“Model is not watertight” in Cura.
Cura reports this when the mesh is not a sealed solid, and it usually arrives alongside a failed slice. The word is doing real work here — it means exactly what it sounds like.
What it means
Pour water in and it would leak out. On a sealed solid every edge is shared by exactly two triangles; an edge used only once is a gap in the surface. Cura cannot tell inside from outside across that gap.
The gaps are often far too small to see. A hole a thousandth of a millimetre across breaks the model just as thoroughly as a missing face.
What breaks it
| Export tolerance | The mesh was written at a coarser tolerance than the model was built to, so neighbouring faces no longer quite meet. |
|---|---|
| Surfaces, not solids | The CAD model was a collection of faces that was never stitched into a closed body. |
| Deleted faces | A face removed during editing and never rebuilt. |
| Un-merged bodies | Overlapping solids that touch but were never combined. |
Three ways to fix it
- Re-export at a finer tolerance. If your CAD package offers a deviation or chord-height setting, tighten it and export again. This alone fixes a surprising number of cases and costs nothing.
- Run a mesh repair. Cura’s Mesh Tools plugin, Microsoft 3D Builder or Meshmixer will close small gaps automatically. Watch what it does: an automatic fix bridges a hole with whatever surface it thinks belongs there, which is not always what you drew.
- Send it to us. Repair from £15, redraw from £45 if the mesh is beyond it.
Find out how bad it is first
The checker counts the open edges and draws them in red on the model. Six open edges along one seam is a different job from four thousand scattered across the surface — the first is a quick repair, the second usually means going back to the CAD.