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pfonline.com/experts Laser Engraving on Anodized Aluminum, Revisited Q. I came across a response you gave to a question on laser etching an anodized surface (short.pfonline.com/laser- anod). 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 nano- meters). This type of laser is capable of scribing right through the anodic coating to the bare substrate under- neath. 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 thick- ness 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 thick- ness. 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 indi- cates the coating has been removed. Calculation for Adding Aluminum to a New Anodizing Bath Q. We can only buy non-hydrated aluminum sulfate [Al 2 (SO 4 ) 3 ] to add dissolved aluminum to a new sulfuric acid anodizing bath. I have read your article in the Products Finishing archive (short.pfonline.com/scabm) about the amount of aluminum sulfate to add in the hydrated form [Al 2 (SO 4 ) 3 • 13 H 2 O]. Would I have to add the same amount of non-hydrated aluminum sulfate, or would it be a different formula? How is that calculation done? A. The amounts of hydrated versus non-hydrated aluminum sulfate to add to the bath would definitely be different. Here is how the calculation goes: • Start with the atomic weights for each element in Al 2 (SO 4 ) 3 Aluminum= 27 Sulfur= 32 Oxygen= 16 • Al 2 + (SO 4 ) 3 2(27) + 3[(32 + (4 × 16)] = 54 + 3(32 + 64) = 54 + 3(96) = 342 The result is that the aluminum sulfate molecule (no water attached) has an atomic (or molecular) weight of 342. We want to know the ratio of the aluminum to aluminum sulfate. • The aluminum in this molecule is 54 and the entire aluminum sulfate molecule is 342. • Divide the aluminum by the aluminum sulfate to find the ratio: 54 ÷ 342 = 0.158 So 1 ÷ 0.158 = 6.33, or 6.33 g of Al 2 (SO 4 ) 3 added to a one liter bath of brand new sulfuric acid will raise the concentration of dissolved aluminum by 1 g/l. • In metric terms: Add 633 grams of non-hydrated aluminum sulfate per 100 liters of sulfuric acid bath to raise the dissolved aluminum by approximately 1 g/l. • In English terms: to raise the concentration of 100 gallons of sulfuric acid bath by one gram per liter using non-hydrated aluminum sulfate: 6.33 × 379 = 2399 g or 2.4 kg 2400 g ÷ 454 g/lb = 5.3 lbs • Remember: 379 liters = 100 gal and 1 lb = 454 g Important Factors in the Light Fastness of Organic Dyes Q. We are working on changing the structure of one of our outdoor products to aluminum and have been having difficulty getting the UV colorfastness to satisfactory levels with a gold and an olive-green color. The blue we are using is OK. I am having trouble determining if this is a thick- ness issue, dye issue or another process. What are the most critical aspects of colorfastness in anodized aluminum in order of priority? A. The most important factors in determining the light fastness of any organic dye are as follows: • Anodic coating thickness. 42 SEPTEMBER 2015 — pfonline.com p f o n l i n e . c o m / e x p e r t s LARRY CHESTERFIELD Anodizing Technologies, Inc. aluminumfnishing@pfonline.com ALUMINUM ANODIZING C L I N I C