Significance of strain path in conditions of variable blank holding force in deep drawing

Abstract

The process of deep drawing is influenced by many factors. The only two factors that could be modified in course of the process, by using special devices, are holding force and position of draw beads. In this paper the results of investigations of galvanized sheet metal zinc coated at one side (marked TyZnI) will be given. Material is 0.8 mm thick. Work piece is of a cylindrical geometry with diameter 50 mm. Blank diameter is 110 mm. For that geometry, variable holding force (VBF) dependence was obtained by empirically-analytical procedure. Functions were obtained at given condition of constant specific holder pressure (VBF decreasing dependence) and at condition of wrinkling prevention (VBF increasing dependence). In the first case, VBF was applied, with change of contact conditions (dry, oil, and oil plus polyethylene foil) without strain history change (one phase procedure). In the second case, two phase procedure was performed. At first phase sheet metal has undergone uniaxial tension towards rolling direction. At the second phase, out of the strained sheet metal blanks were cut out for the operation of deep drawing. The effects of the VBF and variable strain history influence are monitored through principle plain strains distributions, thinning strain distributions, depth and drawing force. In the conclusion we should emphasise that decreasing VBF shows obvious advantages in case without strain history change. With tension at the first stage, decreasing holding force does not bring to improvement concerning the drawing depth, but more uniform thinning strain distribution is obvious. Improvements brings increasing VBF (enlarging drawing depths, better strain distributions). By computer simulation, theoretical and experimental investigations it is possible to define the VBF which would improve results in each particular forming case.