ABSTRACT
In many manufacturing processes the relationship between the quality characteristic of interest and one or more of the process variables may be non-linear in nature. This is very likely to be the case when the levels for the process variables are close to the optimum operating conditions for the process. There are a large variety of designs available for dealing with non-linear relationships. One obvious possibility is to extend the 2k factorial design to the 3k design. That is, to study k factors each at three different levels. These designs are discussed this chapter and an illustration using the friction welding case study is provided. More efficient and statistically pleasing designs include the central composite design, which is explained in this chapter. The chapter discusses the Box-Behnken design. Under certain conditions these two designs are in fact fractions of the 3k design and can be combined to give the full 3k design. Factorial designs with mixed levels are then discussed.
