Effect of global and local stiffness on blankholder pressure in draw die forming

Abstract

This work investigates the effect of elastic deformation of the press and die in a double action press on system performance during draw die forming. A simple linear elastic model of the complete press system was developed to show that the relative stiffness of system components controls blankholder force variation during the stroke. Calibration of the analytical model with data taken from a production press shows that the simple model explains real global system behaviour very well; and that varying system compliances can desensitise the press system to changes affecting the binder gap during the stroke. The experimental data suggests that real press and tool systems do not have an appropriate combination of system compliance, which causes large variations in blankholder force during the stroke. This model will be a useful tool for die designers and process engineers for assessing the potential impact of blankholder force variation during die design. It can also be included in finite element forming simulation models. The main focus of this work is the global model, however it is important to recognise that local structural stiffness of the stamping tools also has an important effect on restraining force variation around the binders. Contact pressure contour plots generated from a finite element model of a production blankholder are used to illustrate the significance of local structural stiffness variation. A simple model is developed to explain this result, which agrees with information available from the tryout process for that tool. Show more