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From Fig. 9, it is found that the temperature distribution of the cavity is non-uniform after 25 s of heating time although the average temperature has satisfied the RHCM requirement. The temperature in the middle of the cavity (P5 and P6) reaches at 133 and 135 , respectively.Meanwhile, the temperature of both sides still keeps lower,especially the temperature at the right side of the cavity(P8) is only 91 . Obviously, temperature differences at different positions of the cavity surface are large. The
middle of the cavity is rapidly heated while the heating effects at both sides are bad. If the polymer melt begins to be injected in this situation and then cooled down to the ejected temperature, the surface temperature of the polymer melt must be inhomogeneous.Consequently,defects such as warp and poor surface of the parts are inevitable.
In order to obtain the plastic products with high-quality appearance and increase production efficiency, the surface temperature of the mold cavity must reach the required temperature within the shortest time. On the other hand,good temperature distribution uniformity of the cavity
surface must be also ensured. Therefore, the layout of heating/cooling channels is very important. The numbers of channels, space between them, and the diameters and distance from the center of the channels to the cavity surface must all be taken into account. High heating/cooling efficiency and good heating/cooling uniformity on the cavity surface are ultimate aims on the premise that the mold strength should meet the requirement. To study the effects of different layouts of channels on heating/cooling efficiency and temperature distribution on the mold cavity surface, the optimization theory and algorithm must be introduced and used. Therefore, the optimization design of the layout of the channels should be carried out in order to make sure of production efficiency and the part’s quality.However, this optimization design will be the further
research work of the authors. In this paper, we only analyze the heat transfer process and its influence on the thermal stress and fatigue behavior of RHCM mold.
4 Thermal stress and expansion deformation of the RHCM mold
During the injection molding process, the high-temperature melted polymer is injected into the mold cavity with high pressure and velocity through the nozzle. Then, the molded plastic part is ejected after the packing and cooling stages.On the one hand, the cavity surface of the mold suffers from erosive and flow wear again and again in the whole process, and the stationary and movable mold inserts contact each other repeatedly due to the opening and closing of the mold. On the other hand, the melted polymer perhaps decomposes to corrosive gas and it has a bad influence on the mold cavity. As a result, the RHCM mold suffers great mechanical, thermal, and chemical shocks which will lead to excessive deformation of the mold. So,fatigue cracks are easy to generate and expand on the mold until the mold fails. Meanwhile, the mold also fails when the shape or the dimensional precision or surface quality of the plastic parts is out of the requirements caused by mold wear. The failure process of the mold is described in Fig10.
In Fig. 10, there are lots of factors that influence the lifetime of the RHCM mold. In order to improve the mold lifetime and reduce the cost, the RHCM mold material must be chosen rightly. Except for good polish ability which is required for the cavity surface, it must have a good thermo-elasticity,thermal fatigue resistance, abrasion resistance,
and corrosion resistance so as to enhance the mechanical and fatigue resistance of the mold. In addition, when the mold material is determined, the structure of the mold and its assembly and fixing mode must be carefully considered in order to reduce the thermal stress and deformation and improve the mold lifetime.
摘要：快速热循环注塑（RHCM）是一种新型的塑料注射成型工艺。它是可以有效防止生产常规产品的诸多缺陷的一种注射成型工艺。在文中，用面板大尺寸的液晶电视作为的例子。为了对热，变形，和疲劳分析建立了RHCM注塑模具模型。首先，通过分析RHCM模具的传热过程，然后在模具型腔表面的研究温度的分布。其次，通过数值模拟，得到该RHCM模具的应力和变形趋势。结果表明，热应力和变形影响下对固定模式之间固定模和固定模板的影响很大。作为一个结果，一个新的固定方式的固定模，模具插入可有效减少由温度变化引起的变形。最后，对于在不同的固定模式中模具的寿命进行了评价，并提出合理的能改善RHCM模具的寿命建议。工程应用证明，通过使用建议的固定模式是可以改善RHCM模具的寿命非常有效的一种方法。