How it works
The checker examines how triangles meet. Open edges indicate an unsealed boundary, while non-manifold edges can make a closed volume ambiguous.
What the results mean
Watertight means the mesh forms a closed surface. Separate shells can be intentional, but should be checked in your design.
FAQ
Can a watertight model still fail to print?
Yes. Wall thickness, supports, material and printer setup are separate concerns.
What should I do with open edges?
Repair the mesh in your modelling or mesh-repair software, then check it again.
STL Manifold Checker · STL Quality Checker · How to Tell If an STL File Is Watertight · All STL tools