Optimizing Core Layouts for Polymer Flow and Sink Prevention
Balancing standing steel thickness below sixty percent of nominal wall and maintaining turbulent cooling prevents cosmetic sink marks.
Post-Gate Freeze
Meaning
The injection moulding process segment occurring immediately after the gate freezes solid isolates the cavity polymer mass from screw pressure within the runner system. Prior to post-gate freeze, the machine screw holds packing pressure to feed supplemental molten plastic into the cavity, offsetting volumetric thermal shrinkage as the polymer cools. Once solidification blocks the restricted gate orifice, cavity pressure drops autonomously as the isolated melt cools and crystallizes against the tool steel.
Process engineers determine this transition point through gate seal studies, establishing the precise hold time required to reach peak part weight and dimensional stability. Any adjustments made to holding pressure or machine pack time after this closure occurs produce zero physical effect on the final molded part.
Cooling Dynamics
Trapped molten polymer inside the isolated cavity block contracts volumetrically as it drops from melt temperatures down toward the tool temperature setpoint. During post-gate freeze, the internal cavity pressure decay curve dictates whether the cooling part remains pressed tightly against cavity walls or pulls away prematurely. Amorphous polymers like polycarbonate shrink gradually, whereas semi-crystalline polymers such as polyethylene and polyamide undergo sharp volume drops as polymer chains organize into dense crystalline lamellae.
If the gate freezes before complete cavity packing, cooling melt draws unreplenished material inward, creating internal voids, deep sink marks over structural ribs, and excessive part shrinkage.
Shrinkage Control
Part weight stabilizes precisely when the packing phase extends up to the exact threshold of post-gate freeze. Extending hold times past gate seal wastes electrical energy, increases cycle time unnecessarily, and accelerates machine hydraulic component wear without improving part density. Conversely, cutting pack time short allows pressurized melt to backflow through the unsealed gate into the runner, producing under-packed, lightweight parts with out-of-tolerance dimensions.
Moulding technicians establish this critical balance by weighing consecutive parts while incrementally raising pack time until part mass plateaus on a gate freeze curve. Tracking this stabilization threshold ensures uniform part densities across multi-cavity hot runner moulds.
Warpage Prevention
Non-uniform gate solidification across multi-cavity tooling induces uneven pressure distributions that trigger component warpage upon part demoulding. Cavities experiencing premature post-gate freeze exhibit low final pack density, higher total shrinkage, and poor dimensional repeatability compared to slower-freezing positions. Unequal thermal dissipation within the tool or unbalanced runner dimensions causes gates to seal at different times, locking variable residual stress patterns into individual parts.
As thermal cooling continues toward ejection temperature, internal stress differences cause thin-walled covers and flat frames to twist or bow out of design specification. Balancing gate geometry and tool cooling circuits synchronizes freeze timing across every cavity, delivering predictable part dimensions across long production runs.