Separate parts in an STL and why they matter
An STL is one list of triangles, but that list does not have to form one connected object. A single file can contain several separate pieces, called shells or bodies, that do not touch each other. Sometimes that is exactly what the designer wanted. Sometimes it is a mistake that will ruin a print.
When separate parts are intended
- A plate of parts. A kit exported as one file, laid out to print together.
- Print-in-place assemblies. Hinges, chains and articulated toys are printed as separate pieces with a small gap between them so they can move.
- Multi-part models meant to be split in the slicer and printed one by one.
In these cases the separation is the point. The gaps are deliberate, and the pieces should stay apart.
When they are a problem
- Floating fragments. Tiny bits of geometry left in the air after editing. They print as blobs, or not at all, and can knock the part over.
- Hidden copies. A duplicate of a part sitting inside the original. The slicer may print both, double the walls or get confused about what is solid.
- Parts that should be joined. Pieces that were meant to be one solid but only touch, or sit with a tiny gap. They print as separate bits that break apart. Where they touch along an edge, you also get non-manifold edges. See what is a non-manifold edge.
How to see them
A shaded view rarely shows the difference, because separate parts look like one surface where they meet. You need a tool that groups connected triangles and counts the groups.
MeshView Pro detects separate parts, so you can see how many pieces a file holds before you slice it. A cross-section helps too, since doubled outlines show hidden copies. See what a cross-section shows inside a 3D model.
What to do
- Count the parts and compare with what you expect. A single bracket should be one part. A plate of six pieces should be six.
- Delete fragments you do not need in a mesh editor or your slicer.
- Merge parts that should be one solid with a union in your modelling tool, after making sure they overlap properly.
- Split on purpose in the slicer when you want to place or orient pieces separately.
A quick sanity check
If the number of parts surprises you, stop and look before you slice. It takes a minute to find a stray fragment on screen, and much longer to find it in a failed print.