Identifying, Predicting Core-Gas Defects through Simulation, Part 2

Aug. 8, 2022
Simulation results help to identify the sources – and point toward methods for addressing – core-gas defects in low-pressure diecastings. Part 2 of a series.

Core-gas defects are an issue for foundries working with sand cores and molds. These defects occur when the resin used to bond the sand particles undergo a phase change from solid to gas, due to the high temperatures of the casting process. This report – the first part appearing in FM&T July 2022 – reviews a study that used simulation to identify core-gas defects in a brass casting produced using low-pressure diecasting. Through simulation results, new solutions were evaluated and implemented, toward establishing a new methodology to reduce core gas defect risks.

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The result shown in Figure 10 is a summary of all locations on the core where the gas pressure exceeded the metallostatic pressure during solidification. In the location where the defect is present (Figure 1) core-gas pressures exceed the metal pressure by over 0.1 PSI. 

Core-gas pressures could exceed metal pressures even after the metal reaches 100% solid, which is why it is important to understand how the casting solidifies.

A small pocket of metal at the casting and core interface is observed in Figure 11. We already know the direction of travel for the gas shifts toward the casting, and this pocket of unsolidified metal will offer little resistance and allow the gas to enter. Since the metal surrounding this pocket is 100% solid, the gas has no way to escape after it enters, resulting in the void observed in Figure 1.

With a root cause determined, multiple options are available to eliminate the core-gas defect. Two of these options include solidifying the metal at the core and casting contact as fast as possible; or creating a path within the core for the gas to escape. 

Iteration 1: Hole drilled in core

The first attempt to reduce the core-gas defect on the casting was a simple approach of drilling a hole in the core near the problem area. Gases will follow the path of least resistance, and adding a path for the gas to follow could remove the gas from the core and reduce its pressures. This was a quick and easy modification for the foundry to replicate, which is why it was chosen for the first attempt.

Figure 12 shows the changes incorporated to the core in Iteration 1. A small, 3-mm diameter drill hole was created and placed 50 mm deep into the core. Due to process restrictions, this was the maximum depth and width the drill hole was allowed to contain. 

The first variable that needs to be confirmed is the gas is traveling through the drill hole and out of the system. The core-gas velocity result in Figure 13 shows the relative core-gas velocity throughout the entire cycle. The vectors applied show the flow behavior of the gas. In the drill-hole, high velocities can be observed due to the hollow location applying no resistance to the gas flow, allowing it to flow toward venting within the permanent mold. Vectors also can be seen traveling to the upper section of the core. This is because vents were placed in the die in this location, too. 

Gas is flowing out of the core with the drill hole applied, however the core-gas defect risk result in Figure 14 still shows some potential for gas to enter the melt. The amount the gas pressure that exceeds the metallostatic pressure is reduced (compared to Figure 10), however the risk is still present and the drill hole is not enough to prevent gas from entering the melt. 

Iteration 2: Reduced vent contact width

The first potential option for reducing tendencies of core gas entering the melt did not work. Gas pressures still exceed the metallostatic pressure in the upper core below the vent. The next available option is to try to solidify metal at the core and casting interface as fast as possible.

The thick vent was originally designed to mitigate the late-solidifying metal observed in the upper casting. In reducing one issue another was created, with the increased heat this thick geometry brings not only to the metal but also the core. The goal of the thinner vent was to act as a chill to solidify the upper casting as quickly as possible (Figure 15.)

Adjustments were made to the core geometry in Iteration 2 to reduce thick sections in the upper casting where high core-gas pressures were observed, as well as to compensate for mold adjustments made by the foundry (Figure 16.)

The 2D plot in Figure 17 shows the total core gas to cast (lbs.) during the solidification cycle comparing the baseline iteration (blue line) to Iteration 2 (red line.) The adjustments made to both the upper vent and the core results in a lower mass of core gas entering the casting during the solidification phase. 

Figures 18 and 19 show the core temperature comparison of the baseline (Figure 18) and Iteration 2 (Figure 19) at the end of filling. A reduced vent thickness results in a 27.8°C (50°F) temperature decrease on the core where high core-gas defect risk tendencies have been observed. 

The vent volume reduction in Iteration 2 eliminates the risk of gas entering the melt in the problem area of the casting. This is due to the new vent bringing less heat to the core shown in Figure 20, which reduces the gas generated and the resulting pressures.

Figure 21 shows the casting containing the smaller vent and adjusted core geometry from Iteration 2. There is still a small depression at the core and casting contact due to the isolated metal at the end of solidification, but this small depression is no longer present after machining. 

Conclusions

Four questions were answered in the baseline analysis in order to determine if the defect was core gas: Is core gas being generated in the process? Is the core gas generated entering the melt? Where in the melt is the core gas entering? Is there a way for this gas to escape?

1.  The first question was answered by using the Binder Content result to observe the binder behavior. During solidification binder was degrading into core gas, shown in the total core gas produced 2D plot.

2.  The next question was answered by looking at the core gas to cast 2D plot, where a large mass of core gas could be seen entering the melt during the solidification phase.

3.  To determine where the core gas was entering the casting, the Core-gas Pressure result was analyzed. A high-pressure front shifted the flow direction of the gas toward the casting. This led to a risk of core gas entering the upper casting under a vent contact. as seen in the Core-gas Defect Risk on the cast result.

4.  The final question was answered by understanding the solidification of the casting. A small pocket of metal at the core and casting contact was isolated late during solidification. This allowed the gas to enter, however because the surrounding metal was 100% solidified the gas could not escape, leaving the void on the finished casting.

With this understanding, a path forward was determined to reduce these defects. It was found that the vent in the upper casting was overheating the core, which was leading to high gas pressures exceeding metallostatic pressures. To reduce the temperature of the core, the vent thickness was decreased. As a result, the severity of the defect was reduced.

This is the second and final installment of a report first presented in FM&T, July 2022.

Taite O. Gallagher is Technical Support Engineer; Matheus M. de Oliveira is V.P. Operations; and John W. Hambleton is Non-Ferrous Application Manager - and all are with MAGMA Foundry Technologies Inc.