The production of hollow-ware by deep drawing and bulge forming
Results of a study of the production of hollow-ware by deep-drawing and bulge forming are presented. Axisyrnrnetrical and asymmetrical shapes were successfully produced from soft aluminium flat blanks in one stroke of a punch. The process consists of drawing, ironing and bulging inside a closed die cavity. The constituent operations are studied individually. In deep drawing without a blank-holder, an approach to convex type die design is presented. The investigation evaluates the effect of die profile geometry on the drawing performance. Three dies of the second degree spiral type,one near to the tractrix shape and the other two with larger radius of curvature, are considered. The materials tested include mild steel, stainless steel, soft aluminium and brass. The drawing process through tractrix, exponential spiral, second degree spiral and conical type dies is analysed using a numerical solution formulated earlier and the theoretical results on the punch load and the strain development are compared with the experimental results. Good correlation is obtained on the development of strains. The theoretical prediction of the punch load is reasonably good except for mild steel which is highly anisotropic. It is shown that by modifying the die profile, the 'punch load can be significantly reduced. The reductions predicted by theory are in good agreement with experiment which means that optimum die design for minimum load is possible. In ironing of cups, using soft aluminium blanks, it was found that punch speeds in the range 7-45 rom/sec have negligible effect on the drawing load. The ironing load decreases slightly as the speed increases in this range. The reduction is more significant with higher degrees of ironing. Measurement of ironed cup wall thickness showed that thickness variations are attributed to planar anisotropy of the blank and geometrical errors in tooling. Free bulge forming is used as a simplified approach to closed die forming. The bulge profile modes under different loading conditions of internal pressure and axial force, the effect of the unsupported cup length and the effect of the cup wall thickness on the bulge ratio were investigated using a specially designed test rig. For bulging of as deep-drawn cups, the bulge ratio increases with increase of cup wall thickness, and it decreases with increase of length. For bulging of annealed cups the length effect is negligible and the bulge ratio increases slightly with increase of cup wall thickness. The procedure used in producing different hollow-ware. shapes is described together with typical failure examples. The thickness reduction and bulge ratio distributions are shown and comparison is made between annealed and as deep-drawn cups.