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Abstract. The need for car body structures of higher strength and at the same time lower weight results in serious challenges for the stamping process. Especially the use of high strength steel and aluminium sheets is causing growing problems with regard to elastic springback. To produce accurate parts the stamping dies must be adjusted more or less by the amount of the springback in the opposite direction. For this purpose well-known software solutions use the Displacement Adjustment Method or algorithms which are closely based on that method. A crucial issue of this method is that the generated die surfaces deviate from those of the target geometry with regard to surface area. A new Physical Compensation Method has been developed and validated which takes geometrical nonlinearity into account and creates compensated die geometries with equalin-area die surfaces. In contrast to the standard mathematical/geometrical approach, the adjusted geometry is generated by a physical approach, which makes use of the virtual part stiffness. Hereby the target geometry is being deformed mechanically in a virtual process based on the springback simulation results by applying virtual forces in an additional elastic simulation. By doing so better part dimensions can be obtained in less tool optimization loops.

Further increasing requirements to the quality of car body components represent a huge challenge for the process planner. As the elastic energy is changing in each stamping operation, it is quite challenging to decide how to compensate which stamping operation. The belonging process definition is known as the compensation strategy. A key prerequisite of an appropriate compensation strategy is a proper location of the part in all stamping operations along the process chain. Proper means that the distance of the part to the die must be minimal to prevent any undefined shift of the part in the die and thus unwanted deformations when closing the dies. Both shifting and deformations would cause dimensional deviations and surface defects. Today different strategies are applied to meet these requirements at least approximately. Strategies, which are published in different papers yet, can only provide suboptimal results. Here a new compensation strategy is being presented, which fulfils the requirement to a proper part location not only in the trimming dies, which corresponds to the so-called Drawshell Strategy, but also in the restriking dies. An important advantage of the respective strategy is that it generally can be applied on all multistage car body stamping operations.

The stamping of dimensionally accurate sheet metal body components still represents a huge challenge in the automotive industry. This is partly caused by the multi-stage production process which complicates the design of an appropriate compensation strategy dramatically. A key prerequisite of an appropriate compensation strategy is to eliminate any undesired influence of elastic energy being potentially induced when closing the blankholders in the several operations throughout the production process. Here, a universally applicable compensation strategy is presented which fulfils this requirement thoroughly; by applying this process, the involved strain energy is reduced to a minimum due to proper part position in all operations. Technically this compensation strategy is achieved by, first, simulating all operations of the stamping process individually, second, by individually calculating the springback after each operation and, third, by accumulating the calculated deviation vector fields for each operation appropriately, which are then used for springback compensation. The process is time and cost effective and the required efforts are moderate even for complicated multistage stamping operations.

Dimensional deviations of stamped car body components caused by elastic springback still represent a significant problem for the development of the stamping tools. First, the springback causes deviations of the part in vertical direction to the part surface due to bending stresses. Second, the surface area of the part contracts (and also bends) due to elastic membrane stresses. The precise manifestation of the contraction depends both on the part stiffness and on the – usually heterogeneous – distribution of the membrane stresses. Usually, the resulting deformations cannot adequately be compensated by globally homogeneous scaling approaches. In order to carry out a locally correct compensation of the stamping tools, Birkert et al. have recently presented a physical scaling approach based on the inverted membrane stresses gained from the part in the closed die to compensate the active die surfaces. This approach delivered significantly better results but left potential for improvement. It is shown in the present paper how the existing approach and thus the scaling values can further be improved by adapting the inverted membrane stresses in an appropriate way. This is done by comparing the amount of strain changes during spring-back with those of the scaling process in a first step and adjusting the compensation stresses, in a second step, in such a way that the considered strain amounts are approximately identical.