Mechanical Finishing Q&A: Blast Finishing for Brightness

Q. We are blasting our aluminum parts in a blast cabinet after machining for a uniform finish. Our current process is blasting with 80-grit aluminum oxide to eliminate machine lines. What media is recommended for a brighter finish?

A. For brighter blast finishes, we recommend a No. 10 to 13 size glass bead or a 120-grit and finer ceramic media. Both media will produce a visually brighter and smoother roughness average (Ra) finish.

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The aluminum oxide you are currently using is an angular abrasive that produces a duller visual matte finish and a courser Ra surface finish. It is a great media for the removal of scale, rust and paint. Aluminum oxide produces an excellent pre-paint and coating adhesion surface and will hide a large tool mark on machined parts.

The glass and ceramic beads are round abrasives that produce a brighter satin matte finish and a finer Ra surface finish. It can also remove light oxidation, rust, scale and paint, but at a much slower speed. Beads are generally used to produce a uniform, bright satin finish on machine and intricate parts.

If the beads do not hide all of the tool marks that the current aluminum oxide hides, try the following recommendations:

• Turn up the air pressure.
• Try a larger size bead.
• Blast first with the aluminum oxide and second with the beads.
• Mass-finish to smooth the lines first and then blast with the fine beads.

Originally published in the April 2016 issue.