PRT and STL files both contain CAD data, 3D design information useful for manufacturing or graphic modeling. Converting from one to the other is simple. STL is an interchange format, meaning it can be used with many programs. PRT is a proprietary format that needs to Pro/ENGINEER. If you want to read a Pro/ENGINEER file in another program, convert it to an SRT.
Open the PRT file in Pro/ENGINEER.
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Select "Save As" from the "File" menu.
Select "Stereolithograph File (*.stl)" from the format menu and click "Save." You will now have an STL file you can use with any STL-compatible application.