Hi Nikola, what settings are you using for your OBJ export, are you using "Output: N-gons" ?

If so try changing that setting to "Output: Quads & Triangles" instead and see if that works better for you.

Some programs do not deal very well with highly complex n-gons, and may fail to triangulate them properly. If you use "Output: Quads & Triangles" mode (it's an option on the meshing dialog that pops up when exporting to OBJ format), the triangulation of complex polygons will happen inside of MoI instead of inside of the receiving program.

- Michael

Have you tried to use the quad and tris, on even just tries export option ?

Looks like Simlab or Sketchfab have trouble to redraw the super long ngones when using .obj format.
If you dont want triangulate model, you could try the 'divide larger than' option ... a value of 0.5 seems good here ... that will help to avoid very complex ngone.

.Obj is not very great for ngones export, probably the worse format in fact.
The model is rendered fine in Modo, using .lwo or .fbx format, with default export settings.

Thank you PaQ and Michael for your advices.The important thing is that some polygonal exporters works for my model,like lwo and fbx.
Thanx again for your answer and for your advices,I will keep trying with other extensions too.

Best Regards from Nikola