심입/박벽 구조를 가진 제품 제작용 사출 성형 공정에서의 금형 설계 조건과 공정 조건이 제품 특성에 미치는 영향

Abstract

Recently, the injection molding process of thin-wall parts with a deep penetration has been considered as an important manufacturing technology in automobile industries due to the reduction of the overall weight and the fuel consumption. In order to fabricate thin-wall parts with a deep penetration, high injection pressures and clamping forces are needed. Moreover, multiple runners and gates are used to fill whole cavities of the mould. The multiple gates cause weld lines in injection parts. The high injection pressure give rise to a core deformation phenomenon. The core deformation results in non-homogeneous thickness distribution of the injection parts. The objective of this research work is to investigate the influence of the design of the mould and process parameters of injection molding process on part characteristics in the injection molding of deep and thin walled parts to obtain a proper mould design and an optimum injection molding condition. Target mould and product were chosen as light-weight battery case and its mould. In case of the light-weight battery case, the ratio of the channel thickness to the channel depth was larger than 1 to 100. In order to investigate the effects of mould design and process parameters on molding and part characteristics quantitatively, three-dimensional injection molding analysis was performed using MOLDFLOW MPI V6.1. In terms of the mould design, the effects of runner system design and the mould temperature on filling characteristics, the weldline formation, the injection pressure-injection time curve, average shrinkages, and the post deformation characteristics were examined. Through the results of the examination, a proper mould design and mould temperature were deducted. In order to investigate the influence of process parameters and the core deformation on the product quality, the core shift analysis was carried out. Design of experiments (DOE) was utilized to estimate an optimum injection molding condition reflected core deformation effects. In the core shift analysis, process parameters were selected as the injection time, the injection pressure and the packing time. In the DOE, L9(34) orthogonal array was used. The signal-to-noise (S/N) ratio was estimated using smaller-the-better characteristics to minimize the core deformation. From the results of the DOE, optimum injection molding conditions were 6.0 seconds of injection time, 30 MPa of injection pressure and 5 seconds of packing time. In addition, it was shown that a dominant influence parameter is the injection pressure. The injection mould to fabricate the light-weight battery case was manufactured using the proposed mould design by injection molding analysis. In order to examine the accuracy of the proposed mould design and the optimum injection molding condition, injection molding experiments were performed using the fabricated mould set. In the injection molding experiment, the variation of the product quality according to the injection pressure was investigated. The results of the injection molding experiments showed that the best product quality is obtained when the injection pressure is 31.87 MPa. In addition, it was found that the maximum post deformation regions of the product is almost the same as those of analysis with the core deformation model. Through the comparison of the results of the experiments and those of the analyses, it was confirmed that the injection molding analyses with the core deformation model can properly estimate the optimum injection molding condition and the post deformation of the product.