Increasing Productivity by Reducing Ejection Times

This article was contributed by Eugene Moore of Hellebusch Tool & Die

Simulation software is a valuable tool that helps designers and engineers understand the details of the casting process, enabling them to consistently create high-quality parts faster and with lower costs than their competitors. In high pressure die casting, simulation software is used to help design better gating systems to feed the metal into the casting, improve the timing of the shot sleeve tip to prevent air entrainment due to turbulence, and identify the most effective locations for overflows, among other things. In this article, we will look at how to reduce the time before a part can be ejected from the die in order to reduce the process time.

The biscuit is a natural place to focus our efforts since it is the last place to solidify in the casting and, therefore, determines when the part can be ejected. So, if we can reduce the solidification time of the biscuit, then we can reduce the overall process time. One way to do this is to remove more heat from the metal through the shot tip by increasing the amount of area in contact with the fluid. While not exactly applicable in this case, the basis for this approach is most easily shown using the equation for steady-state convection, shown below.

$Q=hA\Delta T$

In this equation, $Q$ is the heat flow, $h$ is the convective heat transfer coefficient, $\Delta T$ is the difference between the metal and shot tip temperatures, and $A$ is the surface area of the shot tip in contact with the metal. There are different shapes of plunger tips available in the market today that are designed to increase the surface area in contact with the metal, as shown in Figure 1.

Another method for increasing the heat removed from the biscuit is to moderate the temperature difference between the shot tip and the metal in the biscuit. This is done by adding cooling lines to the tip, as seen in Figure 2. The main drawback with this approach is that it adds considerable complexity to the piston assembly.

New design

For this article a new plunger tip design was analyzed using FLOW-3D Cast and compared to a standard, unmodified cylindrical tip. The modified tip, consisting of a cylindrical tip with a star-shaped cutout on the end as shown in Figure 3, has 20% more surface area than the unmodified shot tip. Neither tip will be water cooled for the analysis.

Analysis

A simulation of the filling (including shot tip motion) and of solidification (without flow) was run for each shot tip design; all other parameters were identical between the cases. There were two primary results of interest: the flow pattern during filling and the overall solidification time. The flow pattern during filling is important because, if the shot tip design were to cause breaking waves and air entrainment, then the tip or the shot sleeve profile would had to have been redesigned.

The first comparison is of the flow patterns in the shot sleeve, shown in Figure 4. This figure shows an image of the fluid during shot sleeve with and without the modified tip and it is seen that the shape of the tip is not significantly affecting the flow patterns. Since there is little effect on shot profile we can focus on the solidification.

The second comparison is of the solidification time. Figure 5 shows the comparison of the average temperature of the tips as a function of time, the heat flux from the metal to the tip as a function of time, and the temperature profile of the liquid metal at time of extraction.

As can be seen in Figure 5, the graphs show that the average temperature of the modified tip is higher because it extracted more heat from the metal. This is also shown in the plot of heat flux; notice the negative value indicating energy removal. The images below the graphs show the liquid metal at the interface of the biscuit and the shot tip. The data shows that there is a 12.7% increase in heat removal using the modified tip.

Conclusions

The shot tip design does have a noticeable effect on the solidification time of the cast part. Simulation software provides a way to analyze its effects and use this knowledge to optimize process parameters.