Meaning
Flow thresholds dictating local pressure drops suppress vapor bubble formation in liquid polymer streams passing through narrow gate passages. High shear rates at restrictive hot runner tips or pin gates push fluid pressures below local vapor pressures, initiating dynamic cavitation limits. Transients exceeding these limits generate micro-voids, polymer degradation, and cosmetic splay in finished injection moulded components.
Shear Limit
Velocity gradients inside narrow runner channels dictate local pressure drops during injection. Exceeding dynamic cavitation limits degrades sensitive polymer chains and produces surface void defects.
Gate Optimization
Channel cross-sections dictate shear rates and local fluid acceleration during high-speed injection moulding. Operating within dynamic cavitation limits requires expanding gate diameters or introducing streamlined radius transitions at runner intersections. Smooth flow geometry prevents sharp pressure drops that trigger dissolved gas expansion and micro-bubble nucleation.
Adjusting injection velocity profiles allows moulders to maintain high volumetric flow rates while keeping peak shear stresses safely below critical cavitation boundaries.
Melt Quality
Dissolved moisture and volatile organic compounds exacerbate void generation under high fluid shear conditions. Exceeding dynamic cavitation limits releases trapped gases into the melt stream, creating silver streaks on molded part surfaces. Proper resin drying combined with controlled gate velocity maintains homogeneous polymer melt.
