STL file errors and how to fix them
An STL file is a list of triangles. It has no units and no exact surfaces. A mesh with defects can look fine on screen and still fail when it is measured or sliced.
| Defect | What it is | What it causes | How to fix it |
|---|---|---|---|
| Open edges (holes) | triangles around a gap are missing | volume and wall checks become unreliable and slicers have to guess | re-export, or close the gap in a mesh editor; weld vertices for tiny gaps |
| Non-manifold edges | three or more triangles share one edge | slicing errors and broken solids | rebuild in CAD and remove internal faces |
| Flipped normals | some triangles face inward | inside and outside swap, the volume gets the wrong sign | recompute normals so every triangle faces outward |
| Duplicate triangles | the same triangle twice, or two opposite ones that make a double wall | wrong wall thickness and double surfaces | remove the duplicates |
| Zero-area triangles | three points on a line, or two equal points | usually harmless in a closed mesh, can break some calculations | remove them, the shape does not change |
| Separate bodies and internal shells | more than one closed shell in the file | a check treats them together, an internal shell can be a cavity | export one solid per part |
| Self-intersections | surfaces cross each other | unreliable thickness | merge bodies with a boolean operation in CAD |
| Wrong units | STL stores numbers, not millimetres or inches | a part 25.4 times too small or too large | state the units when you upload |
What Dokimion reports
- The number of separate bodies in the file, with a note to confirm they are separate parts.
- Open edges, with the count and a warning that volume and checks are less reliable: "Export a closed solid".
- Zero-area triangles in a closed mesh are not reported, because they rarely matter. For metal they are removed before the support calculation, and the shape does not change.
- Units: the file is read as millimetres unless you choose otherwise on upload.
Use STEP when you have it
STEP carries the exact CAD surfaces, not triangles, so it avoids most of the defects above. Dokimion reads STEP, STL and 3MF files up to 100 MB. Export one closed solid per part.
Scanned parts
Meshes from a 3D scanner come with noise, holes and millions of triangles and need cleaning before any check.
Check your file
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