Shell
A shell of a 3D part is a collection of triangles that are connected to each other. Basically, it represents the outer wall of your 3D part.
Interlocking part
If you have an interlocking design, your file will consist of multiple shells that intersect each other. You can upload and order these interlocking shells as one model.
Merged part
If your design is not interlocking but still contains more than 1 shell, then you have a merged part which we cannot put into production this way. Please upload each shell as a separate model to order your parts, see picture below.
Ordering tip for identical shells
In case multiple separate shells are identical, then upload one of the models and order several copies of it, this will give you the best price.