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Recent global efforts to resolve environmental issues are required in sheet metal forming. For this reason, some research projects have been focused on unlubricated processes as well as on the use of volatile lubricants. However, low formability due to poor lubrication conditions is still a matter of concern. In this study, in order to improve the friction conditions, a new algorithm for controlling blank holding force (BHF) and punch speed (SPD) was proposed. The concept is to separate the wrinkle eliminating process from drawing process. That is, the process proceeds with an extremely low BHF until a wrinkle sufficiently grows, and the wrinkle is eliminated by BHF loading without punch penetration. Its effectiveness was investigated by finite element (FE) simulation, in which two blank models (0.5 mm and 1.0 mm thickness) were used. As a result, the maximum forming forces decreased by 5.33% (0.5 mm) and 1.55% (1.0 mm) compared with those of the uncontrolled model at a constant minimum BHF. In addition, a thinner blank sheet is better for the algorithm on the basis of the result that cup height decreases and thickness distribution increases in the case of 0.5 mm thickness.

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In the sheet stamping of strain-rate-sensitive materials, the forming limit is governed by punch speed (SPD) as well as blank holding force (BHF). In previous research, a combination control algorithm for simultaneously varying SPD and BHF was proposed to achieve high-speed deep-drawing. In this study, finite element (FE) simulation is conducted for circular-cup deep-drawing with combination SPD/BHF control in order to reveal the deformation mechanism of the improvement of deep-drawability. The numerical results demonstrate that even though the blank material has a low strain-rate dependence at room temperature, the drawability and production rate can be improved by properly varying SPD/BHF. It is found, from the simulation results, that a relatively low SPD with a low BHF in the early forming stage causes less thickness reduction at the punch shoulder. In the middle-to-last stage, therefore, the remaining blank can be successfully drawn into the die cavity with SPD and BHF higher than the critical constant SPD and BHF.