What Is a Shell?

A shell is a collection of triangles that are connected to each other. Basically, it represents the outer wall of your 3D model. Typically a part has only one shell because every triangle of the part is (indirectly) connected to every other triangle. However, if you have an interlocking design, your model will contain multiple shells and normally corresponds to the number of links you designed. If your design isn’t interlocking but containing more than 1 shell, you may need to check that the different shells are intersecting each other so they can print as one part. 

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.

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