PETG nozzle clearance and extrusion multiplier

  • I've started printing PETG recently and I'm happy with results so far, awesome strength and good looking (except for stringing). But I've noticed that PETG prints better with more distance nozzle-plate than usual, and under-extrusion make parts looking better than both normal/over-extrusion.




    • What distance nozzle-plate is optimal for PETG? (i.e. the distance between nozzle and build plate during calibration)

    • What extrusion percentage is optimal for PETG?


    Your second question seems a bit strange, any value lower than 100 % will cause under-extrusion, so what is optimal? Any value different than 100 % means that you have not calibrated the nozzle well.

    Is it solely the clearance on the first layer that's a problem here? If so, adjust the Z-axis stop. If the nozzle appears to be running into every layer, then as @Oscar suggests, your "underextrusion" value is really a proper calibration for the nozzle in use. BTW: what is "dirty" ? If you mean burnt bits, just drop the temperature.

    @CarlWitthoft Your comment about the temperature makes perfect sense in light of of the OP's mentioning of stringing also!

    Clearance is machine, not material dependant. You also speak about *initial layer thickness*, not clearance, which is *space between parts*

    By dirty I mean that the nozzle seems to gather all strings (or whatever) and gets dirtier and dirtier during the print. The print itself remains good, I mean that there are no trace of nozzle dragging over the surface.

    @FredoCorleone I have assumed that with "clearance" you meant the paper distance when bed levelling, also known as Z offset, maybe it is better you make that more clear. Both questions are now unclear. Please update by [edit], thanks!

    I've tried to make my question clearer. If not I'm more than willing to edit again.

    @FredoCorleone So you mean the calibration distance between nozzle and bed or first layer height, it is still not clear what you mean.

    Calibration distance...

    How long of a print it is and does ot show "buggers" from material collecting on the nozzle? I mean PETG does stick to nozzle somewhat no matter what you do. That's why it's recommended to clean the ouside of the nozzle before each print.

  • anttix

    anttix Correct answer

    3 years ago

    Here is the mental framework that I use to reason about PETG: In a nutshell you want to avoid nozzle contact.



    Unlike most other plastics, PETG sticks to hot brass really well and every time the nozzle moves through material it will pick up some of it. Material around the the nozzle then sticks to a random place creating a blob. It can also cook, turn transparent brown and drop into the print. Investing in a plated nozzle or silicone socks helps but doesn't eliminate the problem completely.



    Now to the questions.



    1) Nozzle Distance



    Distance to the plate has to be such that the plastic is laid down precisely without the nozzle dragging through the material (remember, avoid nozzle contact). Precise lines require the build plate to be level and the flow perfectly calibrated. If nozzle is too low and/or the layer is over-extruded then PETG will stick to the nozzle and rip the lines off the plate again. Inspecting the first layer is required for best results. I like to print a layer test pattern after the flow has been calibrated and tweak Z offset in 0.02mm increments until it's perfect.



    With many other plastics it's ok to have a large amount of "squish" in the first layer as it helps to work around minor leveling issues. This is where the cookie-cutter recommendation to raise the nozzle when printing with PETG is coming from.



    2) Extrusion percentage



    Flow has to be near perfect. Down to one percent perfect. Even a slightest over extrusion and some of the excess material will end up on the nozzle when it makes the next pass. Under extrusion isn't great either as this can lead to holes and affect overhangs where thinner strands of a previous pass may not be enough for the next line to stick to.



    There are two critical parameters: diameter of the filament and extrusion multiplier. This is how to determine the settings:




    1. Measure filament diameter. I use an average of ten measurements over about a meter (yard) of filament taken in multiple orientations.

    2. Calibrate the extrusion multiplier using a method described in Prusa
      manual
      :
      I print a 40x40x40 cube in vase mode with extrusion multiplier set
      to 1 and fixed extrusion width (e.g. 0.45mm), measure the wall
      thickness in three spots on every side, average the results and
      compute the correction factor.



    I perform flow calibration for every new roll of filament.


    To calibrate flow percentage I make a mark on the filament itself, about 10cm then I extrude 10cm within the move axis panel and then divide 10cm for whatever it has extruded. Is it good?

    @Fredo Corleone - this is a different process called E-steps calibration. It is a prerequisite for a good setup. However it's done once when you assemble the printer. The process described here is fine tuning for every individual roll of filament.

    @fredo-corleone The reason additional calibration is needed is that e-steps calibration handles the length of filament going into the machine. However the volume that is extruded also depends on the shape (how oval it is) and diameter of the filament (measured first to get close).

    Also where did you enter that number? This should go to printer firmware and **not** into slicer.

    I've just finished calibrating the extruder stepper, now it extrudes exactly the right amount! And I'm currently calibrating the flow. It seems that my printer over-extrude a bit (printed wall is around 0.45mm while line width is 0.4mm).

    @FredoCorleone If you get 0.45 instead of 0.4 then it's ~12% over-extrusion. Not unheard of but doesn't smell right. Assuming 0.4mm nozzle, the default extrusion width should be more than 0.4mm. Are you sure 0.4mm is correct? If you're using slic3r then set everything under Print Settings -> Advanced -> Extrusion Width to 0.45 and print that cube again. This will make sure that there is no automagic involved in calculating extrusion width.

    @FredoCorleone I don't know what you meant by line width here. However if it's the line width of the model (STL) then it may not match the actual extrusion width the printer can achieve. Extrusion width is determined by slicer settings and in the simplest case slicer will use a default extrusion width (generally larger than the nozzle diameter) and attempt to fill the walls with the number of lines that best approximate the desired thickness. E.g. a 0.4mm wall will come out ~0.45mm thick if this is what the extrusion width in the slicer was set to.

    By line width I meant extrusion width, it's called that way in Cura. Anyway I've successfully calibrated my printer. Thanks

    Measuring the wall width is a very bad way to tune extrusion multiplier. Your caliper may be accurate down to 0.05 mm, the wall width is 0.45 mm. Your uncertainty (optimistic) is 11%. Extrusion multiplier should be accurate down to 1-2%, therefore you cannot get there with a measuring technique accurate to 11%.

    Yes calipers are not accurate enough. I don't see calipers mentioned anywhere in the answer nor the linked pages. Prusa docs linked above call for using a micrometer.

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Content dated before 7/24/2021 11:53 AM

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