Shell
Use Shell to hollow a solid out and leave a wall. Pick the faces to remove, press O, and set the thickness.
Faces to remove, not faces to keep
The picker asks which faces to take away. Those become the openings, and everything else becomes wall. A box with a lid off is one face removed. A tube is two.
A shell with no faces removed is refused rather than producing a sealed hollow. For a closed cavity, shell with an opening and cap it afterwards.
Thickness goes inward
The wall grows into the solid, always. The outside of the part stays exactly where it was, and the cavity is the part minus the wall. The number has to be positive. There is no outward shell in this release, and a negative thickness is rejected rather than reinterpreted.
Anything above zero and up to 100 mm is accepted. What actually survives a print is a different question.
A thickness that survives a print
The number you type has to be built out of extruded lines, so start from your nozzle.
A 0.4 mm nozzle lays lines about 0.4 mm wide, and a wall thinner than one line either vanishes or comes out as a wobble. Two lines, around 0.8 mm, is the practical minimum for a wall that holds its shape, and it is still fragile. For a part that gets handled, go to three or four lines: 1.2 to 1.6 mm. How FDM shapes your design goes into why.
Pick a thickness that is an exact multiple of your line width. A 1.0 mm wall leaves a sliver of infill between two 0.4 mm lines.
When it fails
Shell geometry failed — common causes: thickness too large for local feature size, or selected faces leave the boundary disconnected. Try a smaller thickness or fewer faces.
The first cause is the common one: a 3 mm wall cannot fit inside a 5 mm boss. The second means the openings you asked for would cut the wall into pieces that no longer join up. Lower the thickness first. If that does not clear it, remove fewer faces and check which opening was the problem.
See also
- Fillet: round first, hollow after
- Offset Body and Offset Faces: changing a wall you have already shelled