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No retraction on (multi-extruder) nozzle switch is causing BLOBS


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Posted (edited) · No retraction on (multi-extruder) nozzle switch is causing BLOBS

Hi all. 


Not sure if this is a bug or simple mis-config somewhere.   New Geeetech A20T (triple extruder, single nozzle)  Cura 5.2.1. 


Tried a simple test print with 3 disk shapes (each in a different color PLA from the 3 extruders) and a prime tower.


Getting large BLOBS on print just at the extruder switch (finished printing one color on layer X, switching to next color.)

Just before moving over to the prime tower, it drops a big blob of the current color. (see image below)


I have "Nozzle Switch Retraction Distance" set to 16mm (as part of the A20T profile,)

yet there is no "G1 E-16" found in the gcode anywhere near the Tx command that switches extruders.


When i mock up a similar print on Repetier Host, it does generate the "G1 E-16" and I don't get the blobs,

so it appears Cura is not applying the 16mm retraction I think I've configured at tool/extruder change.   


I suppose I could try doing it manually in the extruder end & start gcode sections,

but what's the "Nozzle Switch Retraction Distance" for, if not this?


TIA for any tips or explanation...  Paul.


attaching image showing the blobs dropped before moving to prime tower, the .3mf file and the generated gcode.




GA_A20T_test_disks.gcode GA_A20T_test_disks.3mf

Edited by pozzello
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    Posted (edited) · No retraction on (multi-extruder) nozzle switch is causing BLOBS

    Within the project file you have the Extruder #1 "Nozzle Switch Extra Prime Amount" set to 40mm³.  That's 16.63mm of filament on top of the 16mm regular prime amount.  That will leave a mark.

    Your "Dual Extrusion" settings for Extruders 2 and 3 look reasonable and neither have any extra prime (much less 500mm worth of extrusion in a blob).  You need to set Extruder #1 up the same as 2 and 3.

    There is no G1 F2400 E-16 because you set Cura up for Absolute Extrusion rather than Relative extrusion.

    This is what is happening every time Extruder #1 gets over the print and starts its first extrusion:

    G1 F1800 X120.359 Y116.04 E203.41115 --- Last extrusion of the previous tool and the filament is right at the nozzle

    G1 F1200 E187.41115 --- Nozzle switch retraction of 16mm for the previous tool
    G92 E0 --- Reset previous extruder to 0
    T0 --- Switch to Extruder 1
    G92 E0  --- Reset Extruder 1 to 0
    G1 F2400 E10 --- 10 mm prime (16 minus your regular retraction distance of 6.0) The filament is 6mm behind the nozzle.
    G1 F2400 E32.63007 --- Here is the problem.  For the other extruders this is E16.0 but for Extruder 1 it's adding that extra prime of 16.63mm (40mm³) that you have asked for in your settings.


    PS:  I just had another look at the project file I posted in the alternate thread and it's fine.  That "40mm³" of extra prime came from somewhere else.


    Edited by GregValiant
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    Posted (edited) · No retraction on (multi-extruder) nozzle switch is causing BLOBS

    Thanx for the insights!


    On 1/5/2023 at 6:37 PM, GregValiant said:

    Within the project file you have the Extruder #1 "Nozzle Switch Extra Prime Amount" set to 40mm³. 


    Fixed that, but was still having  tool-change blobs (and was previously blobbing on all extruder changes, not just when engaging that one). 


    Played around with the various retraction settings, and it seems the culprit was the relatively large *difference* between the 'regular' and the 'tool change' retractions. 


    With the 6mm (regular) retraction and default 16mm 'nozzle switch retraction', you showed how Cura was first doing the E-16, Tx (tool change) then E+10 (=16-6, pre-priming?) of the new color.  It's that E+10 on the new tool/color/extruder that causes the blob before moving to the prime tower, where it un-retracts 6mm before printing/priming on the tower.   I set the 'nozzle switch retraction' to 6mm (matching the 'regular' retraction distance), and the blobs disappeared.   This means I need more priming after the tool change to clear out the previous color, but increasing prime tower volume seems to help compensate, as well as printing walls from inside out to hide the still-transitioning color...


    It's a mystery to me why Cura does the new tool un-retraction/pre-priming BEFORE moving over to the prime tower, but there it is.  (probably just easier to program that way for the case where no prime tower is in use...)


    Thanx again for the pointers!  I think i can at least try a real multi-color print now with some hope of success...


    PS, I think I grok the math of how it's trying to maintain the working nozzle pressure by pre-priming the new extruder by as much as it does, but perhaps the idle extruder sitting at temp leads to a very molten first squirt of the new extruder?    ie, why else would not even equalizing the previous pressure cause a blob?    (as you pointed out E-16, E+10 should leave the filament 6mm behind the tip, but it doesn't, for some reason)    I may play around with a NEGATIVE value for "Nozzle Switch Extra Prime Amount" (instead of a giant positive one!)  to see if that helps...

    Edited by pozzello
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    Posted · No retraction on (multi-extruder) nozzle switch is causing BLOBS

    It sounds like you are making progress.  Every printer will be at least a little different.

    Ultimaker can't test these sorts of settings on every machine out there - just on their own.  General "default" settings that are aimed at Ultimaker machines might not be optimized for "all those other" printers.  In your case - the "3 in 1 out" nozzle is a relative new comer and so some experimenting will need to be done to dial it in.

    In the case of the E+10 and where exactly it's occurring in the toolpath, if you can't fix it then you should probably put up a bug report over on GitHub.  Ideally it should occur away from the model.


    There was a similar problem with Pause at Height doing the same sort of thing.  It was a question of moving a couple of lines so the prime occurred at the park position rather than over the print.  A little change to a post-processor I can do.  I'm not skilled enough at Python to attempt anything more ambitious.


    Continue to post your updates here so the next Geeetech owner can find them.  




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