Hello Henrik,
Most probably you selected all objects prior to exporting to STL, which is a mistake almost every newcomer do.
FreeCADĀ is a history-based parametric modeller. Hence what you see in the Model tree is the history of the part: each object should be seen as a step in creating the part. So you should not select everything before exporting, but only the end result.
If you created your tube by applying a Boolean Cut between two cylinders, then
only select the "Cut" object before exporting.
If that's not the case, please attach your FreeCAD file so I can get a look at it.
BTW, the best place to get help on FreeCADĀ is the
FreeCAD forum. Over there you would have probably got a reply in less than an hour.
Edited 3 time(s). Last edit at 07/03/2016 10:19AM by NormandC.