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    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.10802
    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.
    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 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
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