Analysis of the Elastic Behaviour of the Press System in Draw Die Forming

摘要

Finite element (FE) simulation is capable of eliminating some of the trial and error involved in stamping engineering and die manufacture by identifying potential forming issues at the die design stage. However, to predict forming behaviour, FE models must accurately represent the behaviour of the real press system. Currently this is not always the case. The work presented here illustrates the effects of global and local stiffness on the press system performance and demonstrates the inadequacy of traditional try-out practices and forming simulation models in coping with these. The work addresses the elastic behaviour of a double action mechanical press and draw die because these are the standard for forming medium to large panels in most automotive stamping plants. Tonnage curves recorded in a production press are used to illustrate the effect that the relative stiffness of system components can have on the blankholding force and a linear expression relating the change in force in the inner and outer rams is developed. Local stiffness is analysed using a finite element model of a production tool, which clearly shows that the current modelling practice of representing tools with rigid surfaces is inaccurate. The problems associated with the current try-out process are demonstrated using finite element models to capture the effect of draw beads and stop blocks on contact pressure distribution at the binders. Adjustments made to the tool surfaces based on a static and incomplete assessment of the forces acting on the hinders will result in poor correlation between the FE simulation results and the performance of the real tool. The problem requires a new approach to die design and try-out that reflects a better understanding of elasticity within the press system. The behaviour of the system must be altered by design so its representation in the FE model is both accurate and repeatable. Coupled with a more structured and analytical approach to try-out this will make FE analysis an even better tool for developing die designs and stamping processes.