Thermal Runaway with my E3D Chimera

I've been struggling with this on and off for over a week now but can't get to the bottom of it, one of the extruders on my Chimera suffers from Thermal Runaway whilst the other is fine ?

I'm printing the 'Icicles' retraction test (thingiverse 607968) in PLA at 205degC. It fails everytime at layer 7, the temperature gradually falling from 205 to 198 after the fans start on layer 4. When printing with the other extruder it drops momentairly to 204 when the fans start but quickly recovers and sits happily at 205 for the rest of the print.

Cooling is by a couple of 30mm fans attached to the Chimera using thingiverse 1717604.

The failing extruder was originally on the right and I had built it with the heater block upside down so that it doesn't block the view of the nozzle from the front. I thought this was the problem and rebuilt it the right way up, this made no difference. I've also swapped the extruders left to right so know its nothing to do with wiring, electronics (RAMPS) or the fans. There's something different about the V6 heater blocks but I can't figure out what it can be ? The next step would be to strip down both extruders and swap heaters and then thermistors to see which part is causing it but I would like to try and avoid doing that.

I mostly print with the left hand extruder and have only occasionally used the right but have previously printed smaller objects e.g. Marvins with it with no problems. In the attached photo the offending extruder is now on the left.

Here are the results of a PID tune, do these differences provide any clue ?

OK Extruder: Kp 46, Ki 5.4, Kd 99
Failing Extruder: Kp 56, Ki 6, Kd 122

Any ideas ?

Chris

After more time spent swapping bits around I've tracked the problem down to a faulty heater cartridge proved by cannibalising my Cyclops extruder. The two good cartridges measure 4.8ohm, the faulty one is 6.1ohm's.

Wonder if E3D will replace the Cyclops cartridge ?

Chris

Try sliding the insulation back and measuring at the solid conductors. If it's 4.8 there the problem is likely a bad crimp connection (two out of three of mine have had bad crimps). If you have a crimp tool then the fix is obvious, if not, you can carefully use a pair of side cutters to improve the crimp.

When I bought my chimera I checked the heater cartridge resistance before assembling the hot end. One was at 6.3 ohms and the other was 6.5 ohms. 6.1 doesn't sound too far out of line. Is it a 25 watt cartridge? I think the 25 watt has blue leads and the 40 watt ones are red.

Edited 1 time(s). Last edit at 12/13/2016 05:10PM by Shank man.

I measured the faulty cartridge right at the point where the wires disappear into the ceramic and it reads 6.1ohm so the crimps are fine.

The leads are blue, E3D currently list 12V/30W and 24V/25W cartridges with blue leads. They don't list a 12V/25W cartridge although as you suggest it measures as if it was ?, perhaps they used to sell 12V/25W and this one has got mixed up with new stock ?