Anodizing Q&A: Laser Engraving on Anodized Aluminum, Revisited
Is there a way to determine if the laser is going all of the way through the anodized coating and compromising my part?
Q. I came across a response you gave to a question on laser etching an anodized surface (short.pfonline.com/laseranod). I am examining the possibility of using this technique for some labels. We have a laser in-house I’m looking to use. Is there a way to determine if the laser is going all of the way through the anodized coating and compromising my part? Does the type of laser matter? We are using a gas type laser and, from what I’ve read, it sounded like this type of laser wouldn’t penetrate down to bare aluminum.
A. There are two types of lasers used for etching or scribing (laser engraving) metal surfaces. YAG (for yttrium, aluminum, garnet) is a crystalline, solid-state laser that emits a wavelength of 1.064 micrometers (1064 nanometers). This type of laser is capable of scribing right through the anodic coating to the bare substrate underneath. Turning the power to maximum will determine how deep the engraving is and how rough it leaves the metallic surface. The anodic coating can be removed in this process. Operating on a very low power setting, the YAG laser may be capable of removing “color” from an anodic coating without removing the coating.
The other type of laser engraver uses a combination of gases (carbon dioxide, nitrogen and helium) to emit a wavelength of 10.6 micrometers, which is virtually the same as that of aluminum. In this case, the engraving mechanism absorbs the dyed color in the anodic coating. The color is removed, but no anodic coating is actually removed. The rate of absorption of the color from the anodic coating is determined by the anodic coating thickness and the power output of the laser. The color is more quickly absorbed in thinner coatings and less quickly in thicker coatings. If you had a black anodized coating, for instance, the laser absorbs the black color, leaving the aluminum color “engraved” anodic surface. If the coating is clear, the engraved results may be a mixed bag. With clear anodic coatings, the depth of color (usually shades of gray) is dependent on both the alloy and temper of the part being engraved, as well as the anodic coating thickness. Generally, the more coating thickness, the darker the clear color.
Test the integrity of the anodic coating of the engraved areas with a continuity meter. No continuity indicates the anodic coating has not been removed and continuity indicates the coating has been removed.
Originally published in the September 2015 issue.
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