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

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.

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.

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.