Influence of the connection between forming die and heatpipe on the heat transfer

Abstract

Hot forming tools are exposed to cyclically changing thermal loads. These conditions are caused by the heat exchange between tool and workpiece during forming followed by spray cooling. This can lead to crack initiation and tool failure. A continuous cooling with heatpipes (HP) inside the active tool components could prevent this. HP use a circular flow of a cooling fluid inside a closed tube, often made of copper. Previous studies showed an influence of the connection by thermal paste between the forming die and the HP, its orientation, as well as its inner surface structure. The use of paste proved essential for closing the contact by filling the microscopic air pockets between the surfaces. Only sintered inner structures can be used for force fit, since others are damaged by deformation and thus lose their efficiency. This research paper deals with the influence of the form and force fit between die and HP. To test the impact, HP were connected with heated model dies on one side and an aluminium block (AB) on the other. Thermocouples were used to monitor the temperature of both, the AB and the model dies. The measured temperature and time difference, the weight and the thermal capacity of the AB were used to calculate the heat flow. Different inner surface structures of HP were varied in addition to their fitting type with the model die. The best heat transfer was achieved by using HP with sintered inner structure and force-fit, resulting in nearly full-surface contact.