Relation between width, nozzle size, extrusion width and slicing  

Modifiers

In PrusaSlicer, you can specify different extrusion widths for different parts of your model with modifiers. See https://help.prusa3d.com/en/article/modifiers_1767

Thicker walls are easier to print, wall thickness is "interesting" in PrusaSlicer/Slic3r

Posted by: @spud

[...] I've tried to keep my seals to a 2mm width, but in some places I get a odd number like 1.2mm.  My plan was to go with a 0.5mm nozzle, as most of my walls for the box are 2mm in width. However, I do wonder how the Prusa slicer will handle the 1.2 mm with a 0.5 nozzle.

This has come up a few times. From that thread...

The Slic3r flow math page provides the details. You walls will not be an even multiple of the perimeter extrusion width thick. Walls are made up of one or more perimeter extrusions of your specified with but they overlap slightly (and non-intuitively). To make life more interesting, the amount of overlap is dependent on layer height. The slicer calculates the number of perimeter extrusions to make the wall, and any "leftover" is filled in with gap fill if the area is too small for solid infill. 

At one point, PrusaSlicer/Slic3r would show a tooltip pop-up message to this effect when you hovered over the extrusion width fields IIRC, but that seems to have disappeared.

Questions:
1. I somehow doubt that the slicer is so smart that it prints the 1.2mm with 3 lines of 80% extrusion width? At least not adaptively. That means it will probably just print 2 lines of 0.5 mm for a total of 1mm wall? Or would it round up and go for 3 lines? Are there some other rigid rules that I should know of, so I can construct my cad models to the fastest and most accurate print?

At present, PrusaSlicer and indeed most slicers will fit as many full-width perimeters as possible within the printed wall. For a stand-alone wall (one comprised entirely of perimeters with no solid infill), this may be up to the specified number of perimeters. The exact results depend on your perimeter width settings and the printed part.

• If the wall is thinner than even a single perimeter and you have enabled thin wall detection (under Print Settings->Quality) the slicer may try to produce narrower extrusions. Note that this has mixed success and often produces very weak walls.
• If the wall is thinner than even a single perimeter extrusion and you have not enabled thin wall detection, the slicer will simply not produce any extrusions for that section of the print. You'll see a lot of "why is my part not being completely sliced?" posts here.
• If the wall is thick enough for at least one full-width extrusion, the slicer will fill the desired wall thickness with perimeter extrusions.
• If the space between extrusions is smaller than the solid infill threshold area (set under Print Settings->Infill->Advanced), the slicer will insert gap fill (shown in white in preview mode). There is now an option in PrusaSlicer 2.4.0alpha to disable gap fill, or you can set the gap fill speed to 0 in print settings to disable it in older versions. 
• If the space between extrusions is larger than the solid infill threshold area, it will be filled with solid infill.

2. I have not played with the extrusion width yet, but I understand that you could increase this to get some stronger prints. But does it take into account the width of the extrusion when determining  the number of perimiters to print? Would it for instance print 4 or 5 lines with a extrusion width of 125% on a 2.5mm object with a 0.5mm nozzle?

You can tweak the extrusion width for specific circumstances, but in general, wider extrusions produce stronger parts and better overhangs. With most quality nozzles, you can print extrusion widths of up to roughly 200% of the nozzle size without too much fuss, though be prepared to slow down speeds when doing so. There are many cases where increasing perimeters actually speeds up prints since it will avoid automatic infill generation with the associated zig-zag moves and just print smooth perimeters. It can sometimes improve overhang results as well.

I guess there's no magical way of handling half widths, other than to go down to smaller nozzles to get a better hit ratio for the various lines. But if there is, please let me know.

You can twiddle around with widths in the slicer, but it gets annoying unless you really enjoy doing so. Ideally, 3D printed designs take these factors into account so slicing is not an adventure. If you are designing your parts, thicker walls usually print more easily. If these are storage boxes, it might be worth using the slic3r math page formula to determine the best thickness for a perimeter-only wall. 

You might also want to see if vase mode printing will work for your boxes. A single wide extrusion (e.g., 0.7mm with a 0.4mm nozzle) may be strong enough. You might also consider stepping up to a larger nozzle for simple boxes. I've used a 0.6mm nozzle with 1.0mm extrusion widths with very good results.

In general, keep your layer heights below 80% of your extrusion widths for good wall strength.

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The custom modifier is a good idea, sylviatrilling. I'll be sure to try it for my seals or walls, so I can optimize the fit or speed.

Bobstro, thanks a bunch for the detailed answers. It's worse than I thought then 🙂 No simple rules. You gave me a lot of ideas of how to solve this. Guess I'll just have to play around with bigger nozzles, extrusion width and other slicer settings and test a few quick prints. Looking closely on the slicer and trying different settings will hopefully give some ideas of how fast and solid the print will be.

More a question of awareness

Posted by: @spud

[...] Bobstro, thanks a bunch for the detailed answers. It's worse than I thought then 🙂 No simple rules.

Oh, it's not all that complex. If you're designing your own parts, just be aware that a bit thicker wall is usually much easier to print. The slicer does a pretty good job but is constrained by physics. 

You gave me a lot of ideas of how to solve this. Guess I'll just have to play around with bigger nozzles, extrusion width and other slicer settings and test a few quick prints. Looking closely on the slicer and trying different settings will hopefully give some ideas of how fast and solid the print will be.

Spend some time in preview mode and you can quickly get an idea for what settings do and don't impact print time and quality. Once you decide what settings work best for you, save a custom preset. 90% of the time, nothing will need any sort of adjustment.

Slicer testing

You're right. It'll probably be jus fine with some testing of the settings in the slicer to get a good preview and understanding of the settings 🙂 Thanks again for the advice.