What stringing is and how to confirm it
Stringing happens when molten plastic keeps leaking from the nozzle while the print head moves between two separate areas of the part. The leak gets stretched into a thin thread that cools in mid-air. It is mostly cosmetic, but heavy stringing can fuse into lumps that get in the way of moving parts or snap off and leave marks.
The easiest way to confirm it is a retraction test: two or more thin towers about 30 to 50 mm apart. If wispy threads cross the gap, you have stringing. Very fine "angel hair" that brushes off is mild; thick strands with beads on them point to a more serious oozing problem.
Causes, most likely first
- Printing too hot. Hotter plastic is runnier and drips more during travel.
- Retraction too short or too slow. The filament is not pulled back far enough to relieve pressure in the nozzle.
- Moisture in the filament. Water turns to steam in the hotend and pushes plastic out no matter how well retraction is tuned. Popping or hissing sounds are a giveaway.
- Travel paths that cross open air. Every gap crossed is a chance to ooze.
- A worn or partly blocked nozzle that leaves residue on the tip.
Fixes, cheapest and fastest first
1. Lower the nozzle temperature
Drop the temperature in 5 °C steps and reprint the test. Many PLA filaments string at 215 °C and print cleanly at 195 to 205 °C. Stop lowering when you see weak layer bonding or under-extrusion. A temperature tower makes this quick.
2. Tune retraction
Know your extruder type first. A direct drive extruder usually needs only 0.5 to 2 mm of retraction. A Bowden setup needs 4 to 7 mm because the tube has slack. Start in the middle of the range and change in 0.5 mm steps (direct) or 1 mm steps (Bowden). Retraction speed of 25 to 45 mm/s works for most machines. Going too far causes its own trouble: grinding, gaps at the start of lines, and on all-metal hotends, jams from pulling molten plastic up into the cold zone.
3. Dry the filament
If temperature and retraction changes barely help, dry the spool. Rough guides: PLA 45 to 50 °C for 4 to 6 hours, PETG 60 to 65 °C, ABS 70 to 80 °C, TPU 50 to 55 °C. Use a filament dryer or a food dehydrator; household ovens are often inaccurate enough to soften the spool.
4. Change travel settings
Turn on combing or "avoid crossing perimeters" so the nozzle travels over already-printed areas. Raise travel speed to 150 to 200 mm/s if your printer handles it, which gives less time for oozing. Enable wipe or coasting in small amounts, for example 0.2 to 0.4 mm of coasting.
5. Inspect the nozzle
Heat it and clean the outside with a brass brush. If it still drags strings after everything else, replace it; nozzles are cheap and a brass nozzle wears noticeably after a few kilograms of abrasive or filled filament.
Material notes
- PLA: usually the easiest to fix with a temperature drop and standard retraction.
- PETG: strings more than almost anything else. Expect some fine hairs even when tuned. Keep retraction modest, print near the lower end of its range (about 230 to 240 °C), and dry it often.
- ABS: strings less than PETG; an enclosure helps more with other problems than with this one.
- TPU: use very little or no retraction, often 0 to 1 mm on direct drive, slow retraction speed, and rely on low temperature and slow travel instead. Aggressive retraction can jam soft filament.
Prevention checklist
- Run a temperature tower for each new filament brand.
- Run a retraction test after changing nozzles or hotends.
- Store open spools in a sealed box with desiccant.
- Keep combing enabled and travel speed reasonably high.
- Clean the nozzle tip before long prints.
- Remove the last few wisps with a quick pass of a heat gun; do not linger or the surface will soften.