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 拉中拉深壁起皱的分深模设计析英文文献和中文翻译(5):http://www.youerw.com/fanyi/lunwen_80828.html