Capability Analysis for a Multi-Process Product with Bilateral Specifications

Process capability indices (PCI) can be viewed as effective and excellent means of measuring product quality and performance. Several capability indices including Cp, Cpk, and Cpm are well accepted and the implementation of these techniques is successful. The indices Cpu, Cpl, and Cpk have been used previously for evaluating a multi-process product with smallerthe-better, larger-the-better, and nominal-the-best specifications. For a multi-process product with only the nominalthe-best specifications, the evaluation method should be improved, as index Cpk cannot reasonably reflect the expected process loss. In this paper, index Cpmk is selected to replace Cpk and the application is extended to evaluate simultaneously a multi-process product with both symmetric and asymmetric tolerances. An integrated process capability index for a multiprocess product with the nominal-the-best specifications is proposed. The relationship between the process capability index and the process yield is introduced. A multi-process capability analysis chart (MPCAC), reasonably reveals the status of the process capability for the entire product, is constructed for practical application. A case study of a Sea island micro-fiber product is provided as of a practical application.