Ultimaker Cura 3.6 choosing to fill in narrow walls with diagonal striping 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&nbsp;mm nozzle = 1.2&nbsp;mm brick walls, vs the standard 1.4&nbsp;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&nbsp;mm/3 lines. And yet, in the layer view, I see one wall line, with diagonal striping between those walls:



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&nbsp;mm thick.

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



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?
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.