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Repeatability means the ability to produce parts that have identical properties. It is therefore often necessary to assess production systems or their components in advance according to whether they ensure a repeatable process. But how? High repeatability over a relatively long period generally requires high robustness to compensate for sudden or gradually changing disturbances. Such disturbances may include fluctuating ambient conditions or the wear of machine and mold. Material properties, too, may be subject to fluctuations from batch to batch, representing disturbances for the process. During the lifetime of a process, who knows what disturbances may emerge, what their extent will be, and—most important—how much they will affect the quality of the part? In practice, these future scenarios are usually neglected; processors restrict themselves to making an inventory of the current repeatability. But on closer examination, even this proves to be a non-trivial challenge. By Josef Giessauf and Christian Maier Engel Austria Machine capability index has become a popular measure of process repeatability—but it's susceptible to a number of pitfalls. Process repeatability is not a constant, but varies over time in response to a number of variables. That variation is not captured by a snapshot in time—measuring a discrete sample of parts. And the only way to manage that variation is with process controls that adjust each and every cycle. How Repeatable is Repeatability? 46 SEPTEMBER 2017 Plastics Technology PTonline.com INJEC TION MOLDING