TPU (Thermoplastic Polyurethane) is one of the most popular flexible filaments for 3D printing. It’s strong, durable, elastic, and perfect for making phone cases, seals, RC car tires, vibration dampers, and wearable items.
However, printing TPU comes with a major challenge: stringing.
Because TPU is soft and elastic, it doesn’t behave like rigid PLA or PETG when retracted. If your settings aren’t tuned perfectly, you’ll end up with prints covered in fine hair-like strings, blobs, or even failed sections.
In this comprehensive guide, you’ll learn how to fine-tune your retraction and advanced settings to achieve beautiful, string-free TPU prints—even on Bowden and Direct Drive setups.
Why Does TPU String So Much?
Understanding why TPU strings is the first step toward eliminating the problem.
Main reasons TPU causes stringing:
- It’s highly elastic, meaning it compresses and decompresses during extrusion.
- Retraction commands cause filament to stretch rather than pulling cleanly back.
- TPU resists retraction forces, making traditional retraction settings unreliable.
- It oozes more easily when heated, especially at slow speeds or high temperatures.
Thus, typical retraction settings that work for PLA or PETG can actually make TPU stringing worse, not better.
Key Factors to Control TPU Stringing
There are five primary areas you must optimize:
- Retraction Settings
- Extrusion Temperature
- Print Speed
- Coasting and Wipe Settings
- Filament Dryness
Let’s dive into each with precise adjustments for success.
1. Advanced Retraction Settings for TPU
Retraction must be gentle, short, and slow.
Critical Retraction Settings:
| Setting | Recommended Range for TPU |
|---|---|
| Retraction Distance (Direct Drive) | 0.5–2.0 mm |
| Retraction Distance (Bowden) | 2.5–4.5 mm |
| Retraction Speed | 15–30 mm/s |
| Minimum Travel Distance | 1.5–2.0 mm |
| Retraction Extra Prime | 0–0.1 mm³ |
| Z-Hop | Optional (small, 0.2–0.5 mm) |
Important Notes:
- Shorter and slower retractions prevent the filament from stretching inside the extruder.
- Higher speeds cause filament stretching inside the tube, making stringing worse.
- For Bowden systems, you need slightly longer distances because of tube friction, but keep speeds low.
Cura Retraction Settings Example (Direct Drive TPU):
- Retraction Distance: 1.0 mm
- Retraction Speed: 20 mm/s
- Minimum Travel Distance: 1.5 mm
- Enable Retraction: Yes
- Z-Hop When Retracted: Optional (0.4 mm)
PrusaSlicer Retraction Settings Example (Bowden TPU):
- Retraction Length: 3.5 mm
- Retraction Speed: 20 mm/s
- Minimum Travel After Retraction: 1.5 mm
- Lift Z: 0.4 mm
2. Extrusion Temperature for String-Free TPU
Temperature directly impacts stringing behavior.
Too hot: Filament becomes too runny, oozes easily.
Too cold: Poor layer bonding, underextrusion.
Temperature Guide:
| TPU Brand | Ideal Temp Range |
|---|---|
| eSUN TPU | 210–225°C |
| SainSmart TPU | 220–240°C |
| NinjaFlex | 225–235°C |
| Overture TPU | 210–230°C |
Tips:
- Start at the lower end of the filament’s recommended range.
- Perform a temperature tower test to find the sweet spot where stringing drops without losing adhesion.
- Lower temperatures tend to minimize oozing, but you must balance it with sufficient bonding strength.
3. Print Speed for TPU Success
Slow down for flexible filaments.
Recommended Speeds:
| Setting | Suggested Value |
|---|---|
| Perimeters | 20–30 mm/s |
| Infill | 30–40 mm/s |
| Outer Wall | 15–25 mm/s |
| Travel Moves | 90–120 mm/s |
Slower perimeter speeds allow TPU to lay down smoothly without excessive back-pressure that causes blobbing or oozing.
4. Coasting and Wipe Settings for TPU
Coasting and wipe help control filament pressure before a travel move starts.
What Is Coasting?
- Coasting stops extrusion slightly before a travel move, using residual pressure to finish the current path cleanly.
- This reduces the chance of ooze starting the string.
Suggested Coasting Settings:
| Setting | Recommended Range |
|---|---|
| Coasting Distance | 0.2–0.6 mm |
| Coasting Volume | 0.02–0.08 mm³ |
| Wipe Distance (Optional) | 0.5–1.0 mm |
Slicer Example (Cura):
- Enable Coasting
- Coasting Volume: 0.05 mm³
- Coasting Speed: Match outer wall speed
- Enable Wipe Before Retraction: Optional (use with caution)
Test small prints with coasting enabled, as over-coasting can lead to under-extruded seams.
5. Filament Dryness for TPU
TPU is hygroscopic—it absorbs moisture from the air rapidly.
Wet TPU causes:
- Increased stringing
- Popping sounds during extrusion
- Rough, uneven print surfaces
- Blobs and inconsistent flow
Drying TPU:
- Dry at 40–50°C for 4–6 hours in a filament dryer or food dehydrator.
- Store in an airtight dry box with desiccant between prints.
Never skip drying TPU before important prints—moisture dramatically increases stringing.
Additional Advanced Tweaks for TPU Printing
For those chasing perfect, clean prints:
1. Use Negative Restart Distance (Advanced)
- In some slicers (like Simplify3D), you can set a negative extra restart distance.
- This slightly under-primes after retraction, reducing ooze.
- Try -0.05 mm to -0.2 mm.
2. Tune Pressure Advance or Linear Advance (Firmware)
- Pressure Advance (Klipper) or Linear Advance (Marlin) compensates for pressure build-up inside the hotend.
- Correctly tuned, it drastically reduces stringing, blobbing, and over-extrusion.
- Calibrate it with a Pressure Advance/Linear Advance tower.
3. Enable “Avoid Printed Parts When Traveling”
- Forces the printer to travel around printed perimeters instead of over them.
- Reduces the risk of strings dragging across the model.
4. Minimize Retractions on Short Moves
- Some slicers allow skipping retractions for small travels under 1–2 mm.
- Useful for TPU, which is prone to stringing when retracted unnecessarily on tiny moves.
Common Problems and Solutions for TPU Printing
| Problem | Cause | Fix |
|---|---|---|
| Heavy stringing | Wrong retraction settings, wet filament | Reduce temp, slow down retraction, dry filament |
| Blobs or zits | Over-retraction, wrong coasting | Reduce retraction distance, fine-tune coasting |
| Under-extrusion at start | Excessive coasting or under-priming | Lower coasting slightly, check negative restart distance |
| Poor layer bonding | Too low temperature or airflow issues | Increase nozzle temp, reduce fan |
| Layer shifting | Travel too fast | Slow travel speeds to 90–100 mm/s |
Best Practices Summary for String-Free TPU Printing
✅ Dry filament properly before printing
✅ Use short, slow retractions (0.5–2.0 mm, 15–30 mm/s)
✅ Lower print temperature slightly if stringing persists
✅ Slow outer walls to 20–30 mm/s
✅ Enable coasting and fine-tune for clean travel starts
✅ Use wipe-on-retraction carefully
✅ Keep travel moves fast (but not too fast)
✅ Tune Pressure Advance or Linear Advance if possible
By systematically following these best practices, you can produce flawless, string-free TPU prints—even on challenging models with complex geometries.
Conclusion
Flexible TPU printing doesn’t have to mean fighting endless strings and blobs.
With careful retraction tuning, smart slicer settings, proper filament handling, and printer calibration, you can achieve professional-quality flexible prints that are clean, strong, and visually impressive.
Start slow, test methodically, and you’ll master TPU faster than you ever thought possible.