PET-G is the main filament I use. I can achieve high quality of my prints with my Prusa i3 mk3s printer. Recently however, I've recently changed my nozzle from the default 0.4mm to the 1.2mm. I watched the online video tutorial on how to do it properly and did the whole process carefully.

Temperature: 250 °C/100 °C Speed: max 200 %, but results are the same even all the higher ones . Layer height: 0.35 mm

With my first print (and the next ones too) on the new nozzle I noticed lots of under-extrusion with infill (holes). LOTS of stringing although I've increased retraction. What's the reason ?

  • $\begingroup$ What speed, layer height and temperature are you using? You have just increased the cross-section of the nozzle by 800%. It is possible that the hot end is not able to extrude the filament at the rate that you are requesting. $\endgroup$
    – Mick
    Jul 10, 2019 at 21:37
  • $\begingroup$ @Mick I'm sorry, I've forgotten to add this, I usually include such information. I will edit my question $\endgroup$
    – StLuke5
    Jul 10, 2019 at 21:41
  • $\begingroup$ Can your hotend melt PETG at the rate you're printing? You need a hardcore hotend to take advantage of that nozzle size without really low speeds. Assuming you don't need the wall thickness, 0.4 can probably even print faster with a normal hotend. $\endgroup$ Jul 11, 2019 at 0:09
  • $\begingroup$ "200%" doesn't seem like much of a valid speed - unless the Prusa i3 has some sort of default speed everyone is using? At the same layer height, you can only move 1/3 as quickly as with a 0.4 mm nozzle - your hot end has to melt much more plastic at the same speed. Since you also went to a layer height of 0.35 (with the default presumably being 0.2mm), you're now extrusing 5.25x as much plastic in the same time. Your hot end probably just can't keep up. $\endgroup$
    – towe
    Jul 11, 2019 at 8:09
  • $\begingroup$ @Towe Prusa says that speeds up to 200% are OK. But I'll try switching the layer height to 0.2mm $\endgroup$
    – StLuke5
    Jul 11, 2019 at 10:28

1 Answer 1


Piecing this answer together from the comments on OP's post. Myself and R.. noted that a layer height of 0.35 mm, nozzle width of 1.2 mm, and a fairly high feedrate (200 % according to OP, no reference to what 100 % is), is an exceptionally large amount of plastic to try and melt through almost any hotend on the market except possibly the Volcano, or the high-throughput version of the Mosquito.

OP responded that lowering the layer height to 0.2 mm seemed to fix the issue. That's still an impressive amount of plastic, but it's a 43 % decrease in total flow compared to 0.35 mm layer height.

  • 1
    $\begingroup$ Addendunm: e3D Volcano and Supervolcano have doube/quadruple the meltzone length to gain access to extreme flow for extreme speeds. The only other type of extruder, that I could think of that manages this is a pellet type extruder, where the arbor is heated and offers similar access to high flow. $\endgroup$
    – Trish
    Jul 14, 2019 at 11:45

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