There are more than one way to create the DXF export from the unfolded Sheet Metal part. An option to the drawing export is to use the IronCAD Mechanical add-on and its Face to DXF tool. Drop it on the face of the unfolded part to quickly create the DXF file. See more here: http://ironcad.it/learnICM/en/tools-catalog/0250_face_to_dxf/