Flexible filaments like TPU (Thermoplastic Polyurethane) are amazing for printing durable, bendable parts. However, one of the most common problems when printing with TPU is stringing. Tiny strands of filament stretch between parts of the print, leaving webs that ruin surface quality and appearance.
Fortunately, with proper tuning and understanding, stringing in TPU prints can be greatly reduced or eliminated. This guide explains the causes of TPU stringing and the best ways to prevent and fix it.
Why Does TPU String So Easily?
TPU is inherently more prone to stringing than rigid materials like PLA or PETG because of its physical properties:
- Elasticity: TPU stretches easily, making it harder to cleanly stop extrusion during travel moves.
- Low viscosity: Melted TPU tends to flow more readily, oozing from the nozzle even when not extruding.
- High temperature requirement: Higher nozzle temperatures encourage oozing if not controlled properly.
Understanding these challenges helps in applying the right strategies to control stringing effectively.
Key Techniques to Remove Stringing in TPU Prints
1. Optimize Retraction Settings
Retraction pulls filament back into the hotend during travel moves to prevent oozing. However, with TPU, retraction must be tuned carefully.
Best Practices:
- Retraction Distance: Lower than rigid filaments. Use around 2mm to 4mm for Bowden setups and 1mm to 2mm for direct drive systems.
- Retraction Speed: Slow and steady. Set around 20mm/s to 30mm/s. Fast retractions can stretch TPU and cause jams.
- Extra Restart Distance: Sometimes a small negative restart distance can help prevent blobs at the end of retraction.
Testing small changes and running simple retraction towers help dial in the best values.
2. Lower Printing Temperature
Higher temperatures make TPU flow too easily, encouraging stringing.
Ideal Temperature Range:
- Start around 210°C to 220°C for standard TPU brands.
- Lower by increments of 5°C while monitoring extrusion quality.
However, too low a temperature may cause under-extrusion. Find the balance where flow is smooth but oozing is minimized.
3. Enable Coasting
Coasting is a slicer feature where extrusion stops slightly before the end of a print move to allow pressure in the nozzle to finish pushing out filament.
Advantages for TPU:
- Reduces blobs and strings at travel starts and stops.
- Eases pressure buildup inside the nozzle.
Typical Settings:
- Coasting Volume: Start with 0.05mm³ to 0.2mm³ depending on flow rate.
- Adjust based on small tests until stringing reduces without causing gaps.
4. Increase Travel Speed
The faster the nozzle moves during non-printing moves, the less time filament has to ooze out.
Recommended Travel Speeds:
- 120mm/s to 150mm/s for most printers handling TPU well.
- Avoid very low travel speeds like 60mm/s which give TPU time to leak.
Quick travel moves snap any stretched filament and leave less residue between parts.
5. Minimize Unnecessary Travels
Smart slicer settings reduce the number of travel moves across open spaces.
Techniques:
- Enable Combing Mode: Keeps travel moves inside printed areas where stringing is less noticeable.
- Optimize Travel Path: Slicers like Cura and PrusaSlicer can reduce unnecessary nozzle movements.
Reducing travel distance automatically lessens opportunities for stringing.
6. Dry the TPU Filament
Moisture is a major contributor to stringing. TPU absorbs water from the air faster than many rigid filaments.
Symptoms of Wet TPU:
- Excessive stringing
- Popping noises during printing
- Rough or inconsistent extrusion
Solution:
- Dry filament at 50°C to 55°C for 4 to 6 hours before printing.
- Store in airtight containers with desiccant when not in use.
Dry filament prints cleaner, stronger, and with much less stringing.
7. Shorten Retraction Distance on Direct Drive Systems
Direct drive extruders require much shorter retraction settings compared to Bowden setups. Beginners often mistakenly use Bowden-style retraction on direct drives.
Typical Settings for Direct Drive:
- Retraction distance around 0.8mm to 2mm maximum
- Retraction speed of 20mm/s to 25mm/s
Fine-tuning direct drive retractions solves many stringing problems without causing jams.
8. Check for Nozzle and Hotend Leaks
Minor leaks around the nozzle or heat break can cause extra filament to ooze and string.
How to Inspect:
- Heat the nozzle to printing temperature.
- Manually extrude filament and observe for leaks at the nozzle throat or hotend junction.
Solutions:
- Properly torque the nozzle while hot to ensure a tight seal.
- Regularly maintain and clean hotend assemblies.
Preventing leaks reduces random drips and improves print cleanliness.
9. Slow Down Overall Print Speed (If Necessary)
While faster travel speeds help reduce stringing, printing perimeters too quickly with TPU can cause poor surface quality and uneven extrusion.
Balanced Strategy:
- Fast travel moves
- Moderate perimeter speeds around 20mm/s to 30mm/s
Slow perimeter speeds allow flexible filament to extrude smoothly without pressure inconsistencies.
10. Fine Tune Z-Hop Settings (Optional)
Z-Hop lifts the nozzle during travel moves to prevent dragging across printed areas. While not always necessary, it can slightly help in certain stringing cases.
Best Settings:
- Z-Hop height around 0.2mm to 0.4mm
- Use only if dragging artifacts appear during prints
Overusing Z-Hop can lead to longer print times, so use judiciously.
Step-by-Step Checklist to Prevent TPU Stringing
Before starting a TPU print, quickly verify the following:
- Filament is dry
- Retraction settings are customized for TPU
- Coasting is enabled
- Printing and travel speeds are optimized
- Nozzle and hotend are leak-free
- Slicer settings reduce unnecessary travel
- First layers extrude cleanly without blobbing
Taking five minutes to check these parameters can dramatically increase print quality.
Common FAQs About Removing Stringing in TPU Prints
Why does TPU string more than PLA?
TPU is more elastic and has a lower viscosity when melted compared to PLA. These properties make it harder to control the flow during non-printing movements, resulting in more stringing.
How important is filament drying for TPU?
Extremely important. Even a few hours of exposure to air can introduce enough moisture to cause stringing and popping during printing. Always dry TPU before use.
What is the ideal retraction distance for TPU?
In Bowden setups, 2mm to 4mm is typical. In direct drive systems, 0.8mm to 2mm works best. Always test small changes to find the perfect value for your machine.
Can travel speed alone solve stringing in TPU?
Increasing travel speed helps reduce stringing but is not a complete solution. Travel speed must be combined with optimized retraction, coasting, and temperature control for best results.
Should I use part cooling fans when printing TPU?
Minimal cooling is recommended. Around 20 percent to 30 percent fan speed can help with surface finish without causing layer adhesion problems. Avoid strong cooling unless absolutely necessary.
Will coasting settings affect layer bonding?
If overused, coasting can create gaps at the ends of perimeters. Start with small values and increase slowly while checking for extrusion consistency.
Conclusion
Stringing in TPU prints can be frustrating, but it is entirely manageable with the right approach. By carefully optimizing retraction, temperature, travel moves, coasting, and filament drying, users can achieve smooth, clean prints even with flexible materials. Each printer and filament brand behaves slightly differently, so patience, fine-tuning, and small incremental changes are key to success. Mastering TPU opens the door to durable, functional, and professional-quality flexible 3D printed parts.