The objective of this study is to set up a more general and practical formulae for backward extrusion of shaped tubes.
Instead of using conventional extrusion ratio, an "effective extrusion ratio" is defined in order to take into account the effect of non-circular shapes by taking extrusion ratio through infinitesimal meridional sector at any angular position and by summing up the values throughout the region.
Thus an empirical formula is derived by using the effective extrusion ratio. Through various experiments, the constants for the empirical formula is determined. It is found out that the formula utilizing the effective extrusion ratio is widely applicable to various shapes including tubes with internal polygonal holes.
In order to extend the applicability of the formula, e.g, to include the backward extrusion of tubes with similar internal and external contours, a new shape factor of "effective extrusion ratio" is defined in addition and is effectively applied to the backward extrusion of similar-shaped tubes.
Variation of punch pressure with respect to the change in the length of land is also studied in order to establish the effective punch design in backward extrusion. Experiments are carried out for commercially pure aluminum billets. The use of the proposed empirical formulae for various shaped tubes proves the wide applicability of the formulae.
