Looking at answers this bug seems to have been around for a while:
I tried importing an obj mesh with 2 materials and a third was created called ‘no name’ was assigned to all but 1 poly of another material’s section.
OBJ is a very simple text format mesh so I don’t understand how Unity can not import it correctly - is this a known issue or problem with the format?
It`s a known bug. Reported more than once. And ignored since eons. All we can do is to report it again and again, in hope that Unity gives the Obj Importer a bit love.
Official import format is FBX though. So it is very unlikey that they will fix the Obj importer.
well that’s bad, how about a fix Unity?
In the mean time I’ll have to convert to fbx: Autodesk | 3D Design, Engineering & Construction Software
In all my work with Unity Ive learned that working with OBJ always ends with errors (bad normals, bad materials). Better change your workflow to FBX files, your life will be more easier.
Well, it is a pipeline question here and there. For animation you dont have a choice. Obj cannot deal with animation. So thats fbx. But for static stuff i prefer obj. I can reopen and change Obj files very easy. fbx is much harder to reopen and modify when you don`t have the big boys. Blender has no FBX importer yet. And even with the big boys you may run into trouble with fbx.
The nice thing at obj is the easiness. You can edit the files by hand when you want, its pure text. And Obj makes much smaller files compared to fbx. This doesnt affect the game size though, only the project file size. Nevertheless, when you make daily backups of your project folder, then it makes a little difference.