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It is my first time printing flexible filaments.

I have been trying to print a flexible case for my mini-PC using the TPE 83A by eSun on my KP3S pro.

Whenever I try to print it starts well, then after printing a few strokes, the print systematically fails. It is not an adhesion/levelling problem as I installed a BLTouch probe and made a 12x12 points grid of my 200x200 printing bed, which works perfectly with PLA. It seems that after printing a few lines, the nozzle just stopped extruding filament anymore (see photos); although the extrusion gear still seems to be turning.

FYI my nozzle is a hardened steel nozzle, that I recently acquired to replace the original brass nozzle (because I wanted to print abrasive filament). Could it be the problem? I read that a hardened steel nozzle may not conduct heat as well. I did try a wide range of temperatures and saw the same results; also given the small size of the MK8 nozzle used by the KP3S, I would wager that heat losses due to poor heat conductivity of the nozzle may not be as important no? (I guess that the delta in temperature between the contact point with the heating also depends on the length of the nozzle)

Other information:

  • I print in an enclosure
  • I try to keep my filament dry by using a heating box + desiccant
  • Also the filament doesn't seem to warp, the filament that is on the bed seems to be well adhered. The problem seems to come from the "extrusion system", whereby the filament just stops extruding.
  • I used the recommended printing profile for flexible filament (e.g. reducing speed to 35 mm/s for top speed and to 20 mm/s for the first layers, retraction distance at 1 mm)

Photo of a 3D model printed with TPE filament failing at the edge

Any idea of what might be happening? I gleaned a lot of info from Proper settings for printing rubber and rubber-like materials? on printing flexible filaments. I will perform a bunch of tweaks based on that question to hopefully troubleshoot my way to a proper print through trial and error; however, it would be nice if someone has a better idea that could direct me as to what could be the problem given the symptoms observed.

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2 Answers 2

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After several months of trial and error with this filament, I finally managed to understand the issue. It was a mechanical issue due to the filament sensor of the KP3S Pro.

The out of the box setup of the KP3S pro is with the filament roll being set on rollers and passed into a filament sensor to anticipate situations where filament is running out.

Out of the box setup for Kingroon KP3S

However, this sensor applies pressure on the filament, and it creates a mechanical strain that prevents more elastic materials from being pushed through the extruder.

In order to lessen the mechanical strain on the filament I mounted it over the machine with a spool holder, and stopped using the filament sensor when printing my eSun eLastic TPE 83-A. I found this video, Kingroon KP3S Pro Top Mount Spool Holder in Detail , which uses a KP3S spool mount model which can easily be adapted to the KP3S pro.

Thus the filament feeds directly in the drive, and I have no problem printing this flexible filament.

Top mount for the filament spool

I may (or may not) explore a bit more in the future to see if I can tinker with the pressure of the filament sensor so that I can take advantage of it also with flexible/elastic filaments. But for now I will be printing flexible filament without using the filament sensor.

I won't go in detail about the parameters I used yet for this filament, because I am still exploring the best parameters for this filament. But this issue really was the core of my problem and I could already obtain very satisfactory results with my KP3S Pro printing this eSun TPE-83A.

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Depending on your extruder geometry, you might have to slow down even more. I can't print TPU at 35 mm/s on my extruder, at least.

The problem with flexible materials is that they really don't like to be pushed into a narrow hole. Instead, the filament between the extruder gear and the heatbreak will buckle and bend to the side, forming a loop inside your extruder housing. If that happens, there is no more pressure and the extrusion stops. See Proper settings for printing rubber and rubber-like materials? for a very detailed analysis and tips.

If you use PrusaSlicer or a derivative, a good option to limit the extruder speed for a specific filament is Max Volumetric Speed. That way you can limit the speed for all types of extrusion with a single number, instead of many speed options in the print profile. I'm not sure, but I don't think that Cura has a comparable setting.

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  • $\begingroup$ Thank you for the reply. I am using Cura, I will see about MVS. I will try to further tweak the speed, however, the initial layer speed is 20 mm/s (35 mm/s is top speed, will clarify the post). Thank you for the buckling tip, I had noted it as one of the possibility. However I noted during my tests that when I run multiple extrusion tests with the TPE back to back I have no problem. Is it common that extrusion tests would work but extrusion would fail? Also my extruder [ wiki.e3d-online.com/Titan_Assembly ] is designed with a drive to avoid buckling. $\endgroup$ Dec 14, 2023 at 7:04
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    $\begingroup$ Well, squeezing the molten filament to the bed does create some back-pressure on the extruder. So it is absolutely possible that printing extrusions fail but extruding in thin air works. I guess you will just have to experiment what works, rather than theorizing. $\endgroup$
    – Fritz
    Dec 14, 2023 at 7:53

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