1. Introduction  Injection molding is the most popular method for producing plastic products because of its high productivity and the manufacturability for making various complex shapes. The injection molding process includes six stages: mold closing, mold filling, packing, cooling, mold opening, and part ejection. Among these stages, cooling stage is the most important phase because it significantly affects the productivity and the quality of molded part. Normally, 70%~80% of the molding cycle is taken up by cooling stage. An appropriate cooling channels design can considerably reduce the cooling time and increase the productivity of the injection molding process. On the other hand, an efficient cooling system which achieves a uniform temperature distribution can minimize the undesired defects that influence the quality of molded part such as hot spots, sink marks, differential shrinkage, thermal residual stress, and warpage.1,2  Traditionally, mold cooling design is still mainly based on practical knowledge and designers’ experience. This method is simple and may be efficient in practice; however, this approach becomes less feasible when the molded part becomes more complex and a high cooling efficiency is required. In addition, conventional straight cooling channels are machined by hole-drilling as close to the cavity’s surfaces of the mold as possible. The free-form surfaces of the cavity surrounded by straight  cooling lines and the molded part will be cool unevenly because of the variation of the distances between the cavity’s wall and cooling lines. This not only results in potential defects of molded part but also increases the cooling time. Alternative cooling device such as baffles, bubblers and thermal pins that are used to cool areas being far from main cooling channels can improve the cooling quality. However, this method is not always effective due to the high pressure drop in cooling channels system, especially for medium-sized and large-sized parts with free-form surfaces. 48118
The importance of cooling process in injection molding has drawn a great attention of plastic engineers and researchers. Some researches have focused on analysis of the cooling system and on how to optimize the cooling channels layout in terms of cooling channel size and location by the mathematical calculation and analytical method.3-5 These practical approaches were reported to be more convenient and faster than finite difference and finite element method. They demand less effort of plastic designers than   Besides the solid free-form fabrication technology, milling operation is an alternative applicable method to make complex cooling channels conform to the surface of the mold cavity. This paper presents the U-shape milled groove conformal cooling channels and proposes the design optimization process in order to obtain an optimal cooling channels’ configuration and target mold temperature. The relation between the cycle averaged thermal behavior of the mold cavity and the two-dimensional configuration of cooling channels was first investigated thoroughly by an analytical method. Design of experiment and 2D simulation were done to obtain the mold wall temperature and to check the feasibility of the analytical method. The optimization process of the free-form conformal cooling channels is based on the combination of both analytical method and 3D CAE simulation. The analytical step relies on explicit mathematic formulas, so it can approach the neighboring optimal solution quickly. Subsequently, the three-dimensional heat transfer simulation is applied to fine-tune the optimization results. A case study for a plastic car fender was investigated to verify the feasibility of the proposed method. The results show that conformal cooling channel gives a uniform cooling, reducing the cooling time and increasing the molded part’s quality with less effort of plastic designers and high computational efficiency.  Manuscript received: January 5, 2010 / Accepted: October 24, 2010  those of numerical and simulation software. However, these methods can be applied to simple molded parts. Other approaches used 2-D boundary element method and sensitive analysis in conjunction with the gradient optimization algorithm or hybrid optimizer to optimize cooling channels system.6,7 More recently, applying 3-D CAE and 3-D FEM computer-aided simulation have been widely used to investigate the thermal behavior of the cooling system and the configuration of conformal cooling channels.8,9 These researches’ results showed that the conformal cooling channels give a shorter cooling time and better temperature distribution than those of conventional cooling channels. Park and Pham10 introduced an optimization strategy and investigated some types of conformal cooling layout for an automotive part by applying analytic formulas. Subsequently, they used CAE flow simulation for verifying the optimization results and showing the cooling effect of conformal cooling channels.  To obtain a uniform cooling, the cooling channels should conform to the surface of the mold cavity. This type of cooling system is called conformal cooling channels. The application of this new kind of cooling channels is based on the development of solid free-form fabrication (SFF) technology. Some of works studied the advantages of the conformal cooling system and how to optimize the conformal cooling channels in injection molding tools.11,12 The results were reported that conformal cooling channels offer better temperature uniformity within the mold cavity and better molded part’s quality compared to straight cooling channels. SFF or rapid prototyping technology brings the opportunities to fabricate very complex conformal cooling channels inside the mold core and mold cavity,13 but this technique is still expensive, especially for large-sized mold. On the other hand, the kinds of metal powders used in rapid prototyping techniques, for example 3D printing, selective laser sintering, electron beam melting, and laser engineered net shaping, are limited. It means that the choice of mold material with appropriate thermal and mechanical properties for making conformal cooling channels by SFF is narrower than that of conventional mold. These issues hinder the popular use of rapid prototyping technique for making large injection mold. Although a great of attention has been paid to improve the performance of the cooling system, most of the published works are only feasible for simple plastic parts. One of the evidence is that all the examples used in various case studies are very simple.8 Some of works3,6,7,12,14 relied on 1-D or 2-D heat transfer analysis meanwhile the geometry of plastic parts are usually complex, and they need a 3-D analysis. Moreover, sensitive analysis, finite element or boundary element coding for optimization of some specific cases are still academic and lack of generalization, so they are inconvenient to apply in reality. On the contrary, some of works8,9,15 applied 3-D CAE tools to solve the cooling problem for some more complex parts. Yet, the method of reducing the cooling time and optimizing the configuration of conformal cooling channels of these studies were not adequate. This paper is intended as a contribution to solve this on-going problem by introducing U-shape milled groove conformal cooling channels fabricated by CNC milling machine instead of rapid prototyping method. This approach is suitable for medium-sized and large-sized molded part with free-form surface. The relation between the configuration of cooling channels and cycle averaged thermal behavior of the mold cavity are investigated thoroughly. The size, location, and layout of cooling channels for a quite complex molded part are optimized by the combination of an applicable analytical model based on the equivalent model, computer-aided 3-D heat transfer analysis, and effective optimization strategy. This approach would therefore offer a more feasible and practical way to design an optimal conformal cooling channel and to meet the requirement of reducing the cooling time and increasing molded part quality with less effort of plastic designers.   2. U-shape milled groove conformal cooling channels  Milled groove cooling channels in spiral form has been used for flat parts with round and circular shape in order to obtain better temperature control. This kind of cooling channels is more expensive to make comparing to straight-drilled one, but produces high-quality and distortion-free parts such as precision gears and compact discs.16 Sun8 proposed a modified milled groove method applied to medium free-form parts with two case studies of mouse cover and iron cover. Intensive studies on this kind of cooling channels have not been carried out adequately. In this paper, milled groove cooling channels method is continued to investigate for larger plastic part in automotive industry. Further more, the influence of cooling channels’ configuration to the thermal behavior of the mold cavity and the way of optimization are addressed in the upcoming sections. The conformal cooling channels are different from straight-drilled conventional cooling channels. In conventional cooling channels, the free-form surface of mold cavity is surrounded by straight cooling lines machined by drilling method. It is clear that the distance from the cooling lines and mold cavity surface varies and results in uneven cooling effect. On the contrary, in the conformal cooling channels, the cooling paths match the mold cavity surface well by keeping a nearly constant distance between cooling paths and mold cavity surface. It was reported that this kind of cooling channels gives better even temperature distribution in the molded part than that of the conventional one. Figure 1 shows an example of a conformal cooling channel for a supposed free-form  塑料注射模具设计U型铣槽英文文献和中文翻译:http://www.751com.cn/fanyi/lunwen_50472.html