Under extrusion when starting outer wall

  • See the pictures below. I have a severe under extrusion when the printer starts the outer wall, which is resolved by the time it finishes the outer wall. It starts the layer in the same place every time, so it results in this vertical line, on one side of which is fine (where it finishes the layer) and the other side has bad gaps and the wall is much thinner.


    In this picture, the problem is on the outer wall (see red outer line, the print head is moving counter clockwise.


    G-Code visualization


    And here is the print showing the issue. Just fine on one side, but terrible on the other, precisely where it starts the layer. Strangely, this only occurs on the layers with infill. The top layers seem fine (despite starting in the same place). I have disabled retraction with no effect.


    Here you can see it start at the tip and get gradually better as it progresses.
    enter image description here


    Here you can see that by the time it finishes, it's just fine, and also what a contrast is between the start and end. That should be a flush edge, there.
    enter image description here


    Another view
    enter image description here


    Is there some setting that I should be tweaking? I've exhausted my own ideas of different tweaks to no avail.


    The printer is a Monoprice Maker Select V2. I'm using Ultimaker Cura 4.1.0.





    • Material: PLA

    • Layer height: 0.24 mm (in the pictures, but replicated with 0.16 mm also)

    • Temp: 205 °C, here (but tweaking this hasn't had any effect)

    • Retraction: Disabled


    This doesn't happen on layer changes. Although it does line up with the Z seam, you can see from the G-code visualization below that the outer wall is the very last thing it does. The issue occurs at the start of the outer wall (still on the same layer) that it has the issue, but by the time it ends the wall (just before switching layers) it is fine.


    I've tweaked a few other settings, one by one, and seeing if any have any effect. So far, not really:



    • Outer Wall Wipe: 0.2 (default), 0.8, 0.0

    • Jerk Control - Wall Jerk max velocity change, 5 mm/s

    • Outer Wall before inner (Yes instead of No)

    • Wall Line Count (3 instead of 2): This improves it some, but I suspect just by making it a little more difficult to see


    What have I done since...


    I reset all settings in Cura to a default "Draft" setting and then set layer height to 0.24 mm, and turned off Infill. Then I have tried prints with different settings for "Retract Before Outer Wall" and printing temperature.


    Here are those results:


    These pics seem to suggest a very clear lag in extrusion. 1 and 2 are different temps. 2 and 3 are different retraction.


    205 °C, 0 % Infill, Retract Before Outer Wall: Off
    205 Celsius, 0 Infill, Retract Before Outer Wall: Off


    195 °C, 0 % Infill, Retract Before Outer Wall: Off
    195 Celsius, 0 Infill, Retract Before Outer Wall: Off


    195 °C, 0 % Infill, Retract Before Outer Wall: On
    Note: the retraction setting resulted in a noticeable pause before printing the outer wall. Retraction distance is 6.5 mm, and this is not a Bowden fed device.
    195 Celsius, 0 Infill, Retract Before Outer Wall: On


    There doesn't seem to be anything strange about the G-code, either. Here are the G0 travels just before the outer wall followed by the wall.


    ...
    G0 F7200 X106.319 Y93.413
    G0 X106.26 Y93.909
    G0 X107.213 Y93.658
    G0 X107.8 Y92.542
    G0 X107.286 Y90.844
    G0 X107.509 Y90.394

    ; (outer wall of outside)
    G1 F1328 X107.985 Y90.707 E116.98713
    G1 X108.38 Y91.128 E117.01098
    G1 X108.658 Y91.623 E117.03444
    G1 X108.813 Y92.18 E117.05833
    G1 X108.832 Y92.751 E117.08193
    G1 X108.713 Y93.315 E117.10575
    G1 X108.463 Y93.837 E117.12966
    ; (first curve complete, on to straightaway)
    G1 X99.631 Y107.716 E117.80936
    G1 X98.912 Y108.59 E117.85612
    ...

    Even more done...


    These are retraction off, and 205 °C


    If I set the "print speed" in Cura to 20 mm/s (normally 60 mm/s), the outer wall speed is reduced from 30 mm/s to 10 mm/s. The result is quite good.
    enter image description here


    If I leave the "print speed" at 60 mm/s and adjust only the outer wall to 10 mm/s, it's still quite good.
    enter image description here


    So it seems like an acceleration thing. If I can figure out how to get it to slow down in just the right spots or compensate in some way, then perhaps I can make this problem go away with minimal sacrifice in total speed.


    Welcome to 3dPrinting.SE!

    @Warren Are you using a Heated Build Plate (HBP)?

  • Based on other comments, answers, and question edits so far, in addition to your original question, I believe there are possibly two things going on here: incorrect retraction settings, including a misunderstanding of which settings are relevant and what they do, and issues related to slow acceleration. Both relate to misdepositing/loss of material.



    First, some basics. When the filament is advanced to the point needed to extrude material and print at the intended volumetric rate, it's under significant pressure, compressed between the extruder gear and the nozzle. My understanding is that your printer has a direct drive extruder, not a bowden, so there's far less compression than with a bowden setup but it's still there. This means that, if you try to stop extruding, it's material will continue to come out of the nozzle, just at a decreasing rate, until the pressure dissipates. This effect is reduced but still present if the nozzle is held-against/moving-over already printed material, and heavy if moving over empty space, even moreso if moving across sparsely-filled space like infill where it will bond with the already-deposited material and get "stretched"/"pulled" out.



    The idea of retraction is to pull the filament back when the print head is moving to a new location without trying to deposit material, to relieve this pressure and prevent unwanted misdepositing/loss of material, and to reverse the process, putting the filament back exactly where it was when the last printed line ended, the next time it starts trying to deposit material.



    The relevant options in Cura are:




    • Enable Retraction - must be on

    • Retraction Distance - should be at least 5-6 mm for bowden setups, probably more like 0.5-2 mm for direct drive.

    • Retraction Minimum Travel - should be 0

    • Combing Mode - try different settings. Off is probably the best relative to your issues, but hurts your print time a lot for certain models, and can hurt quality in other ways.



    Everything else related to retraction is fairly irrelevant, especially "Retract at Layer Change" is a niche option and not typically useful. As I understand it, turning just "Retract at Layer Change" on does not mean retraction is on.



    Now, your other issue may be acceleration. Extrusion works best as acceleration speed approaches infinity, because the extrusion rate and pressure needed to extrude will be fairly constant for the entire line/curve. If acceleration is very slow, pressure will be wrong during the start and end of lines. It's likely that, due to high pressure, excess material will get deposited at the end of one line while slowing down, then after moving to start the next line, even if you retract the filament, you'll have insufficient pressure at the nozzle after reversing the retraction to start the next line.



    A jerk limit of 5 mm/s is really low. I'm used to more like 20-30 mm/s. You don't say what your acceleration limit is, but it's probably also low. Slow acceleration has minimal impact if your max speed is slow, because you quickly reach the max speed and most of the print speed (and thus extrusion rate) is steady. But if you want to print at high speeds, you need high acceleration. Try and see if you can increase it. Or accept printing at slow speeds.



    Another option, if you're open to hacking on your printer, is replacing the stock firmware with a recent version of Marlin with the Linear Advance feature. It does the math to model the filament pressure as a spring, with a spring constant you can tweak, so that it can compensate for varying print speed and end lines with approximately no pressure remaining.


    I have managed to reproduce this behavior under much more controlled conditions and was able to definitely see that the issue is plastic leaking out of the nozzle during travels. The longer the travel, the longer the under-extrusion persists once printing resumes. This was also correct: "including a misunderstanding of which settings are relevant and what they do". In particular, retraction in Cura only occurs when traveling outside the model. If you want retraction at other times, there are other settings for this. I still have not found the best settings, but I'll work on it further.

    @Warren: "Combing" is what prevents retraction from happening inside the model. I have an open issue with CuraEngine which explains a lot about what might be going on for you: https://github.com/Ultimaker/CuraEngine/issues/1084

    In the absence of a fix for that issue, if you're getting oozing inside the infill area and it's causing problems, you probably just need to turn combing off entirely. If you do that, it might help to increase min retraction distance from 0 up to roughly your nozzle size or slightly larger (maybe 10-25% more); otherwise you can get retractions at each line direction reversal on top/bottom "skin" fill and the print will be *very* slow.

License under CC-BY-SA with attribution


Content dated before 7/24/2021 11:53 AM

Tags used