high speed retraction causing thermal runaway (I know it sounds silly)
October 19, 2018 09:14AM
Hi,

I am experienced with 3D printers, I have build 8 of them.

But I am facing a new problem with the one I recently made. I have used high torque steppers while building it. It is getting near perfect prints, all set and runs perfectly.

Yesterday, I decided to play with the retraction settings to see how faster can it retract. Stepper manages to get 200mm/s retraction without problems. But when I set the retraction distance longer than 1mm it gets thermal runaway on hot end and stop the print. It still gets 8 hour prints without any problems with old retraction speeds (50mm/s - 3mm), I tried it over the night.

Could not find this documented anywhere and wanted to have some thoughts.

Looking forward to have your suggestinos.

Edited 1 time(s). Last edit at 10/19/2018 11:02AM by VuRaL.
Re: Retraction causing thermal runaway (I know it sounds silly)
October 19, 2018 09:23AM
are you using anything like octoprint that shows a temperature graph you can monitor?

What controller/firmware combo are you using?

Edited 2 time(s). Last edit at 10/19/2018 09:25AM by obelisk79.
Re: Retraction causing thermal runaway (I know it sounds silly)
October 19, 2018 10:28AM
Currently I am printing from SD card on RAMPS 1.4 with final 8bit version of Marlin. I monitored temperature on screen. I print at 220C and it goes down to 211 and goes back up but it shows 190 for a brief moment and goes back to 215 instantly. This made me think if it was a loose cable on temp probe but it is not the case.

I am testing on letice cube with 20% size, lots of retracts and a small print with 15min. Same settings, same model, same spot on bed.

With retract speed 200mm/s and 2mm distance, it goes thermal runaway.

With retract speed 200mm/s and 1mm distance it completes part but it requires more retraction.

With retract speed 50mm/s and 3mm distance it completes but print quality goes lower as part is very small and nozzle stays longer on part.
Re: high speed retraction causing thermal runaway (I know it sounds silly)
October 19, 2018 12:42PM
Sounds like a coupling issue. Maybe the motor currents propagate to the controller board because of unshielded cables, improper grounding, poorly regulating power supply etc.

Try powering the controller separately from the motor and bed power supply.

If you've got an oscilloscope, try monitoring the 5V rail on the controller and see how much noise can be seen. Adding more capacitance to the controller board could help. Shield your cables and use a single earth ground reference point and wire all ground leads directly to it. Make sure if you're using LED powersupplies, especially of the Chinese variety that the ground connection isn't coupled directly to mains. If a light bulb lights up between its case and earth ground, you've got a problem.

Did you also try increasing the acceleration and jerk? Make sure the stepper motor can keep up with high rpm. If the motor stalls it may draw more current. Retraction is more about acceleration than speed. If it needs to ramp from 0mm/s takes longer than ramping from say 20mm/s (set jerk at 2). Maybe you don't need 200mm/s velocity, 50mm/s may be fast enough, but getting up to 50mm/s quickly is key, so experiment with acceleration and jerk.


--
Kind regards
Imqqmi

NFAN CoreXY printer:
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Re: high speed retraction causing thermal runaway (I know it sounds silly)
October 19, 2018 05:01PM
Thank you for the reply.

Electronics and motors are powered seperately already, they are behind a PSU with ground and battery.

I will check the jerk (it was set properly), if seperating extruder motor cables from hot end thermistor cables. They are tied to each other (about 40cmlong) and this would be making the false reads on temperature as you suggested.

I will update as soon as I can test.

Regards
Re: high speed retraction causing thermal runaway (I know it sounds silly)
October 23, 2018 09:14AM
After spending some quality time with the printer and testing your suggestions; my latest conclusion is that the retract speed increases print speed and hot end cant keep up.

I will try to update PID values with the ones I will get while heating bed, running fans at 100% and extruding at 200mm/s

Or maybe I should try using PID_EXTRUSION_SCALING


Else, I dont know what to do, I might go back to 50mm/s and 3 mm retraction but going back sucks when you are tinkering with your toys...

Edited 1 time(s). Last edit at 10/23/2018 05:23PM by VuRaL.
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