dioo

You need your model to be a solid, not just a surface. It must have a second, "inside" parallel face for every face of your shell. If it's not that, look into possible model problems. In Blender, select your model, go into edit mode and use the Select -> Select All By Traits tools (Non Manifold, Interior Faces, Loose Geometry, etc). There are Youtube videos explaining how to use the tools and how to fix the various problems.