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Metal Casting Technologies : March 2006
level required for this process. The variables are the parameters of the CAD-design of the cooling/heating lines, too. Local squeezing Many castings have heavy sections between thin sections. In order to avoid shrink- age cavities it is necessary to perform local squeezing. Purpose of optimization is to avoid heavy sections with pores. The vari- ables are the exact position and the volume of the squeezers as well as the exact point in time, when the squeeze pin is pressed into the solidifying melt. Die filling simulation The die filling is often seen as the most criti- cal and for the casting result the most influ- ential sub-process in high pressure die cast- ing. Apart from some exceptions that require a slow die filling (like infiltration of inserts made of ceramic fibers) the ingate velocities lie in a range between 30 and 140 m/s and the filling times are between 20 and 200 ms. These conditions lead to a turbulent flow, where, due to the geometries of the castings, the melt fronts are nearly always uneven. The flow consists of at least two phases (liquid and gas) and in some cases additionally of a solid phase during the die filling. Flow in the gating system Due to various reasons, the gating design is very important in high pressure die casting. Regarding the design of the gate, the follow- ing needs to be taken into consideration: ● Turbulences in the melt should be reduced in order to avoid entrapped gas in the casting. ● The melt flow through the gate needs to be timed in order to allow the controlled merging of the melt fronts. ● The flow velocities need to be consistent, also when using fan ingates. ● The desired ingate velocities need to be met. ● The desired direction of the melt flow into the cavity needs to be met. The die filling simulation based on an existing design of the gating system allows to evaluate all these problems, and thus to decide if the design is usable or needs to be modified (Fig. 2 through 5). Consideration of casting parameters The die filling is primarily determined by the defined shot parameters. Thus, the plunger velocity and switching points need to be considered very precisely for the simulation (R. Fink, 1999). Generally it is assumed that the machine hydraulics is able to implement the defined parameters. In this case the vol- ume flow of the melt as a function of time is exactly known and will be considered in the calculation accordingly (Fig. 6). Figure 5 Figure 6 Figure 7 www.metals.rala.com.au