The part of the extruder that has all the impression of being a heat sink is just that! Its function is to keep the wire cold. The contact with the hot end if not mitigated could melt the wire for several centimeters and then block the extrusion process. The radiator must be accompanied by a fan that creates the air flow and optimizes the cooling performance.
There are various forms, mainly based on the original design of the printer on which they are mounted. The main difference is on the input filament. If it is a direct extruder, the heatsink will have a small hole (2 or 3 mm depending on the filament used by the printer. If 1.75 mm or 3 mm) that will only allow the entry of the wire.
If we have a bowden extruder (deferred) then the heatsink will have on the top a thread. Be careful it can be of different sizes! For the quick coupling of the PTFE and pipe.
As mentioned before, the heat sink is coupled to a fan. When purchasing a particular fan, special attention must be paid to the side dimensions, which range from 2 cm up to 20 cm.
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The sizes of the fans can also vary in depth, the small fans of the heat sink of a 3D printer are usually 1 cm, but there are also 2 cm and this thicker shape gives a greater flow of air, therefore more cooling.
Beware of voltage!
Obviously in the choice of the fan you will have to follow the present power supply with some freedom. It will always be advisable to choose the appropriate voltage (12v if we have a printer powered at 12v, 24v if … and so on).
The only “poetic license” granted is to put in series 2 12v fans on a 24v power supply, or always for multiples. It might be the right choice if there are conditions including the easiest finding of 12v fans or their lowest cost.
You can find radiators and associated fans in all major e-commerce, they are a fairly common spare product.