Meaning
Dynamic differences in flow resistance within polymer feed channels cause molten resin to fill multi-cavity injection molds at unequal rates. A runner imbalance occurs when geometric differences, temperature variations, or non-uniform shear profiles cause certain cavities to fill ahead of others. The defect creates part weight variations, dimensional instability, and localized flash or sink marks across mold positions.
Boundary covers multi-cavity hot and cold runner moulding systems, excluding single-cavity molds or direct-gated tooling. Mold engineers perform short-shot studies to identify which cavities experience premature filling.
Rheological Variance
Non-uniform shear distribution inside runner intersections alters polymer viscosity across branching flow paths. A runner imbalance develops even in geometrically symmetric runners due to asymmetric frictional heating along channel walls. Higher melt temperatures lower resin viscosity, accelerating fluid velocity toward specific cavity gates.
Standard dimensional inspections fail to detect thermal sources of filling disparity between identical channels.
Processing Consequence
Mold cavities receiving melt earlier experience prolonged packing pressures while lagging cavities are still filling. A runner imbalance causes density variations, warpage, and dimensional non-conformance between parts produced in the same moulding cycle. Flash occurs around early-filling cavities, while short shots occur in under-filled locations under identical machine parameters.
Scrap rates rise as process windows shrink to accommodate extreme cavity variations.
Diagnostic Remediation
Incremental fill studies isolate specific channels suffering from premature or delayed resin arrival. Correcting a runner imbalance requires adjusting channel diameters, rebalancing thermal zones, or installing shear-flipping runner inserts. Mold modifications restore uniform flow front arrival times across all cavity gates.
Verification audits confirm that part weight variation drops within acceptable quality limits post-remediation.
