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Most people, articles, videos, etc. refer to printing speed by linear speed (mm/s), but a lot of YouTubers prefer to talk about volumetric flow (mm3/s) (mm cubed per second). I suspect that at some point in the past year or three, some of the more engineer-y types switched to this new measurement standard, but I'm not entirely sure what happened. What is the practical difference between using linear speed vs flow rate to determine print speed?

For a follow-up, how can you go about changing your print speed as a flow rate in a slicer? It's easy to find the linear speed, but I have not found the flow rate speed. I use Cura and will start using Prusa Slic3r soon.

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    $\begingroup$ flow rate will need to account for layer "thickness." flow rate is easier to relate to the material needs while linear speed is easier to relate to motor capabilities/limitations. $\endgroup$
    – Abel
    Dec 12, 2021 at 22:59

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Linear print speed is widely written in marketing material and in filament manufacturers' official print setting recommendations, and is the value talked about by naive users, including a number of popular YouTube personalities. However, in most contexts it's at best the wrong number, and more often, meaingless.

For example, you will see printer manufacturers bragging that they can print at "250 mm/s", and it turns out what they mean is that, when you set the print speed in the slicer to "250 mm/s", it works. However, their profile has the acceleration set to 500 mm/s², and you need a straight line all the way across the bed to accelerate up to that speed momentarily before starting to slow down again, and overall average prints come out 5% faster than when you set it to "90 mm/s".

If you set this kind of lying (or incompetence, if you accept Hanlon's Razor) aside, and only talk about the actual velocity achieved under an acceleration profile, linear speed becomes meaningful, but probably not the right number, because it's missing any information about the actual limiting factors. Provided the speed isn't beyond the max RPM of your stepper motors (around 650 RPM for typical motors on 24V printers), any printer can print at that speed as long as your layer height is thin enough or your lines are narrow enough. In fact if you go look at a few of the top "1000 mm/s" YouTube videos, they're using 0.1 mm layer height or thinner - which is easy to do, but not interesting to most people. It's not helping you print faster because now you have twice as many (or more) layers to print.

Volumetric flow rate takes all of that into account. If you print at 20 mm³/s and I print at 10 mm³/s (and if these are actual average flow over the whole print, not just a speed limit that's rarely achieved due to acceleration profile limits), your print will finish in half the time of mine. That is a meaningful speed - one that can't be used to lie/mislead by printing thin layers or whatever. Moreover, it's the limit you need to know, for your particular hotend, extruder, and filament type, in order to be able to decide what speed to print at (whether you control this via volumetric flow limits or computing the corresponding linear speed limits yourself). If you're reading the filament manufacturer's recommendations, they didn't say it but they probably wrote those for 0.4 mm line width and 0.2 mm layer height, so you should multiply by 0.08 mm² to get the volumetric rate they intended.

This gets more important with slicers, particularly the upcoming Cura 5, which use varying line width (and thus varying relationship between the linear speed and volumetric speed) to extrude your layers. Now a single linear speed doesn't suffice to give you max performance while also staying within the physical limits of what you can extrude. Cura 5 has "Flow Equalization" to speed up or slow down to keep the volumetric rate matching what it would be at the nominal line width and speed.

Now, linear speed in mm/s is meaningful sometimes - particularly, if you're showcasing the printer's motion system. High voltage steppers, larger pulleys, servo motors, etc. all can achieve some very high linear speeds (over 1000 mm/s) that ordinary 24V steppers (much less 12V ones on older printers) with normal size pulleys simply cannot do. Even if these speeds are not usable for print moves (because the volumetric flow rate becomes the limiting factor for what you can do), they're always usable for travel moves, which can easily be 25% or more of the time spent in complex prints.

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Flow rate adds a dimension to the regularly used printing speed. Note that maximum volumetric flow is determined by the hotend (unless your extruder is under dimensioned or highly geared) as it cannot supply more molten filament than it can melt in a certain time.

So, instead of specifying the print speed, you should include the amount of material it can process. Volumetric flow includes nozzle diameter, layer thickness and hotend type.

E.g. a 60 mm/s of a 0.4 mm nozzle at a 0.2 mm layer height has a very different volumetric flow (4.8 mm³/s) from a 60 mm/s 0.8 mm nozzle at a 0.4 mm layer height (19.2 mm³/s). The latter may require a different hotend to get that flow, but usually it is advised to print slower with a larger nozzle.

Most practical is to use the linear printing speed. This is the value you find in the slicers. But, for a given hotend design it is good to keep the maximum volumetric flow into account to determine whether you are within the specifications of the hotend when you change certain printing/slicing parameters.

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