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Fig. 7. Temperature distributions by steam heating and water cooling.
efficiency because the volume for heating was limited near the cavity surface (section F-F). At the end of heating process, the temperature range at the cavity surface is from 100 °C to 112 °C (section E-E). However, with the same simple plane, in the TV housing mold plane, simulation results showed that the temperature at the center of the channel system is higher than the cavity surface (section D-D).
5.3. The comparison between steam heating and water heating
To compare the efficiency of steam heating and water heating, based on the 5 steps in Fig. 1, first the steam heating system was applied for the mold plate. Further, in the heating process, the steam was replaced by hot water at 120 °C. The target of the heating process is 80 °C, 1 cycle of temperature at point P (Fig. 4) was measured and compared together when steam and hot water were used for the heating stage. In these cases, cool water at 20 °C was used for cooling the mold. Fig. 9 shows the temperature history of these cases. By steam heating, the temperature at P can increase from 50 °C to 80 °C in 8 s, and then cool down until 50 °C in 12 s. However, when hot water was used for heating, it takes 18 s for raising the mold temperature from 50 °C to 80 °C, and 16 s for cooling down to 50 °C. Therefore, when steam was used for the TV housing mold, the time for the heating process can be reduced from 18 s to 8 s. This may be due to the fact that the heat transfer coefficient between the
steam and channel walls is much higher than the water and channel walls. On the other hand, the steam temperature can easily reach to the high temperature, conversely, with the high temperature, water stays at a high pressure. Therefore, the steam temperature is often higher than the water temperature. This is another reason for the higher efficiency of the steam heating method. For the cooling process, although the same 20 °C water temperature and the same flow rate are used, with the steam system, the cooling time is shorter than the water heating. This is because the steam heating rate is faster, and the volume heating, which is effected in the heating process, is smaller than the water heating, therefore, in the cooling step, the first is easier to cool down than the second. By simulation with the same parameter in Table 1, the temperature history at the center point was compared with experiment. In general, the simulation result and experiment had the same value.
5.4. Steam heating with different target temperatures
Variations in target temperatures with heating times for the TV housing mold plate under 5 steps in each cycle can be found in Fig. 10. The result shows that the mold surface temperature can increase from 50 °C to 70, 80, 90, 100 and 110 °C after 7, 9, 13, 17 and 19 s, respectively. It can be seen that when the heating time increases from 7 s to 19 s, the mold surface temperature increases significantly. Both simulation and
Fig. 8. Temperature distributions of various mold surfaces at the end of the steam heating process (Target temperature is 110 °C).
Fig. 9. Comparison of temperature history for the steam heating/water cooling and water heating/water cooling.
Fig. 10. Steam heating and water cooling with different target temperature.
experiment results show their positive agreement. In general, the rate of steam heating is about 3 °C/s. However, mold surface temperature will reach to the saturated value when the heating time is longer. This is due to the limit of the steam temperature which in this case is only 150 °C.
5.5. Effect of steam heating on the product quality
Finally, the PMMA + ABS at 245 °C were injected into the cavity, with the target temperature at 80 °C, and initial mold temperature at 50 °C. After that, by using the Multi gloss 268 m and based on the standard of ISO 2813, the gloss of the product was measured at location P (Fig. 4) with the range of 85°: 0–160 GU and a resolution of 0.1 GU. Fig. 11 shows the comparison of gloss when steam heating was used with the target temperature from 70 °C to 100 °C and melt temperature was fixed at 245 °C. It was found that with the higher target temperatures, the gloss was clearly improved. This can be explained in the effect of the heating