A variation of lubrication condition on die surface during square cup sheet hydroforming was investigated experimentally. The lubrication condition on the die surface became worse in the late stage of the process. This would be because the thickening at the sheet edge in the lateral directions became larger than the film thickness of pressure medium; thus the contact area between the sheet and the die surface increased. To further investigate the difference in the lubrication condition depending on a position on the die surface, some holes were made on the die surface to let the pressure medium flow out through the holes during the process. The lubrication condition became further worse when the pressure medium was released through the holes in the diagonal directions, while remained almost unchanged when released in the lateral directions. This difference might be because the lubrication condition in the lateral directions became worse owing to the thickening in either case, whereas that in the diagonal directions became worse only when the pressure medium was released in the diagonal directions. The above results showed that the lubrication condition was different depending on the position on the die surface.
Hama, T., Kojima, K., Nishimura, Y., Fujimoto, H., & Takuda, H. (2014). Variation of lubrication condition during sheet hydroforming. In Procedia Engineering (Vol. 81, pp. 1029–1034). Elsevier Ltd. https://doi.org/10.1016/j.proeng.2014.10.136