hard anodizing

A while back I had the need to get some sheetmetal (every spell check in the world keeps telling me it’s “sheet metal” and not “sheetmetal”, but I digress) and machined parts hard anodized.  I’ve been meaning to post my notes on the process for some time… and now I am.

what is hard anodizing?

A hard anodized finish or “hard coat”  is shorthand for MIL-A-8625F Type III, and is an aluminum oxide film on an aluminum part.

The film is typically .002″ but can range from .0005″ to .0045″. Keep in mind that the film penetrates the material as much as it builds up on the surface, so a typical .002″ film will have only .001″ surface buildup (It doesn’t sound like much, but toss in a few anodized parts, and thickness can become relevant to your tolerance analysis real quick).

benefits of hard anodizing

There are few reasons to consider getting your aluminum parts hard anodized:

hardness

Like you might imagine, a hard anodized coating is hard. Specifically, it’s hard to the tune of ~70 Hardness Rockwell C ( that’s in the same ballpark as tungsten carbide).  This will make your part significantly more scratch and wear resistant.

electrically insulative + thermally conductive

Electrical current is a surface phenomenon, and the aluminum oxide coating (by virtue of it’s high dielectric constant) does a great job of insulating against current.

What’s more, the thermal conductivity of aluminum oxide is actually pretty high (and it’s thickness is only a few thousandths anyway), letting you get the best of both worlds when seeking an electrically insulative heat sink.

Due to their extreme hardness, hard coated parts are exceptional in wear applications.  The aluminum oxide layer also provides great protection against corrosion and acts as an electrical insulator.

finish

Another virtue of the minimal thickness of the film is that the finish on your pre-anodized part will be the finish on your anodized part.  Brush or grain marks on sheetmetal will show through and a bead blasted part will have a nice substrate to it.

design considerations

Before you send your parts off to the bath house, consider the following:

tapped holes

If your part has tapped holes, beware.  That ~.001″ surface buildup can mess around with the fit of the screw.  For this you have two recourses:

1. To ensure there is no buildup, plug the hole before the part goes into the anodizing bath.  The downside here is that you lose the corrosive protection of the hard coat, which you may or may not care about. It’s also worth noting that it takes less labor ($$$) to plug a through hole than it does a blind hole, so keep that in mind if you’re designing for large volumes.
2. Most anodizing houses have oversized taps that are specifically designed to be used in conjunction with anodizing.  They’ll tap the hole large, fill it in with some anodic surface buildup and call it a day. (I’m  a little suspect of this method, and would only use it if anodizing the threads was absolutely necessary)

color

Like MIL-A-8625F Type II (sulfuric anodizing), hard coating gives you the option to dye the part. (“Class 1″ for not dyed, “Class 2″ for dyed).

The good news stops there. Unlike what you see when you google “anodized aluminum“, the colors you will get with hard anodizing won’t be as vibrant.  What’s more, some places will only dye black, brown, or not dye at all (“Class 1″ remember?).  Some others will toss a green and a red in there, but approach with caution.

It’s also important to keep in mind that, given the same dye, 6000 series aluminum will come out a different color than 7000 series aluminum.

It’s a cheap enough process (the lot charge is $150 for a ~3′ x 5′ tank), so it’s probably worth getting some samples to see if it meets your needs.

sheetmetal inserts

Almost all inserts are steel, which means they can’t be put in before the parts get anodized (steel is a big no-no when it comes to anodizing).

But alot of those same inserts aren’t rated to be pushed into something as hard as 70 HRC (I’m shooting from the hip here but I want to say that most are rated ~60 HRC).

You can ignore the spec, and most of the parts will likely turn out fine… depends on your appetite for risk.

I try to avoid it when I can.

burn marks

Like everything, it’s an imperfect process.  All that varying voltage and temperature and potential surface imperfections, failures are bound to process.

Just understand that some of your parts may get “burned” and leave a not-so-insignificant mark on the surface.  A mark certainly enough to render the part a “fail” on an aesthetic basis.  Most  shops that do both the fabrication and the anodizing of metal will make a few extra parts (especially if they are cheap, like sheetmetal) so that they are covered if one or two of them “burn”.

racking

Your parts are going to be suspended in a bath by a fixture.

That means the fixture will be touching the part.

Where the fixture is touching the part anodic solution will not touch the part.

Where the fixture is touching the part, the surface will not get anodized.

There is no way around it that I know of, just be sure to tell your vendor where you want the parts to be fixtured (hooks looped into thru-holes seem to work well and leave a very small mark).

Those are my notes.  Any of you guys have anything to add?

P.S. If you’re looking to get some parts plated around Lawrence, MA check out CIL Anodizing.

P.P.S. Apple has filed a patent for a cool process called dual anodization, if you liked this post you’ll want to hit that jump.

 

Related posts:

1. black chromium: black hard anodizing for stainless steel
2. apple dual anodization
3. design guide: memory foam manufacturing

Tagged as: aluminum, anodizing, hard anodizing, hard coat, plating