Damp TPU: strings, bubbles and a rough surface; how to dry it and print it with fewer strings

Symptom

With TPU there are lots of strings and blobs between the parts of the print, the filament pops or bubbles as it leaves the nozzle, the surface is rough or full of holes and the layers bond worse than usual.

Likely causes, from most to least likely

1. Moisture: TPU is very hygroscopic and, when damp, causes strings, bubbles and weaker layer bonding (Bambu Lab). Creality asks for it to always be dried before use and kept dry while printing.
2. A spool left out or stored without desiccant: Bambu Lab asks for it to be stored below 20% relative humidity and dried again if it has been stored for a long time.
3. High temperature: TPU flows very easily and, with the nozzle too hot, it oozes (Bambu Lab).
4. Many travel moves between parts: printing several at once layer by layer multiplies the strings (Bambu Lab).
5. Too little retraction for the oozing, or residue of other materials inside the nozzle (Bambu Lab).

Checks to tell which one it is

• Purge a little: if it pops or bubbles, the TPU is damp.
• Think about how long the spool has been open and where you keep it; a hygrometer in the box tells you the real humidity.
• See whether the strings appear mainly between separate parts: then travel moves play a part.
• Compare your profile's temperature with the filament maker's.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Dry the TPU carefully

   Dry the spool before printing at its maker's temperature: Prusa, 60 °C for 4-6 h for its TPU; Bambu Lab, 70 °C for 8 h in a convection oven for its TPU 85A and 90A; Creality, 70 °C for 8-12 h in its dryer. Use a dryer or a temperature-controlled oven, never a microwave or kitchen oven (Bambu Lab), and do not exceed what the spool can take (Creality).

   Check: It no longer pops when purging and there are fewer strings.

   If that does not fix it: Print from a dry box (next step).

2. Print from a dry box

   Print with the spool inside an airtight box with desiccant, taking the filament out through its side outlet, below 20% humidity (Bambu Lab). When drying, TPU expands and the spool may widen slightly: Bambu Lab says this is normal and suggests placing a heat-resistant weight on top.

   Check: Quality is the same at the start and end of the print.

   If that does not fix it: Lower the temperature and flow slightly (next step).

3. Lower the temperature and flow slightly

   If it oozes, lower the nozzle by about 5 °C and, at the same time, the maximum volumetric speed by about 0.4 mm³/s (Bambu Lab): TPU flows very easily and, slightly cooler, it oozes less.

   Check: There are fewer strings and the part still bonds well.

   If that does not fix it: Reduce travel moves (next step).

4. Fewer travel moves and a little more retraction

   Print one part at a time (print sequence by object) instead of layer by layer, or put fewer parts on the plate (Bambu Lab). If it drips a lot, raise retraction by 0.2 mm over your current value (Bambu Lab) and do not overdo it: with flexibles, less retraction means less risk of jams (Prusa).

   Check: Strings between parts disappear or drop a lot.

   If that does not fix it: Clean the nozzle (next step).

5. A clean nozzle

   Before TPU, do several cold pulls or fit a new nozzle, especially if you used carbon or glass fibre-filled materials before (Bambu Lab).

   Check: TPU flows continuously with no dark residue.

   If that does not fix it: If strings persist with dry filament and a clean nozzle, Bambu Lab suggests a larger nozzle and touching up the part afterwards.

Prevention

• Always keep TPU in an airtight box with desiccant and dry it if it has been left out.
• Dry it again if it has been stored for a long time (Bambu Lab).
• If there are several parts, print them one at a time.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Material: TPU.

Official sources

This guide summarises the official documentation linked here:

• TPU Printing Guide (storage and stringing) · Bambu Lab
• TPU 85A&90A printing guide for P series · Bambu Lab
• Filament Drying Recommendations · Bambu Lab
• Drying filament · Prusa Research
• Filament Pre-Printing Guide: Drying · Creality
• Flexible materials (material guide, retraction) · Prusa Research