What are response surface designs, central composite designs, and Box-Behnken designs? – Minitab
A Box-Behnken design is a type of response surface design that does not contain an embedded factorial or fractional factorial design.
- Temperature: 190° and 210°
- Pressure: 50Mpa and 100Mpa
- Injection speed: 10 mm/s and 50 mm/s
For example, you would like to determine the best conditions for injection-molding a plastic part. The factors you can set are:
- Temperature: 190°, 200°, and 210°
- Pressure: 50Mpa, 75Mpa, and 100Mpa
- Injection speed: 10 mm/s, 30 mm/s, and 50 mm/s
For a Box-Behnken design, the design points fall at combinations of the high and low factor levels and their midpoints:
Box-Behnken designs have treatment combinations that are at the midpoints of the edges of the experimental space and require at least three continuous factors. The following figure shows a three-factor Box-Behnken design. Points on the diagram represent the experimental runs that are done:
These designs allow efficient estimation of the first- and second-order coefficients. Because Box-Behnken designs often have fewer design points, they can be less expensive to do than central composite designs with the same number of factors. However, because they do not have an embedded factorial design, they are not suited for sequential experiments.
Box-Behnken designs can also prove useful if you know the safe operating zone for your process. Central composite designs usually have axial points outside the “cube.” These points may not be in the region of interest, or may be impossible to conduct because they are beyond safe operating limits. Box-Behnken designs do not have axial points, thus, you can be sure that all design points fall within your safe operating zone. Box-Behnken designs also ensure that all factors are not set at their high levels at the same time.
