Converting solid parts to shell parts

[FEAnalyst]

@Marco

Last clarification: why there is this simbol “!” on that parts?

See this from the announcement about v3.0.0 RC1 version:

@Matej

Parts with small features

A CAD part now shows an icon with an orange exclamation mark in the model tree if it has an edge shorter than 1/1000 of the part’s bounding box diagonal, or a face narrower than that (a thin strip or sliver). Such features force very small elements around them, which slows meshing and spoils the element quality. They are often left behind by imported CAD, splitting or defeaturing. When you select the part, its small edges and their vertices are highlighted. Remove them with Geometry → CAD Part → Repair (remove small faces and edges, fuse faces, join edges), using Preview to see what will be removed. Other options are Fuse Faces and Defeature Solid.