Maximum Bridge Length

The maximum horizontal unsupported span that can be printed reliably in mid-air between two supported walls.

Theory

Bridging is a special case of an extreme overhang where the filament or cured layer spans horizontally between two anchor points. In FDM the melt is extruded in tension: the already-deposited bead cools quickly enough to stay in place if the span is short. Cooling fan speed is the dominant variable: without cooling, even 5 mm bridges sag. SLS has no bridging concern (powder supports everything). DMLS powder bed also eliminates bridging issues but thermal gradients remain. SLA is the most sensitive: resin has near-zero tensile strength before UV exposure and peel forces act perpendicular to the span.

Application

Design parts with internal channels instead of flat-bottomed cavities to avoid bridges. If a bridge is unavoidable in FDM, use the bridging flow/speed/fan override settings in the slicer. For SLA parts with spans >5 mm, add supports even if the slicer considers them optional: verify with a cross-section view. Replace bridges with arches: a catenary arch of the same span needs no support.

Common mistakes

Slicers vary widely in bridge detection. Always inspect bridge layers in preview. PETG and flexible materials bridge worse than PLA due to higher melt temperature and lower cooling. In SLA, micro-cracks can propagate from insufficiently supported bridges during post-cure UV exposure.

Theme	geometry
FDM	<= 10 mm: 10 mm is reliably bridge-able with standard FDM settings; spans up to 30 mm are possible with optimal part-cooling fan speed and reduced print speed.
SLA	<= 5 mm: SLA resin cures one layer at a time while submerged; unsupported spans beyond 5 mm flex under peel forces and cure with sag or delamination.
Also called	bridging span, unsupported span, bridge distance
Source	Wiki/tech/fdm-fff.md, Wiki/tech/sla.md, Wiki/concepts/support-generation.md