2.2.19. Using Max Volumetric Speed with PrusaSlicer

PrusaSlicer is one of the few slicers to support a maximum flow rate for the hotend. This setting is your best friend when printing with larger nozzle sizes or tricky filaments.

Note

These notes are based on my experiences with the Prusa i3 Mk3 and Artillery/Evnovo Sidewinder X1 printers. If you are using a different printer, please verify the hardware details are similar.

The amount of filament that a hotend can process is specified in cubic millimeters per-second (mm³/s). PrusaSlicer allows you to specify this value with the Maximum volumetric speed (MVS) setting in filament and print settings profiles. Calculating maximum volumetric rates is straightforward:

$Max. Volumetric Rate = Layer Height \times Extrusion Width \times Speed$

The reason that this setting is so powerful is that it allows you to specify a single setting per filament and PrusaSlicer will ensure that print speeds are throttled to stay below your specified volumetric rate. PrusaSlicer also supports setting MVS in print settings profiles as a master limit. I can set my preferred speeds for each feature and the volumetric limits will throttle when – and only if – necessary to keep the flow of filament within the hotend limits. This allows me to use a single print settings profile across all nozzles and filament types. I just need to set an appropriate MVS value for each filament.

Life is a lot more difficult if you’re using another slicer that doesn’t support volumetric limits. You have to set your max speeds to ensure you never exceed the hotend capacity, which means you have to determine the worst case for flow rates and reduce speeds accordingly.

The max volumetric speed you should use depends on your hotend. The E3D V6 hotend that ships with the Prusa i3 Mk3 is advertised with a rate of 15 mm³/s, though in practice 11.5 mm³/s is realistic for printing PLA through a 0.4mm nozzle. If you need to push more filament, the Volcano hotend upgrade increases this to roughly 25 mm³/s but does require some printer and software modifications. Other hotend designs allow even higher flow rates.

Individual filaments also have different characteristics. Viscosity, content and other factors may greatly reduce effective throughput. Most PETG, for example prints well at a recommend rate of 8 mm³/s as compared to PLA at 8-15 mm³/s.

Much of the time, neither linear speeds nor volumetric throughput matter when doing small prints. The nozzle never comes up to full speed and spends most of the time speeding up and slowing back down at lower speeds due to acceleration and jerk limits. It’s when you start doing larger parts or moving fast for things like infill that it matters. “Obeying the speed limit” with the volumetric setting avoids extruder clicks and jams that can spell disaster mid-way through a large print.

2.2.19.1. Adjusting filament profile max volumetric speed

MVS can be set on a per-filament basis under Filament Settings. This allows each filament selection to control overall print speeds.

Fig. 2.20 PrusaSlicer advanced filament settings

2.2.19.2. Adjusting print profile max volumetric speed

MVS can also be set under Print Settings I like to set a maximum “master” value of 11.5 mm³/s here as a safeguard.

Fig. 2.21 PrusaSlicer print setting profile speed adjustments

2.2.19.3. Viewing the effects of settings changes

Slice, save, then look around in preview to see the impacts on speed and volumetric flow:

Fig. 2.22 Viewing effective speeds in PrusaSlicer preview mode

Fig. 2.23 Viewing effective volumetric rates in PrusaSlicer preview mode