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processes of the RHCM mold in order to increase quality of the parts and decrease the production costs further.
2 RHCM technology and mold structure for the panel of LCD TV
2.1 RHCM technology
In conventional injection molding process, both moving and stationary molds usually do not need heating. When the polymer melt is injected into the molds, the temperature of the mold cavities is increased to about 50 . When the mold cavities are completely filled by the melt, they are cooled
down to about 30 by using cooling water. Then, the molds are opened and the injected plastic parts are taken out. Up till now, one cycle of conventional plastic injection molding process is finished. But the quality of the injected polymer part by using the conventional injection molding process usually has many problems, such as weld mark, flow mark,
and so on.
The RHCM process is different from the conventional injection process. At first, in the heating stage, the surface of the mold cavity is heated up to a high temperature which is above the heat distortion temperature of the polymer.Then, the polymer melt is injected into the cavity. The
temperature of the cavity is kept through injection and packing stage. Next, the mold is cooled down rapidly in the cooling stage, then the mold is opened, and the product is ejected. Before the next injection process starts, the surface of the cavity is quickly heated again. As the rise and fall of mold temperature is operated in a short time, the molding
cycle is nearly equal to the conventional molding process.The principle of the rapid heating cycle injection molding process can be shown in Fig1. 1.
During the working process of RHCM, the mold cavity must be heated up to the heat distortion temperature of the polymer, which is usually more than 100 . Nowadays, there are many ways to heat the mold. Chen et al. [13] researched the electromagnetic induction heating on a mold plate.Through computer simulation, this method of heating was proved very effective to heat the surface of the mold plate.Chang and Hwang [14] proposed the infrared heating method to heat the mold cavity surface, and transient thermal simulation was developed to evaluate the thermal condition of the surface. Kang [15] investigated the electrically heating methods and it was also proved to be a very effective way to heat the surface of the mold cavity.In this paper, the steam-assisted heating method was researched and the mold designed by the authors for the RHCM technology can be only suitable for this heating method.
Compared with the conventional injection molding process, the initial temperature of the mold cavity in the RHCM process is much higher. The cavity of the mold is heated by steam and cooled down by cold water in the current research. The structure of the RHCM mold is different from that of the conventional injection mold.Considering the heating or cooling efficiency and the temperature distribution uniformity of the RHCM mold cavity, the structure and the distribution of heating or cooling channels are designed specially. Usually, the production efficiency of the RHCM process is almost equal to that of the conventional injection process. As high temperature, rapid heating, and cooling methods are employed in RHCM, the polymer melt is blended completely in the injection molding process, the melt flow ability is increased, and the viscosity of the polymer melt is decreased. In addition, the mold cavity surface is manufactured into a high gloss surface. So, the surface of the products produced by RHCM technology is very bright and smooth, and there are no weld mark, flow mark, and other defects on the surface. Figure 2 shows part of one type of LCD panel produced by the technology. However, the
surface of conventional products is very coarse as shown in Fig. 3 a, and there are many defects. The comparison between the panel injected by using conventional injection technology and that by RHCM technology is very obvious in Fig. 3.