To the viewers of the tread. jf pion has the only real answer to the Original Poster's question based on the proper review of this model. ANSWER: "because you have some surfaces inside the volume which make a faulty stl when exported in sketchup choose the wireframe view and inspect your cube you wil see a lot of surfaces inside the volume use the eraser tool to erase the useless lines and youby TheZionPhil - Slic3r