Bambu 85A TPU sudden under-extrusion issues

Bambu Lab H2 Series Troubleshooting
1

Hello community!

I would appreciate some input from users with experience in TPU printing to narrow down this issue that has been bugging me for a few days now, and I am running out of ideas to try next.

I have done some successful printing with a Bambu 90A TPU spool on a TPU-dedicated 0.6mm nozzle, and recently got a Bambu 85A TPU spool. I did a couple of small prints, and then I tried to print this model that I have published here: Wheel rim protection pad for Thule bicycle rack - Free 3D Print Model - MakerWorld

This is a very slow print on the 85A - takes more than 10 hours. As you can see from the timelapse below the print was progressing beautifully, but towards the end (roughly at the 7-8 hour mark) I started experiencing severe under-extrusion issues. But my biggest problem is not this single failed print, but that this issue seems like it is now persistent and shows up from the very first layer of any subsequent attempt to use this specific 85A TPU spool.

output

last-frame

I suspected and tried the following, but I am still unable to figure out what’s wrong here.

  1. Filament drying: The filament was dried in an AMS-HT at 75C for 18 hours (that’s the default recommended setting when you select the TPU drying option) right before I started the print, and I feed the TPU directly (no PTFE tubes involved) with the Retracting Spool Holder Pro: Retracting Spool Holder Pro (+SPiNE for P1/X1/H2) - Free 3D Print Model - MakerWorld

  2. Friction that hinders pulling of the filament in the extruder: I ruled this one out as the direct feeding with the Retracting Spool Holder doesn’t involve any PTFE tubes and the spool is nearly free rolling - sometimes too much!

  3. Partial nozzle clog: I did a several cold pulls with TPU, and the nozzle looks in pristine condition: it is a dedicated TPU nozzle that has printed less than 0.5kg of Bambu TPU 90A so far.

  4. Extruder clogging: based on BambuLab’s trouble shooting guide in How to Disassemble and Assemble H2D Extruder | Bambu Lab Wiki :

    You can start by removing the hotend and observing if the filament can be extruded properly. If the filament extrudes normally after the hotend is removed, it indicates a hotend clog rather than an extruder clog.

    I did this, and the filament extrudes normally after the hotend is removed - so I also ruled out an Extruder clogging issue.

  5. TPU heat creep: I could potentially explain the failed print in the time lapse above with TPU heat creep issues after several hours of printing, but I doubt that TPU heat creep is the issue here as the problem is now persistent from the very first layer even after I re-dried the filament and tried to print again the next day.