Raw Tooling Trial Witnessing Protocols for Multi Cavity Injection Moulds
Press side multi cavity trial witnessing mandates independent volumetric fill verification, gate seal determination, and 24 hour conditioned metrology
Rheological Curve
Meaning
Viscosity measurement plots map the shear stress or viscosity of a polymer melt against increasing shear rates to define the fluid behavior of a resin. A rheological curve identifies the pseudoplastic nature of molten plastics where molecular chains align under deformation. This graphical representation differentiates between Newtonian flow and shear thinning regions across multiple decades of shear rate.
Data points derived from capillary or rotational rheometers build this profile. The resulting plot establishes the boundary for processing windows during injection moulding or extrusion.
Viscosity Prediction
Engineers assess how a specific material grade thins under the high speeds found at a gate or through a narrow nozzle. A rheological curve allows for the simulation of pressure drop across complex flow paths in a tool cavity. High viscosity at low shear rates prevents resin drool at the nozzle tip during the pause between cycles.
Decreased viscosity at high shear rates facilitates mold filling by reducing the force required from the screw. Production variability emerges when the actual shear thinning performance departs from the technical data sheet. Operators calculate the energy input required for stable part geometry based on the slope of this profile.
Thermal Sensitivity
Temperature shifts move the entire curve along the vertical axis for amorphous polymers and crystalline resins. An increase in heat input lowers the baseline viscosity and shifts the onset of shear thinning to lower rates. Moulders compensate for shifts in batch consistency by adjusting barrel temperatures to match the known curve.
Degraded material from excess residence time or moisture contamination alters the molecular weight distribution. This shift manifests as a lower torque requirement or a change in the injection pressure profile during the filling stage. Stable processing depends on maintaining the thermal state of the polymer to prevent the deviation of the viscosity from the target curve.
Flow Control
Regrind usage changes the shear thinning behavior compared to virgin resin due to shorter chain lengths from previous thermal cycles. The rheological curve serves as the primary diagnostic tool for characterizing the impact of filler loading on flow resistance. High glass fiber concentrations force a steeper profile that requires higher injection pressures to achieve complete mold filling.
Precise control over the shear history of the polymer prevents excessive degradation in the hot runner system. Proper characterization of these curves defines the capability of a tool to replicate a part design across different production environments.