Experimental and Numerical Investigation of the Effect of Process Conditions on Residual Wall Thickness and Cooling and Surface Characteristics of Water-Assisted Injection Molded Hollow Products

Hindawi Publishing Corporation Advances in Materials Science and Engineering Volume 2015, Article ID 161938, 11 pages http://dx.doi.org/10.1155/2015/161938 Research Article Experimental and Numerical Investigation of the Effect of Process Conditions on Residual Wall Thickness and Cooling and Surface Characteristics of Water-Assisted Injection Molded Hollow Products Hyungpil Park,1 Baeg-Soon Cha,2 and Byungohk Rhee3 1 Mold Technology Team, Hyundai Mobis, 460-30 Sam-dong, Uiwang-si, Gyeonggi-do 437-040, Republic of Korea Molding & Forming Technology R&D Group, Korea Institute of Industrial Technology, 7-46 Songdo-dong, Yeonsu-gu, Incheon 406-840, Republic of Korea 3 Department of Mechanical Engineering, Ajou University, San 5 Woncheon-dong, Yeongtong-gu, Suwon, Gyeonggi-do 443-749, Republic of Korea 2 Correspondence should be addressed to Hyungpil Park; Received 18 September 2014; Revised 17 February 2015; Accepted 18 February 2015 Academic Editor: Aiguo Xu Copyright © 2015 Hyungpil Park et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Recently, water-assisted injection molding was employed in the automobile industry to manufacture three-dimensional hollow tube-type products with functionalities. However, process optimization is difficult in the case of water-assisted injection molding because of the various rheological interactions between the injected water and the polymer. In this study, the boiling phenomenon that occurs because of the high melt temperature when injecting water and the molding characteristics of the hollow section during the water-assisted injection process were analyzed by a water-assisted injection molding analysis. In addition, the changes in the residual wall thickness accompanying changes in the process conditions were compared with the analysis results by considering water-assisted injection molding based on gas-assisted injection molding. Furthermore, by comparing the cooling characteristics and inner wall surface qualities corresponding to the formation of the hollow section by gas and water injections, a water-assisted injection molding technique was proposed for manufacturing hollow products with functionality. 1. Introduction Gas-assisted injection molding (GAIM) is the major injection molding technique used to reduce the thickness of thickwalled products [1, 2]. For this purpose, some researchers applied GAIM to the manufacture of automobile oil and coolant tubes by implementing a complex process (extrusionwelding), such as the process adopted to make metallic pipes. However, the manufacturing process failed because of the formation of irregular hollows and interior surface defects (air bubble) caused by the action of the compressible gas used in the process. Lately, water-assisted injection molding (WAIM), in which water is used instead of gas, is being applied especially in the automobile industry to achieve high productivity, cost reduction, and lightening of functional hollow products [3]. Water has an excellent effect in terms of enhancing the productivity by reducing the cooling time by 30–40%; this is because water has an excellent cooling characteristic, with its thermal conductivity and heat capacity being 40 and 4 times, respectively, those of a gas. It has an advantage over GAIM in that the hollow products with functionality obtained by WAIM have a thin and fine interior surface and the residual wall thickness is regular unlike in the case of GAIM products [4, 5]. However, in WAIM, unstable flow is generated when the water passes through nozzle, and boiling effect is caused when the water is injected into high temperature molten plastic. The 2 flow instability and boiling effect of the water cause several defects such as nonuniform RWT, void in the wall, fingering, and double wall which can decrease the strength and stiffness of final products [6]. Yang and Chou and Liu et al. applied GAIM and WAIM in the forming of curve-shaped hollow products and evaluated nonuniform RWT through experiment [7, 8]. They proposed that the difference in growth speed of frozen layer affected the nonuniform RWT. Liu et al. analyzed penetration length of water for the resin in which glass fiber was added and proposed the changes in RWT affected by viscosity and shrinkage of resin [9]. In particular, the effect of crystallization of RWT that appears due to rapid cooling of resin by high heat capacity of water was examined through experiment [10]. Liu and Lin have analyzed fingering effect in the formed product having rib shape according to process condition in WAIM [11]. However, most of researches were conducted by experiments for process conditions are extremely difficult. This is because the hollow shape formed by water injection is sensitive to changes in the viscosity of the polymer, product shape, and molding process. In this research, the residual wall thickness distribution of the hollow section created by the passage of water and the molding characteristics were studied by performing a WAIM analysis of the filling behavior of the water that forms the hollow. A GAIM-based WAIM system was developed to test the WAIM; the WAIM analysis results and the residual wall thickness distribution of the hollow formed in accordance with the process conditions were compared and analyzed. In addition, the cooling characteristics and interior surface qualities of GAIM and WAIM products were compared to analyze the effectiveness of WAIM in molding hollows with functionality. 2. Water-Assisted Injection Molding Analysis 2.1. Governing Equation. Moldex3D of CoreTech Inc. is the only commercial injection molding software for the WAIM analysis. It was base governing equation of gas-assisted injection molding. The injected fluid applied water instead of gas. The viscosity of the resin at the flow front boundary, which is in contact with water, changes because the boundary is cooled by the water injected into the polymer melt for forming a hollow. It affects the residual wall thickness distribution. Hence, the simulation analysis is necessary for predicting water’s filling behavior, hollow residual wall thickness distribution, and the shrinkage distribution in WAIM [12]. In WAIM analysis, the core area where water flows is assumed to have constant pressure. The polymer melt filling process is defined as the process of introducing a nonisothermal, incompressible, and non-Newtonian fluid. Water is injected at a low pressure for forming a hollow, and, hence, it is defined as an incompressible Newtonian fluid. The surface tension at the flow front of the water is not considered. The mass, momentum, and energy conservation equations of the three-dimensional transient nonisothermal process for WAIM analysis are as follows. Here, 𝑝 represents pressure; 𝑇, temperature; 𝑢, velocity; 𝜏, stress (...truncated)