Extrusion multiplier is applied after Max volumentric speed

[yschroeder]

Version

2.2.0

Operating system type + version

WIndows 10

3D printer brand / version + firmware version (if known)

Prusa i3 MK2.5S Firmware 3.9.0-RC3

Behavior

PrusaSlicer applies the Extrusion multiplier after Max volumetric speed of the filament. This leads to situations where Max volumetric speed is not the hard upper limit on extruded filament per second.

I use the Max volumentric speed to print at high speeds close the maximum melting capability of the hotend. It my case it is configured to 15 mm3/s, anything higher and the extruder starts losing steps. My print takes 20 minutes in this case and print settings are configured to constantly extrude at 15 mm3/s, verified by the volumetric speed preview. If I now set the extrusion multiplier to anything higher than 1.0, the extruder starts losing steps.

You can easily reproduce this by setting the extrusion multiplier to e.g. 2. The print should take twice as long, as you extrude double the amount of material and volumetric speed is at its limit already. However, PrusaSlicer gives the same estimated print time.

I think PrusaSlicer should apply the Max volumetric speed after the Extrusion multiplier. In the example above, the print time should be doubled when setting the Extrusion multiplier to 2.

[TeaFlavoredTea]

This is causing me some trouble too. I'm trying to dial in some 85a TPU on the CHT nozzle with a mk4S on PS 2.8.1. Since the soft TPU jamming is the limiting factor and not printing speeds, I expected to be able to use Max volumetric speed like a throttle. Its unintuitive that once I have the Max volumetric speed dialed in to avoid jamming, if I want to correct for under/over extrusion I'll need to modify both Max volumetric speed and Extrusion multiplier.

It seems that the correct procedure is to first tune the under/over extrusion with Extrusion multiplier and then push Max volumetric speed up.