Hi Dooki. I don't really use Carrara. But to me it looks like the vertex normals are smoothed. This is preferred on a CAD model when all surface joins are C1 (for example when all joins are filleted), but can be a disaster if your surfaces are C0 (for example with a chamfer or a left with a sharp edge). at C0 joins, smoothed normals will create a banded effect like you have here. Check the export options of that other program (ViaCAD?) and see if you can turn off normal smoothing. Of course this might make all the mesh faces noticably faceted in your render (at least will not have the black banding). Or you can explode the object before exporting, then reassemble the mesh in Carrara. Or you can fix it properly and fillet all your joins before export (NOT fun though
!)
shorty