What Could Be Wrong
Drop the STL onto the page and the diagnostic panel reports five categories of issue:
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Boundary edges — edges that belong to only one triangle. These are holes in the mesh; the slicer cannot determine "inside" from "outside".
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Non-manifold edges — edges shared by 3 or more triangles. Caused by overlapping geometry; produces unpredictable wall slicing.
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Flipped normals — triangles facing inward when they should face outward. The slicer will treat their solid as empty.
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Duplicate vertices — same XYZ position stored multiple times. Bloats file size and confuses welding.
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Degenerate triangles — zero-area triangles (two vertices in the same place). Often produced by careless boolean operations.
Read the Visualisation
Boundary edges are highlighted in red wireframe directly on the model. Rotate around to find them — they are usually clustered around an obvious hole, a missing cap, or a place where two boolean meshes did not quite meet. Once you know where the damage is, you know whether automatic repair is enough or whether you need to go back to the source model.
Hit Repair
The Repair & Download button runs four passes in order:
- Deduplicate vertices using a small tolerance.
- Weld coincident edges to close micro-holes.
- Drop degenerate triangles.
- Recompute outward-pointing normals from connectivity.
You get a cleaned STL file in seconds. Re-upload it to confirm the diagnostics turn green.
When to Give Up and Re-Model
If the boundary-edge count is in the thousands, the mesh has structural damage that no automatic repair will fix — some triangles are simply missing. Open the original in Blender or Meshmixer, regenerate the broken section, and re-export. STL Doctor is for fixing dozens of issues, not rebuilding a model.
Run It Before Slicing
Make this the first stop for any STL you did not author yourself — especially Thingiverse downloads and CAD exports. A 30-second repair pass saves hours of debugging "why does this print have a missing wall on layer 47".