Fix
My slicer says the model has errors (non-manifold edges)
Published · How we check our guides
Short answer
The model's surface isn't a clean closed skin: it has holes, triangles facing the wrong way, or edges shared by too many triangles. Slicers fix small errors themselves; if the sliced preview looks complete it will print. Otherwise repair the file once and the warning goes away.
What the warning means
A model for printing has to be a closed skin around a solid: every edge of every triangle shared by exactly two triangles, all of them facing outwards. That’s what “manifold” or “watertight” means. When the skin has a gap, a triangle facing the wrong way, or an edge shared by three or more triangles, the slicer can’t always tell inside from outside, and it warns you.
| Problem | What it is | Typical source |
|---|---|---|
| Open edges / holes | Gaps in the surface | 3D scans, game assets, a missing face in a model |
| Reversed (flipped) faces | Triangles pointing inwards | Exports from modelling programs, mirrored parts |
| Non-manifold edges | An edge shared by more than two triangles, or by only one | Parts touching at an edge, sloppy joins, holes |
| Self-intersections and overlapping shells | Parts pushed into each other, surfaces crossing | Models built from several pieces, AI-generated models |
| Degenerate triangles | Triangles with no area | Low-precision CAD exports |
What your slicer says, and what it does about it
Bambu Studio
An exclamation mark appears next to the object, with a message such as “Error: 12 non-manifold edges”, “Error: reversed faces detected, which may affect rendering” or “Info: 4 open edges”. Bambu Studio repairs some errors by itself on import and reports “N errors repaired”. For the rest, right-click the object and choose Fix model (or right-click the warning icon). Fix model uses a Windows repair function, so it’s only available on Windows 10 and 11; on macOS and Linux, Bambu Studio suggests repairing the file with another tool first.
OrcaSlicer
Shows “Error: N non-manifold edges.” and the tip “Use ‘Fix Model’ to repair the mesh.” Since version 2.4, Fix Model uses its own repair code and works on Windows, macOS and Linux. Version 2.5 moved it into the right-click Modify menu and added Fix All; it handles flipped faces, non-manifold edges, self-intersections and loose “triangle soup” meshes.
PrusaSlicer
Repairs simple errors automatically on import. A warning icon next to the object shows “Auto-repaired N errors” with the details (degenerate facets, edges fixed, facets reversed) and any “Remaining errors” such as open edges. On Windows 10 and 11, right-click the icon and choose Fix by Windows repair algorithm for the rest. Older versions and guides call this “Fix through Netfabb”.
UltiMaker Cura
Cura doesn’t list errors; it quietly works around them while slicing. With Display model errors turned on in Preferences, it highlights missing or extra surfaces. The Mesh Fixes section of the print settings controls how it copes: Union Overlapping Volumes and Remove Mesh Intersection are on by default; Remove All Holes and Extensive Stitching are off and can rescue badly broken models (but Remove All Holes also fills holes you want). The free Mesh Tools plugin adds Check models, Fix simple holes and Fix model normals.
Can I just print it anyway?
Often, yes. Slicers handle small problems well. Check the sliced preview layer by layer: if every layer looks complete, with no missing walls, odd gaps or stray lines, it should print. Fix the model when the preview shows missing layers or holes, when the warning lists many errors, or before you split the model or turn it into a STEP solid, because those steps need a closed solid.
Repair it in your browser, on any computer
- Drop the STL, OBJ or 3MF into Repair STL. The problems are drawn in red.
- It closes holes, turns flipped faces the right way, joins corners that should touch and removes junk triangles, then lists exactly what it fixed. Compare the original and repaired versions.
- Download the repaired STL or 3MF and load it in your slicer: the warning should be gone.
It works the same on Windows, macOS, Linux and Chromebooks, and the file isn’t uploaded. Parts pushed into each other and walls with no thickness still need a 3D editor such as Blender; most slicers print the overlaps fine.
What happened to Netfabb and 3D Builder?
Many older guides send you to Microsoft’s 3D Builder or the free Netfabb online service. Neither is available any more. Microsoft removed 3D Builder from the Microsoft Store in July 2024, so it can no longer be downloaded. Autodesk retired the free Netfabb online repair in 2021, and Microsoft’s online repair service ended in April 2022. The “Netfabb” and “Windows” repair buttons in slicers use a repair function built into Windows itself, which is why they don’t exist on a Mac.