2 questions relating to extrusion width in Prusa Slicer
Hello,
I have 2 questions regarding extrusion width in Prusa slicer:
a) How does Prusa slicer behave when you print a wall that isn't a multiple of the extrusion width (including overlaps between lines)?. So let´s say, we have a 0.4mm nozzle and ALL our extrusion widths are 0.45mm just as an example. I know that when Prusa Slicer prints a 2-line perimeter there will be an overlap between lines, so a 2-line perimeter wall wouldn't be 0,9mm exactly, let say with the overlap its 0.8mm wide in total for a 2-line perimeter and 1.1mm for a 3-line perimeter.
My question is, what happens if I want to print a 0.9mm or 1 mm thick wall for example, or a 1.2mm wall?...Does PS reduce or enlarge the extrusion width to adapt? or does it maintain the width constant and modify ad-hoc the amount of overlap between lines?..I would imagine the printed wall will be as true as possible in total thickness to the thickness of the wall in the model, right?
b) I've seen that it's technically possible to print with a smaller extrusion width than the nozzle diameter (although not recommended). How is that physically possible? I get how you would get a bigger width than the diameter by lowering the nozzle and obtaining a "hamburger" shaped profile for the line, through squishing, but how can you get a narrower line than the nozzle diameter? Is it just by barely pushing the plastic with any force and just letting the melted filament sort of ooze down?.....I can't physically imagine it, and I'm very curious.
Many thanks!
RE: 2 questions relating to extrusion width in Prusa Slicer
In answer is 'it depends'. By default now PS uses a slicing engine called Arachne (same as Cura etc). It used to use a different one that is still there but you have to select Classic slicing.
Arachne tries to dynamically adjust extrusion width as its going. PS doesn't 'Squish' extrusions to get fatter ones as that would alter layer height. It simply pushes more plastic through to deliberately over extrude which has the same effect. This of course depends on the printing speed too as its volume of flow so the calculations get a bit hectic. Well Arachne tries to do the same for thinner parts, it will deliberately under extrude to attempt to get a thinner wall. Note the tries. While generally it does an ok job it doesn't always.
Also the Arachne settings in the default profiles are a starting point. They are not a one size fits all. Every setting should be reviewed on a case by case basis in the slice preview. Not enough time is spent doing that usually before attempting to print.
Classic doesn't do any of that it just uses your defined extrusion width. For thin walls if they were thinner than that they were simply not printed. Go back far enough and you will see complaints about 'feature x is missing on my model' . Walls were just not thick enough to be sliced. That's why there's a setting for Classic called detect thin walls. That would force the slicer to print them (mostly). So you got a perimeter but it was over sized as it was always the extrusion width thick minimum. That setting causes as many issues as it solves though. Once again slice, preview and change things as needed.
Theres also gapfill setting for smaller voids between walls but again that's a setting that has had a long history of its own issues.
This is why it is always best to have walls thick enough to your desired number of perimeters and to have a bit of space for infill if at all possible when you design things. You cant really adjust the distance between the walls in the hope of getting a true thickness, the plastic extruded has to go somewhere, so even if you overlapped more it wouldnt do anything. You can overlap more but you have to couple that with extruding less plastic, hence we have Arachne.
RE:
That made it very clear Neophyl, thank you so much for such detailed answer! So my only remaining doubt would be, in classic mode, how did the slicer adapt for walls thicker than the minimum extrusion width, but not a multiple of it? Would it be with the 'gap fill' technique? So, if you had wall that was 1.5 times the minimum extrusion width, it would extrude one perimeter and fill the remaining width based on the 'gap fill' algorithm to reach the desired width?
Is classic mode still used on specific use cases?
RE: 2 questions relating to extrusion width in Prusa Slicer
With classic it wouldn't. If you have say a 0.5mm thick wall and your extrusion width is set to 0.45 then you would get a single wall that is 0.45 thick. This is why you always see the advice to design your walls at least in multiples of the extrusion widths if you can.
Gapfill is only used inside perimeters.
It also depends on the slicer, theres a fork of PS called Super Slicer that has for reasons allowed gapfill to go to truly ridiculous widths, much wider than the normal extrusion width for a perimeter, and probably the result of a coding oversight rather than deliberately. It can go so large is its trying to fill in a gap thats wider than a single perimeter but not big enough for 2 perimeters. Currently the subject of at least one issue to try and get that limited.
There are differences between slicers in how they handle situations like this and so much of it is geometry dependent.
Even now with either Arachne or Classic you still get problems with single wall thicknesses depending on the other geo in the model.
RE:
Understood. Thank you so much for the crystal clear answer. I'm printing this answer to pdf and saving it in my 3d printing knowledge folder. This forum is a gold mine, with answers like these and those from other users like Swisscheese (filament whisperer) I'm learning so much! Thanks!!