8   that   the  mesh   at  the  top   edge  of   the  part   is  stretched

Fig. 6. Cross-section lines at different heights of the draw wall for different  blank-holder forces.  (a) 100 kN.  (b) 600  kN.

Fig. 7. Split and wrinkles  in the production    part.

Fig. 8. Simulated shape for the production part with split and   wrinkles.

significantly, and that wrinkles are distributed at the draw wall, similar to those observed in the actual     part.

The small punch radius, such as the radius along the edge A–B, and the radius of the punch corner A, as marked in Fig. 1(b), are considered to be the major reasons for the wall breakage. However, according to the results of the finite- element analysis, splitting can be avoided by increasing the above-mentioned radii. This concept was validated  by  the actual production  part manufactured with  larger  corner  radii.

Several attempts were also made to eliminate the wrinkling. First, the blank-holder force was increased to twice the original value. However, just as for the results obtained in the previous section for the drawing of tapered square cup, the effect of blank-holder force on the elimination of wrinkling was not found to be significant. The same results are also obtained by increasing the friction or increasing the blank size. We conclude that this kind of wrinkling cannot be suppressed by increasing the  stretching force.

Since wrinkles are  formed  because of excessive  metal flow in certain regions, where the sheet is subjected to large com- pressive stresses, a straightforward method of eliminating the wrinkles is to add drawbars in the wrinkled area to absorb the redundant material. The drawbars should be added parallel  to the direction of the wrinkles so that the redundant metal can be absorbed effectively. Based on this concept, two  drawbars are added to the adjacent walls, as shown in Fig. 9, to absorb the  excessive  material.  The  simulation  results  show  that  the

Fig. 9. Drawbars added to the  draw   walls.

wrinkles at the corner of the step are absorbed by the drawbars as expected, however some wrinkles still appear at the remain- ing wall. This indicates the need to put more drawbars at the draw wall to absorb all the excess material. This is,  however, not  permissible from  considerations of the part   design.

One of the advantages of  using  finite-element  analysis for the stamping process is that the deformed shape of the sheet blank can be monitored throughout the stamping process, which is not possible in the actual production process. A close look at the metal flow during the stamping process reveals that the sheet blank is first drawn into the die  cavity  by  the  punch head and the wrinkles are not formed until the sheet blank touches the step edge D–E marked in Fig. 1(b). The wrinkled shape is shown in Fig. 10. This provides valuable information for a possible modification of die    design.

An initial surmise for the cause of the occurrence of wrink- ling is the uneven stretch of the sheet metal between the punch corner radius A and the step corner radius D, as indicated in Fig. 1(b). Therefore a modification of  die design  was carried out in which the step  corner  was  cut  off,  as  shown  in Fig. 11, so that the stretch condition is changed favourably, which allows more stretch to be applied by increasing the step edges. However, wrinkles were still found at the  draw  wall  of  the cup. This result implies that  wrinkles  are introduced  because of the uneven stretch between the whole punch head edge and the whole step edge, not merely between the punch corner and