Filament droops into the gap instead of spanning cleanly between two points. The underside of the bridge looks rough, sagged, or has loose strands hanging from it. On a bad bridge, the sagging filament gets dragged by the nozzle on the next pass and piles up into a failed layer.
Confirm the Span Length First
Bridges under 30mm print cleanly on most printers with good cooling. Spans of 30–60mm require proper bridge settings. Spans over 60mm are genuinely difficult and may need supports regardless of settings.
Print a bridging test before changing anything — it tells you where your printer’s limit actually is instead of guessing.
The Most Impactful Fix: Maximum Fan Speed
Bridges need filament to solidify before gravity wins. 100% fan on bridging layers is not optional. If your slicer has a separate “bridge fan speed” setting, confirm it’s at 100% — it often defaults lower. This single change fixes bridging on most printers with adequate cooling.
Exception: ABS and ASA. These materials crack with aggressive fan use. For ABS bridges, limit to 15% fan, slow to 15mm/s, and accept that spans over 30mm need supports.
Fix Sequence After Confirming Fan Speed
1. Increase bridging speed. Counter-intuitively, faster is better. At 40–60mm/s, the filament stretches taut across the gap before it has time to sag. At 20mm/s, gravity pulls it down while it’s still depositing. Most slicers default bridging speed to match infill speed, which is wrong.
2. Reduce bridge flow rate to 85–90%. Less material means less weight on the unsupported span. Don’t go below 80% — you’ll get gaps in the bridge layer.
3. Drop nozzle temperature 5–10°C for bridging. Some slicers (OrcaSlicer, PrusaSlicer) support per-feature temperature. Lower temperature means lower melt viscosity, which means less sag. This helps but matters less than fan speed and bridging speed.
4. Check cooling hardware. If bridging still sags after max fan, your part cooling fan may be underpowered. A 5015 blower fan outperforms a 4010 axial fan for bridging significantly.
When to Use Supports Instead
Spans over 60mm with curves or irregular geometry. Bridges where the underside surface quality matters for the final part. Any bridge in ABS or ASA over 40mm.
What Doesn’t Help
Reducing print speed below 20mm/s — slower speed on bridges is worse, not better. Adding more perimeters — perimeters don’t help bridges since bridges are the flat unsupported span between perimeters.