Walls and Infill: Strength Comes From the Outside, Not the Middle

A wall 1.0 mm thick printed with a 0.42 mm line cannot be made of whole lines. The slicer either leaves a gap in the middle or over-extrudes to fill it, and both are weaker than a wall that is an exact number of lines. So design walls at multiples of the line width: 0.84 mm for two lines, 1.26 for three, 1.68 for four. It is not fussiness — it is the difference between a wall that is solid and one with a void running through it. Modern slicers handle odd thicknesses much better than they used to, varying the line width to suit. It is still worth designing to the grid where you can, and it is worth knowing this is why a 1 mm wall sometimes prints badly.

The top surface is printed over infill, which is mostly air. Too few solid layers and the top sags into the gaps — the 'pillowing' that makes a top face look dimpled. Four to five solid top layers is a normal starting point, and more infill helps by giving them less distance to bridge. That is one of the few cases where raising infill is the right answer, and it is about surface quality rather than strength. The bottom needs solid layers too, and for a different reason: they are what the part stands on and what a screw head bears against. A part with one bottom layer tears through if anything is bolted to it.

Gyroid, cubic, grid, triangular — the differences between patterns at the same percentage are small compared with the difference between two walls and four. Choosing a pattern is a late optimisation. Where it does matter: GYROID is isotropic and prints without the nozzle crossing its own path, which makes it quiet and good for flexible parts. GRID is fast but the nozzle crosses itself every layer, which can knock the part. TRIANGULAR is strong in plane and used where the load is known. Pick one, use it consistently, and change the wall count instead. That is the change-one-thing-at-a-time rule applied to a slicer: a hundred settings are available and about four of them matter.