If your CAD model looks fine but CAM can't generate toolpaths, a vendor rejects your file, or meshing fails, there's a good chance you're dealing with an open shell.
An open shell is a geometry issue that's easy to miss in normal shaded views and painful to diagnose after the fact—especially after exporting to STEP or IGES.
What is an open shell?
An open shell is a surface body (or a "solid" that should be watertight) that has a gap in its boundary—meaning it does not form a closed volume. In practical terms, the model has edges that are not stitched/connected where they should be.
If a part is intended to be a solid, it should be closed (watertight). When it's open, downstream tools can't reliably treat it as a solid.
Why open shells break manufacturing
Open shells often cause downstream failures because manufacturing pipelines assume they're dealing with closed volumes.
Common failure modes:
- CAM toolpath generation fails or becomes unstable
Many CAM operations expect a clean solid to compute boundaries, offsets, and intersections. Gaps can prevent face recognition and boundary detection. - Meshing breaks (or produces garbage)
Mesh generation (for simulation, slicing, or inspection) often assumes watertight geometry. Open boundaries lead to holes, non-manifold artifacts, or unexpected triangulation. - Vendors reject the model or quote delays
Suppliers may need to repair geometry before manufacturing. That adds time, introduces ambiguity, and increases the chance the manufactured result diverges from intent. - You waste time debugging the wrong thing
People often blame export settings or CAM parameters when the underlying issue is simply that the geometry isn't closed.
How open shells happen (the usual suspects)
Open shells are commonly introduced by:
- STEP/IGES import artifacts — Tiny gaps appear at stitched edges, trimmed surfaces, or fillets—especially across systems.
- Boolean operations and trims — Subtract/union operations can leave sliver surfaces or unstitched boundaries when tolerances stack up.
- Fillets/chamfers near thin features — Edge blends on complex geometry can produce micro-gaps or failed surface patches.
- Manual surface modeling — Surface workflows require deliberate stitching; it's easy to leave an edge unjoined.
Signs you might have an open shell
If you see any of these symptoms, check for open shells:
- CAM can't select faces the way you expect
- Toolpaths fail on "random" geometry
- Exported STEP imports as surfaces instead of solids
- Meshing generates holes or refuses to complete
- A vendor says "model isn't watertight" or "needs healing"
How to detect open shells (the fast way)
The fast approach is visual detection of the open boundaries—highlighting exactly where the model is not closed.
You're looking for:
- highlighted boundary edges
- gaps that break surface continuity
- areas where the "solid" is really a surface patchwork
Example: open shell boundary highlight
Open shell boundary highlighted in a CAD model
Preventing open shells (before they hit CAM)
You won't eliminate this class of problem entirely, but you can reduce it:
- Prefer STEP over IGES for solids (when possible)
- After import/export, run a geometry sanity check before CAM
- Be cautious with fillets near thin walls and complex intersections
- Avoid stacking tolerances across multiple conversions
The key is not "never export," it's catch the issue early.
Check open shells visually before manufacturing
If your workflow regularly touches STEP/IGES handoffs, open shells are a predictable failure point.
Use a visual geometry check to identify open boundaries quickly—before CAM, CNC, or vendor handoff.
Try Open Shell Detection in Odin:
Geometry Error Checker →