In injection molding, economical production can only be achieved with the highest possible process speed and consistently high quality. This is the subject of various studies on the machines, the materials and the process itself. To implement process time reductions, sensors are used to record process parameters in real time. This data can be used to optimize the process in terms of time and quality. If corresponding interactions between machine parameters, process variables and quality characteristics are known, it becomes possible for the machine to design the process independently on its own – so called self-optimizing processes. In this study, a concept for optimizing the residual cooling time is presented. For this purpose, a special mold is used, which allows to detect the thermal contraction on a mold core by means of internal pressure sensors. A new evaluation method allows an automated determination of the time of the completed thermal contraction, which should also represent the ideal time for ejection. The principle is demonstrated using polypropylene. It is also shown that the measuring principle can be used to characterize material additives and how processing parameters influence the residual cooling time.