Hot End Temperature Rises Then Falls Rapidly

[pcenginefx]

Hi everyone,

Just had my first issue with my M2 (been working great since Jan.). When I start pre-heating the bed and hotend, the temps on the hotend start dropping rapidly after reaching 170 or so. Obviously temps can't change this rapidly so it must be the temp sensor in the extruder assembly? (or maybe something wrong with the Rambo board?). Before I go ordering replacement parts, which one would more likely be the culprit?

[ednisley]

so it must be the temp sensor in the extruder assembly

Got it in one: a wire on the thermistor has broken.

You can verify that by poking the wiring a bit, but everybody seems to converge on a thermistor lead after a while. The leads seem to break very close to (or, perhaps, just inside) the thermistor body, probably due to motion of the heavier wires + sheaths going to the connector.

It's good practice to immobilize those wires so they can't move, which can be difficult...

[innkeeper]

a few other possibilities:

Could be a bad Thermistor, or even a Thermistor not making good thermal contact.

I had a similar problem that your describing, temperature would curve way down, then overshoot. turned out to be the rambo board.

I've seen this also happen with a bad PID tune, but, since it had been working fine, and unless you have been messing with that, PID seems unlikely the cause.

[pcenginefx]

Yeah, I literally haven't been messing with *anything* on the printer as it has been working perfectly till last night - but it *has* been printing non-stop since January....

With that being the case, I may just start off by buying the Single V4 Extruder Upgrade and seeing if that fixes it....

[insta]

uh guys make sure you pay attention to this. I just lost a v4 (and nearly my printer) because I ignored those fluctuations. It was entirely my own neglect, not a fault of the hotend itself.

[Jules]

uh guys make sure you pay attention to this. I just lost a v4 (and nearly my printer) because I ignored those fluctuations. It was entirely my own neglect, not a fault of the hotend itself.

Uhhhhmmmmm......in what way did you nearly lose the printer?

[insta]

uh guys make sure you pay attention to this. I just lost a v4 (and nearly my printer) because I ignored those fluctuations. It was entirely my own neglect, not a fault of the hotend itself.

Uhhhhmmmmm......in what way did you nearly lose the printer?

All-metal design of the v4 saved it from a full-blown fire, but it got hot enough to carbonize the PTFE liner, burn the rubber insulation off, and turn the thermistor into dust. The hotend has that "steel got really hot" kind of blueish/greenish colorization.

[pcenginefx]

uh guys make sure you pay attention to this. I just lost a v4 (and nearly my printer) because I ignored those fluctuations. It was entirely my own neglect, not a fault of the hotend itself.

Uhhhhmmmmm......in what way did you nearly lose the printer?

All-metal design of the v4 saved it from a full-blown fire, but it got hot enough to carbonize the PTFE liner, burn the rubber insulation off, and turn the thermistor into dust. The hotend has that "steel got really hot" kind of blueish/greenish colorization.

So what was the cause of the overheating then?

[jimc]

i gotta tell you it has always made me nervous that these machines have no thermal safety switch or backup of any kind. i realize thermal switches tend to be large so might not work well but even a second backup thermistor which monitors temp independently from the rambo. temp goes above 300-325 or so and it should shut off the power supply.

fluctuating temps are the result of a failing thermistor or wire harness from the thermistor to the rambo.

[ednisley]

it has always made me nervous that these machines have no thermal safety switch or backup of any kind

You and me, both!

Recent version of Marlin can compare two thermistors and shut down if the temperatures diverge, but that assumes the firmware will continue running and I wouldn't depend on that.

When I replaced the "heater resistors" in my Thing-O-Matic with cartridge heaters, I put a completely separate thermal switch on the hot end:
http://softsolder.com/2011/03/23/thing- ... tch-block/

Then wired up a relay-operated lockout circuit to disable the power supply:
http://softsolder.com/2011/03/14/thing- ... t-circuit/

One time the firmware jammed the heaters fully on, but I caught the TOM before the thermal switch tripped:
http://softsolder.com/2011/06/07/thing- ... l-runaway/

I haven't figured any way to add a thermal switch and power supply lockout on the M2, though, although that project has been nagging me with increasing urgency. I tend to run the M2 when I'm puttering in the basement shop and very rarely let it run when I'm not around, just on principle.