For multiple quality indices, the above two classes of optimization focus on searching the single optimal com- bination of process parameters. However, it is known that the trade-off relationships exist among multiple quality in- dices, so the searching task of the single optimum is not easy. Castro et al. (2007) used DEA technique to find   the

efficient frontier when six quality indices (related with the part’s geometry) were considered. This research proposes to combine several techniques including experimental de- sign, analysis of variance (ANOVA), multiple regression analysis, and DEA to find the efficient frontier of the process parameters when three essential quality indices, warpage, shrinkage, and volumetric shrinkage at ejection, are under consideration simultaneously.

Materials  and product

This study demonstrates how to determine the efficient frontier of process parameters for injection molding by taking an example of the thin cover of a digital camera. The CAD software, Solidworks, is  the  first  to prepare the geometry of the product as shown in Figure   1.

Next, the commercial CAE simulation software tool, Moldflow, is utilized to create the finite element model and uses the finite element and finite difference method to solve pressure, flow, and temperature fields of injection molding (Walsh 1993; Mackerle 2005; Shoemaker 2006).

Table 1 Material properties of GE Cycoloy C2950 PC/ABS

Material property Value

Recommended mold surface temperature (°C) 70

Recommended melt temperature (°C) 275

Melt density (g/cm3) 0.97618

Solid density (g/cm3) 1.1161

Eject temperature (°C) 113

Maximum shear stress (MPa) 0.4

Table 2 Key process parameters and their ranges of operation

to affect products’ quality. Table 2 shows nine process parameters  and  their  ranges  of  operation  suggested  by

Parameter Variable Initial value

Range of operation

Moldflow. Three indices of quality, warpage, shrinkage, and  volumetric  shrinkage  at  ejection,  are  considered in

Injection time INT (x1) 1 s 0.5 to 1.5 s

Injection pressure INP (x2) 120 MPa 100 to 140 MPa Packing pressure PP (x3) 100 MPa 80 to 120 MPa Packing time PT (x4) 10 s 7.5 to 12.5 s Cooling time COTI (x5) 19 s 14 to 24 s

7

Melt temperature MET (x8) 275°C 270°C to 280°C

this paper.

The warpages are measured at four  reference  nodes, and the shrinkages are measured at four edges in this paper; these nodes and edges are depicted in Figure 4 of the XY horizontal plane. In Figure  4,  four  reference nodes N1991, N1962, N1955, and N2261 are shown, while four edges are formed by these four nodes and are denoted by E1, E2, E3, and E4, respectively.