Stringing and Oozing

Likely causes, most common first

1. Wet filamentMost common
   Prusa's drying guide says polyamide (nylon), PVA and TPU need drying more often than PLA, and that wet filament releases moisture in the hotend and causes stringing. PETG is also in Prusa's drying table. Spools left open in a humid room are the usual source.
2. Nozzle temperature too highVery common
   Hotter plastic is runnier and oozes more during travel. Prusa names very high printing temperatures as a main cause and suggests lowering the temperature in 5 to 10 °C steps Starting point, staying within the range on the filament label.
3. Retraction not tuned for your extruderCommon
   Retraction pulls the filament back before a travel move. The right length depends heavily on the extruder: Prusa's own presets use a longer retraction on its Bowden printer than on its direct drive printers Maker spec. Copying a number from a different machine is a common cause.
4. Travel moves over open spaceCommon
   Every move across a gap is a chance to leave a string. Slicer options that avoid crossing perimeters, retract only when crossing them, and wipe while retracting reduce how often the nozzle travels over open air.
5. Dirty or partly clogged nozzleOccasional
   Prusa notes that residue in the nozzle, for example after a long run of one material such as PETG, can make strands stick to the print. A partial clog can also make extrusion inconsistent.
6. Hotend heat or assembly problemsLess common
   Prusa lists poor heat dissipation in the hotend and missing or misplaced parts after a hotend change as causes, and suggests checking the assembly against the instructions.

Fix it, one change at a time

1. Dry the filament using the filament maker's drying temperature and time, or swap to a fresh, sealed spool. Do not exceed the stated drying temperature: Prusa warns the filament will soften and stick together. Store it sealed with desiccant afterward.
2. Print a temperature tower. The OrcaSlicer calibration guide describes the temperature test as quick, easy to read and usable with any material on any printer. Pick the coolest temperature that still gives good layer bonding and surface, inside the filament maker's printed range.
3. Clean the nozzle and remove residue from previous materials.
4. Run a retraction test on your own printer. OrcaSlicer's retraction test prints a tower with a different retraction length at each section. Its defaults of 0 to 2 mm in 0.1 mm steps Starting point suit most direct drive extruders; for Bowden extruders it suggests widening the range to about 1 to 6 mm in 0.2 mm steps Starting point. Use the shortest length that gives a clean section. OrcaSlicer recommends doing this after flow and pressure advance calibration.
5. Tune travel settings. Turn on the options to avoid crossing perimeters and to retract only when crossing perimeters, and keep retract on layer change and wipe while retracting enabled if your slicer has them.
6. Check retraction speed. Prusa suggests a higher retraction speed can help, but not so high that the extruder motor skips. If you change it, retest.
7. Check the hotend if nothing above helps: confirm it is assembled correctly, with nothing loose or leaking, following the printer maker's instructions.

Confirm it is fixed

Print a small stringing test (two or more thin towers a short distance apart) with the new settings and the same filament. The gap between towers should be clean, or have only a few fine wisps that brush off. Then print the real part. If stringing comes back weeks later on the same spool, the filament has picked up moisture again.

Why it happens

When the nozzle finishes one section and travels to another, it stops extruding but the plastic in the hot nozzle is still molten. Some of it oozes out under its own weight and pressure, touches the part, and stretches into a thread as the nozzle moves away. Everything that makes the plastic runnier (more heat), adds pressure (steam from moisture), or gives it more time and distance to ooze (long travel over open space) makes stringing worse.

Retraction fights this by pulling the filament back a short distance before the travel, relieving the pressure in the nozzle. On a direct drive extruder the motor sits right above the hotend, so a short retraction moves the plastic in the nozzle immediately. On a Bowden extruder the motor pushes filament through a long tube, and some of the retraction is taken up by slack and compression in the tube, so it needs a longer retraction to have the same effect. That is why there is no universal retraction number and why the retraction test is worth the few minutes it takes.

Moisture is the cause people most often miss, because nothing about the settings has changed. Water absorbed by the filament boils in the hotend, which pushes extra plastic out during travel and can leave a rough, bubbly surface. A spool that printed clean when new can start stringing after a few weeks open in a humid room.

Fine wisps that brush off with a fingernail are cosmetic. Prusa notes that a quick pass of a heat gun can remove them from a finished print, but keep it brief: longer heat can deform the part. For details on fine relief prints where strings between raised features are hard to clean, see Printing a CNC Relief STL.