of the barrel.
Process Effect
Molders must account for this phenomenon during high-speed injection cycles to prevent thermal degradation of shear-sensitive polymers. While moderate heating lowers the injection pressure required to fill the mold, excessive local temperatures can degrade the polymer chains, resulting in reduced tensile strength or cosmetic splay on the molded part. The effect is particularly pronounced in thin-walled parts where high injection rates are necessary to fill the cavity before the gate freezes.
Thermal Correction
Adjusting the injection speed profile allows the molder to manage the shear rate and control the temperature spike. Lowering the initial fill rate decreases the friction at the wall, while raising the melt temperature reduces the baseline viscosity to minimize heat generation. These adjustments prevent material degradation without extending the cycle time of the machine.