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The slicing software knows exactly (given accurate E-step, filament diameter, nozzle diameter values) how much cold plastic is pushed in the extruder and how much mass is extruded.

Why should the extrusion multiplier be changed from 100%? Shouldn't the slicer extrude already enough to fill the gaps left by the neighbouring perimeter (see rectangular vs recto-oval extrusion profile).

As result of the "why" question, implicitly comes up the question "how" should I (scientifically, not try and fail) define the extrusion multiplier? it would appear that said value should not be touched if the rest is done properly.

The only idea I can have is to print at 100% infill and then saw the part, to check how big are the voids between 4 neighbouring extrusions. Measuring the wall thickness seems very prone to errors (but again, is such calibration really needed?)

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In an ideal world, you will not need to change this parameter once it is properly calibrated.

In a non-ideal world, some filament may be out-of-spec, and some filament may slip when it passes through the extruder. So for a flex (or more flexible) filament, you might increase the extrusion multiplier a little to compensate. If the compensation is material specific, it should be consistent (and might even be advised by the filament manufacturer).

You might want to increase/reduce the multiplier whilst printing the first layer (in lieu of fixing your bed leveling properly).

Maybe you want to fine-tune the top surface (which will be 100% fill) and you prefer to slightly under-fill this (because over-fill results in more noticeable surface defects).

It is for quick hacks or fine tuning, there is probably no scientific approach to setting any value other than 100%.

When it comes to material specific variations (due to melt viscosity, thermal expansion or drive efficiency), there are many factors which affect the tuning. Machine geometry, temperatures, colourants (and other additives in the filament), as well as the base material.

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  • $\begingroup$ Ok so that value is just a hack when nothing else works, for example to compensate minor differences in the nozzle diameter which cannot be otherwise measured. $\endgroup$
    – FarO
    Sep 26, 2019 at 10:32
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    $\begingroup$ Nozzle diameter shouldn't have a 1st order effect on the extrusion volume required in a bulk region. $\endgroup$
    – Sean Houlihane
    Sep 26, 2019 at 10:39
  • $\begingroup$ atop that, different materials behave differently. Some slicers know how and adjust for it, others don't $\endgroup$
    – Trish
    Sep 26, 2019 at 12:01
  • $\begingroup$ Your note about the filament dependency is a good thing but they don't usually tell what you should set. What you need to set is dependant on what you print at, your gear geometry and it can be dependant on the color too. It is pretty much a "compensate agaisnt my benchmark by X" factor $\endgroup$
    – Trish
    Sep 26, 2019 at 15:35
  • $\begingroup$ @Trish did see it on a recent product, and was quite surprised. Not sure which vendor. I'll expand a little. $\endgroup$
    – Sean Houlihane
    Sep 26, 2019 at 15:47

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