How to hollow a model

Although a model may be printed solid, the price is dependent upon the amount of material that’s bound.  For larger models,

 it’s more cost-effective to hollow them out (leaving openings for non-bound material to be removed).

Each model is different, but the typical method for hollowing a model is to offset its surfaces to the interior by selecting each surface or the entire model and typing OffsetSrf. 

 If the surface normals are pointing out, type F to reverse the direction of the offset.

 Make sure that the walls are at least 1/8” thick to scale.  If all of the offset surfaces intersect and form a closed volume, type CreateSolid to remove any excess surface.  If the surfaces do not form a closed volume, they’ll need to be extended (ExtendSrf or ExtendTrimmedSrf) or reconstructed until they do.  Once the model is hollow, create openings for excess material to escape.

Remember to check that the interior of the model is manifold and rebuild it if necessary.  The surface normals for the interior should point in, while those for the exterior should point out.  To check this, select each and type Dir.  If the normals are pointing the wrong way, type F to flip them.