Capillary Rheometry Limits for Post Consumer Resin in Multicavity Tools
Capillary rheometry establishes binding high-shear viscosity limits that prevent multicavity tool imbalance and part weight variance in post-consumer resins.
Cavity Mass Variance
Meaning
Gravimetric distribution across individual impressions within an injection mould defines cavity mass variance. During multi-cavity processing, cavity mass variance quantifies the discrepancy in component weight between different mould locations under identical machine setpoints. Thermal imbalances across the hot manifold and uneven gate wear generate these weight spreads between shot locations.
The metric applies strictly to simultaneous multi-impression production where all components share common injection and cooling phases. It stops applying when comparing consecutive moulding cycles or evaluating single-cavity tools across multiple machine cycles.
Rheological Divergence
Non-uniform shear heating inside runner bifurcations creates melt viscosity gradients that feed cavities at disparate pressures. When molten resin flows through consecutive runner branches, fluid layers adjacent to steel walls experience severe shear rates, driving localized temperatures higher than the stagnant central core. Downstream branches divide these thermal layers unevenly, sending warmer, lower-viscosity material to specific mould cavities while colder resin fills peripheral cavities.
Consequently, cavities receiving higher-temperature resin fill faster and pack more densely before gate freeze occurs. Virgin polymers show moderate consistency under this shearing, but regrind additions widen melt flow variability and accentuate mass differences.
Packing Asymmetry
Pressure transmission during the holding stage deteriorates when gate dimensions or water channel layouts deviate across the mould frame. A gate freezing fractions of a second earlier than an adjacent gate terminates cavity pressurization prematurely, leaving that moulded part deficient in mass and vulnerable to sink defects. Conversely, delayed gate freeze allows additional resin packing, yielding heavier parts and elevated residual stress.
Molders cannot eliminate this imbalance solely by altering overall injection pressure on the machine controller, because intensifying packing pressure expands the absolute mass spread between the tightest and loosest gates.
Mass Tolerance
Specification sheets establish permissible part weight limits to maintain mechanical performance and continuous assembly fit. Uncontrolled cavity mass variance forces moulders to run high packing pressures to satisfy the lightest cavity, which generates flash and part sticking on the heaviest cavity. Rejection rates escalate whenever the gravimetric spread exceeds three percent of the nominal target weight.