The print is very solid except for the 4 walls.

From the top, I can slide a paper down to the bottom. This is ONLY between the walls, the rest of the print is solid. The filament is PLA 1.75 mm.

But the bottom is solid, no gaps.

I have checked the usual problems on Ultimaker troubleshooting photo gallery, but I can find anything similar.

Any advice to fix this would be very welcome.

3d print wall separation

Print settings:
cura settings 1 cura settings 2 cura settings 3

  • $\begingroup$ Increase the infil overlap. $\endgroup$ – Perry Webb Jun 16 '19 at 12:18
  • $\begingroup$ Where do you put the paper in between: between infill and the walls, or in between the walls? If the first is applicable, the previous comment holds, else I'll write an answer, I've fixed this for my printer. Please add some basic information on temperatures, speeds, filament type, etc. $\endgroup$ – 0scar Jun 16 '19 at 14:08

Now that print settings are shared we can see that this problem is not related to too fast printing (only 20 mm/s) or too low print temperature (210 °C should get PLA fluid enough). To explain this, a low temperature and too fast printing cause under-extruded lines.

There are 2 other causes that might be worth investigating:

  1. Under-extrusion. From the top layers one can see that there may be insufficient material printed. Calibration of the extruder helps in this respect.
  2. Inaccurate positioning. This may for instance be caused by loose belts or a mechanical defect.

To fix this, I had results with the following way:

  • Change your extrusion width from being equal to your nozzle size (0.4 mm) to slightly larger (I use 0.45 mm). That way you better combat the shrinking of the filament.
  • Having the Print thin walls setting activated to force the printer to print intermediary walls if there are areas where less than the prescribed wall thickness for a single wall fills in spaces that are as a result of the wider outer walls left. The result for a 1.2 mm wall, the central part is a 0.3 mm zigzag.
  • Lower the extrusion temperature a tad as hotter filament shrinks more on cooling! For PLA about 200 °C is my sweet spot.

Additionally, there are extra steps that could be taken: * Finally, you could play around a little with the extrusion multiplier to try to get rid of the tiny bit of under extrusion you have. * Calibration could help too.

  • $\begingroup$ Are you sure "Print Thin Walls" (fill_outline_gaps) does what you think it does? My understanding is that it has nothing to do with filling a "remainder" between walls. Cura uses low-extrusion zigzag for this in place of an additional low-extrusion wall, and this is controlled by fill_perimeter_gaps, "Fill Gaps Between Walls", which is on by default but omitted for really tiny gaps due to filter_out_tiny_gaps. AIUI, the behavior of fill_outline_gaps is to keep regions of the sliced outline narrower than the nozzle width, extruding w/ low flow. $\endgroup$ – R.. GitHub STOP HELPING ICE Jun 19 '19 at 16:02
  • $\begingroup$ @R.. I have had the issue before, flicking it on and going 0.45 mm extrusion fixed it. $\endgroup$ – Trish Jun 19 '19 at 16:05
  • $\begingroup$ Is it possible just the 0.45 mm line width fixed it? I've been trying to better understand what the gap options do and think (thought?) I did, so I'm interested in your results here. I actually had very bad results with fill_outline_gaps causing blobs and stringing (mysteriously Cura ignored my retraction settings for this layer?!) on top of a horizontally-printed threaded bolt by printing an extra top layer where just a tiny portion of the thread much smaller than 0.4 mm was present. $\endgroup$ – R.. GitHub STOP HELPING ICE Jun 19 '19 at 16:07
  • $\begingroup$ @R.. reordered, added explanation why it helped for me $\endgroup$ – Trish Jun 19 '19 at 16:15
  • $\begingroup$ Still, are you sure you didn't mean fill_perimeter_gaps, "Fill Gaps Between Walls"? That's the option that seems to be documented as doing what you want, and that seems to do it for me. $\endgroup$ – R.. GitHub STOP HELPING ICE Jun 19 '19 at 16:17

I've experienced this too, especially with flex modified PLA filament. For that, fixing underextrusion and increasing temperature made it go away. Sadly Cura has no option to overlap walls slightly (if printed in the right order, this could be done without affecting dimensional accuracy) except possibly the outer one, so you really have to get extrusion rate calibrated right or this will happen.


Look for the horizontal expansion setting in Cura. By default it should be zero. The description includes this:

Positive values can help compensate for too big holes.

The "holes" here includes these gaps. You can set it to something very small (ie: .01 or .03, probably no more than .05) and that will likely be enough to get it to fill in those gaps.

Unfortunately, I only have a little direct personal experience with this setting, hence the probably/likely weasel words, and I can't give much real guidance on exactly how big or small you can go with this.

  • $\begingroup$ These gaps are not those kind of holes. The holes in question are negative space in the sliced layer, not space between extrusions. $\endgroup$ – R.. GitHub STOP HELPING ICE Jun 18 '19 at 4:44
  • $\begingroup$ The setting "horizontal expansion" is not meant for this purpose. Horizontal expansion should be used when print have a systematic deviation from the actual model size. E.g. a 20 mm or 100 mm box print as 20.4 mm and 100.4, you can use a horizontal expansion of -(0.4/2) = -0.2 mm. $\endgroup$ – 0scar Jun 19 '19 at 6:42
  • $\begingroup$ What @0scar said. Also, I realize my above comment was somewhat terse and maybe subject to misinterpretation. "Horizontal expansion" is an operation on the cross-sectional outlines produced during the slicing process, before any extrusion paths are laid out. It may have an effect on gaps due to introducing new space for extra wall lines, skin zigzags, or infill that presses on the existing wall lines in new ways, but it's not in any way a predictable mitigation for gaps and will alter the dimensions of your print, usually in a bad way. $\endgroup$ – R.. GitHub STOP HELPING ICE Jun 19 '19 at 15:56

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