Extruder too hot! - This is a weird one.

[JohnnyRobot]

Hey all,

It's been awhile since my last post. I've had my M2 for 3 years now and it's been running great. I have an E3D V6 hot end.

So far I've been pretty self sufficient. I've been handling the usual print issues that pop up without much effort. But this one I can't seem to shake.

A few days ago, I walked up to a failed print. This is a piece that I've printed in the past without any issues. It's on my SD card and I print it quite often. There was a bulge in the filament that jammed my extruder and the printer was "ghost printing" a few inches above failed print. "Oh well... It happens".

I retracted the filament, removed the extruder, made sure the spur gear was all clear. I primed the system with 100mm or so of fresh filament. No problems there.

I restarted the print. It looked terrible. the surfaces were bubbly and rough. It had a sandpaper like appearance. Nothing like the previous prints (same g-code, same roll of material)

First thought - bad material. I swapped out the filament with another. Issue still remains.
Second thought - Clogged nozzle. Maybe the material cooked while the printer was jammed? I swapped the nozzle with new. Issue still remains.
Third thought - Bad thermistor. While troubleshooting, I noticed a burning smell and light smoke coming from the nozzle. Also, material was falling out from the nozzle in a way I've never seen before. I replaced the thermistor with new. Issue still remains (No more burning smell or smoking though)
Fourth thought - Extruder tube is gunked up causing resistance. Took everything apart and cleaned very well. Super slick. Issue still remains.
Fifth thought - My new thermistor possibly uses a different config table. I did a test and the print started to look MMMuuucchhh better when I manually reduced the temp to 185. So I've been playing around with different thermistor table values. I'm using the Amazon "RioRand(TM) Upgraded 10 pcs 100Kohm NTC Thermistors" which have mixed reviews but are generally close when "#define TEMP_SENSOR_0" is set to 1, 5 or 6. No such luck for me. Issue still remains.

I even added this -> http://www.thingiverse.com/thing:1482724 to my thermisortables.h file and the issue still remains.

Is there anything else any of you can think of? Might I have damaged my main board? I can't figure this one out. Any help is greatly appreciated.

Here are some microscope shot of my extrusion...

[ednisley]

Might I have damaged my main board?

Some ideas:

0) Restore the original thermistor tables. This was a sudden failure, not a firmware problem, and changing the firmware settings will complicate everything else.

1) A failing thermistor (more accurately, a failed thermistor connection) will produce erratic temperature readings and eventually produce an under-temperature error as the connection opens up. You don't mention irregular temperature readings and suchlike, so the thermistor and cable are probably OK.

Watch the temperature as the extruder heats up. If it always shows something near room temperature with the extruder heating, then the cable has failed with a high-resistance "short circuit".

If wiggling the thermistor cable and connectors causes intermittent temperature readings, you've found the problem.

2) Two red LEDs on the RAMBo board turns on when the extruder and platform heaters turn on. The extruder heater LED will be solidly on during the initial heating, turn off when the extruder is too hot, then pulse rapidly on-and-off as the extruder reaches operating temperature. The platform heater LED goes on solid, then slowly cycles between on and off when the platform reaches the proper temperature.

If the extruder LED goes on and stays on, then (most likely) the thermistor is reporting a too-low temperature and the firmware keeps the heater on forever. If it's never off, even when the heater shouldn't be heating, then the firmware isn't in control of the heater, which suggests a RAMBo hardware problem.

Do the LEDs behave that way?

3) The LEDs show the control input to the MOSFET switches, not the switch output to the heaters. The usual MOSFET failure is "stuck on" regardless of the control input. Does the extruder start heating when you turn the power on, even when it's supposed to be off?

If so, then the MOSFET has failed and you're in line for a new RAMBo. There are ways to work around that failure, but they're probably not worth the effort.

With all that in mind, take a closer look at the symptoms and see if anything suggests itself...

[JohnnyRobot]

Update!

I haven't had much time to troubleshoot this issue.

I took Ed's advice and reset my thermistor table. I agree with him that a sudden mechanical failure does not warrant adjusting the firmware settings.

I still had the issue though. Really nasty looking extrusions. I found that I could still print by setting the temperature significantly below the material's nominal print temp. I was printing PETG at 205c. It looked great. (weird)

I knew that something wasn't right, but I had prints to make so I just kept on going with the lower temps.

after about 16 hours of printer run time, I noticed that my prints had a dull finish and I was getting cracks between layers. Classic 'low print temp' scenario.

I raised my temp up to 245c and, viola! It's printing beautifully again.

So, the issue fixed itself? Welcome to my world.

[ednisley]

So, the issue fixed itself?

Nope.

You have an intermittent thermistor connection and, right now, it's Just Ducky™. Tomorrow, it'll go bad, at least for a while, and wreck another project for you. The day after that, no problem. Lather, rinse, repeat.

Assuming the LEDs on the RAMBo are behaving as I described, that weeds out problems on the RAMBo board. What's left are the thermistor and its cable.

Thermistors decrease in resistance as the temperature increases, so open circuits (the most common failure) produce an under-temperature shutdown, short circuits (less common) produce overtemperature shutdowns, and high-resistance failures (quite uncommon) make the RAMBo sense a lower temperature than is actually out there.

The cables use stranded wire to improve flexibility, except for the leads from the thermistor to the first connector. Even with all but one strand broken, the cable will still work fine. As that last strand (or the solid wire at the thermistor) flexes and fractures, the resistance of the remaining metal goes up and up and up until it finally breaks open.

IMO, that's about where your thermistor wiring is right now: the last strand is ready to break...