How to Cool Extruders with Fans on Your 3D Printer
The golden rule about 3D printing extruders: Use fans. When you start experimenting with printing ultra-tiny objects with fine details or printing objects at great speed, you quickly discover an interesting problem with the 3D printing of thermoplastic materials: controlling layer temperature.
If you print very tiny parts that have little layer surface area or turn out an object at very high printing speeds so each layer is completed in a matter of seconds, often the layer of plastic just laid down doesn't have time to cool. It's still a little molten when the next layer is laid down.
With the radiated heat from the nozzle and more hot plastic being extruded, the model can end up a messy blob instead of the object you had intended. You can slow down the speed, but this may not resolve the problem; you shouldn't have to wait even longer to print an object anyway. This is where a controlled cooling fan can make a massive difference.
The cooling fan used to cool a printed object is usually around 80mm wide. It's controlled by the electronics: In your Slic3r-generated G-code, you can specify how fast a cooling fan runs, and when to turn the fan on or off.
With a cooling fan fitted, Slic3r can still run the print at full speed — even when printing fine details of a model. Without a fan, Slic3r would have to instruct the G-code to slow down to allow for natural cooling of the plastic before adding more. As you can imagine, fine structures can be tricky to print without a cooling fan.
A fan has the ability to allow bridging of extruded material. This is an essential part of many 3D-printed objects. Bridging is when a model has to span a gap, essentially making a bridge in thin air. If you extrude plastic with nothing underneath it, the extruded material will naturally sag down and sometimes break.
Although you can bridge filament without using a fan, you will usually have some strings of snapped extruded filament hanging down, and a little sagging. With a fan to help cool the plastic as it's extruded, you can make a tight bridge and get smart-looking results.
A very important point about fan cooling is to mount the fan so it's cooling the top layer of the part being printed. If you cool the heated-bed, your part will pop off in the middle of the print. If you cool the hot-end, your extruder may jam. It's common for a cooling fan to have a 3D-printed duct to direct a stream of air across the object while printing.
In almost all cases, it's not advisable to use a cooling fan for printing in ABS material. The fan makes a part's edges cool too fast and curl up; the next layer may be worse. Eventually the part can be so deformed and warped that the print head can knock it off the build platform. With PLA, however, the exact opposite is true: PLA likes a fan; ABS does not.