How are a filament's volume and length calculated?
after slicing with PrusaSlicer - Printer: Original Prusa XL 0.4 nozzle (unmodified) - Print settings: 0.30mm DRAFT (unmodified) - Filament: Generic PLA (unmodified)
I get the sliced info:
Used Filament (g): 379.85
Used Filament (m): 127.36
Used Filament (mm3): 306326.68
I was thinking volume and lenght were calculated following the cylinder volume formula
V = pi*r2*l , where r is the filament radius, and l is the filament lenght.
If if solve for r, using V = Used Filament (mm3) = 306326.68 , and l = Used Filament (m) * 1000 : 127360 (mm), I get r = 0.875 (mm)
were is this number coming from? I was expecting to have 0.2, since I'm working with a 0.4 mm diameter printer.
(can't add a screenshot rn, forum's web page isn't behaving properly)
RE: How are a filament's volume and length calculated?
oh ok, I see it now. It's coming from Filament diamater: that is 1.75 mm => radius = 0.875
So it's the lenght of filament used, not the total lenght of the printed material, which can be of a smalled section. Makes sense, it's no more a perfect cylinder after it's melted, layer thickness is usually lower then the nozzle diameter...
RE:
The only thing that the slicer knows is what length of filament it is requesting to be fed through the nozzle. As long as your steps for the E axis is correct, then the length of filament fed into the extruder (the m measurement) is the only empirically known value because that is a sum of all the extrude commands in the gcode. This is determined by how much volume of filament it wants to come out of the tip and takes into account layer height, extrusion width (often in Prusa configurations wider than the nozzle orifice), and configured (not measured) source filament diameter. If the actual filament diameter doesn't average out to be what is configured in the filament preset then the extrusion calculations will be messed up leading to either under or over extrusion.
The volume measurement (mm^3) is a sum of all the desired extrusion amounts out of the end of the nozzle. I don't know if PrusaSlicer adds up all the desired extrusions for this figure, or re-calculates based off of the length and the configured filament diameter (i.e. I don't know if it is a running sum from the slicing calculations, or an after-the-fact calculation).
The mass measurement (g) can only be calculated from the volume and the configured filament density in the filament preset. Nothing in the Prusa hardware (or in the software) can take a weight (or mass) measurement. If your configured density is wrong then the mass measurement in the slicer will be wrong proportionally to the error in the density configuration.
Clear as mud? 😉
See these two links for the intended extrusion geometry in PrusaSlicer (inherited from Slic3r).
https://help.prusa3d.com/article/layers-and-perimeters_1748
https://manual.slic3r.org/advanced/flow-math
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