Development of a novel resistance heating system for microforming by modifying surface of dies

Application of resistance heating in micro forming processes is effective for improving formability of thin foils and accuracy of products. Current flows to thin foils can achieve a rapid heating time with very low heating energy owing to the size effect of heat capacity and electrical resistance. To control the temperature distribution of such as thin foils, the heating property depending on the dimensions and electrical resistance of the workpiece has to be taken into account. Furthermore, the positioning of electrodes and shape of the products might affect the current distribution. In this study, a novel heating system, controlling the electrical resistance of die surface by using high density plasma nitriding and annealing processes, was proposed. In addition, finite element (FE) analysis was applied to simulate the heat temperature evolutions in dies and heat transfer to workpieces for designing this heating system. As results, the nitrided layer in depth of 50 μm with a nitrogen concentration of 9 at.% in a steel die was achieved by this nitriding process. After annealing of this nitride layer, the electrical resistance increased to 3∗10⁶ Ω. By using this nitrided tool steel die a workpiece of a SUS304 sheet in thickness of 0.1 mm was successfully heated to around 700 °C with the heat convection from the die, which a current of 40A was applied.