Problem with inconsistancy - HELP!!!!

[anodizeme4ever]

Hello there,
I'm in the process of trying to anodize 2 diffent sizes of 7075 aluminum. One piece is 19 sq. inches and the other is 35 sq. inches. I have 2 - 5 gallon anodizing tanks hooked up in parallel using a 20 amp/30 volt CC/CV power supply. I'm using a 1" square aluminum bar with tapped holes and drilled holes to tighten the titanium wire on that the parts are hanging from. One of the parts have threaded holes I'm able to bend the wire so the end is bent and pressing into the crest of the thread. The other part(19 sq. in.) only has drilled holes so it's a little trickier but I'm able to do the same thing. In each tank, I have 2 - 8" square lead plates for the cathodes so theoretically should be able to get a minumium of 128 sq. inches in each tank.
I tried wiring 2 parts in each tank of the same part(so either 70 sq. inches in one and 70 sq. inches in the other or 38 sq. inches in one and 38 sq. inches in the other) so the total square inches never exceeded 140. When I anodized the parts at 12ASF, some came out with black streaks through them, others looking like they only got anodized from one side and others that didn't even look like they got anodized.
I later tried 1 part in each tank to try and isolate the problem and still not getting consistant results(usually 1 anodizes and the other doesn't) I'm trying to attain Type 2-1/2 Hard Anodizing and am about ready to crack if I can't get this thing figured out.
If anyone has any ideas on how to solve this problem, please get back to me ASAP please - I have another 50 or so parts to do by Monday!!!
Thanks in advance,
Kevin

[sswee]

Are the parts that anodize always out of the same tank?
Are the cathodes wired in parallel also?
Is the tank temp rising in one tank and not the other?
Is the electrolyte in both tanks from the same mix?
How close are the parts to the cathodes? Especially the ones that had black streaks.

Since you're in a pinch for time, may go with a 10 or 12 gal clear plastic storage bin to eliminate a few variables. I take it that you had no problems with a single tank, the problems started with the addition of the second tank.
SS

[rclint]

Also check out the coolers, they work good as well, and do not give to the weight of the solution

Clint

[sage]

You never really explained why you have two tanks in parallel. With a parallel circuit, you won't be able to guarantee the current will be the same in each tank. Minor differences, even though you have the same parts in each, will result in a current imbalance and therefore unpredictable results in BOTH tanks. About the only way you could run a setup like this is to measure the current in each tank and adjust the current with resistors or something to make them both the same - and also continuously monitor because an on-going changes are going to throw it off too. Too much trouble if you ask me. I suggest you get another supply for the second tank. OR
Why not just do the parts all together in the same tank or in separate runs in the same tank.
Another thought (which I haven't really analyzed so don't flame me guys) try hooking the tanks in series. You'll need a higher voltage - your 30v supply might be enough - but in series at least the currents will be the same and it's the current that is doing the anodizing.
Be aware though that you'd have to calculate the 720 rule on each tank for the same current and different part sizes.

Sage

[anodizeme4ever]

Thanks all. After going back to square one and basics, I went back to one tank and am getting 100% results - except for one thing - the 7075 parts are still streaking with black marks. I've tried 6 ASF, 9 ASF & 12 ASF with all the same results. These marks are apparent after going through the first cleaning process(15 minutes @ 140 deg. F)
Can someone guide me on this so I can get ris of these marks.
Thanks Again,
Kevin

[sage]

Maybe a picture would help. Are you saying that there a black marks showing up after cleaning or you can see where they are going to form even after they have been cleaned. I always use 6061 and so never use the de-oxidizer and de-smut but 7075 may be one of the alloys that could benefit from using it. It removes copper (probably other things as well) from the surface. I'd have to look up 7075 and see if it has more copper than 6061. Have you tried the de-smut?

Sage

[anodizeme4ever]

Sage,
Yes, I've tried both and get the same results. Now I'm thinking what someone else said that it could be due to high speed machining process and coolants entering the structure of the aluminum however I've had several parts turn out perfect?? I'm going to be talking with the customer on Monday to determine from them how they are machining these. More news at 11 !!!!!!!