“…In order to fit the response surface better, the central composite design (CCD) method is used to select the initial sample points. Model of CCD consists of eight runs at the corners of a cubic (±1, ±1, ±1), plus six runs at the star points, (0, ±α, 0), (0, 0, ±α), and one run at the cubic centre (0, 0, 0) [17][18][19]. The CCD model is shown in Figure 9.…”