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Sometimes fewer walls are better
(media.kbin.run)
I think I found a counterexample to the common wisdom that more walls always create a stronger part.
The pictured S shape is 1.5mm thick, so printing with 2 walls leaves no room for infill. My testing wasn't very rigorous, but it seems that the hybrid structure of walls + rectilinear infill is 10-20% more rigid than walls alone. The infill adds strength by cris-crossing between adjacent layers.
I think it's fine to include a concentric top/bottom layer, but multiple identical layers weaken the part. I also tried 0 walls (infill only) and that was garbage.
I haven't done any tests where the tensile/breaking strength is relevant, just rigidity. Maybe it's possible to optimize the infill based on finite element analysis or something, but that's not a rabbit hole I'm looking to go down.
I have tested that an all-walls sandwich (PrusaSlicer "solid infill every 3 layers") does not improve rigidity. So far nothing beats 1-wall + rectilinear in that department.