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Pause between moves and extraction

Posted by Antslake 
Pause between moves and extraction
December 27, 2015 02:44PM
I have a makerfarm i3 with ramps and marlin. It seems to me that a lot of problems could be solved if when an extraction happens there was no pause. That millsecond when the x/y movement stops and extraction happens, a little filament can ooze out. Is there a way to get rid of this? Printing with filaments that require greater pressure to push through the hotend are more affected by this, like nylon.
Re: Pause between moves and extraction
December 27, 2015 06:30PM
Antslake,
This is something I have researched extensively.

Like you described, the retraction happens at the end of a segment, when movement has stopped. The longer the retraction takes the more ozze will happen there, causing a blob/stringing.

One way to help this is by adjusting your retraction speed and acceleration to make the retraction happen as fast as possible.
Use only enough retraction to eliminate (most) of the stringing.

Simplify3d has a "Coast" feature that stops extruding a certain distance from the end of the segment. This can help, some people I've spoken to have told me it's great.

I tested Coast with a post processor I'm working on (for testing new features for Slic3r) and found that with my long bowden tube there was enough spring in the filament to keep the pressure high while coasting, so coasting wasn't enough. So I developed and tested "Coast while Retracting" - retraction starts 1mm (adjustable) before the end of the segment - this takes off the pressure so there is much less of a blob, and makes the final retraction time much faster. This is is a pending feature request for Slic3r, among many others.

Hope that helps.


My printer: Raptosaur - Large Format Delta - [www.paulwanamaker.wordpress.com]
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Re: Pause between moves and extraction
December 28, 2015 03:29AM
If the firmware is working properly then there should not be any significant pause between the end of an extruding move and a retraction move.

The effect that Paul describes can be countered by a technique called pressure advance. This increases the extrusion pressure by an amount proportional to the extrusion speed. At the end of an extrusion move when the extrusion speed is reducing, the pressure is reduced and by the end of the move the extruder may actually be retracting filament, depending on the pressure advance setting.

AFAIK the only firmwares that implement pressure advance are Sailfish and RepRapFirmware. I run RepRapFirmware, however I don't need to use pressure advance to avoid blobbing, I just need to configure sufficient retraction. For my E3Dv6 hot end and 660mm Bowden tube, this is 8mm.

Edited 1 time(s). Last edit at 12/28/2015 07:32AM by dc42.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Pause between moves and extraction
December 28, 2015 06:09AM
I should have mentioned that I am using Cura. I've tried all the others but Cura gives me the best results. I wish I knew how to make a script to accomplish what you guys are talking about.
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