When you create an object with Shapeways, the order price is determined by the actual printed volume (not by the model's bounding volume). An easy way to save some money (well, sometimes a LOT of money!) is by making your object hollow. This short tutorial will explain the basics of hollowing objects.

There are of course some trade-offs to this trick:

• Hollow objects will not be as strong as solid objects.
• Objects such as statues may need a solid base to prevent them from keeling over.
• It will increase the number of polygons in your model (of course you can reduce the number of polygons on the inside - it doesn't need the same level of detail as the outside).

Also, you'll have to make sure the walls remain thick enough - we recommend a minimum wall thickness of 2mm.

I'll demonstrate the process of creating a hollow object by taking a simple cube:



In a traditional 3D application this cube will appear to consist only of six faces with empty space in between. For Shapeways however, this represents a solid volume.

The easiest approach is to make this cube hollow is to extrude the outer surface and create a scaled-down version on the inside of the object:



Hint: Simply extruding and scaling may not always give correct results - especially for objects with complicated shapes and sharp creases. You may need to do some manual fixing in these cases.

If you want your object to be truly hollow (i.e.: containing air, not plastic), you'll need to add a few holes in the object wall. These holes will allow us to remove the support or excess material. They should be at least 2mm wide and I recommend you add two or three - more if you have a complex shape that could be more difficult to clean. (The hole in the next image has been exaggerated for clarity)



Finally, inspect the direction of the face normals. It's important that the face normals on the outside of the object point outwards and those on the inside point inward. Most applications will allow you to display the face normals or turn off double-sided rendering - faces with incorrect normals will usually display as a hole in the mesh.



We've just published a tutorial by Bryan Vaccaro that (among other things) show how to do this with Maya.