Your model should be "watertight" (the mesh has a mathematically determinable interior), correct normals, and a manifold topology. Shapeways has a nice [http://www.shapeways.com/tutorials/things-to-keep-in-mind tutorial] explaining this. | Your model should be "watertight" (the mesh has a mathematically determinable interior), correct normals, and a manifold topology. Shapeways has a nice [http://www.shapeways.com/tutorials/things-to-keep-in-mind tutorial] explaining this. |