A Hull Line is a local horizontal contour change that often appears where geometry, thermal mass, solid infill, top surfaces or layer time changes abruptly. Uneven cooling, shrinkage and internal stress can visibly deform the outer wall at that height.
How do you recognize it?
- one or a few horizontal lines at fixed heights
- often exactly at a floor, deck, solid-infill or geometry transition
- the XY position above the line usually remains correct
Common causes
- abrupt layer-time change
- different thermal mass between thick and thin regions
- uneven cooling and shrinkage
- internal stress; flow dynamics can amplify the effect
What should you check first?
- Inspect Layer Time, Speed, Fan and Feature Type at the exact height in the slicer preview.
- Distinguish it from Z-banding, Layer Shift, Ringing and global over-extrusion.
- Change one variable per test; if the defect stays tied to geometry, soften the CAD transition.