Forming Limits for Shrink Flanges of Rubber Formed Parts

Abstract

Rubber forming is an ideal process for the manufacture of a wide variety of flanged parts in small products series, and is attractive for industries like the aircraft industry. The flanges of a formed part are straight, stretch or shrink flanges, each type having its own limits. The forming limits for the straight and stretch flanges are dominated by the formability of the material and the applied strains, which are related to the (local) geometry of the flange. The prediction of the limits for the shrink flanges is much more complicated, since both plastic flow and instability play a role. In recent years, a number of authors developed methods for the prediction of wrinkle formation in metal sheets in different applications. This paper focuses on an alternative approach for the prediction of failure limits of shrink flanges. Shrink flanges wrinkle right from the start of forming process. At first the deformations are elastic, but for the creation of flanges the material has to become plastic. The geometry of the cross-section of the wrinkles can be approximated by sinusoidal shapes having a length and amplitude. During processing the values of both parameters decrease, although the ratio of the two is even more important and should decrease in order to obtain a wrinkle-free part. In the paper the topic is addressed using experimental data, showing the influence of the most important variables like materials properties, strain values, and variables like thickness. Subsequently the parameters are used to provide relations between the different variables, which are used for numerical simulations.