Tonight I learned something very valuable : a black heatsink is way better than a bare aluminium heatsink !
I had a failure with my x-carriage, until recently it worked perfectly fine, handling the heat of the heatsink plate directly in contact of the PLA carriage. But adding the heat of the summer and during a day of printing it slightly deformed and loosen the belt's tension...
Luckily I'm in vacation and when talking with my father he always have very good suggestions. I first thought to improve the printed part but then I showed him my previous experiments around the reprappro hotend :
I had adapted a 40mm heatsink but when seeing that the bare flat plate worked I left it aside, now I think I'will go for it since it have a greater surface of dissipation, but most important and that was the thing that struck him : it is black.
In short we all now that the black color absorb everything and thus became hotter than something white or reflective, but what he teach me is that it works the same in the other way, for emission ! So a black heatsink will be better than any other color of heatsink, by a very noticeable amount.
The good news : beside probably solving the heat problem, it's also a custom part that is replaced by something off-the shelves, that just need a few tweaks (cut some fins, file a flat, and make some drilling like for the actual plate)
Morals of the story : always speak of what you are doing to other people
Black does have higher emissivity, but that is not the whole story. Radiative cooling is most significant in the absence of forced air cooling. A high surface area is generally more important. Anodizing increases the surface area. A polished silver surface is worse. Applying a paint can mess things up, depending on the chemical composition of the paint, even if it is black.
These factors are generally low though, a polished black surface may be worse than an anodized natural silver surface. In practice, a heatsink with higher surface area is better regardles of color.