I am using a design for the fan that combines both the part cooling and heat sink cooling. I ended up using a 40mm server fan, which is really powerful. I ran it at 85% power, which is the lowest it can go without turning off. It ends up cooling the extruder so much that I can't get up to temperature and I would get thermal runaway errors. Should I try to run the PID autotune with the fan on, or would that not help at all? I am currently just putting cardboard over the intake of the fan, which is decreasing the airflow, but I would rather have a better airflow with more aggressive heating.

You need to run pid tune in the same setup you are printing. Everything else will make the pid control work not optimal.

Those combined part and heatsink cooling designs are really bad. Generally, you don't always want part cooling, but you generally always want heatsink cooling.

Maybe thinck of isolating the heater block. I use molded Hight Temp Silicone sleaves that help maintaining a stable temperature regardles of ariflow around the hotend.

---

[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]