Abstract: Being of simple shape and can be easily produced, equilateral frustum shells are often used as initial shells for the dieless hydroforming of small diameter spherical vessels. But in the forming process, equator of the shell tends to withdraw inwards and therefore creates wrinkles here, which badly affects the quality of the vessel. Pointing to this problem, rigid plastic finite element analysis is employed to simulate the hydrobulging process of the shell under internal pressure, then some modifications for the initial shape of the frustum shell are presented to restrict withdrawing of the equator and to prevent it from wrinkling. Results of the simulations show that the modified frustum shell is both simple in shape and effective in anti-wrinkling, and has the capability to obtain a really round vessel which is not possible with an equilateral frustum shell.

Key words: Frustum shell, Hydrobulging, Numerical simulation