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 (non-watertight).
An open shell (non-watertight) 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 (non-watertight)?
An open shell (non-watertight) 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 (non-watertight) break manufacturing
Open shells (non-watertight) 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 (non-watertight) happen (the usual suspects)
Open shells (non-watertight) 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 (non-watertight)
If you see any of these symptoms, check for open shells (non-watertight):
- 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 (non-watertight) (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 (non-watertight) boundary highlight
Open shell (non-watertight) boundary highlighted in a CAD model
Preventing open shells (non-watertight) (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 (non-watertight) visually before manufacturing
If your workflow regularly touches STEP/IGES handoffs, open shells (non-watertight) are a predictable failure point.
Use a visual geometry check to identify open boundaries quickly—before CAM, CNC, or vendor handoff.
Try Open Shell (non-watertight) Detection in Odin:
Geometry Error Checker →