【正文】 he first operation in the twooperation stamping process was sidewall forming as shown in Fig. 4(a), and the second one was the forming of flange of hinge presented in Fig. 4(b), the height of the flange of hinge being 5 mm. Fig. 4(c) shows the thickness distribution obtained from the finite element simulation. The minimum thickness of the deformed sheet was mm and the strains were all above the forming limit diagram. It means the fracture defect could be avoided. In addition, the height of the flange conformed to the target goal to be achieved. However, this process produced a critical defect of 11 wrinkling, as shown in Fig. 4(d), on the flange of hinge, which induces a problem in the subsequent trimming operation. Hence, even though the twooperation stamping process solved the fracture problem at the corner of the bottom and the flange of hinge, a better forming process is still expected to solve the wrinkling of flange of hinge. Fig. 4. Twooperation stamping process. (a) Formation of sidewalls, (b) formation of hinges, (c) thickness distribution and (d) wrinkle. View thumbnail images . Fouroperation stamping process The fouroperation forming process proposed in the present study starts with the forming of three sidewalls and the flange of the hinge with a generous corner radius, as shown in Fig. 5(a). Since the sidewall close to the flange was open and the corner radius was larger than the desired ones, the flange was successfully formed without fracture. Such process successfully avoided the difficulty of forming two geometric features simultaneously, but increased the material flow of the blank sheet. The next step was to trim the blank outside the sidewalls, and to calibrate the corner radius of 4 mm to the desired value of mm. The hinge was thus formed, as shown in Fig. 5(b). The third step was to fold the open side, so that the sidewall could be pleted around its periphery, as shown in Fig. 5(c). The effect of trimming the extra sheet outside the sidewalls in the second step on the third step was studied. 12 When the extra sheet was not trimmed, the thickness at the corner was mm, as shown in Fig. 5(d). The thickness of the corner increased to mm, as shown in Fig. 5(e), if the trimming was implemented in the second step. The excessive material produced by the folding process in the third step was then trimmed off according to the parts design. The last step was the striking process that is applied to calibrate all the corner radii to the designed values. The minimum thickness at the corner of the final product was mm, and all the strains were above the forming limit diagram. It is to be noted that Fig. 5(a–c) only shows the formation of one hinge. The same design concept was then extended to the stamping process of the plete top cover case. Fig. 5. Fouroperation stamping process. (a) First operation, (b) second operation, (c) third operation, (d) without trimming and (e) with trimming. View thumbnail images 5. Experimental validation In order to validate the finite element analysis, an actual fouroperation stamping process was conducted with the use of mm thick LZ91 sheet as the blank. The blank dimension and the tooling geometries were designed according to the finite element simulation results. A sound product without fracture and wrinkle was then manufactured, as shown in Fig. 6(a). To further validate the finite element analysis quantitatively, the thickness at the corners around the hinge of the sound product, as shown in Fig. 6(b), were measured and pared with those obtained from the finite element simulations, as listed in Table 1. It is seen in Table 1 that the experimental data and the finite element results were consistent. The fouroperation process design based on the finite element analysis was then confirmed by the experimental data. 13 Fig. 6. The sound product. (a) Without fracture and