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AJB2K3

Paint &electronics.

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Everyone at some point has to decorate the project cases but what paint to use.
A project im working on (in car PSU for the PS) has to look factory fitted to what i want to do is, have an exposed finned heatsink painted white or blue with a ford badge on it but will paint trap the heat in?
If so is there a conductive paint that wont insulate the sink? :-\

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...also, if the heat sink is actually doing some work, it will get hot enough to cause the paint to peel off. Not a good idea. Ante has a good suggestion with anodizing. Otherwise, if you do not need to actually change the color, you might be able to get that brushed aluminum effect with some type of abrasive tool.

MP

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Another vote for anodizing. It can be done at home, it makes the surface harder and you can use lots of colors.

If you worry about conductivity be shure to drill and tap the holes for connectors and components after anodizing (the oxide layer created during the anodizing proccess isn't conductive).

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Cool i can do it at home.
I asked because i have a pile of computer ally and a large uncoated heat sink. if i every get the parts together ill post pics but -
@GreekPIC when you done post pics of your setup and write an artical, fellow enthueast(sorry for bad spelling) will want to know how.

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Guest Alun

If a non-anodized aluminium heatsink is anodized black, does this improve its effiency? What kind of improvement can be expected?


Black is a very good colour for radiating heat, that's why you'll find lots of heatsinks are anodized black. So anodizing a heatsink black will increase it's efficiency.

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You have that backwards....black actually absorbs heat. Put that black heat sink in the direct sun and watch the efficiency rapidly drop as opposed to to raw aluminum one.

Anodizing on heat sinks was purely introduced for the color aspect. The buyer thinks it is cool to have color. Thus, more sales.

The benefit is protection from corrosion. Anodizing protects from the corrosion you see on bare aluminum when it is aged or in the elements of the weather.

MP

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Guest Alun

You have that backwards....

No.


black actually absorbs heat.


Yes it does absorb heat more efficiently but it also radiates heat better too.


Put that black heat sink in the direct sun and watch the efficiency rapidly drop as opposed to to raw aluminum one.


True, but that's only because it's absorbing energy from the sun, a black heat sink in the dark will radiate more heat than a reflective aluminium one.


Anodizing on heat sinks was purely introduced for the color aspect. The buyer thinks it is cool to have color. Thus, more sales.

Incorrect, they are black to increase the radiating efficiency. Car radiators are black and so are the pipes on the back of freezers and refrigerators and all for the same reason.

Dark surfaces are nearer to an ideal radiator or black body than reflective ones.
http://en.wikipedia.org/wiki/Black_body
http://www.timedomaincvd.com/CVD_Fundamentals/xprt/rad_xfr.html


The benefit is protection from corrosion. Anodizing protects from the corrosion you see on bare aluminum when it is aged or in the elements of the weather.


True.

MP,
The black anodised vs untreated heatsink debate is not a simple one, when a heatsink is in still air a black one is best because lots of the heat transfer is by radiation. In situations where forced air cooling is used an untreated one is probably better because the anodised surface has a slightly higher thermal resistance. As you correctly pointed out black objects absorb surrounding radiation more effectively too so it depends on how much heat is being radiated from neighbouring components too. Other factors also include the shape of the heatsink and the components mounted to it, not to mention whether the surounding surfaces are just reflecting the heat back.

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I will stand on my original comments....

BTW: The radiator theory is bogus. The black paint had nothing to do with heat. An unpainted radiator was ugly due to the way it was manufactured years ago. Black was the standard under the hood. The newer ones are manufactured differently and do not need the paint to hide the brazing. Now most car manufacturers have switched from these to the aluminum non painted radiator cores. (Which in itself would blow your theory out of the water).

MP

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Guest Alun

Now most car manufacturers have switched from these to the aluminum non painted radiator cores.

Yes the paint on the radiator probably increased the turmal resistance too. To be pickey the turm radiator is wrong anyway because they do not emit most of the energy by radiation they do by either convection (in the house) or force air cooling (in the car). You could argue the same about heat sinks but too all my experiance this isn't the case.


(Which in itself would blow your theory out of the water).


Not my theory, see the attached image of an extract of a heatsink datasheet. A black anodised heatsink of the same dimensions has a significantly lower thermal resistance than an untreated one.

post-0-1427914212871_thumb.gif

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Hi,

Matt/flat black, was preferred over gloss black in regards to painting a radiator, because Matt black has more surface area.

When it comes to the colour of the paint you choose, I doubt it would make any difference in regards to the heat being disbursed from within an object, what can make a difference is the type of paint you use and its formula.

Paint your radiator any colour in a matt/flat and it will disbursed heat just the same, although if you add sunlight, black will be less efficient.

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Alun,
That is not a datasheet. What you have posted is a "Sales Brochure". Where does it say the efficiency is derived from the color? There is simply not enough information in this document to make such a determination.

MP

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Guest Alun

But look at the graph:
heatsink.gif
The X-axis is the lengh.
The Y axis is turmal resistance in temperature rise per watt or

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Guest Alun

One very interesting way of proving the blackbody theory is by filling two plastic bottles with hot water, wrap one in aluminium foil and leave the other unwrapped.

Now as long as you don't leave them in the sun or in front of a fire the unwrapped bottle will cool down more quickly than the wrapped one. Aluminium foil is a very good thermal conductor and as it's in thermal contact with the plastic bottle so you would think it would act as a heat sink, but this isn't the case. Plastic may seem transparent but it's opaque in the mid-infrared region where objects at these temperatures emit the most heat. The bottle with the reflective surface will cool down more slowly and thus radiate less energy.

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....and the moral of this story is.....
Do not use plastic as a heat sink?..

;D ;D ;D

but seriously, you have a real problem accepting this, so I will quit. In my last post I was just trying to point out that the difference is not color, it is the difference in the metals.
Here is a link that also explains: http://www.anandtech.com/showdoc.aspx?i=1115&p=2
Some good reading regarding heat sinks in this link.
It is a common myth about the color.

We stopped anodizing the aluminum black in our manufacturing house about 10 years ago. Since then we have used clear anodizing. Works really nice and does not look like crap years down the road.

Congratulations on your home-made aluminum foil thermos.

MP

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Guest Alun

....and the moral of this story is.....
Do not use plastic as a heat sink?..

;D ;D ;D

but seriously, you have a real problem accepting this, so I will quit. In my last post I was just trying to point out that the difference is not color, it is the difference in the metals.

Here is a link that also explains: http://www.anandtech.com/showdoc.aspx?i=1115&p=2
Some good reading regarding heat sinks in this link.
It is a common myth about the color.


Oh I do accept this, the colour isn't the main thing that determines the efficiency of a heatsink I wasn't meaning to imply that it is. In some circumstances the colour doesn't make much difference where forced air cooling is used. I've done a bit more research now and it seems the reason the colour makes a bigger difference with the heatsink I was using is because of it's construction. It's a large piece of flat aluminium with fins on the back, the large flat surface is the part that dissipated the energy more by radiation than convection.

According to the article the difference in metals doesn't matter to much either, aluminium is use despite being a poorer thermal conductor than coper which is just used as a conductive pad on some heatsinks. I believe the shape is more important and whether convection or forced air cooling is used, I'd imagine that the use of heat conductive paste and insulating washers and rubber makes a big difference too. Perhaps the radiator argument applies here too, radiators don't emit most of their energy by radiation they do by convection or forced air cooling, this also depends on the construction too.


We stopped anodizing the aluminum black in our manufacturing house about 10 years ago. Since then we have used clear anodizing. Works really nice and does not look like crap years down the road.


Are you sure it wasn't just to save money?


Congratulations on your home-made aluminum foil thermos.


I did this experiment in way back GCSE sience class. :D

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Are you sure it wasn't just to save money?



No, like I said, we still anodize. Just not black. The White powder that ante has mentioned is exactly what I was talking about in an earlier post. The metal must have a finish to withstand the elements of the atmosphere. Otherwise, you are just wasting your money. Not to mention, you would not keep long term customers by selling something that is bound to fail later on.

MP

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Hey Guy's,

Can I add my 2 cents?

Andodizing and painting are just one aspect.  Here is a thought:

If color is not a major concern, why not get some gun bluing or browning for steel parts, and/or, gun aluminum bluing for aluminum?  I had to check this out, but I threw my meter across the barrel of the rifle I just did and it conducted!  Depending on the amount of bluing you want, you can get a dark blue to almost black, just by repeating applications.  Won't come off due to heat.  My 8mm Mauser gets hotter after 12 fast shots than any of my heatsinks.


The cost for bluing is $4.00 a bottle.

oldgrandpainmi

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Guest 65ShelbyClone

Now as long as you don't leave them in the sun or in front of a fire the unwrapped bottle will cool down more quickly than the wrapped one.


Are we assuming that the foil is making an unrealisitic 100% full contact with the entire plastic bottle, or just as good as one can practically do in the five minutes to conduct the experiment? Since I tend not to use "theoreticallies" for support, I'll address the practical aspect. Not accounted for is the layer of air between the foil and the bottle. This will noticably slow the heat rejection of the bottle.

Something else to ponder is the fact that most aluminum car radiator cores are not painted at all. Brass and copper radiators are soldered and brazed together in manufacturing and look horrible until they are painted with black paint specifically for heat exchangers.

To answer the original question of this thread, you can use high-temp engine enamel from the autoparts store. It can withstand more heat than whatever the heatsink is cooling.

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