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.
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.
If your design is not interlocking but still contains more than 1 shell, then you have a merged part which we can only put into production in the case of grid containers, see ordering tip below. In all other cases please upload each shell as a separate model to order your parts.
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.
Ordering tip for merged parts
For most materials, designs with non-intersecting shells will be rejected. For polyamide and alumide you are allowed to upload multiple shells that do not intersect but these shells have to be gathered in a so-called grid container. You can find detailed info in the design guide of polyamide under the section of grouped models.