Hollow

Shell a solid to a wall thickness, and open faces to make a tray or lid.

Hollow shells a solid to a chosen wall thickness so it prints with less material. By default the shell is sealed, and drainage holes can be drilled up through the bottom so trapped resin or supports escape. **Open faces** turns that sealed shell into a tray, lid, or open enclosure: switch on a named side (Top, Bottom, Left, Right, Front, Back) in the Properties panel, or press Pick faces and click a face in the viewport to remove the wall over it, leaving the surrounding walls flush at that face. Named sides are the quick path and work on any shape — Top on a cylinder gives you an open can. Picking is for faces a side name cannot describe, like an angled one. Pick as many faces as you need — opening two opposite faces gives you a tube. A wall too thick for the part is clamped, and no faces are opened while it is (there is no shell to open). Like Round, all of it is parametric and reversible.

1. Select a single solid.
2. Open Modify → Hollow.
3. Set the wall thickness in the Properties panel.
4. To open it up, switch on a side (Top, Front, …), or press Pick faces and click each face you want removed — use Close all to seal it again.