Current/Voltage for Aluminum Anodizing

A. The rectifier and chiller are not sized to the tank, you size the rectifier to the surface area you put in the tank. Come back, tell us the surface area and thickness you desire, the kind of anodizing (II or III), and we will advise your requirements.

A. Reynaldo,

I'm assuming you are doing decorative anodizing, as it doesn't sound like your current power supply is enough for hardcoat. You should be anodizing at 12-15 amps per sq ft of surface area for decorative anodizing, so size your rectifier accordingly (20 volts should be more than sufficient). If you want to increase your load size, you'll also want to make sure your cooling system can handle your needs, as well. If, indeed you want to hardcoat your parts.. you'll need to size your rectifier for 25-40 amps/sq ft (you'll probably need 50 volts, or more). Check Metal Finishing magazine, or PF magazine, there are a lot of used equipment ads in there.

A. The required voltage is largely dependent on the distance from the anode to the cathodes. If the spacing you presently use will be adequate for the larger parts, then 12 volts will remain adequate, although it sound low to me and I'd feel more comfortable with an 18-volt rectifier.

The amperage required is proportional to the surface area of the load. If it will be 50 times larger, the amperage should be 50 times higher. As a guideline if you don't have enough data to do the proportioning, figure that you will probably not anodize at more than 20 amps per square foot.

Suppliers of used rectifiers can be found in finishing.com's Equipment Directory.

A. Dear Reynaldo

Sizing a rectifier is easy, if you are going into serious production and want to have up to date equipment. For type II you will need 20 to 30 amps per sq ft. at 18-24 volts. For type III it will be 30-40 amps sq ft at 75 volts. Automatic ramp and timer are also essential. Cooling and agitation are extremely important to maintain quality production work. I also highly recommend using aluminum cathodes to save energy and eliminate a lead source

? Drew...20-30 ASF for decorative anodizing? hmmmm..sounds a little high to me.

A. Whew! That's a lot of questions. I'll try a few of them

-You can pretty much crowd as much work into your tank as you want. Shielding isn't much of a problem in anodizing, but stay a few inches away from the cathodes.

The maximum voltage you will need for type II will probably never be more than about 19 volts.=, maybe less.

400 amps is plenty for your size tank.

You can ramp up manually over a period of about 5 minutes. Most T-II won't require any ramp up at all.

Ripple isn't a major concern in anodizing. 5% or 10% should work okay. yes you need voltage/amperage control. I suggest you buy from a reputable manufacturer of plating rectifiers. they will help you select what is correct.

The biggest problem you're likely to have is cooling. Budget at least as much for a chiller as you do for the rectifier.

A. Hi Daniel,

I've been reading the questions that you've been posting for a while now and it sounds like your shop is almost ready to go. It seems like you want to make sure that your setting things up correctly and that you are willing to do a little research to make sure that you don't make any errors that are easier to avoid in the beginning - that is a very good thing.

That being the case, and I'm sure that someone (probably Ted) has said in the past in response to one of your letters, trying to learn EVERYTHING from posting questions on the Internet CAN work - but it is a lot more efficient to crack a book for a lot of the information. Get a copy of "The Surface Treatment and Finishing of Aluminum and Its Alloys" by Wernick, Pinner and Sheasby and you will agree that it was the best investment that you could make. It will more than pay for itself (and the time it takes to read it all the way through - it's about 1000 pages) just in preventing mistakes.

It has, for example, detailed sections describing the effects that voltage, amperage, ramping times and the manner in which the power is turned off after anodizing is done have on the finish. If you already have a copy, check out pages (fifth edition page numbers) 380, 387, 398, 421, and 969-971. I checked my notes and those pages should have some really relevant information to some of your questions.

Good luck!

Thank you so much for that Jim. I have already indeed looked into purchasing this pair of text books. It is without a doubt an investment as much as it is an aid I'm sure. Unfortunately, as dollars are spread thinner and thinner every day while purchasing equipment and supplies, these books are forever pushed to the back burner. To date this entire operation has been built on our own dollar, no bank loans or grants have been issued here. This of course calls for strict budgeting as I am sure you can imagine.

You were correct in assuming that my shop was nearing completion. It may be for this reason that I procrastinate on purchasing these texts. I realize this may in fact be my downfall as I to believe that no single resource should ever be one's sole center for knowledge. For this reason I have been dabbling within our local libraries and speaking with others in the field. I must admit, however, I do find www.finishing.com to be a tremendous help and even perhaps a crutch. Should I find myself misunderstanding something I read or confusing something I hear, I can always come here and have myself straightened out. For that I am very appreciative - you have no idea the progress that I have made here in the past few months. Though I realize I have many more struggles ahead I am willing and able to overcome them - particularly if Ted and people like you are behind me! Cheers all!

A. Hi Daniel,

I can definitely understand not wanting to drop the full price on the book - the $500 typically asked for it is pretty steep. However, every once in a while you can find it on sale and it becomes a lot more affordable (not to long ago I picked up a copy of the 5th edition through the ASM website for $200).

That being said, if my notes are correct you may want to look at questions 51573 and 12130 - the first deals with sizing a chiller and the second deals with sizing a rectifier.

Also, if you haven't yet, you may want to check out the "100 Q and A" at the Anodizer's Plaza reached through the "WWW links of interest" link on the home page. It has some very informative and interesting information that helped me a lot before I bought my copy of "The Surface Treatment and Finishing of Aluminum and its Alloys".

Good luck,

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Ed. note Nov. 2013: Unfortunately, "Anodizer's Plaza" is long gone.

A. Hi Josh. It will probably be closer to 10 pieces than 20 if you figure two 54" high rows, with gaps between the 6" high pieces.

You should probably figure 18 volts and 18 amps per square foot for the rectifier sizing.

Regards,

A. Hi Josh.

I would rack them like this --

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Anodizing is an insulating coating which therefore tends to be rather uniform.

But people with no anodizing experience trying to design an anodizing installation would be a serious mistake. You should retain an anodizing consultant, and the sooner the better because the most important decisions are made early on. I did plating and anodizing line installation consulting for over 30 years and in many cases I felt more like a damage-control specialist than a designer because people had traveled a path of no return for too long before seeking help :-)

Good luck,

A. Hi Josh. Advertiser support is what makes this site possible, and we have at least two world-class anodizing consultants listed in our Consultants Directory.

Regards,

A. Hi, Aijazullah. Ramp control for anodizing can be as complex as you want to make it. I've been to shops which use very complicated digitally controlled voltage/amperage profiles in their belief that it makes for hard anodizing that is better in some fashion, and that may be.

But the basics are simple: Ohm's Law tells us that I=E/R, that is, Current equals Voltage divided by Resistance. The hard anodizing film is a very good electrical insulator; that is, it creates a very high Resistance. If Resistance is very high, we need a very high Voltage to drive Current to continue to build the anodize film. However, when you start anodizing, Resistance is very low, so if you apply a high Voltage, then a very high Current will result and the parts will fry/burn. So you must slowly ramp up the voltage, in proportion to the thickness of the anodizing to maintain a reasonable current throughout the cycle. Or, more simply, just control the process with constant current.

Regards,

A. Hi Aijazullah. As I said previously, the answer can be as complex as you want to make it. If you want to make it complex, the answers depend on the alloy being processed, what temperature you can truly hold at the work surface, which additives (if any) you are using, and several other factors which take many pages of graphs and tables to present. Jim Gorsich described on what pages in the industry's most important reference book to find all those charts and tables and research answers to your particular situation. On top of that, some specialists have implemented additional proprietary current-time profiles they use which they feel are superior, and which they can't be expected to share.

But on the other hand, I think you may not be taking the time to carefully read and truly understand the very simple and important concept that has been explained, i.e., that in lieu of complex current-time profiles you can simply anodize at constant current. Hold your 20 Amps per square foot from start to finish and see how your hard anodizing works; then try again at 30 Amps per square foot. Good luck.

Regards,

We'd like to give credit for this graphic explaining Ohm's Law (A = V / Ω) but see it on many websites without info of whose work it actually was :-(

A. Hi Jason. Electricity can be hard to understand and visualize because it is invisible and rather abstract.

Amperes is the flowrate of the electricity, actually the number of electrons traveling past a point in a given time period, analogous to the flowrate of water; Niagara Falls would be analogous to very high amperage, the amount of Pepsi flowing through a plastic straw would be analogous to very low amperage.
Voltage is analogous to water pressure; it takes strong pipes and high pressure pumps to provide water to sinks & toilets near the top of skyscrapers, even if only a slow drip of water is wanted, whereas a duct tape patch can hold on an above ground pool even if it holds thousands of gallons.

Although you may understand some aspects of electricity very well, I think you have your understanding of Volts and Amps interchanged based on your question. Voltage does not come into play in the 720-Rule, which is simply Faraday's Law of Electrolysis with a couple of constants related to aluminum factored in. I'd suggest that you review our introduction to Faraday's Law as a starting point for a deeper understanding of it. Your operational parameters sound fairly right. 5A for 44 sq in. equals 16.36 ASF, and that's the number for your 720-Rule calculation. The 18V seems somewhat higher than you ought to need for 5A but maybe not outrageous especially if your meters aren't precise ... but you can't independently control it (voltage and amperage are tied together by resistance).

Luck & Regards,

• Topic 0967 "Increasing Type III (Hardcoat) Anodizing Capacity"