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5. Results and discussions
5.1. Effect of steam heating on simple mold plate
The variation in mold temperature (at the center of the surface) versus time under different step in 2 cycles was shown in Fig. 6a. By simulation with the same parameter in Table 1, the simulation and experiment result gets high agreement. Base on this result, the change of temperature at the center point has shown the effect of steam, air and cool water in 2 continuous cycles (from the heating stage until the final cooling step). It can be seen that with the first step, when hot steam is at 150 °C, it is ready to start for heating the mold, it will mix with the air which is still inside the channel, so that, the heat transfer coefficient will increase in time. In this time, the change of mold temperature is not clear. By simulation, the heat transfer coefficient for this period was detected at about 3000 W/m2K. However, when all the air was pressed out (step 2 in Fig. 1), the heat transfer coefficient will reach to the max value, and the temperature of the mold will increase very fast. When the heat transfer coefficient was set at 15,000 W/m2 K, the result will be the same with experiment. Next step, when the temperature at the center point of the mold surface reaches to the target value, the valve will be switched for 20 °C water flow into the channels. In this step, because hot steam is still inside the channel, there is a short time that water will mix with steam. This is the reason for the reduction of energy moving from hot mold to cool water. Moreover, the result is the lower cooling rate at the beginning of the cooling process (step 3 in Fig. 1). After that, when all steam was pressed out, the cooling rate was increased very quickly. At this time, the heat transfer coefficient between cool water and the
Fig. 4. TV housing model and measured points.
channel wall was set at 5000 W/m2 K for simulation. When the temperature at the mold surface decreased to 40 °C, the cool water will be stopped, and the valve will be opened for the air flows into the channel for cleaning the water and preparing for the next cycle. Fig. 6b shows the temperature history between the surface (point C) and the near channel wall (point D). By 3D simulation, this result shows the
main property of volume heating; it is the delay of the temperature history between the heating location and the cavity surface. In this case, the different temperature between point C and D is about 5 °C. In the heating process and in the previous time of cooling, the temperature at point D is higher than point C; however, at the end of the cooling, the temperature at point C is higher than point D. The temperature
Fig. 5. Mold plate of the TV housing.
Fig. 6. Temperature history at point C and D of the simple plate.
distributions (Fig. 7) also show these differences when point C reaches to 100 °C and 140 °C.
In general, when combining hot steam, air and cool water into 1 cycle; the heat transfer coefficient will be changed by the change of the fluid type as well as mixing them quickly. In experiments, by using an infrared thermal measurement system, the temperature distribution at the mold surface was observed. Then, simulation and experiment results were compared together in Figs. 6a and 7. This result showed that the center temperature at the end of the period heating could reach to 140 °C. The higher temperature was concentrated at the center of the surface, and near the inlet or outlet connection. For this simple mold, the
mold surface temperature can increase from 50 °C to over 135 °C in 9 s (9 °C/s), and cool down to 50 °C in 44 s (2 °C/s).
5.2. Temperature distribution of the TV housing mold at the end of steam heating
By simulation, the temperature distribution of the TV housing mold plate was observed (Fig. 8). Based on this result, in 3D and cross-section pictures, the high temperature of the mold plate is concentrated near the cavity surface. In this design, the channels were close to the cavity surface, so, both the heating and the cooling processes can get a higher