Massive forming processes cannot be described by quantitative relationships or analysed by plasticity theories without simplification and idealization of the prevailing conditions. This is also true in designing dies for hot extrusion, where complicated tool-geometries and non-symmetrical processes may exist. Experimental investigations in an industrial plant are difficult or impossible to conduct due to the high temperatures and pressures inherent in hot extrusion. In addition, an industrial press cannot normally be reserved for experiments, since the costs of an interruption in production are very high. In most cases the structure of the press and of the die does not allow direct observation or the taking of measurements or samples during extrusion. Further, die design using trial-and-error experiments in production presses is extremely expensive owing to the high costs of die manufacturing and lost production.
Most of the problems referred to above can be overcome by simulating or modelling the process under laboratory conditions. When deforming model materials, the tool geometries can be varied quite freely and rapidly, due to being able to employ tool materials of good machinability. The material flow can be measured from specimens corresponding to various stages of deformation and different sections of the product, or can even be photographed or filmed during the simulation. Such an approach has been used most frequently for verifying plasticity theories or for visualizing the material flow in relatively simple processes.
In the present work, dies were designed for the production of a hallow section having two holes. The section is extruded from copper. The product is required to be very sound, as it is intended for use in an electrical coil, where no major internal non-homogenities are allowed. Thus bridge dies, which cause internal surfaces due to pressure welding of the material after passing the webs which support the mandrel, cannot be used. Consequently, long mandrels are needed, but mandrels tend to bend if the pressure caused by the material flow is unevenly distributed in the die. Further, the displacement of the mandrels during extrusion causes dimensional changes in the product which cannot be removed during subsequent drawing.
Model-material experiments were used to investigate the material flow during extrusion. The bending of the mandrels is simulated by using mandrels made of plastics and extruding modelling wax (plasticine Filia). Practical experimental methods for use in connection with the production plant have been developed: results so far obtained give very useful information on the extrusion process and make it possible to design dies for producing hollow sections of copper with good dimensional tolerances.