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tors for the quality of a setting are mentioned here as examples: First, can the injection drive follow the selected injection velocity profile? That would not be the case if the injection-pres- sure limit chosen for mold protection is reached. Second, is the injection- pressure curve predomi- nantly the same from shot to shot? This would not be the case, for example, if cold slugs in the nozzle caused fluctuations at the beginning of the injection process. Modern machine controls, like the CC300 from Engel, help engineers to find answers to these questions, and thereby fulfill important prerequisites for the robust- ness of the injection molding process (see Practical Tip 2 below). • Repeatability Tests with Process Data Measurement is diffi- cult and expensive. Why not, as a first approximation, use process data—i.e., actual values from the machine—instead of measure- ments on the part to judge process repeatability? Do not a uniform melt cushion and a stable metering time already say a lot about the quality of the machine? However convenient that approach may seem, it is not possible to relate such process data to the part quality. It becomes particularly critical when a machine capability index is calculated from process data. As described above, this requires tolerance limits. For process data, however, such tolerance limits cannot be found in the part drawing. They must be freely invented, which takes us directly to the next point. • Relaxed Handling of Tolerance Limits "The simplest way of increasing the process capability of a given process consists in relaxing the specification limits" (Wikipedia). For instance, the greater the difference between the upper specification limit and lower specification limit, the more standard deviations can be accommodated between them. These obvious truths should not A common error is to sample parts before determining whether the process has settled down into a steady state. QUESTIONS ABOUT INJECTION MOLDING? Visit the Injection Molding Zone. Process Control