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Diference Meshes In Stl Format.

Hello everybody:
My problem is when i convert my two subtools in polymesh and activate the option merge and weld to only obtain one subtool, not works.
I export like stl to Rhinoceros and the piece have two diference meshes, not one. What can i do to solve this? Please help. I need a only piece to Rapid Prototyping.

Thanks in advance.