When we calculate the price for a 3D printed object, the volume of material used is a factor. So if you wanted to save some money the volume of your model could be reduced by hollowing it out. There are of course some disadvantages to doing this, such as the objects wont be as strong as a solid one, it will increase the number of polygons in your mesh (although the hollowed out area doesn’t need the same level of detail as the outside) and you may need to add a base to allow the model to stand. Also bear in mind wall thickness and confined hollows, more information on both can be found in the following tutorials:
Exporting STL files from 3D modelling software
Considerations when designing for 3D printing
The following picture shows a simple cube. In normal 3D modelling software this would be represented as six faces with an empty space in the middle, but to a 3D printer this is a solid volume and so would be printed as a solid.
The next picture shows the cube hollowed out. How you do this will depend on the modelling software that you are using. This is a simple explanation and your models will be a lot more complex, so it may not be a case of simply scaling down the outside shape to hollow it out, but remember the inside does not need to be as complex as the outside.
In its current state the cube will still be solid as the hollow area will be filled with support material, but with no holes this cannot be removed. So you will need to place a few holes into the model. These will allow the support material to be removed. They should be at least 5mm wide and there should at least be two of them, if your model is complex then it will require more. The next picture shows a hole added into the face for this reason.
Lastly you need to check that the surface normals are pointing in the correct direction. Most modelling applications will allow you to display the face normals. The face normals on the outside should be pointing outwards and those on the inside point inward. Faces with incorrect normals will display as a hole in the mesh.
Hollowing out a model is not an essential operation, but if you are looking for the most economical option for creating your model this may be for you.
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