How to fix an STL file (holes, non-manifold edges, flipped faces)
Quick answer: most broken STL files have holes, faces turned inside out, or parts that overlap. Slicers patch small errors on their own. For the rest, use a repair tool. Bambu Studio's Fix model works on Windows only. Blender's 3D Print Toolbox has Make Manifold. The free splicestl repair tool runs in any browser and uploads nothing. If repair can't untangle a mesh, rebuild it as one solid.
- Convert 3MF, OBJ, GLB or PLY to STL in the same tool. Check the size afterwards, because STL has no units.
- Two old fixers are gone. 3D Builder left the Microsoft Store in 2024, and Autodesk stopped developing Meshmixer in 2021.
What the errors mean
A slicer needs a closed surface with every face pointing out, so it can tell inside from outside. "Manifold" means exactly that: every edge joins two faces, and there are no holes.
| Error | What it is | What you see |
|---|---|---|
| Holes (open edges) | Gaps where faces are missing | "Non-manifold" warnings, missing walls |
| Non-manifold edges | An edge with one face, or with three or more | Slicer errors, gaps in layers |
| Flipped faces (normals) | Faces pointing into the model | Dark patches in the preview, wrong infill |
| Overlapping parts | Separate shells inside each other | Extra walls inside the print |
| Self-intersections | The surface passes through itself | Artefacts that simple repair can't fix |
| Duplicate or empty faces | Repeated or zero-area triangles | Error warnings, odd infill |
Fix it in the slicer first
Slicers repair small errors quietly as they load. Bambu Studio marks a broken object with a warning symbol. Right-click it and choose Fix model, or click Repair in the warning. Fix model uses Microsoft's repair service, so it runs on Windows 10 and 11 only. On a Mac or Linux, use one of the tools below. To share a clean file, repair the STL itself.
Fix it in Blender
Blender's 3D Print Toolbox finds and fixes print problems. It ships with Blender up to 4.1. From 4.2, install it from Get Extensions.
- Import the STL and select it.
- Open the 3D-Print tab in the sidebar (press N).
- Click Check All. Solid counts non-manifold edges; Intersections counts faces that cross.
- Click Make Manifold. It fixes bad normals, holes and stray edges or faces.
- Run Check All again, then export the STL.
To see the problems yourself, go to Edit Mode and use Select ‣ Select All by Trait ‣ Non Manifold.
Fix it in the browser (no install)
- Open splicestl repair and drop the STL, OBJ, 3MF, PLY or GLB. Nothing is uploaded.
- As it loads, it welds vertices, closes holes, turns flipped faces, removes duplicates and merges overlapping parts.
- The Manifold geometry engine then checks the result. The mesh card lists what was fixed, and new caps show in green.
- If the mesh still isn't a closed solid, click Rebuild as one solid. Pick the fine grid for small, detailed models.
- Choose a format under Save as and download.
A model that was already sound comes back unchanged.
When repair isn't enough
Self-intersecting meshes, common from scans, sculpts and AI generators, can defeat a normal repair. A rebuild works out which points are inside the model on a fine 3D grid, then builds a new surface around them. Overlaps and small gaps vanish, but detail finer than the grid softens. If you have the original CAD file, export it again instead; that is always cleaner.
Converting 3MF, OBJ, GLB or PLY to STL
| Format | Units | Notes |
|---|---|---|
| 3MF | Millimetres, stored in the file | Comes in at the right size; colours and slicer settings don't survive in STL |
| OBJ | None | Check the size; a tiny model was probably in metres |
| GLB / glTF | Metres | Scaled ×1000 to millimetres |
| PLY | None | Common from scanners; check the size |
| STL | None | The format every slicer reads |
Drop the file into the repair tool and pick STL under Save as. It converts the other way too: STL to 3MF, OBJ, PLY or GLB.
Repair or convert a 3D file
Holes, flipped faces, overlapping parts and self-intersections, with a list of what changed. Nothing is uploaded.
Open STL repairFrequently asked questions
What does non-manifold mean?
A mesh is manifold when every edge joins exactly two faces and the surface is closed. A non-manifold edge has one face, which is a hole, or three or more, like a fin. A slicer can't reliably tell inside from outside on such a mesh.
How do I fix non-manifold edges in Bambu Studio?
Right-click the object with the warning symbol and choose Fix model. It works on Windows 10 and 11 only. On a Mac or Linux, repair the STL in Blender or a browser tool, then import it again.
How do I fix an STL file in Blender?
Enable the 3D Print Toolbox, which is an extension from Blender 4.2. In the 3D-Print tab, click Check All, then Make Manifold. Check again, then export the STL.
How do I convert 3MF to STL?
Open the 3MF in a converter or slicer and export STL. In splicestl repair, drop the file and pick STL under Save as. The size carries over, because 3MF stores millimetres; colours and slicer settings do not.
Does repairing an STL change the model?
Only where it was broken. Holes get caps, flipped faces are turned and duplicates are removed. Rebuilding as one solid does change the surface, and softens detail finer than its grid.
Is there a free STL repair that doesn't upload my file?
Yes. splicestl repair runs entirely in your browser, with JavaScript and WebAssembly. Blender is free and works offline too.
Sources
- Bambu Lab wiki, Fix model — Windows 10 and 11 only; Netfabb-based repair from Microsoft; where to start it. Read 9 Nov 2026.
- Blender 4.1 Manual, 3D Print Toolbox — the checks and Make Manifold. Read 9 Nov 2026.
- Blender Extensions, 3D Print Toolbox — for Blender 4.2 LTS and newer; version 1.5.0, 19 October 2026. Read 9 Nov 2026.
- 3D Printing Journal, A farewell to 3D Builder, 7 August 2024 — no longer available from the Microsoft Store. Read 9 Nov 2026.
- Meshmixer's status: see Meshmixer alternatives.
What changed — November 9, 2026: first published.