Inconsistency in batches

Hello Small Fish in Big Pond......

What type of Al are you processing?

Unfortunately, cast aluminum is much more difficult to achieve a consistent anodized finish than wrought aluminum. The reason for this is different degrees of porosity in certain areas of the part from the casting, or (if present) silicon being present on the surface, which does not anodize - only turns a dark gray color. To obtain the bronze color, are you using a bronze colored dye after the anodize process? If so, is the inconsistency after the anodizing, or after the dye?
You mentioned that the customer wants a "bronze color". Are you certain that the customer wants anodizing, or a chromate conversion coating? A consistent chromate coating may be less difficult to achieve, if this is what they want. Sometimes a customer mistakingly calls this conversion coating "anodizing". A chromate will only give about 1/2 of the corrosion protection that anodizing will achieve, but if this is an acceptable limit of protection, you may want to investigate.

Excellent posts gentlemen and fellow finishingtalk.com members!!!!

JK makes a valid point in measuring the anodic coating thickness. However, standard color anodizing is generally simple and most anodizer's will simply anodize at @ 15 - 17 Volts DC. JK......shoot me a PM and share with me your technology.....I dig it.

However, being you are working with a cast aluminum, you have a few variables that will affect the coloring process. The first, being the cleaning process, which needs to be more consistent than the anodic coating itself. Labrat points out a very important fact, and that being the silicon levels in the casting itself and if "overcleaning" is obtained, it will affect the consistency of the porosity of the coating, thus, affecting the coloring process.

I learned more often than not, the KISS principle applies. "Keep It Simple Stupid", no offense meant. But, making sure your process is consistent is the most practical thing you can do to eliminate the inconsistencies.

First, make sure all of your processing chemistry operate within optimum parameters. i.e. temperature, pH, and time, TPT you may refer to it as.

Second, be conscience of the fact, that it always will look better, the first time through the process. You "DO NOT" want to be stripping the anodic coating, especially aluminum castings, and expect to improve the color without changing the aesthetic appearance of the color anodized coating.

Before I go any further, I want to take the rubber gloves off and applaud you for anodizing based upon amps per square foot (ASF). As I mentioned, most anodizers simply set the rectifier @ 17 volts DC and let 'er rip.

And last, TPT (temp., pH & time) comes very much into play when trying to reach consistent colors when anodizing large production runs. And as you mentioned, you are working with an aluminum casting, hence an already very porous substrate, thus, absorbing a large amount of the dye in your dyeing process. As you are processing from the anodize solution to the dye, you may be dragging in a very acidic rinse, therefore, reducing the pH of the dye solution.

Your technical data sheet for the dye solution will instruct you on the proper additives to control the pH of the dye solution. You may have to use "trial and error" to obtain a standard volume of typically, acetic acid or ammonium hydroxide to lower (most likely not the case) or raise the pH of the dye solution. Maintaining the pH parameters, as well as the temperature, will allow you to keep consistent dye times. Of course, you will also need to calculate the amount of dye additions to me made over a course of a certain quantity of parts or total surface area processed.

This my friend is how our "Platiing & Anodizing Forefathers" figured it out. If you want to try this out.....buy yourself, if you do not already have it (LOL), a Michael Jackson CD with the 80's track "Pretty Young Thing" a.k.a. PYT, and see if you don't just get the whole TPT concept. Not to mention, a very productive and PRETTY color anodize coating.

And to set the record straight, back to my almost offensive "KISS Principle" comment: I used the term "stupid"....I could have used the term "silly", "smartguy" or "scientist", for that matter. Remember plating & anodizing is a combination of science, chemistry, physics, and above all, ART!!!

I just realized that this post is ANCIENT (almost as ancient as the music track I mentioned....haha)!!! I hope someone can take something from my post.

G'day!!

"3"