When I got up last Friday morning my PC had this on the screen: -
The PC had stopped talking to my Mendel about half way through a seven hour build, so the axes had stopped moving but the extruder was left running for a few hours. The result was that the extruder was encased in a solid ball of ABS about the size of a tangerine. Thanks Microsoft! What I actually said at the time was less polite!
Of course I was planning to put some safeguards in the firmware and also run the machine from an SD card, but have never quite got round to it as I have been printing virtually non-stop for months.
I couldn't remove the extruder from the carriage because the blob was too big to go through the gap, so I had to dismantle the x-axis to release the carriage.
I have seen this happen to other people and it wrote off the extruder, but I thought this one should survive because it is mostly metal underneath the blob.
I tried using a loop of hot nichrome to slice bits of it off. The nichrome cut through OK, but the ABS closed up behind it, so it achieved nothing.
Next I tried a small circular saw attached to a Dremel. That worked OK, but threw off sawdust and bits of ABS hot enough to burn, even through light clothing. I got the bulk of it off that way but when I nicked one of the heater wires I decided to stop.
I got some more off by heating it with a hot air gun and pulling lumps off with a pair of pliers. That was OK but the whole extruder got too hot to hold and it was starting to soften the carriage.
I got the remainder off by running the heater up to 200°C and using a knife, wire cutters and pliers. It took me about 3 evenings in total to remove the blob and the machine was out of action for a week while I reassembled it and calibrated it again.
It now takes a bit longer to warm up, and extrudes more filament during the warm up process than it used to. I suspect therefore that the thermistor is reading low and so it is running hotter. It doesn't seem to cause a problem with the ABS that I am using. I had to increase the time I run the extruder to prime it after warm up though.
I also seem to have managed to bend one of my z lead-screws while sliding the x-axis bars in and out. It doesn't matter as the axis is constrained by the z-bars, but annoying as it rattles a bit.
All in all a bit of a disaster. It's running again now though, but I still haven't put a safeguard in my firmware. I will have to develop it on HydraRaptor and load it into Mendel between builds. I have disabled automatic updates!