STL repair guide
How to Repair an STL File for 3D Printing
Substantially updated 2026-10-31. First published 2026-09-05.
Diagnose the actual mesh fault before editing. A duplicate face needs a different operation from a missing surface, and a detached piece may be an intended part. Keep the original, save each derivative separately and compare the exact file you will slice.
| Finding | Narrowest useful action | Stop when |
|---|---|---|
| Exact duplicate or degenerate triangle | Run explicit standard cleanup on the repair page; it uploads this file for server processing. | Geometry disappears or the report retains a serious finding. |
| Orientation conflict on an otherwise closed region | Review a supported cleanup correction and recheck winding. | The shell is open or its intended inside is ambiguous. |
| Detached component | Use local piece review on the same repair page. All pieces start kept; Keep largest part proposes the greatest-area piece, with a near-equal warning. | You cannot identify whether another piece is intended. |
| Missing surface or open boundary | Reconstruct the known surface from CAD or in a mesh editor. | The cap shape or design intent is uncertain. |
| Non-manifold join or intersecting shells | Repair the Boolean or assembly in the source model. | An automatic union would alter a cavity or mating face. |
Start with completed report triage. Removing an intentional shell is a design change; lossy reduction is a separate surface-fidelity decision. Neither should be inferred from a successful cleanup.
For a degenerate-face finding, compare zero-area and narrow triangles before deleting a designed feature. Check the complete cleanup report, including duplicates, orientation corrections and skipped reasons, rather than assuming only that one category changed.
Know which download you are making
Piece review runs in your browser and exports only retained original triangle coordinates and winding. It does not fix duplicate faces or add missing geometry. The example's 20 triangles include a 12-triangle cube and two four-triangle tetrahedra. Keeping largest proposes the cube, but surface area is only a ranking rule, not evidence that the other pieces are waste.
Standard cleanup is separate. It starts only when you press Clean STL safely and uploads the selected file. Its report names removed duplicate or degenerate triangles, orientation corrections and unresolved reasons. It does not automatically apply your local Keep selection. An open-face cube remains open unless a deliberate source edit restores its missing two triangles.
Verify the derivative against its purpose
- Reopen the saved STL and record its triangle count, bounds and supported quality findings. An unavailable volume is not zero.
- Check the region you meant to change. Compare the original and derivative in STL revision comparison when surface movement matters; finite samples do not certify all features.
- Inspect affected slicer layers, dimensions, openings and intended separate parts. Keep the source CAD or original STL for correction.
The generated widened-cube comparison reveals a 4 mm surface change; a retriangulated control has different bytes with zero displayed sampled deviation. These controls show why bytes and triangle count alone cannot establish an acceptable repair. If the intended surface is unknown, do not infer it from a successful download or clean-looking preview. For a broader failed-import diagnosis, use the slice-failure guide.
FAQ
Does Keep largest part also run standard repair?
No. It only changes the local retained-component selection. Standard cleanup is a separate explicit server action.
Can standard cleanup fill a missing cube face?
No. The missing face requires a deliberate reconstruction from the design. Safe cleanup does not create that missing surface.