Failing tower calibration - Prusa

Hi,

Pic: [nicolinux.org]

We have a weird issue with the 50mm calibration tower. I have set the temp to 180/180 (Slic3r). The first layer barely extruded something. Then it worked "well" for a while but then suddenly the temperature dropped to 170° and the layers seemed to sink. I didn't cancel the print right away and noticed that the temperature raised again and the print got better.
We have a heated bed and a fan mounted temporary (and at a bad angle pointing directly down at the heated bed).
I read in another thread that one might need to cool the extruder - although this seems pretty unusual (never seen a Prusa with a fan for the extruder).
[forums.reprap.org]

What could this be?

Thanks,
Stefan

Are you printing PLA or ABS?
What model of exturder do you have?
What model of hot end do you have?

If printing PLA you need a fan on the top side of your hot end when it meets the exturder. You also need a fan blowing on the part right where the extruder is printing for PLA expecially for skinny and small prints like that.

When these two fans are blowing accross the tip of the hot end, they will cool it. Most usually have to wrap an insulator arround the metal part of the hotend to keep the fans from cooling it too much. The few wraps of kapton tape I covered mine with was a enought to start out. Now that I'm trying to do even smaller prints I need to wrap mine.