Extruder fan - when should it run?

[hmanvel]

​I just installed an E3D hot end on my Rostock Max V2, have it working but not yet printing. So far, when heating up, the extruder fan does not run. I cant recall whether that is normal? When should the fan run, only when you are printing? Only above setpoint? Just want to make sure before I try calibrating and printing etc.

[lightninjay]

If you are referring the the fan that cools the E3D heatsink, as it is an all-metal hotend, most would tell you to wire the fan direct to 12v so it turns on as soon as the printer has power.

Many utilize a setup where the fan will turn on after the hotend reaches a certain point, and remains off when the hotend is at room temp.

Depending on your current setup, it would probably be easier to just wire stratight to 12v.

[IMBoring25]

Even if you had it set for thermostatic control the fan should start as soon as the heater does and remain on until the temperature falls below a threshold value (RMax v2 uses 50C by default).

[Zesty_Lykle]

The reason the fan is so important is that otherwise you will get jams.
If the fan is not on, or positioned wrong ( the overhang at the bottom and the lowest fin of the heat break covered) the heat can rise above the heat break and melt the material in the heat sink. That will cause the hot end to jam.

Lykle