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Stopping extrusion a little before retraction jumps

Posted by sourceless 
Stopping extrusion a little before retraction jumps
December 14, 2015 09:25AM
Hi,
I have fabtotum 3d printer. It's with bowden tube feeder and it has a hard time with retractions(oozing, blobs)
The best thing I could do was
- decrease temp as much as I can
- increase jerk speed of the feeder as much as I can
- put a little negative "extra length on restart" for the retraction - but if the retractions are very close it leads to under extrusion.
I don't like wipe while retracting, i think it damages my printer.
Not crossing perimeters is cool.

So instead of trying to hack it with all kinds of tricks, is there an option which would stop the feeder few mm before a retraction jump? So that it could release the pressure in the extruder.

Thanks
Re: Stopping extrusion a little before retraction jumps
December 14, 2015 10:07AM
Simplify3D's coast-to-end feature does that. So does the pressure advance algorithm in my fork of RepRapFirmware. However, if you are getting blobs at the start of travel moves, this usually means that you don't have enough retraction distance configured.

Edited 1 time(s). Last edit at 12/14/2015 10:07AM by dc42.


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Re: Stopping extrusion a little before retraction jumps
December 14, 2015 10:27AM
I made a test, and even while retracting there is still a little filament coming out. I managed to fix this up to a point with the jerk speed but I don't think the motor likes that... I think it was 6-7 Jerk
(1.75mm filament)
I was even doing slow print speed 50mm/s and 0.1 per layer to reduce pressure but still couldn't completely remove it. And after all 50mm/s is a bit slow, I was hoping for at least 80-90.

Edited 1 time(s). Last edit at 12/14/2015 10:29AM by sourceless.
Re: Stopping extrusion a little before retraction jumps
December 14, 2015 11:37AM
Try increasing your retraction distance. 3mm-8mm is usually a reasonable range.

Dave
Re: Stopping extrusion a little before retraction jumps
December 15, 2015 02:38AM
I tested this extensively using a post-processor I created for Slic3r.

I found that with a long bowden tube - just coasting was not enough. For my printer there was enough spring pressure to keep it printing at about the same rate.
So I tested something I called Coast while Retracting. This starts the retraction an adjustable distance from the end of the loop (about 1mm in my case).
This resulted in less of a blob at the end, very little stringing. The small amount not extruded was added to the beginning of the next segment to keep pressure neutral.

Note that normally retraction begins when the extruder has come to a stop at the end of a loop. The slower the retraction the larger blob is formed. So my main advice for now is, if you can increase retraction speed, do so. There is spring pressure from the filament that helps in that regard, so retraction can be faster than unretract. Print two small objects quite a distance apart, increase retraction speed and acceleration until you can hear it's too much, then back it off a bit. You need to adjust those one at a time tho. Adjust retraction length until stringing just stops.

You do need a small amount of "extra length" on unretract, because retraction (any movement really through the extruder gear) deforms the filament a bit. You will see the need for this especially if you have a very large number of retractions close together.

This gets into another issue, a retraction distance that is too long can cause increased delay in extrusion at the beginning of the next loop (for some extruders, maybe all). This shows up as more starvation at the start of a line after increased retraction distance (I tested proportional retraction). Just adding more extra length on restart to compensate doesn't solve this, then you have over extrusion. One thing that is needed is a change in the acceleration at the beginning of the first segment after a retract, proportional to the new pressure level. This is easily done by splitting the first segment as needed, and applying a proportionally slow acceleration to it, with normal acceleration after. Darn, I got started again (sigh). Tons more work to do on this. Long thread here.


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Re: Stopping extrusion a little before retraction jumps
December 21, 2015 07:43AM
Quote

You will see the need for this especially if you have a very large number of retractions close together

This is where I make use of "Minimum Travel After Retraction". For small parts, like 3 cm x 1 cm, I make Minimum Travel After Retraction greater than 3 cm so that there is no retraction inside the part, but there will still be retraction for travelling to the next part.
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