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The organization of this paper is as follows: in Sec. 2, we describe the target models for this study. Then, experimental and numerical procedure along with detection point for comparing product deformation will be followed. In Sec. 3, we present the benchmarking results by numerical simulation. A complete DOE analysis is also provided.
2 Analysis Procedure
2.1 Model Description.
We focused our study of injection molding process on mobile device. We have carefully selected two different geometries, which were named as “Battery cover” and “Front cover,” in order to access the shape effect of the device. As shown in Fig. 1, there is no empty space other than small holes for camera and speakers on battery cover, while front cover is formed by ribbed structure with large hollow space. These two typical but antithetical models in mobile devices enable us to develop a universal numerical model compromising geometrical effects. In order to acquire the reliability of the obtained data, multiple detection points were selected for both numerical and experimental analysis. These distinctive points have been used to measure the dimension of both width and height as shown in Fig. 1. We also indicated top and bottom side of battery and front cover in Fig. 1.
Fig. 1 Initial CAD design and detection points for both numerical analysis and experiment (a) battery cover and (b) front cover
2.2 Experimental Measurement. Both battery and front
cover were manufactured experimentally by injection molding process under specific processing conditions. As shown in Table 1, three different processing conditions were applied for the battery cover. It has been generally accepted that mold temperature plays a critical role on the final deformation in the industry. In order to investigate the effect of mold temperature more precisely, we have separated mold temperature into top and bottom in attempt to assign different values (see Fig. 1). The top and bottom mold temperatures of battery cover have been changed as oppose to front cover where those have been set equally as 100 ℃. For battery cover, the bottom mold temperature has been fixed to 105 ℃ for all the test cases. The case 1 uses 95 C as top mold temperature which is lower than bottom mold temperature. We have increased the top mold temperature exceeding bottom one to see the directional effect from mold temperature difference for case 2. The mold temperature for case 3 remains ame as case 2 but injection speed and packing pressure have been increased, resulting in decreased injection time. Different processing parameters such as injection time, packing pressure, etc. were applied for front cover. While three gates are used for all the cases of battery cover, seven gates are applied for front cover. We think that the above processing conditions including the variation of mold temperature can provide various settings generally encountered in real injection molding process. The final parts produced by injection molding process were captured by a 3D scanner (Laser Design Systems, Inc.) and deformation was measured at all the detection points by comparison with initial CAD design. This procedure originated from reverse engineering makes it possible to compare numerical simulation results with experimental Ones.
2.3 Numerical Analysis.
In this paper, a numerical model was developed to simulate injection molding process followed by thermal deformation. Generally, the numerical analysis for injection molding process of a polymer is governed by conservation equations of continuity, momentum, and energy. Especially, the behavior of the plastic melt is considered as a non isothermal, non- Newtonian flow. he governing equations for non isothermal and non-Newtonian fluid are as follows:
For the numerical analysis of injection molding process, it has been generally accepted that Hele-Shaw type of flow model provides a reasonably accurate description and viscosity of molten polymer can be described by the modified-cross model of Williams-andel-erry (WLF) equation defined as follows: