Process Targeting for Optimal Capability when the Product is Subject to Degradation

Abstract

A performance capability measure is proposed that is useful in a wide class of production processes. Traditional quality improvement methodologies concentrate on getting the process right in order to meet the capability requirements of the product immediately after production, as measured for a key quality characteristic by one of the popular capability indexes. This is not the best strategy when the characteristic degrades over time in use. The implications of this type of degradation on targeting the production process are investigated and illustrated for five typical degradation models. A method is given which enables the optimum settings for the target values of the process mean and standard deviation to be chosen, given a particular model for product degradation. Commonly used philosophies which advocate targeting at the centre of the specification range are shown to be sub-optimal. A useful consequence of better targeting practice is that warranty periods can be extended without increasing the costs associated with supporting warranty claims.