Meaning
Injection moulding process control refers to the systematic adjustment of cooling parameters and pressure holding phases to prevent the localized shrinkage of polymer material upon the inner surfaces of a cavity. Sink mark elimination governs the thermal transition between the melt phase and the solid phase during the packing stage of cycle development. It defines the boundary where internal volumetric contraction ceases to pull the outer skin of the part inward.
Engineers apply this constraint to wall sections where thickness variation creates differential cooling rates. Proper management of these variables ensures the solidified part maintains a uniform exterior finish without geometric depressions.
Pressure Dynamics
Thermal contraction drives the surface indentations observed when a thick cross section cools faster than the adjacent rib geometry. Sink mark elimination prevents these voids by maintaining high cavity pressure until the gate freezes shut. This action forces additional molten resin into the cavity to compensate for the reduction in density as the polymer molecules rearrange.
Moulders calculate the hold time based on the distance from the injection point and the distance to the furthest thin rib. Short hold times leave the interior core molten and susceptible to collapse when the mould opens. Long hold times increase part stress but provide the stability necessary to prevent surface defects.
Resin Selection
Polymer morphology dictates the sensitivity of a formulation to volume shrinkage during crystallization. Sink mark elimination relies on the specific heat capacity and the crystalline structure of the selected pellet grade. Virgin resins offer predictable shrinkage rates compared to regrind because the molecular weight distribution remains stable throughout the batch.
Moulders adjust the temperature setpoints to account for the variation in viscosity when filler content shifts between different batches of glass fibre. High viscosity materials offer better dimensional stability but require higher injection pressures to drive material into the extremities of the tool. Tooling design influences the effectiveness of these adjustments by setting the geometric limits of the rib depth and draft angle.
Cooling Optimization
Efficient energy transfer out of the metal tool prevents the molten center of a heavy wall from pulling away from the cavity wall. Sink mark elimination creates a balance between cycle speed and the uniform removal of latent heat through conformal cooling channels. Faster cooling rates preserve the surface aesthetic if the pressure profile holds the material tight against the steel surface during the final seconds of the sequence.
Moulders evaluate the success of this strategy by checking the gloss level across the gate area and the midsection of the runner system. Variations in the coolant temperature across the length of the tool cause local soft spots that trigger surface imperfections even when the hold pressure remains constant. The final quality of the moulded component depends on the ability of the operator to synchronize the packing stage with the thermal conductivity of the specific resin grade.
