Thermal Runaway While Printing Solved

[RandomNick747]

TLDR: If you’re getting thermal runaways while printing, but not while stationary, check if your thermistor is loose and try to better secure it to the hotend.

During a print, the temperature of the hotend plummeted and the print was automatically cancelled due to thermal runaway. I discovered my thermistor managed to dislocate from the ferule that was crimped around it and which kept it in place in the hotend. I managed to reposition the thermistor in the hotend to the point where it would not fall out when printing, but was still loose in the hotend. I didn’t think this would be a large deal as the temperature throughout the hotend should be fairly constant. Boy was I wrong.

I could not get more than 5 minutes into a print before my hotend temperature would plummet and the thermal runaway protection killed the print. I could not figure out what the problem was as heating the hotend while stationary still worked and would hold temperature indefinitely. I couldn’t readily find any information online other than definitions of what thermal runaway protection is.

I did observe that the thermistor actually peeked out of the hotend while printing and tried to find a way to better secure it. The only way I found that properly secured the thermistor was a hotend that uses a screw and washer to hold the thermistor in place without moving. After installing the new hotend the printer started to work like normal. Something as simple as the thermistor moving a few millimeters caused temperature swings of about 30° C.