A printed object is rarely solid all the way through, and the choices about what fills it are where most of the cost and time live.
Infill
Filament printers fill interiors with a pattern at a chosen density. Low density is fast and light; high density is strong and slow. Most decorative prints need very little; functional parts need considerably more.
The pattern matters as well as the percentage — different geometries trade strength direction against print time.
Hollowing
Resin printing usually hollows the model entirely, leaving a shell. It saves a great deal of resin and introduces a new requirement: drain holes. A sealed hollow traps uncured resin, and trapped resin creates pressure that can crack or explode the print. At least two holes, positioned so liquid can escape as air enters.
Supports
Overhangs need something beneath them. Supports are printed and removed afterwards, and they leave marks.
- Orientation first. Rotating the model to reduce overhangs is better than supporting them.
- Support placement matters. Keep them off visible surfaces where possible.
- Contact size trades ease of removal against reliability.
Wall thickness
The shell has to be thick enough to survive handling and, for resin, cleaning. Too thin and it flexes, warps or breaks during support removal.
Splitting
Large models are often printed in parts and assembled — to fit the build volume, to avoid supports, or to orient each part optimally. Designing the split deliberately, with alignment pins or keys, produces a better result than cutting arbitrarily.
For generated models
Generated meshes are surfaces, not solids, and frequently have zero-thickness regions that hollowing cannot work with. Establishing a consistent wall thickness — usually by solidifying the surface — is a step that comes before any of the above.