Re-anodising heatsinks?
I found out the hard way today that putting anodised aluminium heatsinks in the dishwasher to clean them is a very bad idea. They're now a sort of streaky grey with black areas where the devices were mounted. "Mortified" is the word.
Is it possible to get heatsinks re-anodised? |
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The alkaline solution of dishwasher tablets attacks aluminium, as you've found out!
Depending on how bad the surface is, you may be lucky. Take a few photos and email South West Metal Finishers. No connection apart from a satisfied customer in the day job! |
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Black anodising is essentially cosmetic for a heatsink that is not running stinking hot.
The majority of heat transport from a heatsink is via convection, not radiation, for typical heat rise of say 40C. In that case the difference between a black anodised heatsink and a shiny aluminium one is negligible that for typical heat rise. Rather than pay for a small quantity to be stripped to bare metal and re-anodised, I'd be tempted to spray paint them black. |
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Per Craig: If you use VHT [AKA Very High Temperature] spray paint there is no chance that this will lift or bubble .... this stuff is used by car restorers etc. on exhaust manifolds. (It goes under then name of VHT / 'Firecote' etc. Retailed by CarPlan - made by Tetrosyl Ltd. Bury, Lancs. BL9 6RE. Tel. 0161 764 5981.)
They'll be fine .............. |
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That's interesting -- thank you. Does this type of paint work well on aluminium? I seem to recall reading horror stories about needing etch primer or exotic surface preparation to make paint adhere properly.
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For an outlay of 15 shillings, anodising can be done in the average home...according to PW in 1962 (see attached)!
I wonder how big your heatsink(s) are? PM me if you consider any "homebrew" option. As for dish washers, I have put the odd item of agricultural engineering in them, but never any item of radio/electronics. B |
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Worth trying for some fun!
I believe after dyeing, commercial anodising features a further electrolytic operation which hardens the oxide coating and makes it more dense. This is missing from the PW article. But I may be wrong on this. Hopefully someone will correct me! Look forward to what South West Meatl Finishers come up with. If time is not of the essence, they may offer to wait till a big batch of something arises, and put it in at the same time for a nominal sum. |
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Not sure the homebrew option would go down very well with Madam or the elf 'n' safety people nowadays.... |
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Being amphoteric, it's difficult to predict how the aluminium oxide will have reacted, but there's a fair chance that it's just the dye which has been leached out. I'd be inclined to dump the heatsink in a hot dye bath for a few hours before contemplating anything more drastic.
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John |
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My understanding is that the conditions chosen for anodising intentionally create a porous oxide layer and the dye goes in to the pores. That is then is sealed in by the action of the subsequent immersion in boiling water which "seals" the pores, though I guess the same could be done electrolytically.
B |
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The reason that anodised aluminium cannot be used in ultra high vacuum systems is the pores. These give rise to what is known as a "virtual leak". In other words it has the characteristics of a leak, preventing the vacuum level to be attained, but in fact it is something in the chamber outgassing. And anodised aluminium outgasses pretty much forever.
The aluminium treatment for UHV use is electropolishing. For decorative use (ie not vacuum use!) electropolishing and then anodising (and dying) gives a superb mirror like finish. |
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I've used these guys on a number of occasions, and found them to be pretty good. Perhaps worth a try. Based in Uxbridge.
http://www.metroplating.co.uk/ |
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That's an excellent link Craig - if you click on the various services they offer, they give some very good concise descriptions of each of the anodising processes.
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Hello
This sounds like something that motorbike customizers or restorers would need doing. Perhaps if you could get a read of the adds in a 'bike magazine there might be some firms that would do a small job. Just a ramdom thought. Will |
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A guy I know in Sweden has a plater that takes a black anodised heatsink, strips the anodising and then gold plates it. Must ask him who does this, hopefully in a cost effective way.
This is what his gear looks like |
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We use metro plating, they are a friendly bunch, worth having a chat at tea time (10 am) outside (fag break) for a "private job". One of the cleanest plating plants I have ever seen.
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Andy |
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Didn't you see my earlier post regarding dissipation being almost all convection and hardly any through radiation for a temperature rise of less than 40C? The colour of the heatsink is pretty much irrelevant for a temperature rise of less than 100C.
There is another effect that reinforces this idea called the "view factor". Think about how much each part of a finned heatsink area sees of the environment, and is capable of thermally radiating. Before you get very deep into the profile, each area of the heatsink sees progressively less of the environment, and the more it sees of the adjacent fin. Since the adjacent fin is at the same temperature there can be no radiative energy transfer - it can only radiate if it sees a cooler environment which it can only peek through the gaps between the fins. All of which says: the colour of the heatsink does not matter if you can touch the heatsink without burning yourself. Cooling is close to 100% convection. The colour of a heatsink is traditionally black for decorative reasons only, and alas reinforces the pseudoscience argument that thermal radiation assists dissipation to a significant degree. Here is a link to a heatsink manufacturer that says the same thing http://www.abl-heatsinks.co.uk/heats...ace-colour.htm Quote "The heat transfer from the heatsink occurs by convection of the surrounding air, conduction through the air, and radiation. Heat transfer by radiation is a function of both the heat sink temperature, and the temperature of the surroundings that the heat sink is optically coupled with. When both of these temperatures are on the order of 0 °C to 100 °C, the contribution of radiation compared to convection is generally small, and this factor is often neglected. In this case, finned heat sinks operating in either natural-convection or forced-flow will not be effected significantly by surface emissivity." Craig |
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OK Craig, I retreat defeated!
Andy |
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And from the Aavid heatsink manufacturer https://www.aavid.com/product-group/...ons-na/anodize
"Relatively large extrusions and those used at low temperature rise, as in many high power applications, will only gain up to 10% by the addition of an anodized surface. With forced ventilation (using a fan) convective cooling is about 3 times higher than in natural convection. This changes the proportion of heat transfer due to radiation. An anodized finish will only add 4 -8% to the overall cooling effect in forced air. This percentage again, depends on fin spacing and heat sink dimensions. The color of the anodized finish makes little impact on emissivity since most radiational heat loss occurs at wavelengths higher than visible light. As a thumb rule, if anodize is not required for aesthetic or corrosion protection, we suggest it only for small, open finned heat sinks in natural convection." Craig |
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There is an application that needs 100% radiative cooling though - in space applications (where no-one can hear you scream!). Since it is in vacuum, the only method of getting heat out is via a radiator panel pointed at deep space (which is only about 2 Kelvin, so T1^4 - T2^4 is high). Each circuit board has a thermal layer connected to its housing. Each of those (there might be dozens or up to a hundred on a space vehicle) is coupled to the radiator panel with ammonia heatpipes. Craig |
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From what you say, it seems possible that painting something black could actually be detrimental from a thermal standpoint? Probably not a big effect, just a waste of time? B |
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Well, back when I was involved in this from a production standpoint, anodising - particularly black - was a nightmare. Depending on the condition of the bath, and the repeatability of the dying and sealing processes you could get anything from a nice deep black right the way to almost a blueish tinted black.
What precise colour you get is also strongly dependent on the aluminium alloy - so that is another process to control if you aim to get colour uniformity. In the end we had a sample and the plater had a sample. They (it wasn't Metro Plating, by the way) were told that if the batch was visually different to the sample to strip it and replate. And we did rigorous QC on goods inwards. Powder coating can be much more predictable from a colour point of view, and with a much wider colour palette, but it is difficult to get uniform coating on sharp corners and into deep profiles (like a heatsink). There is another sneaky problem with extruded heatsinks - they are always slighly banana shaped. It is inherent in the extrusion process. Aavid cheerfully own up to that, and say if flatness of the device mounting surface is important (and why wouldn't it be?) that it should be post-machined flat. And they will do it for you - at an additional cost. Craig |
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It's not just space-in-a-vacuum that denies cooling by convection: anywhere in zero-gravity experiences the same effect. There are lots of fans used for cooling in things like the ISS.
[Zero-gravity has other odd effects: a friend of mine is NASA's fire-suppression-in-space supremo. Without convection, fire behaves very differently in zero-G !] |
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That is a good point. My experience is with unmanned space vehicles - I hadn't thought of cooling implications in manned zero g environments, and particularly the hazards of fire and how it behaves.
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I used to work for a well-known power supply manufacturer, their approach with large heatsinks for highly-dissipating linear power supplies was to black powder-coat the heatsinks! As far as I know, there was no study done to determine whether plain, black anodised, or black powder-coated were best. I suspect not very much difference, as others have commented! |
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Old car engine oil is used in metalwork to give a black finish, the only problem is this is done by heating
said metal to cherry red. Easier to achieve with steel and iron than ali. Still might be worth experimenting with a blow torch. How big is said HS? To be honest I'd just spray it black with bog standard spray paint, doubt it'll ever get hot enough to damage the paint. Think of a car roof in summer, gets hopping on one leg waving your hand about hot. If that don't appeal here's - https://www.ebay.co.uk/sch/i.html?_f...paint&_sacat=0 various high temp paints as found on ebay. I've used both the barrel paint and SAS 59, works ok and have also used stove paint. Both the exhaust and stove paint comes off eventually, but then both hospital hot. I've had my burner glowing orange, not blacking can survive that. Andy. |
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There's also a guy in Sweden who takes aluminium electrolytic capacitors, de-cans them and re-homes the innards in little hardwood housings. His web pages are quite lyrical about the improvements to be obtained and the effects of different woods. No mention of life-expectancy. I'm afraid I didn't bother to save the URL when I was pointed to it.
Couldn't be the same bloke, could it? I suppose he'd have gone for Iroko heatsinks. At least the gold plate will have virtually no effect in either direction. David |
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Would the recased electrolytics be any worse than old cardboard cased ones? :-)
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No - the guy I know's day job is at the Karolinska Institute carrying out research aiming at helping stroke patients recover more quickly and better by Transcranial Direct Current Stimulation. His degree is in engineering Physics with majors in mathematics and computer science. So a hard technologist - not a smoke and mirrors guy by any stretch Craig |
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8-\ That guy in sweden should rehouse some 3-1000A's in wooden cases!!!
The sound should be superb, especially in class A Joe |
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As seems to be usual on this forum, I learned a great deal about heatsinks and heat transfer from this thread and many thanks to all for the very valuable suggestions. Every day's a schoolday :) |
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I'd be interested in who the company in Manchester is
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Yes - please share! (With a photo of the refurbished heatsinks!)
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The company is Colour Anodising of Radcliffe, Manchester. I'll put up before-and-after pics of the results as soon as they return
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I did a web search before you posted, found them, and discounted them - because the website looked so swish I was sure they had to be expensive!
But very interesting looking company. Regarding gold plating, I also had a look around the UK and found http://www.goldplating1.co.uk/ in Bolton. Prices look really good - maybe I'll try a heatsink and see what they would quote for stripping and gold plating. |
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