The influence of friction on the prediction of wrinkling of prestrained blanks when drawing through a conical die

This work deals with the deep drawing of prestrained circular blanks into cylindrical cups when drawing through a conical die using flat-bottomed punch. It is well established that the use of a conical die can enhance the limiting drawing ratio compared with that obtainable in a conventional drawing operation. When a conical die is employed, the need for a hold-down or clamping ring is eliminated. However, this enhances the propensity of the blank to fail by wrinkling or buckling, particularly in the early staged of a drawing process in which thin sheet blanks are used. There has been no wrinkling when thick sheet blanks are used for drawing. For the resent work, annealed prestrain sheet blanks or commercially pure zinc, SS301, SS304 and SS316, having thickness 1.6 mm and different diameters are drawn though a conical die under two different frictional conditions (smooth and rough surfaces of blanks), until the appearance of a first stage wrinkle. It is shown that the onset of wrinkling takes place when the ratio of the plastic strain increment (dεr/dεθ) reaches a critical value. This critical value is plotted against the ratio of initial blank diameter to initial thickness of the sheet blank for the above said metals. This shows a straight line relationship with different slope values for different frictional conditions and metals.