Closed Loop Springback Control in Progressive Die Bending by Induction Heating

Large springback of high strength steels in progressive dies hinders accurate manufacturing because a proper prediction of elastic unloading is not possible due to material variations, tool wear, and varying ambient conditions. In order to compensate springback and enhance the forming limit when brittle materials are bent, a warm bending technology for progressive dies through inline induction heating is developed Within a certain temperature range, there is a linear relation to the springback angle, which allows a direct influence on the final bending angle. Based on this principle, a closed loop control with a feedback of the angle after unloading is implemented, which adjusts the sheet temperature before bending. With the developed discrete controller, finally a stable and fast control mode is achieved so that an initial deviation from the target value is reduced to less than Delta theta = +/- 0.30 degrees.