1

I recently upgraded my Ender 3 Pro with a direct drive kit, primarily so I can print soft materials like TPU. I bought a spool of Overture TPU to try it out.

I'm getting really bad stringing on every print, to the point where it's pretty much unusable. This is my first time printing with TPU, so I'm at a loss for what to do.

Direct Drive mod - After modding the printer I did a few prints with PLA, and it works great with very little stringing. So I assume I assembled the mod correctly and that's not the issue.

Material - Reviews of the Overture TPU on Amazon mostly say it has low stringing and are generally positive, so I'm assuming it's not just bad filament.

Temperature - The consensus in the Amazon reviews seems to be that 230°C is the sweet spot. I did a temperature tower from 205 °C to 230 °C by 5 °C, and the print quality and stringing was pretty much uniform throughout.

Retraction - I tried with retraction off, then with 1 mm, 2 mm, and 6.5 mm, in various combinations with temperature and speed. Results varied in details, but all were pretty bad.

Speed - I tried slowing down to 15 mm/s (from Cura's default of 50). Results got even worse.

Following are photos of a stringing test with various combinations of settings.

PLA - 205 °C - Layer thickness 0.2 mm - Retraction 1 mm PLA for reference - 205 °C - Layer thickness 0.2 mm - Retraction 1 mm

TPU - 205 °C - Layer thickness 0.2 mm - Retraction 2 mm - Print speed 15 mm/s TPU - 205 °C - Layer thickness 0.2 mm - Retraction 2 mm - Print speed 15 mm/s

TPU - 230 °C - Layer thickness 0.2 mm - Retraction 1 mm - Print speed 15 mm/s TPU - 230 °C - Layer thickness 0.2 mm - Retraction 1 mm - Print speed 15 mm/s

TPU - 230°C - Layer thickness 0.2 mm - Retraction 1 mm TPU - 230 °C - Layer thickness 0.2 mm - Retraction 1 mm

TPU - 230 °C - Layer thickness 0.2 mm - Retraction 2 mm TPU - 230 °C - Layer thickness 0.2 mm - Retraction 2 mm

TPU - 230 °C - Layer thickness 0.2 mm - Retraction 6.5 mm TPU - 230 °C - Layer thickness 0.2 mm - Retraction 6.5 mm

TPU - 230 °C - Layer thickness 0.12 mm - No retraction TPU - 230 °C - Layer thickness 0.12 mm - No retraction

TPU - 240 °C - Layer thickness 0.2 mm - No retraction TPU - 240 °C - Layer thickness 0.2 mm - No retraction

Is there something else I should try to reduce stringing? Is it possible that I just got a bad spool of TPU? Or is it more likely something's wrong with my printer?

Robert
  • 347
  • 5
  • 14

4 Answers4

1

Due to the flexible characteristics of TPU, stringing is a known "complication" with this material.

is a video from Matterhackers:
  1. no retraction - stringing is expected
  2. dry filament - ensure the filament hasn't been exposed to the air for extended periods - use an oven or dehydrator if needed

There are many other tutorials and videos covering the process.

Tom Sanladerer has also a video covering the use of flexible filament.

  1. ensure correct alignment of hobbed bolt and feed hole
  2. consider to use a piece of PTFE tubing downstream of drive mechanism
  3. use 3 mm (usually 2.85 mm nowadays) filament for better results (if possible)
  4. bed temp - 40 °C with glue stick on bed
  5. print slower
  6. adjust extrusion multiplier by testing various settings

Obviously some of the above references don't apply or are being observed.

0scar
  • 32,029
  • 10
  • 59
  • 135
fred_dot_u
  • 10,532
  • 1
  • 10
  • 24
  • I've deleted my previous comment because it was unnecessarily hostile and not particularly helpful, but I still don't think this is a good answer. The internet, including youtube, reviews of filament, SEO-optimized sites full of fluff articles about 3D printing, and apparently even content from filament manufacturers, is full of unjustified, often contradictory, and outright wrong information about slicing/print settings, and the first video cited by this answer is one of those. It's really disappointing that the vendors selling flex materials don't even know how to use it... – R.. GitHub STOP HELPING ICE Sep 05 '20 at 23:17
  • Tom Sanladerer is a much better source for accurate information, but much of the advice you cited is not particular to OP's problem. Bed temperature and extrusion multiplier (assuming the latter hasn't already been cranked up too far) are not going to be the cause of stringing, and filament diameter is outside the scope of what OP can reasonably change. About the only things in this answer that might be relevant to solving OP's problem are points 1, 2, and 5 on the Tom Sanladerer video. I've elaborated in my answer on how they might be involved. – R.. GitHub STOP HELPING ICE Sep 05 '20 at 23:21
1

Your printer is capable of printing TPU out of the box, and very well if you upgrade the firmware to Marlin 2.0. That you're having problems like this suggests that you messed something up with the direct drive kit. TPU, being so flexible, is very sensitive to having any excess play in the path between the extruder gear and the hotend. I'm not familar with this particular direct drive kit, but knowing how the Ender 3's bowden tube works, with the PTFE extending all the way to where the nozzle seats, it seems likely that it was removed and replaced as part of the conversion, and if this was not done right, there are all sorts of ways the TPU could kink between the extruder gear and the nozzle, messing up the relationship of extruder moves to actual extrusion, and in particular messing up retraction completely.

Speaking of retraction, it must be on, and properly configured to print anything accurately. When you don't retract, or don't retract sufficiently, there is still high pressure at the nozzle even when you're not trying to extrude anything (during travel moves), and material will exit the nozzle, then will be unavailable to extrude in sufficient quantity where it belongs later. Advice to turn retraction off is just wrong, always.

If you use Marlin 2.0 with Linear Advance and calibrate the spring constant right for TPU, you can use the same retraction distance that works for PLA. If you're stuck with the original firmware that doesn't have it, you need to add to the retraction the length that the TPU will compress when printing at the speed you want to print at (and you will get inaccuracies when speed varies from that). I had to use 12 mm retraction with flexible filaments when using the original Ender 3 firmware and printing around 30 mm/s. This was with the bowden, so the distance needed may vary with your modified setup. (And again, check that nothing is wrong with it! Installation errors or product design flaws seem like the most likely cause here.)

Also, see this self-answer from when I first started printing with flexible filaments on an Ender 3. The biggest problem was the severe over-priming in Cura's default start gcode for the printer, which left so much excess pressure (filament compression) after priming that no amount of retraction would stop the stringing. Fixing that was the single biggest ingredient in getting rid of my stringing problems.

R.. GitHub STOP HELPING ICE
  • 10,302
  • 4
  • 24
  • 57
1

I just installed a direct drive myself and noticed it was stringing.

What fixed it for me was lowering the flow rate. Ultimately the E-steps most likely need to be recalibrated but just turning my flow rate down to 85 % got rid of all the stringing. Basically, I'm over extruding, since my slicer and Octoprint both have options for flow rate modifier I might not even touch the E-steps, and swap between Bowden and direct drive depending on the job.

Greenonline
  • 5,831
  • 7
  • 30
  • 60
user26780
  • 11
  • 1
1

Something similar happened to me and it was a combination of E steps calibration and a gap between the hot end and the PTFE tube. fixed both and ran a few temperature and retraction towers to set the values that worked for my TPU (sainsmart) You are charging a lot on how the material is delivered and manipulated by the printer when migrating to a direct drive

This video made it easier for me :) Hope this helps.
Jesus Reyes
  • 11
  • 1