Underextrusion at the beginning of infill

Hi everyone,

I've been playing with my first 3D printer (i3 derivate) for more than a month now and while I'm generally quite happy with the print quality, there are still some issues I would like to address before calling the printer "properly calibrated".

The main issue I'm unable to resolve is insufficient extrusion at the beginning of infill (most notably at the top surfaces):


This picture is only for illustration: the extrusion coefficients were set to 0.75 and 0.8 to make the problem easily observable. As you can see, the infill line is thin at the beginning (in one corner) and the thickness gradually increases. If I set the coefficient to what I think is close to the correct setting, the result is more like "underfilled at the beginning, slightly overfilled at the end".

What I think is happening is that when the extruder goes from the external perimeter (slow and thin → low extrusion speed) to the infill (quick and thick → high extrusion speed), the pressure changes too quickly and the "springy plastic" does not have the time to react.

I tried to play around with various settings in Slic3r (pressure advance, extra length on restart, automatic speed, ..) and with acceleration in the firmware and nothing seems to be able to fix it without causing other issues in different places. Reducing the infill speed to ~10 to 15 mm/s helped, but not nearly as much as I would expect..

Is this a common issue, or am I the only one with this kind of problem? I couldn't find this specific problem mentioned in any of the various print quality debugging sites I know of..

Thanks,
Martin

(Some additional facts: I'm printing PLA at 195 °C and 40 mm/s (and about 20 mm/s for external perimeters), using a direct-drive extruder. Nozzle is 0.4 mm, internal perimeters and top infill width 0.68 mm, external perimeters 0.4 mm (setting to 0.68 mm does not help either). Layer height 0.1 mm. There is no retraction before the infill starts.)

Are these foto's from a top layer or the first bottom layer? Is the under extrusion always at the same position for each object (right top corner)? or does it depends on the direction of the extrusion. Reason for asking is that it almost looks like the bed is lower at one end and therefor there is more distance between bed and hotend on that side of the bed

PS Or one side of the z-axis is higher\lower then the other

Edited 1 time(s). Last edit at 03/01/2016 10:24AM by Frans@France.

Set all your extrusion widths to 0.4mm, same size as your nozzle,(should even out infill and perimeter) and re-calibrate your extruder steps/mm. How did you come up with 0.68 for a 0.4mm nozzle?

Ideal setup would have an extrusion multiplier of 1, being at .75 and .8 shows something being off in your configuration.

Edited 2 time(s). Last edit at 03/01/2016 11:10AM by Dirty Steve.

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Frans@France
Are these foto's from a top layer or the first bottom layer?

It is the top layer.

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Frans@France
Is the under extrusion always at the same position for each object (right top corner)? or does it depends on the direction of the extrusion. Reason for asking is that it almost looks like the bed is lower at one end and therefor there is more distance between bed and hotend on that side of the bed

It depends only on the direction of extrusion. The bed is quite level (I've been printing across the entire bed without problems) and even if it wasn't, the print is very small (15x15 mm for the smaller one) so it wouldn't be even observable.

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Dirty Steve
Set all your extrusion widths to 0.4mm, same size as your nozzle,(should even out infill and perimeter) and re-calibrate your extruder steps/mm. How did you come up with 0.68 for a 0.4mm nozzle?

Thanks, I'll try that. 0.68 is the default Slic3r uses when you don't touch the settings and use automatic width. From [forums.reprap.org] I understand that it may be actually desirable to use larger width in some situations, since the filament extruded into free air is actually thicker than the nozzle itself. If you want to print the same width as the nozzle, you must stretch it a little bit, but then there is more tension in the just-laid filament, it contracts and curls up. For infill that is not an issue, but I assume Slic3r uses the thicker line to make sure the "squished plastic" more easily fills all holes. And also to speed-up printing: the default for internal infill is even larger, 1.28 mm. So I think it is not necessarily a bad thing to use widths that do not match the nozzle..

That being said, reducing it to 0.4 mm (the same as the external perimeter) could be useful, since I can match the speeds easily in this case. And constant speed should mean that the pressure won't change during print. (More or less, there is still a small pause during travel moves.)

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Dirty Steve
Ideal setup would have an extrusion multiplier of 1, being at .75 and .8 shows something being off in your configuration.

As noted in the first post, this is set only to better show the issue. Normally I would use about 0.94. Which is certainly a bit on the low side, but it's exactly where I noticed the problem: I followed Triffid Hunter's calibration guide and I couldn't finish the part where you observe the top solid infill and note when the gaps disappear -- by the time they disappear, the other corner is already overfilled.

Slic3r's auto-calculated extrusion width has never been correct for my printer. I use nozzle size for extrusion width in all the slicers I print with. Slic3r, KISSlicer, Simplify3d, and SkeinForge at times.

You can't get an extrusion width of 1.28mm from a 0.4 mm nozzle.

Edited 1 time(s). Last edit at 03/01/2016 01:48PM by Dirty Steve.

All right, setting all widths to 0.4 mm helped quite a lot; even without calibration and leaving the extruder coefficient at 0.8, the surface is nearly perfect:


(Left is with default (automatic) Slic3r widths, right is all 0.4 mm.)

There are still some signs of asymmetry, but even not considering the problem I'm trying to solve, the surface just looks much nicer. So thanks a lot for the idea -- line widths were one of the parameters I was afraid to touch, thinking that it would be best to leave it at the Slic3r defaults (since you have to enable Expert mode AND even then it is hidden away in the Advanced settings, giving me the impression that it's not something to be concerned about).

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Dirty Steve
...
You can't get an extrusion width of 1.28mm from a 0.4 mm nozzle.

Good point. It is for the internal infill, which it is not that critical, but it is certainly beyond the reasonable limit of my nozzle: I just measured diameter of the flat area around the nozzle opening and it is around 0.8 to 0.9 mm. So if the width is larger than that, there is nothing that would hold the plastic down, i.e. there is no "squishing action" taking place. And without the squish, there is no way the plastic will just expand to the desired width on its own. True. Good to keep in mind..

I'm amazed, how after a month and two weeks I'm still finding new parameters that I thought I shouldn't touch -- and they turn out to be very important for the resulting quality.

If that photo is of the bottom layer, then your bed is NOT level. Extruding more plastic is a sudo fix but the correct fix is to properly level your bed.

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tmorris9
If that photo is of the bottom layer, then your bed is NOT level. Extruding more plastic is a sudo fix but the correct fix is to properly level your bed.

It is the TOP layer. I noted it both in the first post, and in my first reply.
I also explained that it depends on the starting point of extrusion, it would appear in a different corner if Slic3r decided to start there. Bed leveling is absolutely not an issue.

There does seem to be the phenomenon of die swell, in that the free air extrusion width is not the width the filament ends up when extruded onto a part during printing ie there is less swell when the filament of contstrained after it is extruded.

I originally used the free air width as my nozzle diameter now I measure a new nozzle using micro drills (which have been check with calipers first) and use that value as the nozzle width.

Edited 1 time(s). Last edit at 03/05/2016 02:03PM by DjDemonD.

The solid infill / solid top infill / top infill extrusion widths may be too low. If the rest of this part looks fine, these may need to be tweaked.

---

Master Tinkerer

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Montiey
The solid infill / solid top infill / top infill extrusion widths may be too low. If the rest of this part looks fine, these may need to be tweaked.

I tried three different widths for comparison and it seems that 0.5 mm gives me by far the best infill results, so I think I can consider my original issue to be, more or less, solved.

Extrusion widths in sequence from the left: 0.5 mm before calibration, 0.5 mm after calibration, 0.65 mm, 0.8 mm.


Although the result is acceptable for my purposes, it is clear that the problem is still there; setting all extrusion widths to the same number does not solve anything, the extrusion is still uneven when using 0.65 or 0.8 mm line widths. It's just that 0.5 mm is small enough so that the under-extrusion at the starting point is not that obvious.

It also seems that increasing the extrusion width somehow leads to under-extrusion. (Which is.. weird? Since I don't hear the extruder motor skipping steps or anything -- where does the missing plastic go?) So, having different extrusion widths for perimeters, infill, supports etc. looks generally like a bad idea (hooray for the well hidden Slic3r defaults) -- each of them would need their own extrusion multiplier to achieve perfect calibration. Or so it seems to be at least in my case.

Anyway, thank you all for your ideas. I'm now moving from PLA to ABS for a few weeks, so I have a different set of fun problems to play with.

Edited 1 time(s). Last edit at 03/12/2016 01:02PM by H.

That gradual ramp up in the extrusion was a good observation. I wish I understood why it is happening. Ooze or a step in nozzle pressure would seem like possibilities, but I would expect the effect to be much shorter duration. It's almost like you have a much higher degree of springiness in your extruder than is normal.