Polymer Melt Flow Behavior in High Shear Injection Molds
High shear molding relies on pseudoplastic shear thinning, requiring capillary rheometry over melt flow index to control viscous heating and pressure losses.
Pin Gate Geometry
Meaning
Narrow feed orifice configurations restrict polymer melt flow into cavity spaces through small circular entry points. The dimensions defining pin gate geometry determine injection pressure drop, shear heating rate, and vestige height on finished plastic components. Datasheet viscosity measurements fail to reflect the extreme shear rates generated when melt passes through sub-millimeter gate land diameters.
Tool designers select orifice diameters and land lengths to achieve automatic degating during mold opening cycles.
Flow Restriction
Tapered circular channels reduce melt volume immediately prior to cavity entry. This abrupt cross-sectional contraction increases flow resistance, requiring elevated injection pressure from the molding machine. Land length influences both pressure drop and mechanical strength at the gate break point.
Excessively long land sections cause high pressure losses, while overly short lands erode under continuous abrasive melt flow during extended production campaigns.
Shear Generation
Intense shear stress inside pin gate geometry lowers melt viscosity through local shear thinning. Controlled shear heating assists mold filling when processing viscous technical polymers.
Degating Efficiency
Clean fracture during mold separation depends on precise gate land taper and tip diameter. Adding regrind resin increases melt elasticity, which can cause stringing or incomplete gate separation during automatic plate opening. Proper land design concentrates stress at the part surface, ensuring minimal gate vestige without pulling material from the cavity wall.
Controlling runner system geometry preserves automatic operation and maintains consistent part aesthetics.