1. Definition of air vents and overflows
First, the filling simulation was used to evaluate the most suitable position and size of the overflows and air vents in the die. This also made it possible to evaluate the critical areas of the part in terms of air entrapment, and therefore possible porosity in the finished product.
When evaluating the simulation results, areas with a high risk of gas porosity were found, which could be affected by this problem because of inadequate venting. However, it was considered that, under optimised process conditions, this should not be severe and therefore should not cause rejects in production.
The first pictures highlight some internal areas of the part where the air does not escape correctly from the die.
In particular, air entrapments can be seen at the thick areas around the mounting holes, which led to four overflows being made in the die next to these areas.
The next picture highlights an area where an overflow becomes very important: it is one of the last areas to fill, and an air pocket that must be able to escape is clearly visible.
Finally, the last area where air remains in the cavity is highlighted; it is actually inside the part and not on an edge where a vent or overflow could be placed.
However, there are core pins forming the existing holes, which provide a secondary escape route for the air remaining in the area.
2. Gate section sizing
Thanks to our experience we have dealt with many aspects of die casting die design, above all the correct set-up of the feeding of the part in the die, as well as the optimisation of process parameters.
To bring together everything learned and apply it at design and production level, a spreadsheet was developed. It is divided into several modules. The main one calculates the gates according to our experience, using the NADCA formula for the filling time. Other modules provide further information, such as the ideal shape of the fan feeders or of the runners, as well as the machine parameters to use as a first approach to the die.
The main values used in the main module to calculate the gate sections for this part are listed below.
- Number of cavities: 1
- Average casting thickness: about 3.5 mm
- Part volume calculated from CAD: about 265 cubic centimetres
- Percentage of volume taken by overflows: 10%
- Critical casting thickness: 2.5 mm on the rib farthest from the gate area
- Metal temperature at the gate: 665 °C
- "Average" die temperature: 270 °C
- Solid fraction allowed at the end of filling: 30%
- Minimum gate velocity limit: 25 m/sec
- Maximum gate velocity limit: 50 m/sec
Using the NADCA formula, a cavity-only filling time of about 0.048 seconds was obtained, with a required squared flow rate of 31.5 litres^2 / seconds^2.
This made it possible to define an optimal working point with a pressure of 65 bar and a squared flow rate of 40 litres^2 / seconds^2, also to respect the geometric limits on the gate area. The resulting area is 150 mm^2 which, corrected to account for the opening angle given by the real feeder geometry, becomes 183 mm^2. Keeping a 65 mm constraint on the gate length, a thickness of 2.8 mm is required.
3. Ideal feeder sizing
The feeder was also obtained through empirical calculations collected in another module of the spreadsheet.
The graphical result is shown in the picture. The feeder was then adapted to reality to respect the maximum dimensions available on the die. Had it been possible to have a larger distance between the shot sleeve axis and the gate on the part, the filling of the cavity would have improved overall.
4. Plunger velocity profile
Simulation makes it possible to predict the behaviour of the metal during filling inside the shot sleeve too; at present only the ProCAST simulation software, available at Flowservice Srl, can guarantee an adequate simulation of this phase of the process. This is very important to prevent air entrapment caused by the waves that form in the sleeve at unsuitable velocities: velocities that are too high or too low create waves in different directions, but in both cases harmful.
A constant velocity profile gives an adequate filling of the sleeve, as shown by the following sequence of pictures. The value considered adequate for the first phase is 0.19 m/sec.
As the sequence shows, the free surface of the liquid metal moves gradually, in a way suited to a correct filling of the first section of the runner.
5. Second phase velocity
Preliminary calculations give a second phase velocity of 2.8 m/sec, so as not to exceed the critical velocities at the gate. The simulation then checks this value to see whether, during the whole cavity filling, any area exceeds the recommended maximum velocity limits, which depending on the part are in any case not higher than 55-60 m/sec.
In this case a critical threshold of 40 m/sec was considered. These velocity limits are used to guarantee a good die life, avoiding premature wear caused by excessive metal velocity. As the following picture shows, these limits are respected, so the second phase velocity calculated can be considered adequate.
On the colour scale, red shows the areas close to the 40 m/sec limit. The velocity values refer to the actual plunger movement, with a plunger diameter of 55 mm. Changing these settings during the process gives results different from those presented.