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I'm playing with printing some Lego-like bricks. I'm using brick models where the wall thicknesses are aligned to match an exact multiple of my printer nozzle (.4 mm nozzle = 1.2 mm brick walls, vs the standard 1.4 mm), with other adjustments to the interior ridges of the brick so they should still fit with real Legos.

I'm seeing an odd effect when slicing in Ultimaker Cura 3.6. My settings for Wall Thickness/Wall Line Count are 1.2 mm/3 lines. And yet, in the layer view, I see one wall line, with diagonal striping between those walls:

Legos Layer View

Part of it could be the interior ridges creating some thicker walls in places, but if you look at the piece on the left, there are no ridges (it will be a plate, rather than a brick, and Lego plates don't use ridges). Yet it still has the striping. I re-measured the STL in 3 different modelling programs (Meshmixer, 3D Builder, Tinkercad) and all show the walls as exactly 1.2 mm thick.

I then noticed I can remove this by putting something small (even .001 mm is enough) in the "Horizontal Expansion" setting:

Lego Layers view without striping

This is more what I wanted, and it cuts the print time almost in half. I can also fix this by setting enough bottom or top layers to handle the whole piece with a wall thickness of 1 line...

... but why was it necessary? What is going on here?

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  • $\begingroup$ Have you tried increasing the number of shells? Why do you want a wall thickness of one line? $\endgroup$ – Mick Mar 6 '19 at 1:27
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    $\begingroup$ a good note: print quality improves a little if you use a little wider extrusion width than the nozzle. $\endgroup$ – Trish Mar 6 '19 at 1:44
  • $\begingroup$ @Mick I want a wall thickness of three lines, like the second image. $\endgroup$ – Joel Coehoorn Mar 6 '19 at 4:45
  • $\begingroup$ Hmm! It's not a problem with Simplify3D, which is what I mainly use. With Cura, the only way that I could do it is by changing the wall line width to 0.3mm. Could you not create a solid brick, and then print with no infill and no bottom layers? Personally, I would would adjust the wall line width to get the required number. $\endgroup$ – Mick Mar 6 '19 at 12:28
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    $\begingroup$ OK. Try setting the infill percentage to 100%. That should do the trick. $\endgroup$ – Mick Mar 6 '19 at 12:54
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All slicers have their own peculiarities, and one of Cura's peccadilloes seems to be that It likes to generate an even number of shells for narrow structures, even if an odd number of shells would work nicely. Slic3r and Simplify3D have no problem with this, and will automatically generate an odd number of shells if they will fit the model.

Cura, on the other hand, will reduce the number of shells and generate infill. If you have set the infill percentage to less than 100%, it will try its best to fulfil that requirement, and in your case, generate zigzag infill. However, if you set the infill to 100%, it will, in effect, create an additional shell, although it is really infill that follows the direction of the walls as far as Cura is concerned, since there is no space to generate the specified infill pattern pattern.

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  • $\begingroup$ Setting infill to 100% did not change anything. $\endgroup$ – Joel Coehoorn Mar 6 '19 at 14:35
  • $\begingroup$ @JoelCoehoorn That's a pity. It did on the test model that I created (a square pipe with 1.2mm walls). Even reducing the infill to 99% caused it to switch to zigzag infill. Are your sure that your walls are exactly 1.2mm wide? $\endgroup$ – Mick Mar 6 '19 at 16:11
  • $\begingroup$ @JoelCoehoorn Can you proved a link to the STL file so that I can play around with it? This is an interesting little problem. $\endgroup$ – Mick Mar 6 '19 at 17:25
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    $\begingroup$ You can get the stl files here: thingiverse.com/thing:3424550 I used both the 2x2 brick and the 2x2 plate, with the raw stl files included there (no need to use the thingiverse customizer app yet). Remember, I'm looking to match the second image, not the first. $\endgroup$ – Joel Coehoorn Mar 6 '19 at 17:39
  • $\begingroup$ What if you set the infil to a square pattern, essentially limiting it to one pass along each side of the brick? $\endgroup$ – Carl Witthoft Mar 6 '19 at 18:53

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