What is Causing These Tendrils on My Inside Contours?

Few things are more frustrating than a 3D print that looks perfect on the outside but is filled with thin, hair-like "tendrils" or "branches" on the inside contours and holes. Unlike standard stringing, which usually looks like a fine cobweb, these tendrils are often thicker and appear to grow specifically from the inner walls. In the 3D printing community, this is typically caused by a combination of oozing during travel moves and specific slicer pathing logic.

1. The "Combing" Setting (Cura Users)

The most common cause of tendrils on inside contours is a slicer feature called Combing. Combing tells the printer to keep the nozzle within the printed area during travel moves to avoid retractions and reduce stringing on the outer surface.

• The Problem: When the nozzle travels across an "inside hole" (a void), it may still think it is "inside" the part. If it oozes slightly during this move, the plastic sticks to the inner wall, forming a "tendril" every time it passes.
• The Fix: Change your Combing mode to "Within Infill" or "Not in Skin." Alternatively, set a "Max Comb Distance with No Retract" (e.g., 2mm or 3mm) to force a retraction for longer moves across holes.

2. Insufficient Retraction or "Wipe"

If the printer doesn't properly suck the filament back before moving across an open space, gravity and internal pressure will force plastic out of the nozzle.

• Retraction Distance: For Bowden extruders (like the Ender 3), ensure your distance is 5mm–6mm. For Direct Drive (like a Prusa or Bambu), 0.5mm–1.5mm is standard.
• The "Wipe" Feature: Enable "Wipe While Retracting" in your slicer. This moves the nozzle slightly back into the wall while it is retracting, "cleaning" the nozzle tip so it doesn't leave a tendril on the inner edge.

3. Wet Filament (The "Sizzling" Effect)

Filament that has absorbed moisture (hygroscopic) will create internal steam pressure in the hotend. This pressure forces plastic out of the nozzle even if your retraction settings are perfect.

• The Symptom: You may hear tiny "popping" or "sizzling" sounds while printing. The tendrils will look bubbly or inconsistent.
• The Fix: Dry your filament in a dedicated dryer for 4-6 hours. This is especially critical for PETG, TPU, and Nylon, which are notorious for creating "branching" tendrils when damp.

Troubleshooting: Slicer Settings for Clean Contours

Adjusting these specific parameters can help eliminate the "growing" effect of plastic on your inner diameters.

4. Excessive Printing Temperature

If your hotend is too hot, the filament becomes overly fluid (low viscosity). It will leak out of the nozzle like water, making it impossible for retraction to keep the "tendrils" from forming on inner holes.

• The Fix: Run a Temp Tower. Lower your printing temperature in 5-degree increments until the tendrils disappear. Often, dropping just 10°C can solve inner-contour oozing entirely.

5. Avoid Crossing Outlines

Most modern slicers (OrcaSlicer, PrusaSlicer, Cura) have a toggle called "Avoid Crossing Perimeters" or "Avoid Crossing Outlines."

• How it works: This forces the slicer to plan travel paths that go around holes rather than over them. If the nozzle never travels over the empty space of the inner contour, it cannot leave a tendril behind.
• Note: This may increase your total print time slightly, but it virtually guarantees clean internal holes.

Conclusion

Tendrils on an inside contour are almost always the result of Combing errors or moisture in the filament. By ensuring your filament is dry and forcing the slicer to retract when crossing open spaces (instead of trying to stay "inside" the mesh), you can achieve perfectly smooth internal bores. If the tendrils are thick and "branch-like," look toward your temperature and retraction; if they are thin and hair-like, look toward your travel path settings.