Filament 3D printers typically do not produce solid interiors. Instead, slicing software generates an internal lattice called infill to support upper layers and add rigidity without consuming an entire spool of filament. According to Engadget, resin printers usually produce solid models unless hollowed out, but infill is a key setting on the filament printers most beginners use.
Engadget reports that most models can be printed at 10 to 15 percent infill, citing Prusa, while Flashforge recommends 20 percent as a general starting point. The article notes these are not universal rules. Decorative pieces may need very little infill, while load-bearing brackets or parts that take repeated impacts deserve more. In some cases, 0 percent infill can work if the model's geometry supports itself, but wide flat surfaces can sag without internal structure.
Higher infill increases filament use, weight, and print time, and can improve compression resistance, but it is not a magic strength setting. Engadget points out that wall thickness, print orientation, and material choice also matter, and Prusa says adding more perimeters can improve strength more effectively than adding infill. The pattern also matters: grid is a general-purpose option, cubic and gyroid provide strength from multiple directions, and lightning infill branches only where support is needed, saving material and time but offering less reinforcement. For beginners, the default pattern and moderate density are reasonable starting points.