Repairing STL meshes: Inspector, Make Solid and Remesh
Rejection of the file by the slicing system, missing regions in the printed piece and localised failures habitually stem from a single cause: the mesh does not constitute a closed solid. Correction generally requires only a few minutes.
Criteria of geometric validity
For the slicing system, an STL file is printable only if it describes an unambiguous solid — that is, if for every point in space an unequivocal determination exists between interior and exterior. This requires:
- Closed (watertight) mesh. Absence of holes; a discontinuity of a single triangle invalidates the determination.
- Consistent normals. Uniform orientation of all faces outwards.
- Manifold geometry. Each edge shared by exactly two faces.
- Absence of self-intersection. The surface must not pass through itself.
- Single component. Isolated fragments generate debris on the build platform.
Aetiology of defects
- Intraoral scanning: subgingival regions and areas with fluid present generate discontinuities.
- Model scanning: angles inaccessible to the optical system produce unrecorded zones.
- Boolean operations: between meshes of disparate density, generating problematic edges.
- Intensive sculpting: may fold the surface upon itself.
- Export from third-party applications: with variability according to the source system.
Inspector: initial procedure
By means of I the program analyses the mesh and flags defects with spherical markers differentiated by colour according to their nature. Correction may be performed individually or globally by means of Auto Repair All.
The function satisfactorily resolves small-scale defects. Faced with extensive holes, however — a broad subgingival region, for instance — the algorithm generates a geometrically plausible closure surface that does not necessarily correspond to the actual anatomy. Visual inspection of the result is indispensable: an inadequate closure in a critical zone is more detrimental than the original discontinuity, since it ceases to be evident.
Where the Inspector reports hundreds of defects on a freshly scanned model, the problem may be attributed to the scanner export. Re-export is preferable to correcting artefactual defects.
Make Solid: reconstruction
Where the Inspector proves insufficient — severely compromised mesh, multiple overlapping shells, chaotic geometry — Make Solid resolves the matter by reconstruction. It does not correct the existing mesh: it generates a new mesh, closed by definition, enveloping the original geometry.
The procedure entails loss of detail. The higher the resolution selected, the greater the fidelity to the original geometry and the greater the resulting file size. The point of equilibrium varies with the application: a biomodel tolerates losses that a seating surface does not.
- Inspector — few, localised defects, with a need to preserve the original geometry. Always the first option.
- Make Solid — severely compromised mesh, result of complex Boolean operations, or where a guarantee of a closed solid is required.
Remesh and Reduce: optimisation
Remesh (R on the selection)
Redistributes triangles at uniform density. Files derived from scanning habitually present regions of heterogeneous density, which produces unpredictable response from the sculpting brushes. Applying Remesh prior to sculpting corrects this condition.
It likewise constitutes the appropriate correction for meshes with elongated triangles, characteristic of extrusion boundaries, which produce artefacts during slicing.
Reduce (Shift+R)
Decreases the triangle count. A full-arch file at high resolution may compromise the performance of both the application and the slicing system. Moderate reduction relieves processing without perceptible loss.
Reduction over seating surfaces should be avoided: loss of resolution in the marginal zone compromises precisely the region where precision is determinative.
Inverted normals
This constitutes the defect most difficult to identify, since standard display does not reveal it. The characteristic manifestation consists of the piece being represented as hollow in the slicer preview.
Diagnosis is established by alternating the display mode with Q until reaching the mode representing normals. Correction is effected by Flip Normals on the affected faces or by Make Solid, which reconstructs the whole with consistent orientation.
Validation protocol
- Inspector free of defects (I).
- Single component, verified in the Object Browser after Shift+Y.
- Minimum thickness observed throughout.
- Scale in millimetres, explicitly verified.
- Reasonable file size.
- Binary STL, with identifying nomenclature.
Execution of this protocol requires approximately half a minute. Detection of the defect after two hours of printing entails the resin consumed, the machine time and, potentially, rescheduling of the clinical appointment.
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