Establishing Press Side Fill Balance Protocols for Multi Cavity Injection Moulding Tooling
Press-side fill balance protocols decouple injection velocity from hold pressure using short shot weight analysis to equalize cavity fill within 5% balance.
Runner Shear Rate
Meaning
Velocity gradient measurements describing laminar fluid deformation within the delivery channels of an injection mould quantify the mechanical work applied to polymer melt before cavity ingress. Tooling engineers compute the runner shear rate to balance melt flow, predict thermal generation, and prevent molecular degradation during the injection phase. The parameter depends on volumetric flow speed, runner geometry, and polymer melt rheology, scaling inversely with the third power of circular channel radius.
Because molten thermoplastics exhibit pseudoplastic shear-thinning characteristics, higher shear rates temporarily lower apparent viscosity while simultaneously generating friction-driven heat. Excessive shear induces polymer scission, cosmetic discolouration, and gate blush defects in finished parts.
Fluid Dynamics
Mathematical formulations for non-Newtonian flow through cylindrical conduits derive shear rates directly from volumetric flow rate and channel dimensions. In round cross-section runner channels, the runner shear rate reaches its maximum value at the channel circumference, while dropping to zero at the stationary center of the flow stream. Newtonian fluid assumptions yield simplified velocity gradients, but polymer melt calculations apply the Weissenberg-Rabinowitsch correction to account for shear-thinning behavior under commercial injection velocities.
Molders maintain channel shear values within resin-specific thresholds, typically between one thousand and ten thousand reciprocal seconds, to avoid structural breakdown.
Thermal Interaction
Viscous dissipation within narrow feed channels converts mechanical injection power directly into localized thermal energy across the moving melt. High shear rates accelerate this viscous heating, elevating the melt core temperature above the barrel set-point before the flow front reaches the mould cavities. While controlled shear thinning lowers injection pressure requirements, unmanaged shear creates localized hotspots that degrade heat-sensitive resins like polyvinyl chloride, polyoxymethylene and flame-retardant polyamides.
Moulding technicians observe gate splay, surface jetting, and erratic structural warpage when shear heating drives melt temperatures beyond thermal degradation thresholds.
Tooling Optimisation
Runner sizing represents a permanent tooling commitment that sets baseline processing margins for the operating life of an injection mould. Oversized runner channels lower the runner shear rate and prevent thermal degradation, but they extend mould cooling cycles and generate excessive regrind material. Undersized channels minimize material consumption yet require extreme injection pressures, risking polymer chain degradation and flash at the parting line.
Designing balanced runner networks requires coordinating channel diameters against gate geometries to maintain uniform shear rates, consistent filling kinetics, and uniform part densities across all tool cavities.