Gaps between perimeter walls; potential under extrusion?

Question

The cube is a 2 cm x 2 cm with infill at 30 % and layer height 0.2 mm, more details can be seen below.

I'm printing with PETG using an Ender 3 printer.

There seems to be a gap between the perimeter walls, I have already referred to other forums and specifically: " How to fix wall separation in 3D prints (gaps in between wall perimeters)? ", but I still can't find a solution for it.

Most would suggest to tighten up the pulleys, I've tried that, but that didn't work. Others also suggested tweaking to a higher temperature, again I've tried from a range of 230-250 °C, but this also failed.

Infill and initial/top layers seems to be strong and all lines are bonded except for the perimeter walls.

More details regarding the problem:

Here are my print settings:

As an aside, travel speed of 150 will ruin PETG prints if there's any travel over already printed material, including combing. You should set it below 60. — R.. GitHub STOP HELPING ICE, Aug 03 '20 at 12:51
Welcome to 3DPrinting.SE! Hope we can fix it here for you! Regarding: `Infill and initial/top layers seems to be strong and all lines are bonded except for the perimeter walls`. Sorry, but no way does that look normal in your photos! I don't think a slight over-extrusion will help you here, judging the top infill of the cube you are severely under extruding. — 0scar, Aug 03 '20 at 13:47
@R..GitHubSTOPHELPINGICE I always read that high travel speed is useful with PETG to reduce stringing, together with high retraction speed. Why would it ruin prints? — FarO, Sep 04 '20 at 07:31
@FarO: Explanation here: https://3dprinting.stackexchange.com/a/13808/11157. In general, though (with other materials; for PETG it's just counterproductive), high travel speed is at best a workaround for stringing you haven't solved by fixing the root cause. If you don't have errors leaving pressure at the nozzle, it should not be hard to move across the entire bed of typical-sized printers at 30-40 mm/s without any oozing. — R.. GitHub STOP HELPING ICE, Sep 04 '20 at 13:15

score 1 · Answer 1 · edited Aug 03 '20 at 17:13

PETG filament is not entirely rigid and compresses slightly in the Ender 3's extruder gear and Bowden extruder setup. Tightening it will only make this effect greater. Being compressed at the point where mm of extruder advance is applied means less than the desired advance of at-nominal-diameter filament will take place. I find I need a flow of 104% to compensate for this.

When adjusting flow in Cura, make sure you get the main flow setting not the first-layer one (which is an additional factor on top of the main one and can be left at 100%), and that all the derived flow settings for each type of extrustion (walls, top/bottom, infill, etc.) all come out matching the value you set. When I first tried fixing this with flow, they didn't propagate right and I ended up testing changes that weren't actually doing anything.

With that said, you may have something else going on too. The underextrusion looks pretty severe, including in the top layers which you said looked okay. You should not see deep grooves between the lines like that. I suspect they're only bonded to the layer below, not to their neighbors. This could be a result of tightening the extruder pulley, or some other problem.

Very interesting the compressing/biting of the extruder gear! Knowing I'm printing a lot of 2.85 mm PETG, my gears are also very tight biting the filament, but, I've never had to adjust flow. I guess that's a benefit of using much thicker filament? — 0scar, Aug 03 '20 at 13:58
@0scar: I would guess so. FWIW initially I didn't even notice the underextrusion with PETG but it was a big issue for TPU where the factor was 13%. I then thought to check it for PETG to see if it might be related to my layer adhesion issues, and indeed I found I needed an extra 4% for PETG to extrude the nominal amount of material. This made sense in terms of their relative compressability. — R.. GitHub STOP HELPING ICE, Aug 03 '20 at 14:23

score 0 · Answer 2 · answered Aug 03 '20 at 13:42

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I agree with @R.. GitHub STOP HELPING ICE, looks like under extrusion you could try a flow rate test print like this link to try and dial in the value.

But probably worth just trying a flow of 105% and see if it solves the problem. Another thing worth verifying is that your printer is feeding accurately. You can mark out 100mm from the feed hole, then manually command the printer to extrude 100mm, then measure the distance from the 100mm mark. This will tell you if you have a feed problem.

answered Aug 03 '20 at 13:42

Sneaky Puffin

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If the problem is as I described, OP should see a feed problem reflecting the 4-5% difference. Don't try to compensate for it with Esteps mess, though; this leads to all sorts of problems. Just adjust flow or compute a corresponding adjustment to filament diameter - conceptually, the issue really **is** filament diameter, i.e. that the filament is slightly narrower than its nominal diameter where it's squeezed between the extruder gear and pulley. – R.. GitHub STOP HELPING ICE Aug 03 '20 at 13:45
@R..GitHubSTOPHELPINGICE Four or five percent aren't going to help the OP! – 0scar Aug 03 '20 at 13:49
@0scar: I wonder if OP tightened the pulley so much that it's compressed a lot more. OP's pics look like what I had with TPU and no flow adjustment, where the magic number ended up being 13% (because TPU compresses a lot more in the gear). – R.. GitHub STOP HELPING ICE Aug 03 '20 at 13:51
Otherwise I can't think what the issue could be, if OP's not hearing clicking/grinding. My normal go-to cause would be oozing material where it doesn't belong, but that's not going to be happening with 7 top layers - there's just nowhere wrong for it to go. – R.. GitHub STOP HELPING ICE Aug 03 '20 at 14:08
Yes, I have already tried tuning up the flow rate up to 110%, the problem still persists, I have yet to try above that. I have also done some research and many people said that heating the PETG too high could possibly shrink it? Is this one of the factors that could possibly made the gap between the walls? As I have yet to try on decreasing the temperature below 230, below of which it is rated at. – Brilliant Purnawan Aug 04 '20 at 03:06

Gaps between perimeter walls; potential under extrusion?

2 Answers2