Meaning
Statistical performance metrics evaluate the overall capability of a manufacturing process over an extended period without assuming that the process is in a state of statistical control. Quality managers apply the ppk index to assess whether a moulding run can consistently meet specification limits when accounting for all sources of variation. This metric represents the actual, historical performance of the process rather than its potential capability under ideal conditions.
Statistical Calculation
Calculating this value involves using the standard deviation of all individual measurements collected across the entire production campaign. In contrast to other capability indices that use within-subgroup variation, the ppk index incorporates long-term drift and material lot changes. A low value suggests that the process lacks stability, which may require adjustments to the moulding parameters or the resin formulation.
Process Evaluation
Process validation protocols for automotive or medical injection moulding typically demand a value of at least one point six seven before mass production can begin. Using the ppk index allows manufacturers to demonstrate to their customers that the combination of tooling and material can produce parts that are consistently within tolerance. This provides a clear picture of the scrap rate that is likely to occur during future production runs.
Sourcing Strategy
OEMs use these performance indicators to compare the capabilities of different injection moulding suppliers. A supplier with a high performance index can minimize the need for manual inspection, which helps reduce production costs.
