Your 3MF isn’t watertight — here’s the fix

"Model is not watertight" means your mesh has openings — the outside surface doesn't fully close around the inside. The slicer can't tell what's solid material and what's air, so it guesses, and guessed geometry prints badly: missing walls, spongy infill in the wrong places, or failed prints.

Why it happens

Common causes: models assembled from multiple parts that don't quite touch, faces deleted during editing, damaged downloads, or meshes exported from CAD with tolerance errors.

How to fix it (about 30 seconds)

1. Drop your .3mf into the Ferdelance checker — inspection is free, no sign-up.
2. Open boundaries show up in amber in the 3D viewport; you'll see a count of exactly how many openings exist.
3. Auto-Repair welds stray vertices and caps every hole, then re-verifies each part. Your AMS color painting survives untouched — that's the whole point of this tool.

More fix guides

• How to fix non-manifold edges
• Fix holes and missing faces in your model
• Repair a 3MF without losing your AMS colors
• Free STL repair that never uploads your file