On the extent of stretching during deep drawing

The terms 'pure stretch forming' and 'pure deep drawing' are used frequently in the study of sheet-metal forming processes and formability tests. Pure stretch forming involves biaxial stretching of the blank in the absence of flange movement, resulting in severe thinning and a gross increase in the surface area of the blank. Pure deep drawing, contrarily, connotates the entire blank being drawn into the die throat with negligible thinning, so that the final product is, substantially, of the same surface area of the blank. The presence of thickness variations in fully drawn cups, however, implies that stretching does exist. This suggests that the cup-drawing process lies between the two extreme conditions representing pure stretch forming and pure deep drawing. In this paper, a parameter which describes the extent of stretching, relative to the amount of draw-in, is proposed. With the use of this parameter, the effects of drawing parameters, such as the punch nose radius, the blank diameter and the blank-holding load, on the drawability of materials can be better understood.