RepRap researchers have been experimenting with a unique water cooled extruder design. Cooling your extruder is important to ensure that the heat from the hot end that melts the plastic printing material doesn't creep up into the rest of the mechanism, where you risk frozen plastic in the wrong places that gum up the works.
Typically extruders are cooled by fans, but this experiment involves piping cooled water thru a portion of the extruder to maintain correct heat levels. An external reservoir (in this case a beaker) and a standalone water pump ensures cooled water flows continuously.
Why do this? According to researcher Adrian Bowyer:
But the fan is slightly bulky… This is much lighter and more compact, and the cooling is more efficient. It consists of a brass block that replaces our normal aluminium cooling block that attaches to the fan. The brass has water channels drilled in it, and some soft silicone tubing connecting it to a small 12V gear pump. The inflow and outflow temperatures are only a fraction of a degree different, meaning that multiple heads could be chained in series and all cooled by the same flow.
Will we see water cooled extruders appearing on inexpensive 3D printers? Perhaps if the increased efficiency complements the switch from fans to water cooling, which could be more complex to include in a commercial product.