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Metal Casting Technologies : June 2007
www.metals.rala.com.au 46 ABSTRACT he cold chamber high pressure die casting process is one of the most general processes to make a lightweight material alloy casting. This process usually guarantees the short cycle times and good surface quality. However, the gas porosity defect appears easily in this process due to the air entrapment during the injection stage. The flow pattern of molten metal in the shot sleeve is closely related to the air entrapment. Generally, the flow patterns in the shot sleeve are concerned with the various plunger speeds and fill rate of molten metal. To investigate the mechanism of the gas porosity generations in the shot sleeve, the numerical simulation is introduced in this study. The numerical simulation results were shown good agreement with the experimental results using water model. From the comparison results, we conclude that the developed mathematical model for the flow pattern simulation in the shot sleeve of the cold chamber high pressure die casting process is useful to determine the proper condition of the plunger speed in some rage of the molten metal fill rate. INTRODUCTION Many recent research works in material engineering have been carried out to find out more effective casting methods for the lightweight materials such as aluminum alloys. The cold chamber high pressure die casting process is one of the most general processes to produce the lightweight material cast. It injects the liquid molten metal into a mold at high speeds. From this process, we can usually get the short cycle times and good surface quality of casting products. This process, however, has a problem that the gas porosity defect appears easily. The gas porosity is occurred mainly due to the entrapment of air in the molten metal as a consequence of the high speed injection. The gas porosity usually causes the pin- hole defects that are harmful to the mechanical properties and surface quality etc. The flow pattern of molten metal in the shot sleeve is closely related to the air entrapment. The flow pattern of molten metal in the shot sleeve depends on the die casting operation conditions; filling rate and plunger speed. The main aims of this study are the development of a numerical simulation system for the flow pattern analysis in the shot sleeve and the accuracy verification by comparing to the experimental results from a water model. NUMERICAL MODEL A numerical model based on the SOLA-VOF technique was used to simulate the molten metal flow in the shot sleeve. Continuity equation for the mathematical modeling is the equation. where V is the vector of velocity. Volume of fluid equation is written by where t and F are time and fraction of fluid volume, respectively. Navier-Stokes equation is given by where G, ρ and P are vector of gravitational acceleration, density of molten metal and pressure. Energy equation is , where T, c and L are temperature, specific heat of fluid and latent heat. fs is the fraction of solid, and k is the thermal conductivity. The VOF method is using the fractional volume function F(x, y, z, t) for tracking free surface boundaries. When averaged over the cells of a computational mesh, the average value of F in a cell is equal to the fractional volume of the cell occupied by fluid. A unit value of F corresponds to a cell full of fluid, whereas a zero value indicates the cell contains no fluid. The cells with F value between zero and one contain a free surface. The VOF method considers the amount of F to be fluxed through the face of a cell during a time step of duration Δt. The amount of F in one time step is F δ times the face cross section area, where and Investigation on the flow pattern in the shot sleeve of the cold chamber HPDC process By Jun-Ho Hong, Young-Sim Choi, Ho-Young Hwang, Jeong-Kil Choi