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Minimal Flow Rate Limit


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Posted · Minimal Flow Rate Limit

Hi everyone!


Currently I'm working on a project in which I'm researching material mixes that are suitable for FDM printing, while reducing energy consumption by eliminating the need to heat material and printbed during the print. This means that the material mixes that I'm working with are paste-like and are able to be printed at room temperature and they dry during and after the print until they're solid. I've modified a normal printer and fitted it with a syringe that is slowly pressed by a stepper motor (the extrusion motor). In initial tests I found that the minimal flow rate that Cura supports (5%) is actually too high. With my first design a flow rate of 0.44% was needed. Since then I've fitted some gears, which increased that value for 7.0% and since then I have bought different lead screw which could further increase this value.


Although I've solved the issue, I'm still curious why this lower limit is there in the first place. From a technical standpoint, I feel like a lower flow rate would be easier to deal with for the stepper motors, so why is the limit there? I can understand that it would not be realistic to print PLA or PETG at a 5% flowrate, but still that doesn't explain why it should be possible to choose this lower limit. So my first question is what the arguments would be to install the 5% limit for the flow rate.


My second question is if there is any way to surpass this limit and easily edit the G-code to still be able to use lower flow rates. Using the gears for the modification makes the system a little clunkier, heavier and the center point of gravity is less than ideal. A design that doesn't need gears would result in better print result and makes it easier to place the syringe. As mentioned, with the original lead screw, which had a 8mm lead, I would need a 0.44% flow rate and I've now also got lead screws with a 2 and 1mm lead, which would result into 1.76% and 3.52% flow rates. If you have any suggestions how I could easily make G-codes that allow such low flow rates, I would love to hear. Thanks in advance!


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    Posted · Minimal Flow Rate Limit

    Slicers for FDM printers (usually) calculate the extruded volume based on filament diameter, linewidth and layer height.

    I assume in your case it would be much better to create a new material with a more reasonable "filament" diameter and don't touch the flow rate at all.

    Or even better: adjust the value for "steps/mm" for the extruder motor to achieve a suitable material transport for this setup (that's a firmware setting).


    That said - which Cura version and which printer definition is used? If it is meaningful or not - in general one can specify a flow rate down to  0.001 percent in Cura and the extruded material volume will be multiplied by this factor (checked with Cura 4.12.1).

    The 5% limit may be specific to this particular printer and/or material definition?


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    Posted · Minimal Flow Rate Limit

    Dear tinkergnome,


    Thank you so much for your reply. I've checked and you are completely right. I had left the material on PLA, thinking it wouldn't matter much, but that was indeed the reason why I couldn't go below a flowrate of 5%. Now that I made a custom material I can go way lower, just like you said. That means I can already make the printer work without gears, which is great!


    As for the settings you suggest me to change rather than playing with flowrate, would you be able to explain it a bit more? I'm not really sure how to proceed. I calculated the flow rate based on how much material would be extruded from the syringe when the extruder motor would make one full rotation. So I disassembled the original printhead to see what volume of 1.75mm filament would be extruded when the motor made one rotation and compared that with what volume of material would be extruded with the setup that I made with the syringe (this was with the original leadscrew with an 8mm lead). This lead me to a 0.44% flow rate.


    For the filament diameter, I'm guessing I should use the internal diameter of the syringe, which is 13.6mm. Filling that in in the material settings should be enough you think?


    Steps/mm are a bit more complicated for me. I've been looking into it, but I don't really understand it. I found some webpages that explain how to measure the steps/mm for regular filaments but I'm not sure how to measure this with my printer. I guess I would have to check how much the plunger moves down into the syringe, but measuring that would be quite difficult compared to measuring it with regular filaments. I'm not sure if I can do it accurate enough. But I could definitely try. When I found a value for the steps/mm I have to put that into the printer's firmware right? I'm using a Creality CR-10V3 that is running on Marlin 2.0.x that I have already modified and flashed onto the printer. 


    It would be great if you could help me some more. Your suggestion to look into the material has already been great! So thanks again for that. Being able to remove the gears will make the system already a lot more simple to use and sturdy!


    Yours sincerely,








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    Posted · Minimal Flow Rate Limit
    8 hours ago, Gutharic said:

    I guess I would have to check how much the plunger moves down into the syringe, but measuring that would be quite difficult compared to measuring it with regular filaments.


    It sounds like one revolution of the leadscrew moves the plunger by the pitch of the screw (multiplied by the reduction ratio of gear - if present).


    0.44% is a very low factor - but you already determined it. If you know the current value of E steps/mm for your firmware, just multiply it with 0.0044 and you get the new value. The new value will be very small of course.


    For Marlin firmware you can set it with gcode M92 (*) - it's possible to add it to the start script in Cura and test it. Don't forget to reset the setting for flow rate back to 100% if you try this.


    (*) for example:


    M92 E1.72  ;probably something near this value



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