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0735-1933/$ – see front matter. Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2010.07.012
auxiliary heating and is limited to increase in mold temperature of roughly several tens of degrees centigrade. Another research had used the water vapor in heating the mold with the special mold design. This research has increased the mold temperature from 30 °C to 110 °C to help the melt easily fill the cavity to reduce some defects in the product [12,13]. However, in the real mold for the TV housing of LCD (liquid crystal display), the heating and cooling channels are different. Therefore, both the heating and cooling efficiency are affected. In addition, with this design, the mold is much more complex and the cost of tooling will increase.
In this study, a steam-assisted heating system combined with water cooling was applied for a simple model to establish the heating capacity of the steam in injection molding. Furthermore, for the real application, a TV housing mold was operated with the assistance of this system for confirming the efficiency of steam heating on the mold heating process and for product quality. In both designs, the heating and cooling processes were used with the same channel system. To gain efficiency in heating, at the end of the cooling period, an air flow will be blown into the channel for removing all the cooling water. The stages of steam, air and water will be considered. Besides the experiment, a 3D simulation was created for both models under different conditions and compared with the experiment results.
2. General process of steam heating in injection molding
In this research, the water vapor (steam) will be used as the heating source to increase the cavity temperature of the injection mold. Then, the hot melt will be solidified by using cool water. The efficient use of steam heating will increase by the cleaning of water before it flows into the channels because of combining 2 different fluids for heating and cooling. Therefore, in general, the application of steam in injection molding will be achieved using the five steps as shown in Fig. 1. First, by opening the valve, the hot water vapor will flow into the channel. At this time, the air still appears inside the channel, so that the steam and air will be mixed together. At this point, the stage of these fluids is not
stable, and the result is the low heat transfer coefficient between the water vapor and the wall of the channel. At this time, the mold is still open after rejecting the previous part and is waiting for the next cycle. Second, when the steam flow stabilizes, all air is pushed out and only steam appears inside the channels. Therefore, the heat transfer coefficient will increase very fast. The energy transferred from steam to the mold plane will heat the cavity surface. This is the main heating process in steam heating. When the cavity surface is heated to a high temperature, the mold will completely close in preparation for the filling process. Third, when the heating process is finished, the temperature at the cavity surface will reach the target value for assistance in the filling and packing of the melt. At this time, the valve control will be switched for the water flow into the channels, and the cooling process will begin. At the beginning of this step, water will mix with steam and create an unstable stage, so, the heat transfer coefficient between channel and water is still very low. Fourth, after a short time, all the steam will be pressed out, and only water flows remain inside, so, the heat transfer coefficient will be increased, and the cooling efficient will be higher. Because of the heat relieving by cool water, the mold plane and melt will cool down to reject temperature. Fifth, after all melt reaches to the eject temperature, the mold will open for rejection of the part and prepare for the next cycle. At this time, the valve control unit will switch on for air flow into the channel to clean out all water. This step will finish when all channels are clear of water.