Welcome to the SeeMeCNC Community

Was also covering this in my build thread, but trying here as well. At my wits end, been fighting this for months.

Basically, I keep having to increase the temp in my hot end, in order to print. For a few years I had been consistently printing in the 225-230c range. Then, this summer, I started to get some clogs. Eventually I discovered I needed to set my temp to 245c just to get anything to extrude. So I started printing at 245c while trying to find what's going on. Now even 245c isn't enough, tried 270c and still not enough. Finally I had to set my output at 290c just to get anything to extrude and print. I doubt this is correct, but I don't have a way to calibrate it right now. And whatever is happening, keeps getting worse over time anyway, because I keep having to raise the temp to print.

.4mm nozzles at this point are impossible to use, they just clog almost instantly. .8mm works for a while till I have to raise the temp again.

The temp indicated by the thermistor always matches what I set it to, so it's not like a heater that can't keep up, that would indicate a lower temp than expected. No matter what I set the temp to, It'll warm/cool to that temp.

Printer Specs
Rostock Max V2 running firmware 0.92.2 (had an earlier version when this started, since updated)
E3D V6 hot end, with new 'block and sock' upgrade to a cartridge thermistor.
RAMBO board.

Things I've tried to solve this problem, with no success.....
Updated firmware
Installed new thermistor, then updated to the newer style thermistor and heater block.
New nozzles. Many attempts.
Checked that all wiring connections are good.

Could use any suggestions you guys have on this.

Welp, I'm confused.

Did a couple cold pulls, re-checked all the connections, and suddenly it was flowing again around the 200c mark. So set my software back to 250c and she's printing ABS again with a .8mm nozzle. Quite nicely actually.

I did find I had the retract setting a bit too aggressive for the E3D hot-end at 5mm, and apparently that's a BAD setting for this hot end. So I knocked it down to 3mm.

Not going to complain, just curious which of those changes fixed it. Until I go through a full spool without it coming back, I'll still be nervous.

"The temp indicated by the thermistor always matches what I set it to, so it's not like a heater that can't keep up, that would indicate a lower temp than expected. No matter what I set the temp to, It'll warm/cool to that temp."

That's really just an indication on how well your PID is tuned, and not any testament to the accuracy of the thermistor. While it is working fine, I would measure the resistance of the thermistor at room temperature and once it gets heated up (with the power off, of course). Use these as benchmarks should things go south again.

I agree, that's why the first thing I did was change the thermistor and check all the wiring to it.
In fact, I think I changed it three or four times. Results were identical.
Always reads accurate at ambient.

I do wonder if there was a bad connection somewhere along the signal path. So I inspected and re-seated all the connections from the thermsitor all the way to the RAMBO. Seems to be ok now, but I did so many things yesterday I don't know which one fixed it.

-Hans

Personally, I had the same issues whenever I retracted more than 4mm...the filament would get clogged inside the v6. I think the reason you had to increase the temp was to melt the filament inside the cooling chamber...I'm surprised you didn't burn your filament.

Check your hotend fan to make sure it's mounted on right. The first v6 mount I made (design on thingiverse) forced the fan to be a little higher, about half an mm. I don't know for sure if that added to the problem, but since I installed a new mount and kept retraction at 4mm and less, I haven't had a problem since.

Out of curiosity, do you have your rostock in an enclosure?

I did some research, and the resistance range for the 160C - 260C range that our hotends work in is about 500 - 50 ohms. As the temperature goes up, the resistance of the thermistor goes down. So, it should be around 500-ish ohms at 160C, and 50-ish ohms at 260C.

Looking at the Rambo 1.2 schematic (http://reprap.org/mediawiki/images/d/d0 ... c-note.png), you'll see that the thermistor is in series with the bottom half of one of the two 4.7k resistors that make up a voltage divider. So the voltage to the ADC goes down as things heat up.

Self-heating can be a problem, so I did a quick calculation of the maximum self-heating potential (with a shorted thermistor), and I came up with about 2.5 milliwatts (max is around 18mw). This shouldn't be a problem.

For the thermistor to be the problem, it has to vary a heck of a lot at the room temperature (100k ohms) before we would ever even begin to notice. But only a tiny bit at the high end. (http://reprap.org/wiki/Gen7_Research#Se ... ge_Divider)

The other elephant in the room is the cap. And as everyone knows, if a fart is let loose in the room, the dog gets the blame. If a circuit acts wonky for no apparent reason, blame the cap.

Well, I can happily report it seems to be going 'normal' now. Even extruding through my .4mm at a setting of 225c

I think reducing the retract measurement was potentially the big factor there. Some cold-pulls to clear it out, and it's actually extruding very nicely now. I'm also surprised I wasn't burning anything at the temps I had things running at. I was getting seriously concerned about melting components of the printer too. When I went through the initial build a few years ago, I don't think the cold-pull technique was widely known yet, but once I gave it a try things woke up quickly.

-Hans