Why PETG Leaves Strings Between Parts
Fine hairs, wisps, and cobwebs appear when molten PETG continues to ooze from the nozzle during travel moves. It is impossible to stop the flow completely because the plastic remains hot and pressurized. The goal of tuning is to reduce this pressure before travel and shorten the time the nozzle spends moving over open space.
The problem is usually more noticeable with PETG than with PLA: PETG is generally printed at a higher temperature, remains viscous longer, and adheres more strongly to the nozzle. For this reason, stringing should not be addressed by immediately maximizing retraction. Start by checking filament moisture, printing temperature, and the condition of the hotend.
Rule Out Wet Filament First
Wet PETG may hiss, crackle, produce small bubbles, create a rough surface, and leave large amounts of fine stringing. Retraction provides little improvement in this situation because moisture turns into steam inside the melt and pushes plastic out of the nozzle.
How to Check the Spool
- Heat the nozzle to the normal printing temperature and manually extrude 50–100 mm of filament.
- Listen carefully: repeated popping, hissing, and visible bubbles indicate moisture.
- Compare a fresh section of the spool with filament that has been left exposed for several days.
Dry PETG according to the recommendations from the spool or filament dryer manufacturer. Household ovens may overheat relative to the set temperature, so using one without a separate thermometer is risky. After drying, store the spool in an airtight container with desiccant.
Find the Lowest Reliable Printing Temperature
The higher the temperature, the more fluid the melt becomes and the more easily it stretches into strings. However, lowering the temperature too far reduces layer bonding, creates a matte surface, and may cause under-extrusion. The goal is to find the lowest temperature at which the layers remain strong and the surface remains even.
- Use a small temperature tower or another model divided into several sections.
- Configure temperature changes by height in the slicer and verify them in the preview.
- Print at the same speed, layer height, and cooling settings that you plan to use later.
- Evaluate stringing, wall appearance, bridging, and layer adhesion.
Use the working range printed on the spool label. Start near the middle of that range and adjust the temperature in small increments. If the tower is too small, the hotend may not have enough time to stabilize after each temperature change.
Tune Retraction Without Overdoing It
Retraction pulls the filament back before a travel move and reduces pressure in the nozzle. The main parameters are retraction distance and speed. Too little retraction leaves strings, while too much may pull softened plastic into the cold zone, increase the risk of clogs, and cause gaps after travel moves.
Starting Values
| Extrusion System | Distance for the First Test | Speed for the First Test |
|---|---|---|
| Direct drive | About 0.5–1.0 mm | About 20–35 mm/s |
| Bowden | About 3–5 mm | About 25–40 mm/s |
These are not universal settings, but starting points for common designs. The limits specified by the extruder and hotend manufacturers take priority over this table. Excessive retraction is especially undesirable with some all-metal hotends.
How to Run the Test
- Print a model with two or four towers.
- Change only one parameter at a time: first retraction distance, then retraction speed.
- For direct drive, increase the distance in small steps, such as 0.2 mm. Larger steps may be appropriate for Bowden systems.
- If gaps, extruder clicking, or delayed extrusion appear after travel moves, return to the previous value.
Do not try to eliminate stringing only by increasing retraction speed. An excessively abrupt movement may grind the filament and cause the drive gears to slip.
Tune Travel Moves in the Slicer
Cura, PrusaSlicer, OrcaSlicer, and other slicers use different names for these settings, but the underlying approach is the same: move faster, cross open areas less often, and avoid unnecessary retractions.
- Travel speed. Increase travel speed within the mechanical limits of the printer. The shorter the travel time, the less plastic can ooze from the nozzle.
- Avoid crossing perimeters or Combing. These modes try to keep the nozzle inside already printed areas, although they may sometimes create longer travel paths.
- Retract on layer change. This can help when strings appear during transitions to a new layer, but it also increases the number of retractions.
- Minimum travel before retraction. A value that is too low causes retraction on every short move.
- Wipe while retracting. During retraction, the nozzle moves briefly along an already printed line to release some pressure.
After changing these settings, inspect the travel paths in the preview. If the nozzle crosses every open area in the model, rotating the part or printing the objects separately may be more effective.
Check Cooling, Speed, and Flow
PETG cooling should be sufficient for bridges and small features, but excessive airflow reduces layer adhesion. Cooling is a secondary factor in stringing control: moisture, temperature, and retraction should be corrected first.
Check the flow ratio after calibrating the extruder feed. Excessive flow increases nozzle pressure, makes the seam protrude, and produces thicker strings. It is better to adjust flow using a measurable calibration model or a dedicated test rather than relying only on the appearance of the first layer.
Make Sure the Hotend Is Working Correctly
Stringing may result from a mechanical problem. With the printer cold, check whether the heater block is loose and whether there is plastic on the threads. Leakage above the heater block often occurs when the nozzle is not tightened against the heat break inside the heated hotend.
- Clean the outside of the nozzle according to the printer manufacturer’s instructions.
- Inspect the silicone sock: PETG stuck to the heater block may drip onto the model.
- Make sure the hotend heatsink fan is running when required by the printer design.
- If clogs keep recurring, inspect the PTFE tube and the extruder gears.
How to Distinguish Stringing from Seams and Blobs
| Symptom | Likely Cause | What to Check First |
|---|---|---|
| Fine cobwebs between towers | Temperature, moisture, retraction | Drying and a temperature test |
| Thick branches or blobs | Excessive flow, high temperature, dirty nozzle | Flow and hotend condition |
| Small bumps in one vertical line | Layer seam | Seam position |
| Gap after a travel move | Retraction is too long or too fast | Reduce distance and speed |
| Hissing and roughness across the entire part | Wet filament | Dry the spool |
Recommended Tuning Order
- Dry the PETG and clean the nozzle.
- Check the filament feed mechanism and make sure the hotend is not leaking.
- Use a temperature tower to select the lowest stable printing temperature.
- Tune retraction distance first, then retraction speed.
- Increase travel speed within safe limits and configure perimeter avoidance.
- Check flow, cooling, and seam placement on an actual part.
Save a successful setup as a separate profile named after the material, nozzle diameter, and printer. Do not copy retraction settings blindly between different machines: the filament path length, extruder type, hotend, and specific spool can all significantly affect the result.