Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Question About Vertical Anodizing
Q. Dear Sirs, We are a Chemical products supplier for finishing industry in MedellÌn, Colombia. One of our customers has started his new vertical Aluminium Anodizing plant since four months ago. They have had several problems, but the most important has been non uniform colour along the profiles. The upper part appear darker than the lower part for all colours they produce (clear bronze to black). The actual composition of the bath is as follows: sulfuric acid=40 g/l, Tin(II) Sulphate= 15 g/l and Stabiliser Additive=35 ml/l. We would really thank you any recommendation that you can give us to solve this critical problem. Thanking you
Juan JosÈ PavÛn Palacio- Colombia
1999
A. Sorry, Juan, I'm not familiar with the term "vertical anodizing". Are you describing something different than conventional anodizing, or are you simply noting that the parts hang with the longest dimension up-and-down rather than left-to-right?
I guess that I would have expected the opposite result to what you are getting, i.e., I would have expected the bottom of the parts to be exposed to the dye longest and consequently be darker. Since that is not the case, my guesses would include the possibilities that
- The dye tank is suffering temperature stratification, being colder at the bottom due to the heating coils being too high in the tank and the agitation being insufficient.
- Some of the color is being leached out in the seal tank and the effect is more marked at the bottom due to longer immersion time. However, since seal times are ordinarily very long, the additional time should be inconsequential.
- Even though it seems unlikely, because it is easy to test, I think the first thing I would do would be to try to determine if the anodizing thickness is nonuniform from top to bottom.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. Dear Sir
Ted's response was dealing specifically with the use of organic dyes. Your question is dealing with what is now an old system (But the best, especially if the app' is out-doors - U/Violet light-fast required) O.K with tin sulphate light scattering approaches it is very important to have an anode (TIN) equal to the work area (The aluminium work that has been anodized)- The Cathode and space it equally from the work for uniform deposition. Also of importance is the A.C.,Voltage & Current waveform shapes during this process to be equal on positive and negative cycles. Get the Book from BRACE...The Technology of Anodizing Aluminum [on AbeBooks, on Amazon, on eBay" (affil links)affil links]
Robert Fyfe- Gosford, NSW, Australia
2004
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