Anodizing tips please ...

[13DoW]

Hi,
I have several heatsink extrusions which I want to use for a power amp project. They are unfinished so I want them cleaned up and anodized. I have found a local metal finishing shop that can do the work - bead blast to clean and give them a uniform finish followed by anodizing.

I have two questions:
1. The shop can do two types of anodize - type II or type III. They say type III is harder but type II has a more uniform color.
Which type of anodize is used for commerical power amp heatsinks?

2. The anodized layer will add to the thermal resistance of the transistor mounting - does anyone know how much?
I can specify anodize-free areas but this will almost double the cost of the anodizing.

Thanks in advance
13th Duke of Wymbourne

[djQUAN]

you could use type III and mount the transistors directly on the heatsink with only thermal grease.

[tsmith1315]

2. The anodized layer will add to the thermal resistance of the transistor mounting - does anyone know how much?

They way I understand it, anodizing uniformly oxidizes the surface of the aluminum so it will no longer react with oxygen in the air. Given that, you should be able to sand the areas where transistors will be mounted. Be sure the sanded area is covered with thermal compound to keep moisture out.
And of course, you may want to verify my recollections...

Tim

[chipco3434]

I can specify anodize-free areas but this will almost double the cost of the anodizing.

Ask the shop if you can "mask" using a compound they specify. You might ask them if you can use a urethane mask on the areas you don't want annodized.

Secondly, you could abrade back through the anodization layer with silicone carbide sandpaper (wet or dry) or spot face the area with a mill.

All in all, the anodization layer reduces thermal conductivity by 10-15%. (That's what I've read on the internet so it must be true!) That's not much compared to using too much thermal compound under the package.

[kilowattski]

Have your heat sinks hardcoated. Go to thread Anodising Alep5 : what color ??? and look at post #16. Hard coating with no thermal goo and washer has better thermal conductivity than a bare heat sink with thermal goo and washer.

[highbias]

Ya have to becareful with mounting devices right onto a hard coated sink though. Even a tiny pinhole in the hard coating can cause a short. There is a definate limit on how thick the hard coating can be built up and its best to check with your anodizer to see how far thay can take it. Normally one ten thousandth metal penetration and one ten thousandth on the surface is about the capability of most places. This might be fine for a small Aleph with relatively low voltage rails but with anything much larger than an Aleph 3 I would still use thermal pads between the devices and the sinks. Speakers are too valualable to risk in general terms!

I've have had parts bead blasted and you will end up with a matte finish that is very good at holding dust and very difficult to clean. I don't personally recomend it. Polishing the parts has always worked better and they are easier to clean. Panels can can be grained in many ways but taking them to some place that has a Timesavers machine will give you more consistant graining results. A Timesavers machine will even remove minor gouges in the surface.

Important hint!: With these machines be sure to give them a small piece of sacrificial metal of the smae width and thickness for each piece you take in. If there isn't a small sacraficial piece fed in immediately ahead of the panel part then the first edge gets a sort of rolled appearance.

I also forgot to mention that Hard Coating is much more fade resistant than regular anodizing is. Blacks won;t fade out to purple. HArd coating IS more expensive though... you have to decide.
Mark