Study on Induction Heating Coil for Uniform Mold Cavity Surface Heating

Recently, energy saving is one of the important issues for polymer processing industry. Electromagnetic induction heating has many advantages such as fast heating and low energy consumption. Previous studies using electromagnetic induction heating for rapid tool heating have indicated that the temperature uniformity on a cavity surface is not easy to be achieved. In this paper, two different coils were used for heating uniform 7？mm thick hot work tool steel (JIS SKD61) surface. One is a four-row coil with opposite current directions and the other is a two-row coil with identical current directions. Magnetic flux concentrators were used to control magnetic field and heat the workpiece uniformly. The heating experiment results showed that coil with opposite adjacent current directions had more uniform temperature distribution on tool surface. The temperature uniformity was about 94%~95%. The coil with identical adjacent current directions had higher average temperature and the temperature rose from 50°C to 150°C in 15 seconds. 1. Introduction Induction heating is a common heating technique applied in industry. For example, it can be applied for metal welding, heat treatments, annealing, and so forth. In plastic industry, induction heating was applied to injection molding. High tool temperature can improve replicability of microinjection molding [1]. But induction heated tool surface temperature uniformity is difficult to be achieved. By applying different induction coils and an integrated power supply, Fujita et al. [2] showed that the induction heating temperature on different zones could be controlled. In 2006, Yao et al. [3] increased tool temperature by proximity effect which means that currents of opposite directions flow to two different metals and cause the current aggregate on the proximity surface. With the proximity effect, tool temperature can be heated to 240°C in 5？sec. In 2006, Chen et al. [4] applied induction heating to injection molding. The mold temperature could be heated to 100°C~140°C from 60°C in 3？sec. Their results showed that a higher mold temperature can increase microstructure transcription. The above papers also showed that the uniform temperature is very important to heating mold. In this paper the coil design and temperature distribution were the main concern. There are several techniques to modify the magnetic field distribution and uniform heating distribution along the heating length such as reducing the number of turns in the center or turns density [5], adjusting the distance between working coil and workpiece [6],