Technical Article Continued from pg 21


the blue material is air that exist inside the shell cavity. If your permeability is low, the


air can’t escape out of the cavity fast enough through the shell walls causing an increase in air pressure. That increase in air pressure, if high enough, can resist the advancing melt front halting its progress, which can lead to a misrun. One way to help fill out a thin section is to increase the permeability of the shell. If the permeability is increased, the metal can displace the air out of the cavity with relative ease and the local pressure does not increase. Another method is to add vents. Venting isn’t typically in investment casting, but cavities used for dewaxing can serve as vents if placed appropriately. One negative effect of increasing the permeability of your shell could be


surface finish issues. A small sand grain structure on the surface produces a remarkable finish, but at the same time, it doesn’t leave channels open for air to flow through. Changing the makeup to find an appropriate balance of surface quality and permeability is needed.


Pressure


Pressure, usually in the form of metallostatic head in gravity casting application, is often not considered when evaluating misrun defects. As mentioned, increasing pressure can help fill out sections even if the metal has fallen into the solidification interval. The faster you can fill the tree before metal begins to enter the casting cavity for example, the more head you will have while filling the cavity and thus will be able to fill challenging features. At the same time, the lower the metallostatic


pressure during filling, the more difficult it will be to prevent misrun. Conclusion


Misrun defects are complex and there are many factors that play a role in its occurrence.


Evaluating temperature,


velocity, air components, among other things will allow the foundry engineer to discover the root cause of the issue and find more effective solutions to increase the quality of the part but minimize any detrimental effects a solution may cause.


References 1. John Campbell, Complete Casting Handbook Metal Casting Processes, Metallurgy, Techniques and Design (Oxford, UK: Elsevier Butterworth-Heinemann, 2011).


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