The authoritative public forum
  for Metal Finishing since 1989
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What is the maximum etch depth I can get before hard coat anodizing?

I would like to find out what the typical maximum etch depth prior to hard coating anodizing is. I would like to have the machine tool marks removed via etching and leave a nice matte finish. It seems like I was told 0.002-0.004" was a typical etch depth, but now when I call a couple shop they seem to shy away from that. I don't care about the part dimensions decreasing, just standard hard coat after etching.

Scott Harsila
- Kirkland, Washington, USA
2007

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Dissolving metals chemically of any type is never the first option one should consider when looking for a "matte" finish, particularly when you're looking for 2 to 4 mils or better of removal! All the dissolved metal eventually must be waste treated and aluminum is particularly troublesome in the presence of other metal hydroxides formed in waste treatment; it's light and tends to float when all the metals are intended to sink. Rather than chemical removal, mechanically treating the surface by glass bead or alumina blasting would give a nice matte surface and avoid such "heavy" etching. Another possibility would be to avoid using a "macro" etch, typically heavy caustic etch, in favor of an acidic "micro" etch specifically formulated to create a matte appearance with minimal removal. Many firms serving the optics industry offer matte black anodize finishes which employ this type etching - they're used when we have to anodically treat precision machined parts where a matte appearance AND dimensions matter.

Milt Stevenson, Jr.
Syracuse, New York
2007

Can these specialty optical coatings that yield a matte finish also be finished as undyed instead of black?

Scott Harsila
- Kirkland, Washington USE
2007

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Yes. Black or clear matte anodize can be done.

Milt Stevenson, Jr.
Syracuse, New York
2007