Process for aluminium reflectors; anodizing polished aluminum

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A. Hi, Enzo. The majority of reflectors are not electroplated, they are bright dipped and anodized. You may wish to study those topics. I am not sure offhand of the achievable reflectance, but ASM Metals Handbook Vol. 5 suggests the values obtainable from bright dipping and electropolishing depending on the aluminum alloy.

However, it is probably a risky idea to proceed on a project of this scope, about which you "don't know anything", without retaining a metal finishing consultant / consulting engineer. I no longer do consulting but I can tell you from decades of doing so that the earliest decisions are the most crucial ones; don't wait until you've painted yourself into a corner. Good luck!

Regards,

A. Most bright dip processes and electropolishing processes are dependent upon the alloy, I'd hope that for the best coverage you would be working with pure aluminum and not an alloy, because with the alloys your reflectivity degrades as the amount of aluminum in the part is decreased. 99.9% aluminum can easily reach 95% from what I have seen. Other alloys typically range from 85-90% at the most, some even less than this, and in most cases a clear anodize overtop the bright dip finish will dull the surface a few % points, but it can be lessened by using different concentrations of sulfuric in the anodize. The only way to truly guarantee that you could get 95% would be to use pure aluminum, or consider some sort of silver plating over aluminum.

A. I am interested in helping you.

I need more information on what you want to do, that is, with details. I consider that the explanation given by the previous poster is correct. But as you can see they can't do much with it. It is necessary to go into details.

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A. Your anodizing thickness is on the thin side for corrosion resistance, but it could be alright if the potential for corrosion isn't bad. Millions upon millions of reflectors for internal use (dentist lights, indirect lighting sconces) and external use (highway and city lighting) have been built of aluminum. Although 6061 is a good alloy for anodizing, the purer alloys would give a purer (and thus more transparent and smoother) anodized coating.

A. Yes, it is possible...although the silicon content in 6061 may cause you to lose some of your mirror finish. If I were you, I'd change your alloy to one with less Si (5052 comes to mind). And, obviously, keep your coating thickness to the bare minimum. Another option would be to electropolish the Al just prior to anodizing (but again, the 5000 series of Al is more conducive to EP than the 6000 series).

A. "Clear" is an okay description, Mandar; "sulfuric acid anodizing" is fine too; or "Type 2"." Yes, virtually any anodizing shop will be able to do the anodizing.

But if you decide that a chemical "bright dip" is appropriate after your mechanical polishing--or an electrolytic polishing as Marc mentioned--then you will find that many anodizing shops lack that capability.

A. In regards to the topic of material choices. What is this part shaped like? Is it a solid block of billet 6061? Is it plate or sheet? The reason I'm asking is because I'd like to know its uses more than just what was stated originally. As Ted said earlier and Marc followed up with, there are potentially more suitable alloys for a bright finish depending on what your part is shaped like and what it is being used for.

3003 polishes to an incredibly bright finish, and it chemically polishes very well also. But its downside is that its a very very soft alloy compared to the others they suggested, not to mention its only available in sheet/plate type structures that I know of.

IF your part is cosmetic in nature, and its capable of being made from 3003 sheet or plate. I'd seriously suggest you look into it. Its used extensively in the reflector category that Ted talked about.

Not to mention I think 3003 gives a beautiful colored anodize finish also. Outside of the pure base aluminum alloys, its what I would choose above and beyond the others if my project allowed for it.

A. Hi Doug. It's hard to guess what aluminum alloys would have been used.

Most clear coats won't yellow much, especially if not in sunlight (UV rays). Easiest would be the one-component types, which are quite thin.

adv.
You might try Everbrite [a finishing.com supporting advertiser], which is a single-component clearcoat.

Two-component automotive clearcoats are more difficult to apply, but thicker and probably don't need redoing for many years.

It is possible to anodize such parts but, particularly if they are castings, you'll probably lose much of the effort that you put into polishing them. (Anodizing converts the aluminum on the surface to aluminum oxide, which is essentially transparent, but aluminum alloys have other materials in them like silicon and copper, which turn gray or black in the anodizing process).

Regards,

Thanks Ted. I appreciate your insight and quick response. The machine won't be exposed to UV so think I'll give the clear paint a go. At least that way if I screw up it'll be a lot easier to remove than plating or anodizing. Thanks again

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A. Hi Kimberly. Mirrors and reflectors are made by polishing the aluminum, then anodizing it...

You need to use a fairly pure aluminum that anodizes well, and keep the anodizing as thin as possible while still meeting your needs. The ASM Metals Handbook has an excellent chapter on the topic, with suggestions on aluminum grades, thicknesses, polishing methods, etc.

Regards,

A. Hi Phong. The best approach would be gold plating the reflector, but it might be prohibitively expensive. A thin anodizing on the polished surface should be fine, but some alloys make better reflectors than others, and electropolishing or brite dipping would be better than hand polishing (see topic 3584, "Aluminum reflectors: alloy selection and finishing" please).

Regards,