Study on instability of the point bolster sheet metal dieless forming

The point bolster dieless forming, which adopts point rod as bolster instead of the conventional model bolster, eliminates the constraint of the mould in order to realize the real dieless forming. Even though the feasibility of this new forming method is verified by plenty of practical experiments and FEM simulation on the axisymmetric part, there are still several kinds of forming instability flaws. These flaws include collapse, radial ridge and circumferential ridge. The generation of radial ridge and circumferential ridge lies on the dimension of the transient plastic region. Furthermore, the dimension of the transient plastic region lies on the following three factors: the material's mechanical properties, thickness of the sheet metal and shape of the forming tool. The collapse comes from the initial instability under the propagation of stress wave if the following three factors are neglected: orientation error, blank holder's oscillation and material's slip around the bolster. The collapse is essentially one kind of shell dynamic buckling. To overcome these flaws, this paper provides corresponding solutions after the instabilities are analyzed and summarized. Meanwhile, these solutions are experimentally validated.