Innovation in Engineering Computational Technology
Chapter
14 J. Sienz, J.F.T. Pittman and A. Polynkin
Centre for Polymer Processing Simulation and Design, C2EC Research Centre, School of Engineering, University of Wales Swansea, United Kingdom Keywords: fluid-assisted injection moulding, thermoplastics moulding, non-Newtonian flow, heat transfer, moving interfaces, finite elements.
Fluid-assisted injection moulding is an important variant of the traditional technology for injection moulding of thermoplastics, in which a low viscosity fluid is injected under pressure after the polymer melt to displace the plastic and form a hollow core. The gas-assisted version of the process, which uses pressurised nitrogen, is well- established, and is applied in the production of two main classes of components: thin, shell structures with hollow reinforcing ribs, such as TV cabinets; and fully three-dimensional hollow objects, such as tubular components and handles. The potential of the newer, water-assisted process is yet to be fully realised, but it offers a number of important additional capabilities. Cooling is accelerated by heat transfer to the internal water, reducing the moulding cycle time and increasing productivity. Larger diameter, smooth bore mouldings can be produced offering a new route for the manufacture of fluid-carrying ducts with integrally moulded flanges and other functional parts, with important potential in automotive applications. Commercial simulation software is available for gas-assisted injection moulding (GAIM) [1]. In this the geometry is simplified using the two-dimensional Hele-Shaw approximation in shell elements for thin parts, and one-dimensional beam elements for gas channels. This software is used mainly for the simulation of GAIM for shell structures with ribs, where predictions of gas penetration length and percentage of channel cross sections occupied by the gas core are provided. It is less appropriate for fully three-dimensional hollow objects, where the geometrical simplifications are not applicable and the lack of any predictions of plastic wall thickness variations circumferentially or in bends is a significant drawback. For these reasons interest in recent years has turned towards the development of fully three-dimensional simulations of GAIM [2,3,4,5,6,7,8,9]. Simulation of water-assisted injection moulding (WAIM) poses new challenges, including modelling the important heat transfer that occurs between the water and hot plastic, and dealing with inertial effects resulting from high water velocities. Work in this area is only now beginning, but is important for the fuller exploitation of the process and the realisation of its potential in providing improved and alternative manufacturing routes. The present paper provides an overview of work in the Centre for Polymer Processing Simulation and Design at the University of Wales Swansea, developing simulations of fluid-assisted moulding in parallel with experimental moulding trials. Fundamental studies using two and three-dimensional finite element analyses of gas-displacing, shear-thinning, temperature-dependent liquids from prismatic channels of various cross sections are summarised. Applications of the three-dimensional simulation of GAIM for industrial parts with complex geometry are then described and compared with the results of moulding trials. As an alternative to these large-scale computations involving two-phase flow with moving interfaces, an approximate method is presented that requires only a two or three-dimensional simulation of the steady flow of the polymer melt. Finally, preliminary work on the simulation of water-assisted injection moulding (WAIM) is outlined.
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