Upper bound analysis of torsional backward extrusion process

In this paper, backward extrusion process with low die rotation was studied to improve the problems of conventional backward extrusion process: the requirement of large forming machine, the difficulty in selecting the die material caused by high surface pressure, high cost of forming machine caused by improvement of noise and vibration, etc. In this experiment, model material, plasticine, was used as specimen. The result of torsional and conventional backward extrusions experiment were analyzed and compared by two methods. One is upper bound technique computing velocity field, and another is FEM simulation with DEFORM-3D. In the upper bound analysis, the velocity fields were derived using the dual stream functions. The use of stream functions in formulating upper bound solutions has some advantages. The number of unknown is reduced and incompressibility condition is satisfied. These result show that the torsional backward extrusion is a very useful process because this process yields the homogeneous deformations and lower forming load (decrease about 30%). In this paper, the most proper angular velocity of container is found to be around 0.16 rev/s. But, if the angular velocity was over 0.18 rev/s, deformation pattern was non-uniform and forming load was increased. Due to the decrease in forming load, die life can be improved and it is possible to use a press of relatively low capacity. Also this process can reduce corner cavity and improve the initial cast structure, owing to the high deformation and uniform strain distribution. Additionally, this process might be useful for powder compacting.