Pretty simple, Utherane is on a Metal Ring which is threaded, You get the ring and screw it onto the threaded metal core, but it will have a metal ridge on one side of the core, and one side of the ring stopping it from coming off
I could see it working somehow if they had metal threads on the inside of the urethane but it's still more practical to just pour it; and they can make wheels faster if they're poured.