Im using Prusa Slicer 2.1 for my FlyingBear Ghost 4.

I just changed my 0.4 mm nozzle for a 0.2 mm but it seems to jam in the heater probably due to too much filament trying to get out by the nozzle. Where is the setting to reduce the filement speed and how much I should reduce it?

Here are my settings: Config.txt

  • $\begingroup$ permission fixed $\endgroup$ Commented Sep 14, 2020 at 23:18

3 Answers 3


0.2 mm and 0.4 mm are half the diameter, but the maximum flow is not just half: Flow scales with the area. The 0.4 mm nozzle has an area 4 times as the 0.2 mm one:

$\frac{A_1} {A_2}=\frac {0.2^2}{0.1^2}=4$

You need to reduce print speed or the volumetric flow by this factor or make sure your printer can handle the increased flow by reducing the viscosity of the melt - for example by increasing the print temperature.

Also note, that a 0.2 mm nozzle can't be operated with layer heights above 0.15 mm.

  • $\begingroup$ Ok, where do I change the settings in prusaslicer? $\endgroup$ Commented Sep 13, 2020 at 21:30
  • $\begingroup$ @EricBrochu reduce print speed, I think it's usually 60 mm/s, so it would be 30 mm/s $\endgroup$
    – Trish
    Commented Sep 13, 2020 at 21:31
  • 2
    $\begingroup$ @Trish: That should be 15 (1/4). But it might be possible to get by with more if the limiting factor at 60 was melt rate rather than viscosity. $\endgroup$ Commented Sep 13, 2020 at 22:38
  • $\begingroup$ @R..GitHubSTOPHELPINGICE yes, brainfail XD $\endgroup$
    – Trish
    Commented Sep 14, 2020 at 10:13

Here's where you change the speed in Prusa:

Changing Speed in Prusa

As Trish said, you're likely trying to push too much material through too fast.

Steps you can take:

  1. Increase print temp
  2. Decrease print speed
  3. Slice for thinner layers
  4. Don't forget to adjust your nozzle diameter and/or extrusion width(s) as well.

I'd personally start with increasing my print temp 10 degrees and cutting my print speed in half, as well as making sure I had 0.2 mm nozzle and extrusion widths.

  • $\begingroup$ you don't answer the question at all. $\endgroup$ Commented Sep 14, 2020 at 19:45
  • $\begingroup$ I addressed the issue described in your title. Now I've also answered the question in your question. :) $\endgroup$
    – Davo
    Commented Sep 14, 2020 at 19:51
  • $\begingroup$ What about the speed of the filament entering the heater. I do not think that Print speed is a problem, its the speed the feeder pushes the filament. $\endgroup$ Commented Sep 14, 2020 at 20:32
  • $\begingroup$ Filament advance speed is calculated, and essentially equal to the demand (how much output you need per second) (minus offgassing), which is based on your travel velocity (feed rate), layer thickness, and extrusion width. For more details, please see steps 2, 3, and 4 in the part of my answer of which you were originally critical. $\endgroup$
    – Davo
    Commented Sep 14, 2020 at 20:37
  • $\begingroup$ i always print with 0.1mm layers, can't go thinner. $\endgroup$ Commented Sep 14, 2020 at 20:45

The discussion about slowing the print speed is important, but in my experience it is not at the root of the problem, and slowing down printing may make it worse.

If ny "heater" you mean the complete hot-end, then I suspect you are jamming in the cooler part of the hot-end. This cooler part is separated from the heater itself by the heat break, which is often a thin-walled metal tube.

The hot side of the heat break is heated by the heater. The cooler-end, the cool side of the heat break, is cooled by two things:

  1. cold filament being moved through it and
  2. airflow from the fan over the heat sink fins.

It is heated by:

  1. Thermal conduction through the thin metal tube
  2. Convection airflow from the hot-end vertically and over the cooler-end
  3. Hot filament being pulled through the heat break during retraction.

When you are printing with a smaller cross-section of extruded filament, the slicer program should adjust the flow rate and speeds based on the lesser volume of plastic extruded for a given pressure. Unfortunately, this is less plastic, so the filament cools the cooler end of the heat break less than it would if the feed rate were higher. As a result, the temperature of the cooler end goes up. If there is insufficient airflow to keep the cooler side below the softening point of filament, the filament softens and jams inside the cooler end.

Of course, if your jam actually is in the hot end, this discussion is not relevant.

But, I have found with my Prusa i3m3 that I have had problems with jamming in the cooler end, especially when I printed filament with a higher melting temperature, ABS in this case. I reduced the area of the airflow so that no air could pass through other than by passing over the heat sink fins, and the ABS printed correctly.

All of 3D printing is a balance of one factor against another. It is hard to lose, though, by increasing the cooler-end temperature. All will seem well until you print something with a lower flow, or a higher retraction rate, or you try using a higher temperature for the same filament. Then the jam can unexpectedly occur, or, perhaps worse, it can be an intermittent jam, or there can be a lot to sticktion, and printing becomes prone to gaps, or occasional under extrusion.

  • $\begingroup$ Ah, now it makes sense - yes, smaller nozzles do create heat creep, in addition to being challenged with high flow due to viscosity. $\endgroup$
    – Trish
    Commented Sep 15, 2020 at 19:38

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