5. Summary and Concluding RemarksTwo types of wrinkling occurring in stamping processes were investigated using finite-element analysis, and the causes for wrinkling were examined and the methods to eliminate such wrinkles were developed. The first type of wrinkling appears at the draw wall in the stamping of a tapered square cup. The occurrence of wrinkling is attributed to the large die gap, which is the difference between the side length of the die cavity opening and the side length of the punch head. The large die gap results in a large unsupported area of sheet metal when the metal is drawn into the die cavity and an unfavourable stretch between the punch head and die cavity shoulder. The large unsupported area of sheet metal is therefore prone to wrinkling. The finite-element simulations show that this type of wrinkling cannot be suppressed by increasing the blank-holder force. Another type of wrinkling investigated occurs in an actual stamping part that has a stepped rectangular geometry. It is found that wrinkling occurs at the draw wall above the step even though the die gap is not sufficiently large. The wrinkling is due to the uneven stretch between the punch head and the step edge, according to the finite-element analysis. Several attempts were made in the die design to eliminate the wrinkling, using finite-element simulations, and an optimum design in which the step was cut off is finally established. The modified die design for eliminating wrinkles was validated by the production of a defect-free production part. The good agreement between the simulation results and those observed in the drawn production part demonstrates the accuracy of the finite-element analysis, and the effectiveness of using finite-element simulations as a substitute for the expensive method of actual die try-outs is thereby confirmed.AcknowledgementsThe authors wish to thank the National Science Council of the Republic of China for the grant NSC-86–2212-E002–028 that made this project possible. They also wish to thank KYM forproviding the production part.References1. K. Yoshida, H. Hayashi, K. Miyauchi, Y. Yamato, K. Abe, M.Usuda, R. Ishida and Y. Oike,“The effects of mechanical properties of sheet metals on the growth and removing of buckles due to non-uniform stretching”, Scientific Papers, Institute of Physics and Chemistry Research, 68, pp.85–93, 1974.2. T. X. Yu, W. Johnson and W. J. Stronge, “Stamping and spring-back of circular plates deformed in hemispherical dies”, International Journal of Mechanical Sciences, 26, pp.131–148, 1984.
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