The authoritative public forum
  for Metal Finishing since 1989
-----

Color variation in bulk anodizing

2000

My company does bulk anodizing of small cosmetic parts. Recently we have experienced a high degree of variation in our gold-dyed parts (light,dark,iridescence). Is there an additive or technique available for minimizing variation? We have studied the dye and the problem seems to be centered in the anodizing tank. Is lake water turnover as big a problem in anodizing as it is in other types of plating? Has anyone experienced seasonal anodizing problems?

Thanks for your help!

Victor Waldman
- CT,USA

---

2006

This letter is pretty old, but it was one of the first hits on a Google search of "bulk anodizing" - so I figured it was a shame it hadn't ever been answered!

Bulk anodizing, especially for cosmetic purposes, can be a very touchy process. Unlike racking you can't ensure that all the parts are getting close to equal current density, nor can you get good solution flow as easily. The problems most people see with color variation in bulk anodizing is due to different thicknesses in the anodizing coating on the parts - typically it grows thinner (and the color lighter) as the parts move inward toward the center of the basket. These problems are especially noticeable on sensitive dyes like Gold dye.

All of these problems can be overcome, and the methods we've learned after specializing in bulk anodizing of aluminum parts (especially fasteners) for over 25 years allow us to pretty much avoid the issue completely on uniform lots and minimize it greatly on mixed lots.

First, don't mix alloys - the gold dye is very responsive to pore size, and while color matching is possible, you'll never pull it off on two different alloys your anodizing at the same time - you have to run them to different parameters.

Second, keep a TIGHT control over the amp minutes per square foot your applying - if you want the color to match, you've got two put on as identical a coating as possible.

Third, make sure your tanks are AGITATED - the harder the better. Heat will dramatically increase pore size and will have very noticeable effects on the gold dye.

Fourth - it goes hand in hand with #3 that you need a good cooling system.

Fifth - Practice. There is still some art to finishing, after all.

There are a few more techniques that we've developed, but to take advantage of those, feel free to contact us through the link below and ask for a current quote! Good luck!

Jim Gorsich
Compton, California, USA