Translations:Import from STL or OBJ/7/en

Closing holes
You can also manually close holes in your mesh object by
 * Selecting your mesh object in the tree view
 * Choose  →    from the top menu
 * Specify maximum number of edges to be filled (3 is default)
 * Since STL and OBJ are meshes consisting of triangles the default number of edges should be sufficient

Another method of manually closing holes in your mesh object would be
 * Selecting your mesh object in the tree view
 * Choose  →    from the top menu
 * Select one of the edges of the hole in the mesh object in the 3D view
 * Right-Click in 3D view and choose to exit the command