I have a design and I would like to test to 3d print only a section of it so I don't have to print the whole design and waste material incase something goes wrong with it. how can I select that part and save as .stl file. I tried making a section and saving it like that but at the end whole design appears.
If I understand correctly then you could use a couple of options depending on what you need to do.
1. Save the part out as a copy then simply cut away the material of the solid that you don't need leaving just the bit to print. Then svae as translated and select *.stl
2. Save a copy. Use the solid body options and split the part (using surfaces) to leave a body which represents the part you want to 3d print. delete the rest leaving just that body and then save as translated *.stl