Sometimes objects require holes or cuts that result in a gap, (also known as a 'bridge'). As a 3D printer creates bridge features, it essentially stretches the plastic thread across the gap rather than stopping to print directly over the gap.
For small enough bridges (generally smaller than 5mm long), print quality is unaffected, which eliminates the requirement for supports. However, gaps or bridges over 5mm will either begin to sag or have surface marks from support.
Depending on your design and the tolerance (see below), a small amount of sag may be fine.
Below is a summary of the effects of gaps without support, for average desktop FDM printers:
- <5mm no quality loss
- 5-20mm little quality loss (0-0.2 mm sag)
- 25-35mm (0.1-0.5 mm sag)
- 36-60mm (0.5-2mm sag)
- >60mm significant quality loss, potentially unbridgeable (2-10mm sag)