0.9mm or 0.87mm for extrusion multiplier calculation?
I'm printing 2 layers hollow box (no infill) to calibrate my extrusion multiplier. I noticed that recommended thin wall thickness under Prusa Slicer for 2 lines perimeter is 0.87mm. I've been using 0.9mm to calculate my extrusion multplier all this time.
Should I be multiplying the 2 lines perimeter thickness by 0.87mm instead of 0.9mm for calculating extrusion multiplier?
Best Answer by bobstro:
It depends.... and not for obvious reasons.
tl;dr - PrusaSlicer does not produce walls that are exactly the sum of all of your perimeter widths thick.
PrusaSlicer inherited (forked) the code base from the original Slic3r. As such, it still (to the best of my knowledge) uses algorithms that were ported from the original PERL code to C++. Some of the logic in those algorithms was developed in the early days of 3D printing and are a bit mysterious. Or at least "not obvious". None more so that the math used to calculate flow rates.
When you print with a 0.2mm layer height using 0.45mm layer heights, you'd expect the resulting wall to be 0.90mm thick. This is not the case as you've noted with the pop-up tooltip help. The reason is rooted in Slic3r's original flow math calculations. If you print with a single perimeter, the wall width will be a single extrusion thick. If you use more than one perimeter (as most prints do), the math changes. A wall consisting of 2 perimeters each 0.45mm wall will be sliced to produce a wall 0.85mm thick. This is done to fill voids between extrusions for wall strength. To make it even more mysterious, the exact width also depends on your layer height. I am not sure I understand why, other than to "fill voids". If your 2 0.45mm wide extrusions are sliced using 0.15mm high walls, the expected thickness is 0.868mm.
None of this really matters if you're printing thicker parts. You just see the nice, hopefully strong exterior walls. We get into trouble when we assume wall thickness is calculated by simply adding together the width of all the perimeters. The tooltip provides a hint, but I and others stumbled around a bit before realizing the importance of the Slic3r flow math.
So to answer your question: If you are after high precision, use the widths calculated using the Slic3r flow math. I've got a spreadsheet here that (I think) takes all the factors into account. If you want to keep life simple, the "add perimeters" math will get you close and a 2nd print and measurement is usually plenty accurate enough. If you want to stay sane, generate your test prints with another slicer. (I think.)
RE: 0.9mm or 0.87mm for extrusion multiplier calculation?
It depends.... and not for obvious reasons.
tl;dr - PrusaSlicer does not produce walls that are exactly the sum of all of your perimeter widths thick.
PrusaSlicer inherited (forked) the code base from the original Slic3r. As such, it still (to the best of my knowledge) uses algorithms that were ported from the original PERL code to C++. Some of the logic in those algorithms was developed in the early days of 3D printing and are a bit mysterious. Or at least "not obvious". None more so that the math used to calculate flow rates.
When you print with a 0.2mm layer height using 0.45mm layer heights, you'd expect the resulting wall to be 0.90mm thick. This is not the case as you've noted with the pop-up tooltip help. The reason is rooted in Slic3r's original flow math calculations. If you print with a single perimeter, the wall width will be a single extrusion thick. If you use more than one perimeter (as most prints do), the math changes. A wall consisting of 2 perimeters each 0.45mm wall will be sliced to produce a wall 0.85mm thick. This is done to fill voids between extrusions for wall strength. To make it even more mysterious, the exact width also depends on your layer height. I am not sure I understand why, other than to "fill voids". If your 2 0.45mm wide extrusions are sliced using 0.15mm high walls, the expected thickness is 0.868mm.
None of this really matters if you're printing thicker parts. You just see the nice, hopefully strong exterior walls. We get into trouble when we assume wall thickness is calculated by simply adding together the width of all the perimeters. The tooltip provides a hint, but I and others stumbled around a bit before realizing the importance of the Slic3r flow math.
So to answer your question: If you are after high precision, use the widths calculated using the Slic3r flow math. I've got a spreadsheet here that (I think) takes all the factors into account. If you want to keep life simple, the "add perimeters" math will get you close and a 2nd print and measurement is usually plenty accurate enough. If you want to stay sane, generate your test prints with another slicer. (I think.)
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