Tuesday, January 19, 2021

Fix Under-Extrusion Problems by Setting Max Volumetric Speed (MVS)

Fixing Under-Extrusion with a Single Slicer SettingThere are many potential causes for under-extrusion and clogged nozzles on a FDM (filament) printer but, in most cases, the extruder can't grip and push the filament with enough force and speed. There are numerous guides for troubleshooting temperature and speed settings, but it can be extremely difficult to find the magic combination of settings to print fine details with that cheap filament you bought before knowing any better. 

PrusaSlicer to the rescue with MVS 

Fortunately, PrusaSlicer added a setting that's essentially a way to throttle extrusion speed and prevent those moments when the force required is too much for the extruder/filament to handle. 
Note: While this fork of Super Slicer is enhanced and maintained by Prusa, it can be used to generate g-code for non-Prusa printers as well.

Maximum volumetric speed (MVS) sets a limit for how fast the extruder will push filament by overriding any speed settings which would result in more material being pushed than this setting allows. This override can be tied to a specific filament or print setting since it's dependent on layer height and extrusion width (nozzle size). 

Lets visualize how this works in practice 

To simplify, think of the extruder as a vehicle driving from one location to another. That vehicle will travel at different speeds along it's route and we want to be sure it doesn't try to go too fast at any point along the way. This means we don't want to interfere with the times when it's going slower and only restrict it's speed when necessary. If the vehicle's maximum speed would normally be 70 mph and we discover that it's tires lose traction when traveling faster than 60 mph, we're going to prevent acceleration past that point. The rest of the trip under that speed is unaffected so, if 90% of the trip is under that speed, only 10% of the route will take more time. A ten-minute trip would take one minute more. 

How to start using maximum volumetric speed (MVS) 

One way to find an optimal MVS would be to compare estimated print time with and without the MVS limit set. Start by multiplying your layer height (i.e. 0.2mm) × extrusion width (i.e. 0.4mm) × speed (i.e. 40mm/s) for the equivalent MVS (3.2mm³/s). If both print times are the same, that MVS value is higher than the maximum flow of your print. If the difference in print times are minimal, only those moments when the greatest amount of material is being extruded are being limited and most of the print remains unchanged. As this number is reduced, more of the print instructions will be affected by this limit resulting in increasingly longer print times. 

For example... 

In my case, I had a terrible time with the Mika3D filament I purchases being too brittle for the extruder gears to push hard without bits breaking off, clogging the teeth, grinding away an indentation, and eventually slipping instead of being pushed forward. This became particularly problematic when I switched to a 0.25mm nozzle which made it harder to push filament through and made this filament virtually unusable. By setting a MVS of 0.6mm³/s on a temperature tower at a layer height of 0.10mm, my print time increased from 12h 4m to 15h 15m. While a ~25% increase in print time may seem like a lot, I consider waiting a few more hours for an overnight print to be a small price for more reliable extrusion and fewer failed prints. 

Your best MVS setting will vary 

It's important to remember that my setting of 0.6mm³/s will not work the same for a different layer height or nozzle size so some experimentation may be needed to find the appropriate values for various profiles, but each print setting can have it's own MVS which can even be overridden by the MVS for those filaments which require more delicate handling.

While we've focused on how maximum volumetric speed can be used to avoid extrusion issues, what MVS value works best will depend on the temperature of your hot end and even how tight your extruder idler bearing is pressed against the hobbed gear. MVS isn't a magic solution to all potential extrusion problems but, with a bit of tweaking, it could make the difference in being able to use that filament which has caused so many headaches. 

Did this help? 

Whether you're still struggling or this was the key that fixed your problem, leave a comment to let us know. I'd love to hear your results and am happy to help where I can. Someone else may even benefit from reading about your experience!


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