IMPROVEMENT OF PART QUALITY IN STAMPING BY CONTROLLING BLANK-HOLDER FORCE AND PRESSURE

Predominant failure modes in stamping of sheet metal parts (deep drawing and stretch forming) are wrinkling or tearing. Wrinkling may occur at the flange as well as in other areas of the drawn part and is generated by excessive compressive stresses that cause the sheet to buckle locally. Fracture occurs in the drawn material that is under excessive tensile stresses. Failure is influenced by the blank-holder pressure (BHP), and local and instantaneous control can be achieved: (a) directly by various types of draw bead designs; or (b) indirectly by controlling the local pressure acting on the blank-holder ring. Modern mechanical presses are equipped with hydraulic cushions and various advanced multi-point pressure control systems. These systems allow the setting and control of the BHP over the periphery of the blank holder as a function of location and time (or press stroke). This paper summarizes the principles and applications of the latest BHF and BHP control schemes, especially those using multiple-point pressure application techniques, developed around the world. The information presented in this report was obtained from a literature review and industrial visits to selected press manufacturers.