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Problems with retraction and prime
#1
Hello all,

got my Prometheus set up a week ago and first two dual color prints done also. On the second print, dual color dice, I noticed some irregularities about extruded material, some places had too much and some places too little. I guessed that it must be something about retractions and primes.

So I thought that I had something not set up properly and started tuning the setup by printing this retraction model: http://www.thingiverse.com/thing:1240361.

Results are pretty much the same on both extruders, retractions and or primes cause skipped steps randomly, which cause artifacts on the prints (blue is E0, white E1):
https://www.dropbox.com/s/r1qa5uyhja3ie9...7.jpg?dl=0
https://www.dropbox.com/s/slivi20l3usxwy...3.jpg?dl=0
https://www.dropbox.com/s/v4zhms7949807d...5.jpg?dl=0
https://www.dropbox.com/s/8frxcilvu5k8sr...0.jpg?dl=0

Either prime fails and not enough material is printed or retraction fails and too much material is printed. Or both fail and result is actually more or less ok, depending how many skipped steps there are...

Things I've tried so far:
- adjust stepper driver vref 0.45 - 0.93V. On A4988, 0.45 should be the proper vref. Rising over that only makes the steppers more loud, but problems are still there
- change E jerk from 15mm/s to 10 and even 5. Still random errors
- tried retraction speeds 40-80mm/s
- swapped stepper drivers between extruders. No change. Both drivers have been working previously.

Retraction length is 4mm. Bowden tubes less ~35cm
Board is Megatronics 3.1, with active cooling on the drivers. Also stepper motors are cooled as per intructions. Motors are cool to touch.
Running Marlin RCBugFix.

Any thoughts? Not really insterested to do any dc prints (or any prints) as long as I know there'll be artifacts...
 
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#2
@spegelius 

Thanks for the info! The ProStruders use 5:1 geared extruders so they have more than enough torque available if everything is properly configured and tuned. Follow the steps below and let me know how it works out:

1) Tune each stepper driver current to 1.2A:

This is the most likely cause of skipped steps. If you cannot set the motor current in your firmware, increase the pot on the stepper drive manually until you hear a clicking sound from the motors. Then reduce the current slightly so that the clicking sound goes away. This will be the maximum current that the motors can handle and will be about 1.2A.

2) Set the E Jerk to 15mm/s and retraction speed to 80mm/s as outlined in the User Guide

3) Tune the retraction length:

Since you have relatively short 350mm bowden tubes, 4mm may be too much retraction and could be pulling the plastic into the cold zone which would cause temporary jams and skipped steps. Set the retraction length to 2mm and do those test prints increasing retraction by 0.5mm until you don't get any more stringing. This will give you the optimal retraction length.

Also, it would be great if you could provide some more information. What material are you printing? Have you dried your filament before printing? What temperature are you printing at?

Let us know how it goes!
 
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#3
Thanks for the reply.

I did have most of the stuff properly configured, but the problem turned out to be the retraction length; tuning it to 2mm seems to have gotten rid of the skipped steps and artifacts and is still enough for string-free prints, at least with ABS. My vref on the A4988 drivers is ~0.5V at the moment.

I did the retraction test, no artifacts and also printed Dual color Benchy, which has quite a lot of retractions and no jamming. I might need to bump the retraction length up a bit, there were a couple of small blobs, but on the whole, quite clean print Smile.

Thanks for the tip, didn't really think retraction length would cause such big problems.

filaments: ABS @235-240, should be quite dry
PLA @205, also dry(ish). No problems with other printers.

Next to try PETG and soluble support materials. Also need to design a part cooling fan tunnel for printing PLA.
 
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#4
@spegelius 

That's great! Be sure to post pictures of your dual extrusion prints  Smile
 
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#5
Sure thing, I'll start a new thread in Show and tell section.
 
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#6
Been having jamming problems with some PLA brands, they start to stick when the overall flow of the material is really slow. This happens when printing relatively slow (like ~10mm/s) or having lot of small details and retractions. You can see the results here:
https://www.dropbox.com/s/j4w3clm7125kck...2.jpg?dl=0
Bigger parts print fine, as long as the filament moves fast enough to not to heat up too much.

So what to do? Filament heating too much too high seems to be the problem, so I designed a fan shroud for 40mm fan:
https://www.dropbox.com/s/m0w4roxnrm9oc5...3.jpg?dl=0
https://www.dropbox.com/s/j9o4uy45j22qsq...0.jpg?dl=0
There's quite a lot of airflow now (and more noise). It seemed to have helped with a brown PLA that I was printing at the moment, but before trying the really problematic filaments and model (Benchy), I tried another fix for uneven extrusion: seasoning the nozzle. I.e. put some oil into the nozzle Smile https://www.youtube.com/watch?v=shSfZxK-VUk
I've yet to try the problem filaments with the seasoning and the fan. Latest benchy turned out fine with white and brown PLA so apparently I didn't mess up anything at least.
 
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