The Optimization of Variable Blank Holder Force for Sheet Metal Stamping

This paper focuses on the research of blank holding force (BHF) and proposes a new strategy to optimize the variable BHF and determine the drawing limit under the variable BHF. The optimization strategy is based on the analysis of BHF formability window and integrated into FEM code to obtain time or/and spatial optimal BHF that applies on binders. And then a stepped rectangular box of 60 mm drawing height with segmented binders and a cylinder cup without die bottom are adopted to validate the optimization strategy. Firstly, constant BHF FEM simulations through careful experiment design are conducted to discover the drawing limit height under maximum constant BHF. Secondly, FEM simulation combining the new BHF optimization strategy is carried and the optimal profile of variable BHF is determined for both step box and cylinder cup during the whole punch stroke. The result shows this stepped box could be formed successfully using 5754-0 aluminum alloy sheet, while the maximum drawing height under constant BHF is 45mm. And the drawing limit of cylinder box under optimal BHF could reach the drawing height of 61mm, which is greatly increased comparing with maximum of 47mm under constant BHF. Finally, the constant BHF experiment and the derived trajectory of optimal BHF of both geometries are verified on a multipoint variable BHF hydraulic press and the experiment results correspond well with those of FEM simulation. This new BHF optimization strategy not only helps to determine time and spatial optimal BHF for known geometry, but also assists to design maximum drawing height for unknown geometry, which makes it an effective method.