Anodizing tank time

[usao]

I anodized my first piece yesterday successfully, but I was totally confused about how to compute the time. It appears as though you take 900 divided by the current density of 4.5A/sqft and the result is the time in minutes (200). This doesn't appear to relate to the size of the part. I would appreciate some clarification on the time in the anodizing tank.
Thanks

[mcaswell]

Not sure where you got the 900 from.
You anodize at the rate of 4.5 amps per sq ft of surface area, or 30 milliamps per sq inch. You do this for 90 minutes.

[NeoMoses]

I'm not really sure where you got the 900 from either. I have heard of the '720 rule' which states that it takes 720 Amp minutes per square foot to create 1 mil of anodized layer.

How does this relate to the size of the part? 4.5 amps/sq. ft.. You see that the current is divided by the surface area of your parts, thus giving you a current density.

[usao]

I found the following references on a related web-site regarding the figuring of the time:
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Figure out how long to anodize the part using the following rule - 900 amps per minute per square foot of part or 15 amps per square foot of part per hour.

The focuser tube has 42.4 sq " of surface and is drawing 2 amps according to the meter on the power supply. 900/2 =450 amp min, and 42.4/144 = .294 sq'. So 450 times .294 = 132.3 minutes in the tank.
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I was trying to generalize this statement, and I found flaws, such the units on the value of 900 would have to be A*min/sqft to make the math work as follows: 900(A*min/sqft)/2(A)=450(min/sqft)*0.294(sqft)=132.3(min).

When I applied these equations to my part, I found that I was drawing 1.01A and when I incorporated the part size into the equation, it resulted in 120min. The part bubbled for about 110 mins then stopped. I waited for the full 120min, removed and cold-washed the part, and it looked great.

I saw the 90 min in the manual, but it appeard to me as though it was only appropriate for the part being discussed, I did not realize this was a generalized value no matter the part. Perhaps that could be made clearer in the manual. My PS has a volt-meter and an amp-meter. The amp-meter doesn't appear to work properly, so I in-lined my digital meter and got the 1.01 A rate. Over the entire session, the voltage held steady at 7.5V and between 0.99 and 1.01 amps. The reading never changed, untill I removed the part after 120 mins.

Not sure exactly what I did wrong, but the part came out great anyhow. Ill keep experimenting.

[Fibergeek]

I too don't know where you got "900" from. I'm not aware of any other website that has info relevant to the LCD process.

Going by your numbers; since 42.4 sq.in. is about 0.29 sq.ft.:

1A / .29 sq.ft = 3.45 A/sq.ft.

This is toward the lower end of the LCD current density range range where 120 min. would have been appropriate. 90 min. is for 4.5 A/sq.ft. current density.

A voltage or current change that low over the entire session is suspicious. The ammeter on your power supply may not be the only thing not working.

[mcaswell]

If you are using our LCD system, you really should ignore all the OTHER stuff out there. It's simply not relevant.

Using instructions from other websites and other techniques is NOT a good idea.

[usao]

In reference to: "The LCD Anodizing System" manual, on page 8, under section heading "Operating the Anodizing System", paragraph 6, it states that the duration of the anodize is between 1-3 hours, based on reaching PAR. Im trying to determine how to get to this PAR value. Here's my issues:
1) When I anodize for 90 mins at 4.5A/sqft, I don't get a good anodize, I find that I must run longer than this for the part to get a good uniform anodize layer. When I anodize untill the part stops bubbling, I get much better looking results.
2) For reasons that I do not understand, both the current and voltage remain constant during the entire process. The voltage does not increase as expected (due to increased resistance and Ohm's law: I=V/R). I noticed that the bubbles from the part decrease significantly after a longer time than 90 mins, but well within the limits posed by the operating instructions as referenced above.

[Fibergeek]

The bubbling (hydrogen being liberated by the anodizing process) should not stop or even slow down at any point in the anodizing process, LCD or otherwise. Hydrogen liberation starts within a few minutes of applying power and it should bubble at a fairly uniform rate for the entire time the work is under power. If it does not you have a serious electrical problem. Seeing no change in voltage or current also indicates an electrical problem.

First; inadequate electrical connections to the work is the most common problem beginners encounter. Almost everyone gets this wrong at first.

Next is using the crude and nearly useless ammeter on a battery charger (if that's what you are using) to measure low currents (like 1 amp). At low current, a battery charger ammeter is commonly 50% off.

Third is not enough voltage available from the power supply, if you're using a battery charger, the 6V setting will cause this.

If you require any more information you will have to get more specific about your setup.

[usao]

Getting better.

My setup is using 2-gallon tanks with the lead cathode running down the side and across the bottom of the tank. For the electrical end, I bought a current limiting power supply rated at 10A. I hooked the negative to the lead and the positive through my digital ampmeter to the AL tank bar, on which I have AL wire suspended from AL screws. I got 12-guage AL wire from radio shack and tapped a small hole one drill-size down from 12-guage. I then screwed the AL wire into the hole, cutting threads on the wire untill it started to twist on itself (about 1/4"). The parts are rigidly connected to the wire, and the wire is wrapped around the 1/4" AL screws on the tank bar with AL washers.
The PS could be the problem as you say, as it's old. Im going to try and get a new current limiting PS off eBay and try that.
My most recent results were good, and the dye took, but even after 15 mins in the dye tank using the 110 heater, the blue dye didn't fully penetrate, and there are what look like shadow lines on the part where it's lite and not fully dyed.
Thanks a bunch, ill keep working on it.

[M_D]

Re: Getting better.

How are you cleaning the parts? That's another thing which can make a significant difference in the anodizing, both in time required and how even and deep they dye. Even if the water sheets, there may be a native oxide layer. It can also cause an uneven coloring or appearance. The oxide seems to retard the anodizing and takes more time to get equal to parts that are free of native oxide.

[Fibergeek]

Heed M_D's warning about native oxide; you usually can't see it, but it certainly can foul you up.

Regarding your setup:

By "current limiting power supply"; I hope you mean a constant current power supply, it has adjustment knobs to vary the voltage AND the current. Most all fixed power supplies have some form of current limiting, but this only works at the max power limit for the supply, which won't do.

Be suspicious of you connection method to the work; some here have a method like yours working, but they usually thread the wire and the hole. An incomplete thread on either helps the wire cold flow, which forms the required liquid tight joint. You may want to bolt the wire to the work (aluminum hardware) until at least you get things sorted out. You want to crush the wire under the bolt.

Cathode shadow is caused by improper cathode placement for the tank geometry. You might try not running the cathode across the bottom but on all four sides instead. You also must also have at least 3 inches from the work to any cathode surface or shadow effects can appear.

[elton10]

speeding up the process

Mike suggested I post this to the forum:
How can I speed up the process so that we're not spending 90 mins to anodize each batch?

Can I go to 6 A ft 2^.. will that reduce the time proportionately (i.e. from 90 mins to 60).

thanks in advance

Elton

[Fibergeek]

You certainly can.

Recall that the published LCD range is 3 to 6 A/sq.ft. 4.5 A/sq.ft. is in the baseline LCD instructions because its the exact middle of the LCD range. I know this because I selected it.

Anodic coating growth is proportional to both anodizing time and current density. Its almost linear if electrolyte temperature is held constant and the dissolution rate isn't excessive.

The upper limit (6 A/sq.ft.) was selected for four reasons:

1. In most Caswell kit scale anodizing the power dissipated in the electrolyte is still low enough not to require temperature control. (Equipment costs.)

2. The peak voltage is still low enough to permit a typical battery charger. (More equipment costs.)

3. Lower current density is more forgiving about agitation requirements. You still have to have it, but its not as important that it be real good. (Yet more equipment costs.)

4. Pore size is about optimum for easy and excellent dyeing with the (safer than traditional concentration) 5% (vol.) LCD electrolyte.

Some of you here have acquired the skills, knowledge, and experience to rightly be called "small scale" anodizers, you aren't "amateurs" or "hobbyists" anymore in terms of the results you are consistently getting. The patience you show and help you make available to the less experienced guys is appreciated.