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The RHCM process has obvious advantages. But the structure of the molds, the heat transfer and deformation of the molds, and the lifetime of the molds become the major concerns in implementing this new molding technology.because of obvious defects on the product’s surface. As the
additional post-processing, such as spraying process is needed, the whole production flow is prolonged; the cost is increased and the environment is polluted, too. Recently, the RHCM technology is applied to the production of LCD TV panels and other large polymer products, and products
with good appearance are obtained.
The RHCM mold structure is different from that of conventional injection mold. Figure 4 shows one quarter of RHCM mold of a LCD TV panel designed by the authors of this paper. In the RHCM mold, there are several heating/
cooling channels above the mold cavity surface. They are distributed directly along the cavity surface with some distance away from each other. In the heating stage, the channels are used to heat the mold, while in the cooling stage, they are used to cool the mold. Reasonable shape and
layout of the channels can lead to high heating/cooling efficiency and good performance products. Therefore, they are important aspects for successful implementation of this new injection molding technology.
3 Thermal analysis of RHCM mold
During the RHCM process, the mold must be rapidly heated and cooled to designated temperatures in the injection molding cycle. The production efficiency and the part’s quality are seriously affected by the temperature and its distribution uniformity along the mold cavity surface.Weld mark, flow mark, and sink are easy to be formed if the temperature is low or distributed unevenly on the mold cavity surface after the heating process. Therefore, there is a strict requirement on the temperature of the mold cavity surface after the heating process in order to obtain high quality plastic parts. The temperature distribution on the mold cavity surface must be as uniform as possible and the heating/cooling time of the mold must be short enough to ensure the production efficiency. Therefore, it is very necessary to study the heat transfer in the RHCM mold.Based on these facts, the designers can evaluate design plans and optimize the heating and cooling channels. At the same
time, as high and low temperature alternates in RHCM injection process repeatedly, the main components of the mold will suffer much greater stress than conventional injection mold, so larger deformation will occur and fatigue cracks on the mold will form more easily. Therefore, to
consider the heat load caused by the temperature alternating on the mold is also of great importance.
From the structure of the RHCM mold, it is learnt that the heating/cooling channels are straightly distributed along the cavity surface as shown in Fig. 5. Additionally, the major consideration in this paper is focused on the temperature and its distribution uniformity of the mold cavity surface. Therefore, we can select a cross section of the mold as the simplified mathematical model for heat transfer analysis. Figure 6 shows the cross section selected for the simplified heat transfer model.
3.1 Heat transfer models of RHCM mold in heating and cooling stages
In order to make sure that the mold is heated and cooled rapidly, the stationary mold insert is insulated with the mold plate. So in the heating stage, it is assumed that heat transfers only between the stationary mold insert and the steam through the heating channels. Therefore, the model
determined for the heating stage is shown in Fig. 7. For conventional injection mold, the channels are fewer and they are used only to cool the mold. But there are more channels which are used both for cooling and heating the RHCM mold.
In the course of the cooling stage, heat transfers among the polymer melt and the stationary and movable mold inserts, so the structure of movable mold insert and plastic melt must be considered. The established model, which is shown in Fig. 8, is different from that of the heating stage.Besides, there is a larger channel used for cooling in the movable insert. In the cooling stage, all of the channels both in the stationary and movable mold inserts are filled with low-temperature water to cool the mold.