For a school project, my teacher gave me a brand new Longer 3D printer to create parts with. After one good job, the printer cannot get past the raft creation without air printing. The thickness of the printed filament tapers down until no filament is coming out. I do not believe there is a jam in the hot end because if I remove the filament and cut off about 4 inches, the printer extrudes as normal.

I am using 1.75 mm PLA on a Longer 3D LK4.

When the problem occurs, the extruder continues as normal but prints out very little filament. The extruder drive gear also ticks backward as if it cannot supply enough force to move the filament forward.

I have tried decreasing the speed, increasing the temperature, turning off retractions, but none of those options seemed to have any effect on the problem.

If videos or pictures could be helpful let me know what to include.

My working idea of what's wrong is that heat creep is causing the filament inside of the heat sink to soften causing the filament to thicken and Jam. I could be wrong.

Failed Raft

Figure one highlighting how the filament thins over the course of the raft creation until no more filament is extruded.

Thickened Filament

Figure two showing how random parts of the filament is thicker than others. The thickness is noticeable and once I cut this section off, filament flows freely through the Bowden tube without friction. With this section, the filament is difficult to remove and requires more force.

I have tried:

  • increasing the interface thickness to 0.4 from 0.27 mm and from 0.4 to 1.0 mm
  • increasing the print speed from 50 mm/s to 90 mm/s allowing the printer to cool off. decreasing the temperature to 190 °C

During the print, I have noticed that the heat sink is hot to the touch during the raft creation but cools off once the fan starts spinning.

The fan does not spin at all during the creation of the raft. But spins at full speed once the actual print has begun. Longer does not have a setting inside their software to change this.

I have gotten a few really small prints to work with print surfaces smaller than 2 cm wide

The prints themselves turn out great if they get past the raft stage (only for small prints). This additionally leads me to believe that heat creep is the problem and that the fan not being on is the cause.

Let me know if you have any ideas.

  • $\begingroup$ Here's links with heat creep and a Bowden tube: Here's links with heat creep and a Bowden tube:; 3dprinting.stackexchange.com/questions/8193/… : $\endgroup$
    – Perry Webb
    Commented Feb 25, 2021 at 18:37
  • 1
    $\begingroup$ People describing heat creep with a Bowden tube describe the filament widening at the end like your picture $\endgroup$
    – Perry Webb
    Commented Feb 25, 2021 at 18:40
  • $\begingroup$ huh... The filament might be uneven... $\endgroup$
    – Trish
    Commented Nov 18, 2022 at 16:25

3 Answers 3


I have noticed that the heat sink is hot to the touch during the raft creation but cools off once the fan starts spinning.

The fan cooling the hot end should always be "on" when you start the printer, or at least cool the hot end when the print starts (it is possible if you use a Raspberry Pi and OctoPrint to control relays to have the fan switch on just before starting printing).

If not, the cold end (radiator) of the hot end gets to hot and your filament will soften way too soon. This phenomenon is called heat creep. Looking at the filament you pictured, heat creep is definitely a possible cause. Another good hint is the diminishing filament output.

It may be that the radiator cooling fan is accidentally connected to the incorrect port on the controller board. What you describe is a fan kicking in after the first layer of printing, this is typical behavior of the part cooling fan.


Your symptom sounds like heat creep might be stopping the extrusion. See if this discussion helps you troubleshoot: Understanding all the ways to avoid heat creep

Note: decreasing speed and increasing temperature makes heat creep worse.

Your picture of filament shows what people with Bowden extruders describe as typical of heat creep. The Bowden tube isolates the filament drive motor from the temperature of the extruder. On well put together Bowden tube extruder, there is no place for softened filament to bunch up. So, heat creep occurs when the melted filament goes up the region of the heat break in the heat sink. Thus, to determine heat creep one needs to ask, "Is the expanded part of the filament extending into the heatsink.

  • $\begingroup$ Thank you for the information. I believe you may be correct and that post is extremely helpful. I noticed that for some reason Longer does not run the fan at all during raft creation. That may be causing the issue. I also noticed that when I pull the filament out, the section near the hot end is noticeably thicker than the rest of the filament. Would this be a symptom of heat creep? $\endgroup$
    – Chris Howe
    Commented Feb 25, 2021 at 7:33
  • $\begingroup$ A thicker melted end can be normal, especially if you cold pull. What can happen with heat creep is the normal gear marks pushing the filament through the hot end, then a notch where the gear spins and removes the softened filament instead of pushing it. You may want to post a picture. $\endgroup$
    – Perry Webb
    Commented Feb 25, 2021 at 9:55
  • $\begingroup$ I added a picture to the link in the answer 3dprinting.stackexchange.com/questions/15629/… $\endgroup$
    – Perry Webb
    Commented Feb 25, 2021 at 13:41
  • $\begingroup$ Thank you for all your help Mr. Webb, but this issue is still persisting. I have updated my post if you are available to give it another look. $\endgroup$
    – Chris Howe
    Commented Feb 25, 2021 at 17:28
  • $\begingroup$ The Bowden prevents heat creep from getting to the filament where the drive motor pushes it. So, heat creep will look different on your filament. $\endgroup$
    – Perry Webb
    Commented Feb 25, 2021 at 17:37

As you can see on picture 1, on the left-bottom side there is extruded less material than on the right-top side. This, to me, is an indication that your bed might not be flat. When the left-bottom side is a few microns closer to the nozzle than the right-top side, the nozzle will push onto the bed and that prevents it from extruding. A sign of this will be extruder skipping. As you mentioned, keeping the fan at least running slowly during raft creation is always a good idea.


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