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.
Injection Gate Shear
Meaning
Localized rate of velocity deformation experienced by molten plastic passing through an entry orifice defines injection gate shear. During the injection phase, injection gate shear dictates both the apparent viscosity of the polymer melt and the localized frictional heating generated at the entrance to the mould cavity. Narrow gates force the polymer chains to slide rapidly past each other, creating intense velocity gradients between the stationary wall and the rapid central stream.
This parameter applies to all melt delivery gates, including pin and edge configurations under high-speed injection. It stops applying once flow enters open cavity spaces where flow cross-sections expand and shear rates drop precipitously.
Viscosity Thinning
Pseudoplastic polymers exhibit dramatic reductions in dynamic viscosity under elevated shear rates inside the gate constriction. Flow resistance drops sharply, which enables high-molecular-weight resins to flow through tiny gate cross-sections without demanding excessive injection pressures from the machine hydraulic system. Polyolefins and styrenics flow with minimal resistance when shear rates approach several thousand reciprocal seconds.
However, if processing velocities drop, viscosity rises immediately, causing hesitation marks and incomplete mould filling. Resin datasheets publish nominal melt flow rates, but real gate viscosity depends entirely on instantaneous gate shear rates.
Thermal Degradation
Viscous dissipation within the shearing zone generates internal heat that can surpass the thermal degradation limit of the resin. When excessive gate shear elevates melt temperatures beyond compound tolerances, polymer chains break, resulting in reduced tensile strength and degraded impact resistance. Flame retardants and glass fibers suffer severe breakdown under aggressive shear, resulting in silver streaking and surface blemishes on part surfaces.
Slower injection velocities or enlarged gate diameters suppress shear heating and preserve molecular integrity.
Geometric Restriction
Geometric standards typically restrict maximum shear rates to forty thousand reciprocal seconds for unfilled engineering thermoplastics. Exceeding this boundary induces jetting, localized gate blush, and elevated residual stress around the mould entry. Proper gate geometry balances fill velocity against permissible shear limits.