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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.
A shape like an S is going to be highly anisotropic with regards to flexing and rigidity. Yes, more walls won't really help if they are not in the direction to resist the force in question.
Like someone else mentioned, an I beam is basically as strong as a beam with the interior filled in, because that's where the stress goes.