Simulation of injection-compression molding for thin and large battery housing
Introduction
There is an ever increasing demand for thin and large polymeric parts across a wide range of applications in display, medical, mobile, and automotive areas [[1], [2], [3], [4], [5]]. For instance, a plastic window and a light guided plate (LGP) used in display devices have thin and large dimensions. While polymer processing methods such as extrusion and injection molding are currently employed to produce common plastic parts, it is still not easy to manufacture thin and large polymeric components [[6], [7], [8], [9], [10]].
In polymer processing, polymer is melted by applied heat and then solidified during cooling. This phase change necessarily happens depending on the temperature condition. When a molten polymer is injected into a mold cavity in injection molding, a skin layer forms on top of the mold surface. The formation of such a layer may induce incomplete cavity filling, i.e., the so-called ‘short shot’. In this sense, solidification of molten polymer in the cavity needs to be minimized to prevent the short shot phenomenon. In particular, when fabricating thin parts, this is a very important issue to take into account. To handle it, increasing the injection flow rate of polymer into the mold cavity is one method [11]. However, it entails an increase in the filling pressure of injection molding. If the pressure exceeds the clamping force of the injection molding machine, a flash-out of molten polymer occurs in the mold cavity. In some cases, the mold may be deformed. Another method for the issue is to extend solidification time of polymer in the cavity. This requires an increase in the mold temperature or insulation the mold wall [9]. It results in a reduction in the viscosity of polymer melt, thereby decreasing the filling time and pressure. However, it also leads to a longer cooling time and lengthens the overall processing time.
Injection compression molding (ICM) can act as a solution to resolve the issues mentioned above [[12], [13], [14], [15], [16]]. ICM adds compression to the molding processes as illustrated in Fig. 1. When the polymer is injected in the cavity, the thickness of the cavity is larger than that of the final molded part. After the resin injection, compression is applied to the mold to decrease the cavity thickness. Consequently, the injected polymer melt is squeezed until the mold cavity is fully filled [17]. Therefore, it is not necessary to increase the temperatures of polymer melt and mold wall in order to fill a thin cavity. In general, the ICM process has been applied to the production of LGPs [18,19].
Weight reduction of component parts is a very important target in manufacturing engineering. In this study, we analyzed the fabrication method of large and thin polymeric parts for battery cases using ICM. Numerical simulation was carried out to model the manufacturing process, and compared with the experimental results.
Section snippets
Theory
The objective of this study was to estimate ICM for the production of thin battery cases. The ICM process was compared with ordinary injection molding processes. To do this, the fluid flow and heat transfer characteristics of ICM were predicted using a commercial computational fluid dynamics software program, Moldex3D (CoreTech System Co., Taiwan). Three conservation laws were taken into account as governing equations: conservation of mass, conservation of linear momentum, and conservation of
Experimental
For the fabrication of battery cases, a blend of PC and ABS copolymer was supplied by SABIC Corporation (CYCOLOYCX7240). In injection molding, a charging time of 0.3 s was applied, and velocity/pressure switching occurred when 98% of the cavity volume was filled. The detailed processing conditions of injection molding are listed in Table 1. In particular, the melt temperature was 285 °C, which is higher than the glass transition temperature and the melt temperatures of PC and ABS. This implies
Results and discussion
Fig. 2 presents the time when the melt front of polymer reached at a location in the cavity at the filling stage of normal injection molding. In general, it is not easy to experimentally observe the flow front in the cavity, while numerical simulation can help one understand the filling behavior of polymer melt in the mold cavity. In this study, three different short shot tests were carried out to visualize the flow front in the cavity experimentally (Fig. 2 (a), (b), and (c)). Since the short
Conclusions
In this study, we investigated ICM characteristics to fabricate thin and large battery cases. Compared to normal injection molding, ICM uses a relatively low filling pressure. This led to relatively reduced shrinkage and warpage in final products. A mold was prepared for injection molding, and the injection molded parts were compared with the numerical results. Several important characteristics of injection molding (filling pressure, clamping force, filling time, filling pressure, volumetric
Acknowledgement
The present research was conducted by the research fund of Dankook University in 2018. Also, this work was supported by the Basic Research Program (2015R1D1A1A02062233) and 2018R1D1A1B07049173 of the National Research Foundation (NRF) funded by the Ministry of Education, Science and Technology, Korea. This work was supported by GRRC program of Gyeonggi Province (GRRC Dankook2016–B03). In addition, this work was supported by the National Research Foundation of Korea(NRF) grant funded by the
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