My i3 MK3 is printing very well for solid parts of an object, but it messes up with infill.

Infill rough

As you can see in the image, the infill is broken into pieces and bends to that the surface becomes rough and the nozzle touches it next time it moves there, which probably does not make things better.

I've read the Prusa problem page, where they suggest three solutions:

  • change infill type
  • flexible filament (I'm using PETG, so this does not apply)
  • lower printing speed

I'm printing with 20% infill and I believe that this worked before at the same speed, so I'm tempted to say that both remaining options are options, but it should work without them.

It also seems to me as if the infill is thinner that ordinary walls. Is the extrusion speed lower in case of infill? Is there a way of changing the extrusion speed for infill?

Side note: my printer always tells me to upgrade to the latest firmware 3.5.1. Since I have just received the printer back from a warranty repair after the last firmware upgrade, I don't want to upgrade the firmware. Could the issue be related to firmware?

Infos requested from comments:

I'm basically using the Slic3r default settings for PETG. The only thing I adjusted is the temperature, since I'm using HDGlass PETG and there was a recommended temperature written on the spool. Relevant settings seem to be

  • Filament settings
    • fan speed min 30 % max 50 %
    • bridges fan speed 50 %
    • enable fan below 20 s
    • slow down below 20 s
    • min print speed 15 mm/s
  • Speed settings
    • Perimenters 45 mm/s
    • Small perimeters 25 mm/s
    • External perimeters 35 mm/s
    • Infill 200 mm/s
    • Solid infill 200 mm/s
    • Top solid infill 50 mm/s
  • $\begingroup$ what makes smooth infill desireable? $\endgroup$
    – Trish
    Feb 11, 2019 at 21:40
  • $\begingroup$ @Trish: a) less noise of the nozzle scratching over the rough infill, making me feel uncomfortable that it might just tear down the whole thing. b) Stability of the final product as expected. c) After the infill, the will finally be a solid top. That top will look ugly when printed over a rough surface. Do you have that problem as well and you just leave it like that? $\endgroup$ Feb 11, 2019 at 21:43
  • $\begingroup$ have you tried different infils? I like 3D infil, like gyroid or cubic. I know that some linear infils (triangle or square) sometimes do that as the lines pass over themselves. $\endgroup$
    – Trish
    Feb 11, 2019 at 21:45
  • $\begingroup$ Sure, I can try a different infill. I just don't understand why it works once and it does not work now. I'd like to have reproducible prints that work at first print. Otherwise it's a waste of material, energy and money. I don't want to pollute the environment trying new infills which might or might not work. $\endgroup$ Feb 11, 2019 at 21:54
  • 2
    $\begingroup$ how does the infill print speed compare to the perimeter print speed? If the infill speed is appreciably higher you will have failed bonding between layers, which is somewhat how it appears in the image provided. PETG likes reduced cooling, from my understanding (limited) which might also point to cooling fan speed comparison. $\endgroup$
    – fred_dot_u
    Feb 11, 2019 at 22:22

2 Answers 2


From your comments can be read that you print infill at 200 mm/s.

Know that 200 mm/s is ridiculously fast (like high travelling speed), close to the limits of printing on certain machines (for an AtMega)! It is hard for the filament to keep up at this speed. A value of 60 mm/s would be a good value to start experimenting. Your infill is not rough, it just failed printing. I have printed kilometers of PETG, normal print speeds for my PETG are recommended at 30-50 mm/s by my manufacturer; I get good results at 50-60 mm/s. On my Ultimaker 3, 70 mm/s is also feasible.

Note that the filament you use seems to have rather low printing temperatures (195-225 °C) as opposed to the PETG filament (co-polymer) I'm used to. The manufacturer does not specify advised print speeds (other than "high", but what defines "high"?), but this user posted some of his print settings for this material. The overall speed of 60 mm/s seems to support lower than 200 mm/s print speeds.

  • $\begingroup$ Ridiculously fast - well, it was the default setting for PET. Is there a way to save this setting as part of the filament definition? At the moment I can only find this as part of the print settings, which means I always need to adjust this. However, I'd like to ensure that I decrease the infill speed every time I use PET as the filament. $\endgroup$ Feb 11, 2019 at 23:12
  • $\begingroup$ @ThomasWeller Just change it and save it. At least that is possible in default Slic3r or Ultimaker Cura. $\endgroup$
    – 0scar
    Feb 11, 2019 at 23:20
  • 1
    $\begingroup$ 60 mm/s improved much. 50 mm/s is almost perfect. Great to see that I don't have to decrease the overall printing speed. $\endgroup$ Feb 12, 2019 at 11:59

I have dealt with this on infill as well on multiple MK3s.

However, it was not the speed itself, but the hot end having difficulty extruding enough to keep up with the infill.

Some things to try:

  • Raise hot end temperature 5 degrees (melt filament faster)
  • Lower infill speed, it will not affect your overall print time very much (surprisingly)
  • A combination of the two above.

Good luck!

  • $\begingroup$ Yes, temperature does make an effect, certainly when you are using 2.85 mm filament, that needs more time to melt through to the core. But from experience I found that still with raising the temperature you hit a limit causing the line width to be smaller than intended. But a good additional answer! I wondered to add that myself yesterday, but limited the answer to the IMHO high extrusion speed. Welcome at 3D Printing.SE! $\endgroup$
    – 0scar
    Feb 12, 2019 at 8:20

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