Cannot Print Smooth Single Extrusion Curves (S3Dv5)

[NativeOkie]

I'm printing with flexible filament and need the walls of my part to be as thin as possible (essentially the width of the nozzle - 0.4mm in my case).

Single extrusion walls have caused me grief for way too long. I either get less than acceptable surfaces when printing, or completely absent sections here and there after slicing. Most of the "missing sections" go away if I merge my model in Fusion 360, so that's a step in the right direction.

If I select "Allow Single Extrusions" in the Advanced Settings under External and Internal Thin Wall Type, the preview of the sliced part results in a nice smooth curve in dark blue - indicating a single extrusion. However, when printing, the wall is a series of very small and jerky segments that leave a very rough surface.
If I leave that setting on "Perimeters Only" the curve prints smoothly and leaves a perfect outer surface. However, the walls are actually two passes of the nozzle and my print has stiffer/thicker walls than I'd like.

How do I smooth out the curve when printing Single Extrusions?
In the sliced preview the differences are easy to see.
- The Allow Single Extrusions curve - when viewed line by line - is a series of very short extrusions with stops in between.
- With the setting on Perimeters Only the curve is still a series of short lines, but they are slightly longer and the stops in between are gone.

I have fiddled with the thickness of walls when modeling. I have experimented with the Extrusion width in Automatic and Manual mode. Nothing seems to fix the problem, so any help would be greatly appreciated.

Thank you.

[S3D-Jason]

One easy suggestion might be to use perimeters only, set the number of perimeters to 1, and then go to Edit Process Settings > Advanced tab, and set the Slicing Region Repair Mode to "Merge all boundaries". That may fix the problem you mentioned where it prints 2 passes on the thin walls instead of just 1 assuming you have a single bodied part.

If you need more advice, it might help to attach a factory file for the print and tell us the exact layer and line number where you are seeing the issue.

[NativeOkie]

I appreciate the quick response, Jason.

I should have been more descriptive about my part. It needs to be hollow. I have four processes as the print builds - each section needing different settings. I do have perimeters set to 1.

I have used the Merge All Boundaries function from time to time. It's sometimes handy to remove some of what I call "ghosting" - where features of a print above a certain level seem to have shadows below on a level that should be made of smooth, straight lines, but instead have hints of what's to come above.

In this case, when I turn on Merge All Boundaries for the portion of the print that I need hollow, it unfortunately just creates solid layers for the whole section that uses that particular process.

I cannot share the actual part that I am printing, but I can model a cube that might illustrate the problem I'm having. I'll get back to you. Thanks.

[NativeOkie]

Thanks for taking the time to help. Here's a factory file that illustrates my issue. The side walls are 0.4mm thick and the top and bottom are 0.6mm thick.

It's made up of five processes to build a rounded box.

Section 1 - Basic Solid fill with no issues.

Section 2 - Perimeters Only selected under Advanced > External Thin Wall Type.
Results in the print head traveling out and back so to speak, extruding both ways. If you preview the slice by line and scroll through the movement, you'll notice nice, steady movement around the object

Section 3 - Allow Single Extrusions selected under Advanced > External Thin Wall Type
Results in layers where the print head only goes around once, but the movement is jerky and slow. Again, view the slice preview by line and you'll notice the extrude-then-pause behavior.

Section 4 - Extruder > Extrusion Width is set to 0.4mm
In this section the wall completely disappears even though the wall of the box is exactly 0.4mm. On the other thin wall layers I dropped the extrusion width to 0.38mm so they'd print.

Section 5 - Solid fill with no issues.

[S3D-Jason]

For section 2, you could easily use the Merge All Boundaries trick, but set the number of top and bottom solid layers to zero. That way it avoids looping around the outside + inside, but you don't get the solid fill you don't want.

For section 3, the jerky movements you see in the preview are just places where it is adjusting the extrusion width slightly to perfectly fill that corner. When the machine actually prints those movements, it shouldn't actually pause at those positions since the flow rate and the general direction of travel aren't changing. The machine firmware should have look ahead which will smooth all that out.

For section 4, even though the wall might be 0.4mm in your CAD package when you created it, exporting to STL typically adds some faceting error that could easily cause the wall to become 0.38mm or something similar. So it's not surprising you may need to adjust the extrusion width slightly.

Hope that helps!

[NativeOkie]

Thank you, S3D-Jason, for the help. It did, in fact, fix the problem.

I've attached a picture of the resulting print. I printed the model without the top (my Section 5 was not included in the slice) so I didn't have any supports to deal with.
You can see the three wide sections clearly in the picture. The bottom section is so short you don't really notice it, but the three bands of Sections 2, 3 and 4 are visible.

The bottom section is nearly perfect (Section 2 in my previous description). I changed the setting Advanced > Slicing Region Repair Mode to Merge All Outlines like you suggested. For this section the nozzle traveled around once and then moved up a layer. The movement was smooth and the resulting wall is very thin like I was hoping for. You can see ridges where the nozzle changes direction to create a curve, but the resulting print is so thin you really cannot feel them.

The center section (previously described as Section 3) still suffers from the jerky movement caused by the Allow Single Extrusions setting. You can more prominently see the ridges created by the stop-and-start pattern in the slice. My printer does not smooth anything out like you suggested. Maybe I don't have something set right? My printer is a Prusa MK4.

The top section (Section 4) prints smoothly like the bottom one. The main difference here being that the print head travels around and back, extruding twice. You can feel that these layers are thicker than Section 2.

[NativeOkie]

Ultimately I think I would like for the Allow Single Extrusions to work. The Merge All Boundaries function works great in this test piece, but in my actual project there are some features that just disappear because of the "Merge".

You mentioned my printer maybe having some sort of smoothing effect done by the printer itself. Is that a common setting? Maybe mine isn't turned on or isn't set strong enough.

I'm not familiar with it. What might a setting like that be called?

Thanks.

[S3D-Jason]

The lookahead feature in the firmware is typically always turned on by default. It's what allows the printer to look past the single command it is processing right now to see the next 10-20 upcoming commands that will come next. This way it can predict things like sudden upcoming direction changes to know when it needs to start slowing down.

Looking at the gcode generated from the files you attached, I didn't see any sudden direction or flow rate changes, so that would make me think the firmware would be able to smooth that out just fine. It typically only jerks around when there are sudden direction changes.

Maybe you can try running similar gcode on a different machine entirely to see if it's a specific problem with the firmware.