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Coloring aluminum castings

I have a potential customer that wants to change its specification from painting an aluminum casting black to anodizing the casting and coloring it black. We have made samples using different timings throughout the process. The results were what we have seen in the past, a dark grey finish. It was uniform, but was similar in color to what the part looked like after cleaning and before anodizing. Our thought is that the anodizing film is not the best for accepting a dye. Can you recommend anything that might enhance the film to accept a blacker finish? I am still waiting on information on the alloy of the casting.

Thanks

Tony Ianniello
metal finishing - El Paso, Texas
2005

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First of two simultaneous responses --

Castings have high impurity levels. You need to lower the impurities at the surface by the use of certain chemicals if you want to achieve a dark black color.

Bill Grayson
- San Jose
2005

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Second of two simultaneous responses --

All castings are different. You anodize aluminum ONLY. You do not anodize the silicon on the surface. Thin walls cool faster and have more silicon on the surface. Most job shops do not understand how to reduce the area of silicon on the surface and increase the area of aluminum (hence moving from gray closer to black). Such job shops are set up for wrought metal and just run the high silicon castings right through the same process - it will not work. Special technology is required to get a good dye on castings. And you have to start with a good castings in the first place.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2005

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Oil it very sparingly with any edible oil(olive is best for aluminum) and then heat it slowly to 400 °C,repeat if need.Instead oil You can use white of an egg or 20% shellac solution.

Good luck!

Goran Budija
- Cerovski vrh Croatia
2005