I am creating a stl file for rapid prototyping.
But have problem with creating a watertight polygon mesh objects.

I follow the steps in Rhinoceros:
"Use the Join command, then Weld (angle=180), and UnifyMeshNormals to turn a group of meshes into a single watertight mesh object. "

Then all the previous separated mesh objects merged together becomes one.
But after I export the result as a new stl file and open this new one in Rhinoceros, it becomes many parts again.

anyone know why?