Meaning
Polymer viscosity variations occurring across localized velocity gradients dictate the distribution of shear rate micro zones during high pressure injection moulding. Fluid velocity differences inside runner systems and gating geometries establish these distinct regions where polymer chains experience unequal mechanical stress. Processing stability relies on maintaining uniform molecular orientation within acceptable tolerances across the entire part geometry.
Thermal Gradient Mechanics
Frictional heating rises substantially inside localized high velocity fluid streams. Viscous dissipation generates higher melt temperatures directly adjacent to cavity walls while core material remains cooler. Thermal expansion rates diverge between adjacent zones and warp the finished part if cooling channels fail to remove heat evenly.
Resin Specification Limits
Molecular weight distribution profiles determine how polymers react under localized stress conditions. Virgin pellets tolerate higher velocity gradients before chain scission degrades mechanical properties compared to heavily degraded regrind compounds. Melt flow rate tolerances published on raw material datasheets often mask localized viscosity swings that cause dimensional distortion on the shop floor.
Moulding Parameter Drift
Hydraulic pressure fluctuations inside the injection cylinder alter the boundaries of velocity gradients without warning. Screw speed adjustments shift the location of thermal peaks and change internal stress patterns throughout the moulded component. Part specifications demand strict repeatability because minor machine drift expands localized stress zones beyond acceptable engineering margins.
