What the mesh check looks for
A hole your slicer does not report can cost you a failed six-hour print. Info names each fault before you slice, line by line.
| Line in Info | What it means | Why it matters for printing |
|---|---|---|
| Watertight | yes or no; per body for assemblies | a slicer needs a closed volume to know what is inside |
| Holes | gaps bounded by a loop of open edges | layers come out with missing walls |
| Open edges | edges used by one triangle only | the surface does not close |
| Non-manifold edges | edges shared by three or more triangles | inside and outside become ambiguous |
| Flipped triangles | faces wound against their neighbours | shading errors and wrong slicing |
| Degenerate triangles | faces with zero area | they add no shape but can trip up other tools |
| Inside-out normals | the whole shell faces inward | the slicer may treat the solid as empty |
What STL repair fixes
Edit → Repair → Repair mesh runs these fixes: it welds duplicate vertices, removes degenerate and duplicate triangles, makes the normals consistent and outward, then closes simple holes. The message under the button lists what was fixed and what is still open, so you know whether to trust the result.
In the test an open box, a 20 mm cube with one face missing, came back watertight with a volume of 8000 mm³. An IGES cube built from 6 separate surfaces with mixed face orientation also came out consistent after Repair.
How to fix an STL file in the tab
- Drop the STL on the viewer and open Info. Note the watertight line and the counts.
- Press Edit, open the Repair section and press Repair mesh.
- Read the report. If it says the mesh is closed, the watertight line in Info reads yes.
- Save with Convert → STL, or 3MF for a slicer that reads it.
The fixer runs on your computer, in this tab. A supplier's part or a client's file never goes to a server for repair.
Where simple hole filling stops
Holes are closed with a fan of triangles from the edge loop. That works for flat openings and small gaps, such as a missing bottom face or a pinhole from a bad export. A large hole in a curved surface gets a flat patch, which shows as a dent or a facet on the print.
For those, fix the source: re-export from the program that made the model, or rebuild the surface in a sculpting app. Scans with thousands of tiny holes are better served by a scan-cleaning tool before they come here.
Why booleans, cuts and text need a watertight mesh
Combining, cutting and engraving all compute what is inside the part. An open mesh has no defined inside, so these operations stop and ask you to run Repair first. Once the check shows a closed mesh, the STL splitter and the booleans on the STL editor page work.
Assemblies are checked body by body: in the AS1 STEP test 10 of 10 bodies were closed. After a repair, look at the size and weight on the measure page, or save the part as 3MF with STL to 3MF.
Questions and answers
Can Repair fix every broken STL?
No. It handles welded seams, bad triangles, flipped normals and simple holes. The report says what is still open; big curved holes and heavy scan damage need the source program or a mesh tool.
Does repairing change the shape of my model?
Welding and removing zero-area triangles leave the visible shape as it was. Closing a hole adds new triangles across the opening.
Is this STL repair online or on my computer?
The page loads from the web, and the repair runs in your browser tab. The file is not posted anywhere.
Why is the mesh still not watertight after Repair?
The remaining faults are listed in the message: usually a hole too complex for a fan fill or non-manifold edges. Fix those in the program that made the model.
Can I check OBJ, 3MF or STEP files too?
Yes. The mesh check runs on any model the viewer opens, and on STEP assemblies it runs per body.