Cleaning the Mold Release Agent Off of Aluminum Castings

Question:

We are trying to clean the mold release agent off the aluminum castings we purchase. The release agent gets into the pores of the aluminum and resurfaces upon heating—so the cleaning process has to clean into the pores of the aluminum. The process our current supplier uses has yielded a casting without the release agent but we now have aluminum oxide and smut on the casting. Now we need a better way to clean the mold release agent and also a way to clean the aluminum oxide and smut off the casting. Product cost is $100 per casting and we have 1,000 castings with the oxide and smut on them. Your thoughts, particularly on cleaning the oxide and smut? Thanks. E.B.

Answer:

It may be difficult to find a cleaning agent that will be able to penetrate the porosity of a casting. An aqueous process would be hard pressed, but a solvent cleaner would have a better chance since most solvents have a lower surface tension than aqueous solutions. Depending on the volume of your operation, you may want to outsource this cleaning to a company that is using chlorinated solvents. If the operation is large enough, you could consider running test pieces through a cleaning process at a supplier of degreasing equipment (www.pfonline.com/suppliers.html). If that process appears viable and you can make the business case to bring the cleaning in-house you will gain additional control of your process and minimize in-process inventory.

If it is only a very short run of parts, you may have to try something as simple as a paint thinner from your local hardware store. Although I do not think the chances of success are very good with this option, considering how easy it is, it is probably worth a try. If these options are not viable, the last possibility may be to pre-heat the part to vaporize the mold release agent, then deal with cleaning it off after it has been driven from the pores of the casting. Unfortunately, at that point, depending on the composition of the release agent, it may be baked onto the surface and have a varnish-like appearance.

In order to clean off either the baked-on mold release agent or the oxide/smut mentioned in the other scenario above, a caustic etch and nitric acid desmut may be the best option available. The high pH of the sodium- hydroxide-based cleaner will attack and dissolve the aluminum from underneath your surface contamination. The alloying elements of the aluminum will remain as a dark smut. This can be removed by a relatively short (about 30 second) dip in room temperature nitric acid at about 25-50% concentration by volume. This could be the best option if you will need to address castings that use both types of mold release agents in the future. The only difference in processing is that the first type would receive an initial “bake out” prior to the cleaning process.