Process Capability Assessment with Trended Processes: A Case Study

Process capability indices play an important role in process control as they assist in deciding the suitability of a manufacturing process to meet the desired quality requirements. In order to achieve high quality with optimum cost, shifting from product control to process control is essential in manufacturing organizations. Process capability study is a prerequisite before moving in to full scale production. The successful application of traditional process capability indices requires the process to be stable and normally distributed, but it is not the case for trended processes. Interpretation of process capability for trended process is a critical issue. In this paper, the traditional process capability indices were tested for the trended process. Two different sources of process variation, one from the process trend, the other from random variation of individual observation were estimated and used to develop new process capability indices for trended process. The new indices that were developed for trended process showed that the capability of trended processes could be improved through periodic replacement of the tools to the maximum achievable value (1.33). This paper presents the step by step procedure to determine the tool adjustment frequency to meet the pre specified requirements on the capability index when the quality characteristic of interest is represented by a linear trend. A case study was conducted to evaluate the performance of the traditional and proposed indices for trended process. Results of the process capability indices for trended process were found satisfactory.