Wild Hotend Temp. Swings
Recently my I3 MK2 has started displaying large temperature fluctuations while printing, sometimes resulting in Thermal Runaway errors. I have tried running the PID calibration and it finished successfully but did not fix the problem. The problem on presents it self while the printer is printing. During heat up / if I set a temp and leave it it holds rock solid, as soon as it starts printing I can see temperature fluctuations as much as down 100c one second and back up the next. Attached is an image showing the problem, the highlighted area is when the print started. Is this likely to be a failing thermister?
Thanks,
Nathan
Re: Wild Hotend Temp. Swings
Hi Nathan,
Yes, it is very likely a failed thermistor. I had this exact problem and replaced my hotend thermistor just yesterday. Now the temperature is as stable as I've ever seen it (+/- 1 C).
If you have never replaced your thermistor before, feel free to ask any questions you may have.
Some helpful tips:
You are fortunate that the wires haven't broken completely and that you can still preheat the hotend. This makes removing the old thermistor very much easier. To remove the old cartridge, preheat the hotend. Now, loosen the small grub screw right next to the nozzle and beneath the thermistor (not the larger black one beneath the heating element). There is a hole in the heater block on the opposite side from where the blue wires go into the thermistor. You can use one of the hex wrenches to push the thermistor cartridge out.
I don't know if you're handy with a soldering iron, but the E3D replacement thermistor includes a connector between the thermistor wires and extension cable that could possibly interfere with the operation of the printer, so it may be necessary to cut it off and solder the thermistor wires to the black & red extension wires. The problem with the connector is that the blue wires are only about 1 7/8 inches long from where the come out of the thermistor cartridge to the point they enter the connector. So, this fixes the location of the connector on the wire bundle behind the extruder. The first thing you do NOT want to do is pull it taut because the strain will break the wires in pretty short order (I speak from experience). You definitely want to leave some slack, as shown in the photos in the extruder installation instructions. So, if this placement doesn't work for you, as it didn't for me, it will be necessary to cut the connector off. If you aren't able to do the soldering or don't know anyone who can do it for you, maybe you can look into whether Prusa will ship you a replacement that does not have the connector.
One last tip: if you decide to solder the wires, stripping the blue fabric insulation off the thermistor wires was tricky unless you have the right tool. I wouldn't recommend trying to use a knife or blade to strip the insulation. You really need a pair of wire strippers that can go down to 30 gauge.
Good luck.
Re: Wild Hotend Temp. Swings
I ended up having to replace the entire heatblock + thermistor and heat core as the screw holding the heat core in was stripped from the factory. Ordered new parts and installed them last night and it is printing better than ever.
Thanks for all the replies.