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limit. One would then change the setting parameters in order to influence the shrinkage and so reduce the length somewhat. If this is not possible, the mold geometry would have to be changed. PITFALLS OF REPEATABILITY MEASUREMENT If the injection molder has performed a repeatability test, he obtains at least one of the above-described numerical values. This allows the process—or, more accurately, a snapshot of the process—to be assessed. It has been found in practice that the results of a repeatability measurement often do not stand up to closer scrutiny. There are a host of error sources, the most common of which are described below: FIG 2 From measurements of part length over 50 successive cycles and the resulting standard deviation (shown as a double arrow), the machine capability indexes c m , c m,l and c m,u are calculated. Part Length, mm Shot Count 39.3 39.25 39.2 39.15 39.1 39.05 Machine Capability Indexes 0 10 20 30 40 50 6σ 3σ 6σ 3σ 3σ 3σ σ σ Upper Tolerance Limit Lower Tolerance Limit c m =2.32 c m,l =3.05 c m,u =1.59 INJEC TION MOLDING