Optimization of a Die Cast Vacuum System using Numerical Simulation
By Alexandre Reikher, Albany Chicago LLC
It is not every day that you are presented with an opportunity to design a new vacuum system for high-pressure die casting.It is even more challenging when the new design has to be tested on a production die without any interruptions to the production process. That is where numerical simulations play a vital role in accomplishing this task. FLOW-3D was an obvious choice to help with design and testing of the new vacuum system. We have performed many flow and thermal analyses over the last 11 years using FLOW-3D, and have learned to rely on its results in making decisions for the die cast process and die design.
The present work started with introduction of a new project, transferred from another manufacturer. During the usual process parameter calculations it became obvious that the vacuum system that came with the die would impose a serious limitation on the process parameters. A high-level vacuum would be necessary in the cavity (100 mbars) to produce an acceptable level of quality in the cast parts. Parts have to be heat treated after die casting, so a small amount of gas porosity could result in blisters on the surface of the part, rendering it unusable.
Designing the CFD Model
In order to adjust the process parameters to achieve the vacuum level required in the cavity, a computational model was constructed in which the casting, with overflows and runner system, were combined as a cylinder of the same volume. A moving piston was placed on one end of the cylinder and an open flow area equal to that of the vacuum valve was placed on the other end. A 100 mbar pressure was applied as a boundary condition at the open end. Calculations for both slow and fast shot velocities were prescribed to a piston, modeled as a general moving object. Analysis of the compressible air flow in the cylinder was made to determine the time to lower the pressure in the cavity to the pressure in the vacuum tank, which was necessary before a fast shot velocity can start. It became obvious that limitations of the vacuum system dictate the process parameters. Figure 1 shows air pressure distribution in the cavity of the die. Based on the results of calculations, the slow shot velocity had to be adjusted to allow time for the vacuum system to evacuate air from the cavity.
Figure 1. Air pressure distribution in the cavity during die-cast process (initially-calculated velocity profile).
Understanding the Model Results
Figure 2. Original vacuum valve design.
Figure 3. Multi-stage plunger deceleration system. After several iterations, a slow shot velocity was chosen. It allowed time to reach pressure in the cavity of the die equal to a pressure in the vacuum tank. The next step was to open up the air flow area of the vacuum valve to eliminate restrictions imposed by the current design. Initial calculations showed that the minimum size of the air passage has to be 300 mm2. The original valve had only a 90 mm2 cross section area for ventilation (see Fig. 2).
It wasn’t possible to just open up channels in the vacuum valve to a calculated size. Restriction of the metal flow is necessary to build pressure in the vacuum channel to close the valve. It was decided to adapt a previously-developed system used for plunger deceleration (see Fig. 3). Instead of restricting metal flow by reducing the cross sectional area of the channel, a flow loss was generated at turns in the channel. Metal flowing through the channel requires progressively more pressure to maintain the same velocity. As resistance to the metal flow increases, the pressure of the metal in the cavity also increases.
The Final Vacuum Channel Design
After several design iterations, a final model (Fig. 4) was chosen based on the space restriction of the current die design and the metal pressure necessary to close the vacuum valve. Metal flow analyses were conducted using the final vacuum channel design. Pressure generated by metal at the valve push button is compared to the analysis result of the current system (Fig. 5).
Figure 4. New vacuum runner design.
Figure 5. Pressure generated in original and new vacuum system (dyne/cm2).
Analysis Results: Comparing the Old and New Systems
The results of the analysis showed that the new system not only generates more pressure necessary to close the valve, but also gives an extra 5 milliseconds to close it.
Comparison analysis of the air pressure in the cavity between the original and the new vacuum system confirmed previous calculations (Fig. 5). A newly-sized ventilation block will allow adequate time to reach the specified pressure in the cavity. Analysis has also shown that the new vacuum block will allow an adequate air flow from the cavity of the die into the vacuum tank during the fast shot velocity stage as well. Visi-Track plots shown in Fig. 6 compare the pressure curve in the cavity of the die between the old and new vacuum blocks. Subsequent production runs have demonstrated that the new vacuum block achieves the specified pressure before the start of the fast shot stage. A 50 percent reduction in defects attributed to the new vacuum system (see Fig. 7) confirms the results of the simulations that the new vacuum system would be able to maintain air pressure in the cavity within process requirements.