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Weldline, which has an influence on the strength as well as the surface appearance of a plastic product, is one of the crucial defects in plastic injection molding (PIM). Process parameters such as melt temperature, injection time and so on should be adjusted and optimized for the weldline reduction. Rapid heating cycle molding (RHCM) that controls the mold temperature during the PIM is an effective approach for the weldline reduction, but the mold temperature profile is completely determined by an engineering experience or a trial and error method. In addition, some process parameters such as melt temperature, injection time and packing pressure also play an important role for the weldline and cycle time. In this paper, for weldline reduction and short cycle time, the mold temperature profile and several process parameters in the RHCM are determined by using sequential approximate optimization. The mold temperature during heating time is maximized for the weldline reduction, whereas the cycle time is minimized for the high productivity. Through the numerical result, the trade-off between the mold temperature and the cycle time is clarified. Also, it is found that the higher mold temperature is, the shorter weldline is.

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High strength steel (HSS) have several advantages such as lightweight and high shock absorption. In sheet metal forming of HSS, twist springback occurs remarkably, which results in the low product quality. In addition, major defects such as wrinkling and tearing easily occur due to the low formability. Blank holder force (BHF) and blank shape have a direct influence on the twist springback, the wrinkling and the tearing. Variable BHF (VBHF) that the BHF varies through the stroke is valid for the twist springback reduction. In this paper, we propose a novel method to evaluate the twist springback, in which both the torsion angle of top of the product surface and the angle of left/light flange are evaluated. Based on the evaluation, a design optimization to determine the VBHF trajectory and blank shape is performed under several design constraints. The numerical simulation is so intensive that a sequential approximate optimization using radial basis function network is adopted to determine the optimal VBHF trajectory and the blank shape. Thorough the numerical simulation, the validity of proposed approach is examined.