In my experience, most molders do not take the time to determine the optimal cooling time. This should be done after the minimal acceptable mold temperature is determined.
Cooling Time Study: Essentially, you should incrementally step down the cooling time and allow the process to stabilize between each step. The parts produced at each step should be tested for quality to determine the lowest time which produces an acceptable part.
Optimal Cooling Time: The optimal cooling time setting is 10% above the minimal acceptable time. This provides a buffer to compensate for normal fluctuations in mold and melt temperature. You should allow the process to run for a while at this time while checking quality to ensure you have allowed for process stabilization.