Trouble removing broken hotend thermistor
The thermistor on my MINI+ hotend has a broken wire.
I found the article on replacing the hotend thermistor, and ordered a new thermistor. However, when trying to remove the broken one, I am running into trouble. The grub screw won't loosen, and the old/broken thermistor therefore won't come out. I'm assuming that during the various blob print failures the grub screw has become coated in PLA and that's why it won't turn. The solution for this kind of problem is typically to run the hotend temp up to above PLA's melting point, but I can't of course because the thermistor is broken.
Should I use a heat gun/lighter/soldering iron to try to loosed up the hardened PLA cementing it in place? (for the heat gun at least I'm worried about melting the pieces that aren't supposed to get up to the melting point of PLA, like the print fan mount, etc.)
Thanks for any advice,
-- Mitch Patenaude
RE: Trouble removing broken hotend thermistor
I wound up changing the heatblock too. The thermistor and heater wouldn't come off even with the screws removed.
I recommend a Levendig silicone sock:
They keep the hot end nice and clean.
Regards,
Mark
RE: Trouble removing broken hotend thermistor
Thanks for replying.
I did buy some socks, thanks for the pointer.
The person on Prusa's chat support (Rodolfo?) said that one solution is to plug the new thermistor into the board, wedge it next to the heat block, then heat up the hotend so that the grub screw will turn and the thermistor will come out. Once the new thermistor arrives, I'll update with how it went.
RE: Trouble removing broken hotend thermistor
It didn't go well. I heated up the block using the technique described above, and on my 3rd or 4th attempt[*] managed to get the grub screw out, which now appears to be permanently welded to the end of the hex wrench. However, the thermistor still won't come out. I've tried heating the block again and both pushing against it from the right side with the hex wrench and pulling it from the left with needle nose pliers, and all I've done is manage to mangle it a little. I think I'll be replacing the whole block, maybe the whole hotend minus the print fan.
[*] The problem is that the thermistor is wedged between the heater block and the heat sink, so it reads low until the fan turns off, at which point it shoots up and causes the printer to think it's in thermal runaway. It's also getting so hot that it's smoking badly even if I only set the target temp at 150C.
RE:
It didn't go well. I heated up the block using the technique described above, and on my 3rd or 4th attempt[*] managed to get the grub screw out, which now appears to be permanently welded to the end of the hex wrench. However, the thermistor still won't come out. I've tried heating the block again and both pushing against it from the right side with the hex wrench and pulling it from the left with needle nose pliers, and all I've done is manage to mangle it a little. I think I'll be replacing the whole block, maybe the whole hotend minus the print fan.
[*] The problem is that the thermistor is wedged between the heater block and the heat sink, so it reads low until the fan turns off, at which point it shoots up and causes the printer to think it's in thermal runaway. It's also getting so hot that it's smoking badly even if I only set the target temp at 150C.
Yes, I replaced the heater block, heater and thermistor. They were stuck to the block. Maybe replace the heatbreak too while you are at it. Put a silicone sock on and it should not happen again.
It was not hard, just time consuming. Prusa provides good directions for all the replacements.
Regards,
Mark