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:
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:
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?