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Injection molding process is a widely used manufacturing technique to massively produce the components of mobile device with various sizes and complicated geometries. However, the final part quality, especially dimension or geometry, referring to the original design specifications is not often acceptable due to various reasons. This study aims at developing the numerical model to predict the final part quality and subsequently identifying the critical reasons for existing problems. MOLDFLOW and ABAQUS software have been simultaneously used to simulate the injection molding process and thermal deformation arising after ejection step from the mold. In order to validate the model, the deformation predicted by the developed model was compared with experimental results, and both results showed good agreement. We also carried out design of experiment (DOE) to investigate the effect of various processing parameters that affect the final deformation of injection molded product. The developed model and information derived from DOE are expected to provide useful resources to the initial stage of mobile device design.20605
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Keywords: injection molding, design of experiment, numerical analysis, thermal deformation, residual stress
1 Introduction
An injection molding process is one of the key manufacturing techniques to produce parts from polymeric materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools down and hardens to the specific configuration of the cavity. Since it is possible to produce large number of complex parts in a short time, it is widely used for manufacturing components to be assembled for various engineering devices including mobile phones. Usually parts from injection molding process go through a large range of temperature variation, the dissimilar shrinkage of heterogeneous material parts resulting in warpage can be caused by thermal deformation. These phenomena can lead to defects such as size difference in components which ultimately bring about disassembling parts. To reduce defective products in injection molding process, distinguishing the critical factors for final thermal deformation from others would be a very important issue. Controlling them for optimal processing condition should increase the productivity and decrease the manufacturing cost.
Several attempts have already been made to identify the controlling parameters in injection molding process. Kurt et al. [1] studied the influential factors that affect final part quality in injection molding process experimentally and revealed that part shrinkage was strongly influenced by cavity pressure and mold temperature. Min [2] carried out various experiments by injection molding and concluded that injection velocity and holding pressure were critical factors to shrinkage. They finally obtained optimal processing conditions for specific injection molding design. Postawa and Koszkul [3] experimentally investigated the influence of processing parameters on shrinkage and weight of final parts in injection molding. They reported that clamping pressure and mold temperature affect mass and shrinkage of the molded part depending on material and distance from nozzle.
However, due to various parameters involved in injection molding process, experimentally identifying the decisive factors not only requires significant time and cost but also poses several limitations. Along with rapid progress in computer-aided engineering (CAE) software, many researches, which are hard to carry out experimentally, could be conducted numerically. Furthermore, various commercialized tools, which can simulate injection molding process accurately, have been introduced and used for various practical studies. Huang and Tai [4] and Chen et al. [5] carried out numerical analysis of injection molding process for a thin shell feature using commercial software and determined influential factors by DOE. It showed that packing pressure is the most significant factor. Melt as well as mold temperature is also found to be important in their study.