Dynamic Viscosity Curve Generation and Transfer Point Calibration in Technical Molding
Calibrating transfer points via press-side viscosity curves optimizes fill pressure, cuts part mass variation, and locks in DIN 16742 tolerance bands.
Holding Pressure Stage
Meaning
The phase in the injection moulding cycle immediately following volumetric cavity filling maintains hydraulic or electric force on the polymer melt while the part cools. Thermoplastic processing relies on the holding pressure stage to pack additional melt into the mould cavity to offset thermal contraction during solidification. The machine screw holds a designated pressure level until the gate freezes shut, establishing final part density and dimensional tolerance limits.
Pressure control during this phase governs volumetric shrinkage control and internal stress development, stopping once the runner system isolates the cavity from screw pressure.
Volumetric Compensation
Semi-crystalline and amorphous polymers experience volume reduction as melt temperature drops below the transition point during cooling. Hydraulic pressure maintained during the holding pressure stage forces extra polymer mass across the gate, packing the solidifying skin and preventing void formation inside thick wall sections. Crystalline resins like polypropylene shrink up to two percent by volume, requiring prolonged pressure application to maintain nominal part dimensions.
Inadequate pressure transfer leaves unfilled micro-voids and causes sink marks opposite internal ribs. Excess packing pressure increases residual stress, leading to mould ejection binding or post-mould warping.
Gate Freezing
Solidification of polymer within the narrow gate channel establishes the physical boundary for pressure transfer into the cavity. Weight study trials determine gate freeze time by measuring part mass across increasing dwell durations until mass plateaus. Lowering nozzle temperature accelerates gate closure, cutting cycle time while reducing total packed mass.
Defect Mitigation
Precise adjustment of pressure magnitude and packing duration prevents common surface and structural moulding defects. Application of stepped pressure profiles during the holding pressure stage prevents flash at parting lines while eliminating surface depression flaws. Holding pressure levels typically sit between fifty and eighty percent of peak injection pressure.