Closed edges and orientation
A closed triangular surface uses each edge twice, with opposite directions in the adjoining faces. An edge used only once is a boundary; more than two uses indicate non-manifold geometry. The converter refuses these cases because calling the result a solid would hide a real input defect.
One connected surface
Separate bodies require explicit multi-body representation. This bounded converter expects one connected shell and rejects disconnected triangle groups. Repair a working copy in a mesh editor, then export each intended body separately. Do not bridge unrelated pieces merely to satisfy a topology check.
Passing is not a manufacturing certificate
Duplicate and degenerate faces are also rejected. Edge checks still cannot certify every distant self-intersection or every narrow feature. Reopen the result in an independent CAD application, inspect sections, and verify dimensions against the physical requirement.
Use a repair sequence that preserves the design
Start by checking whether the file contains the intended part alone. Separate detached bodies in a mesh editor, remove exact duplicate faces, then inspect boundary loops and face orientation. Save each repaired version under a new name. Filling every hole automatically can close an intentional opening in a hollow design or create a surface that was never present.
A cube triangulated with two triangles per side has twelve faces. Remove one triangle and three boundary edges appear; reverse only one triangle and its adjoining edge directions disagree. These are different defects, even though both prevent the converter from treating the mesh as one valid closed shell.
Review near-coincident and intersecting surfaces
Vertices that merely look coincident on screen may have different coordinates in the file. Decide an appropriate weld tolerance in the source editor using the intended physical dimensions; this converter does not silently move vertices to close gaps. Similarly, two closed volumes can intersect while satisfying many local edge rules. Inspect sections and the receiving application’s geometry checks when an apparently valid result still behaves unexpectedly.