PrusaSlicer is Filling Voids: Why It Happens and How to Fix It
A common frustration for 3D printing enthusiasts is when PrusaSlicer automatically fills in internal voids, holes, or hollow cavities in a model, while Slic3r or Bambu Studio renders the same STL perfectly. While Bambu Studio is based on PrusaSlicer, their default "handling" of broken geometry and internal volumes can vary significantly. This issue is rarely a bug in the software; rather, it is how the slicer interprets "non-manifold" geometry or specific slicing algorithms.
1. The "Slice Gap Closing Radius" Setting
PrusaSlicer has a safety feature designed to ignore tiny cracks in a mesh to ensure a print doesn't fail. However, if this value is too high, the slicer will "heal" intentional voids, treating them as accidental gaps.
- The Problem: The "Closing Radius" is larger than the wall thickness or the diameter of the void.
- The Fix: Go to Print Settings > Advanced > Slicing and look for "Slice gap closing radius." Try reducing this value to
0or0.005.
2. Non-Manifold Mesh and "Auto-Repair"
PrusaSlicer is notoriously strict about manifold geometry. If an STL has inverted normals (the "inside" of the face is pointing "outside"), PrusaSlicer may assume the entire internal cavity is part of the solid volume.
- The Difference: Bambu Studio often runs a silent Netfabb repair in the background, while Slic3r (older versions) might simply ignore the error and slice the "islands" as they are.
- The Fix: Right-click the object in PrusaSlicer and select "Fix through Netfabb" (Windows 10/11) or use the "Fix through 3D Builder" option.
3. "Remove Small Internal Voids" Feature
PrusaSlicer includes a specific toggle meant to save filament by ignoring tiny internal bubbles that don't affect structural integrity. If your design has small, intricate hollow sections, this setting might be the culprit.
- The Fix: Navigate to Print Settings > Layers and Perimeters > Quality and ensure that "Remove small internal voids" is unchecked.
Slicer Feature Comparison: Handling Geometry
Understanding how these three slicers handle complex or broken files can help you choose the right tool for a specific project.
| Slicer | Repair Engine | Void Handling Philosophy | Est. Cost |
|---|---|---|---|
| PrusaSlicer | Netfabb (Cloud) | Strictly follows manifold rules; closes gaps by default. | Free / Open Source |
| Bambu Studio | Netfabb (Integrated) | Aggressive auto-repair; high tolerance for "broken" STLs. | Free / Open Source |
| Slic3r (Original) | Basic Internal | Literal interpretation; slices exactly what the file says. | Free / Open Source |
| Simplify3D | Proprietary | Manual control over "Hollow" vs "Solid" mesh logic. | ~$199.00 |
4. Check "Slicing Mode" (Regular vs. Even-Odd)
In the Advanced settings of PrusaSlicer, there is an option for "Slicing Mode." Most users stay on "Regular," but if your model has overlapping volumes (which often causes voids to fill), changing this can help.
- Even-Odd Mode: This uses a mathematical rule where any area enclosed by an even number of surfaces is considered a void. Switching to this mode often solves the "filled hole" issue in complex architectural models.
5. XY Size Compensation Issues
If you have a very small void and a positive "XY Size Compensation" (used to fix dimensional accuracy), the slicer might expand the walls inward until they touch, effectively "filling" the void.
- The Check: Ensure your XY Size Compensation is set to
0in the Advanced settings to see if the void reappears in the preview.
Conclusion
When PrusaSlicer fills voids that other slicers don't, it's usually trying to be "helpful" by repairing what it perceives as a broken mesh. By adjusting the Slice Gap Closing Radius and verifying the Manifold status of your STL, you can usually match the successful output of Bambu Studio or Slic3r. Always use the "Feature Type" view in the G-code preview to confirm if the filled area is being identified as "Internal Infill" or "Solid Infill"—this will tell you if the slicer thinks the area is supposed to be solid.
