. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Casting innOVatiOns
did not coincide with the simulations (Fig. 2). Custom Castings involved Magma engineers to assist in sorting out the issue. The engineers first thought the
large voids were due to shrinkage porosity. The simulation results were again reviewed and evaluated to check for errors that may have caused the discrepancy in results. Variations of material temperature, cycle times and mold temperatures were run to see if they would result in the observed defects. Through all the variations of parameters, the casting remained nearly defect-free with nowhere near the defect severity of the first sample run. The simulations showed locations of shrinkage in the defect region, but it was minor in relation to the defect size seen in the sample run. The simulation inputs were double-checked and all
Fig. 4. The defect prone region of the casting took 140 seconds after filling to completely form a skin. Pictured at left is the inital skin formation. At right is the final casting skin.
the process parameters were modified to the possible extreme, yet the simulated defects varied only slightly. Eventually, the engineers decided the casting defect was not shrinkage porosity at all, but a condition in the process was not being captured—core gas. Magmasoft currently does not directly calculate the evolution of gas from core and mold binders. However, the conditions that cause off-gassing of cores and their migration into the casting are accounted for by the software’s results. In order for a core to emit a gas, uncured binder must be present in the core or mold, and these core/mold
materials must reach high temperatures. Images from the simulations at Custom Castings showed that the core pos- sessed all the required conditions to produce gas (Fig. 3). The second condition of core gas-related porosity is
an entry point for the gas to flow into the casting. Once a skin has formed on the outside of the casting, the likelihood of gas migrating into the casting is small, as the gas will take the path of least resistance. Simulation software results were used to identify the locations of the casting that formed a skin last and thus identify the entry point of the gas into the casting and the amount of time it was able to do so (Fig. 4). These two results, in combination with a sectioned
core, revealed all the conditions necessary for the core to produce gas and the location within the casting that the gas would reside (Fig. 5). With the true defect identified, Custom Castings’ engi-
neering department worked with its core materials supplier to troubleshoot the variability in the coremaking process. The company reduced the binder content of the sand, which yielded a slight improvement in the defect sever- ity. During the investigation of the cores, Custom Cast- ings discovered the core molds were not heating the core evenly, causing the uncured binder issue. To verify the sand cores were the root cause of the problem, the cores were replaced with an alternate material to completely remove the potential for core gas porosity issues. A sample run of the castings was conducted using the new material, and it resulted in 100% sound castings and verified the cores were the source of the porosity issue.
METAL
Fig. 5. The core gas defect indicated in the simulation correlated to the location of the defects found in the castings.
MarCh/aPril 2010 Metal Casting Design & PurChasing 49
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