Quick answer
Sagging bridges are almost always caused by too much heat and not enough cooling for the short time the filament is unsupported. Set your part cooling fan to 100%, drop the nozzle temperature 5-10°C below your normal setting, slow the bridge speed to roughly 20-40 mm/s, and keep spans under about 50 mm before switching to supports.
Why bridges sag
A bridge is a line of filament stretched between two supported points with nothing underneath it. Every other layer has the previous layer to rest on and cool against, but a bridge line is extruded into open air, so it has to firm up fast enough to hold its own weight before the next line prints on top of it. If the plastic is too hot, moving too fast, or not getting enough airflow, it stays soft long enough to droop before it sets, and each line pulls the next one down with it.
1. Max out the cooling fan
Cooling is the single biggest factor in bridge quality. For PLA, run the part cooling fan at 100% during bridges; this is usually the default bridge behavior in slicers like Bambu Studio and Orca Slicer, but confirm it hasn’t been overridden. ABS and ASA need less aggressive cooling to avoid warping and layer cracking elsewhere in the print, so keep those closer to 50-70% and lean on the other fixes below instead.
2. Drop the nozzle temperature for bridges
Lower the temperature 5-10°C from your normal printing setting so the extruded plastic solidifies faster once it’s in open air. Go too low and you’ll trade sagging for under-extrusion or weak layer bonding, so change it in small steps and reprint a test bridge between adjustments rather than guessing at a big drop.
3. Slow down the bridge speed
Printing a bridge too fast stretches the filament across the gap before it has time to cool, which shows up as drooping or uneven, wavy lines. Slow the bridge speed to around 20-40 mm/s in your slicer’s bridging settings so each line has a moment to firm up before the next one lays down beside it.
4. Tune bridge flow ratio and line spacing
A bridge flow ratio below 1.0 (commonly 0.85-0.95) thins the extrusion slightly so lines don’t sag under their own weight, while values above 1.0 add material so adjacent lines touch and bond into a solid skin. In Orca Slicer this lives under Quality Settings > Bridging as Bridge flow ratio; Bambu Studio exposes the same setting under advanced quality settings. Start near 0.95 and adjust based on whether your test bridges look thin and gappy or fat and sagging.
5. Keep spans short, or add supports past ~50 mm
Unsupported bridges up to about 20-30 mm print cleanly on most machines once cooling and speed are dialed in, and well-tuned setups can stretch that to 100+ mm. Once a span passes roughly 50 mm, treat it as the danger zone: either add supports under the middle of the gap or redesign/reorient the model so the bridge is shorter. No amount of fan and speed tuning reliably fixes a bridge that’s simply too long for the filament to span.
6. Reorient the model to shorten or eliminate the bridge
Before troubleshooting settings further, check whether rotating the part in the slicer removes the bridge entirely or turns a long span into a short one. A model that bridges poorly in one orientation can sometimes print with no bridge at all in another, which is a more reliable fix than chasing cooling and speed settings on a stubbornly long span.
7. Print a bridging test piece for your filament
Bridging performance varies by brand and even by color within the same material, since pigments and fillers change how fast a filament cools. Print a simple bridging test model (20/30/40 mm spans are common) whenever you switch filament so you know your machine’s real limit instead of relying on generic numbers from someone else’s setup.
Frequently asked questions
What’s the maximum bridge distance without supports?
As a conservative baseline, treat 20-30 mm as reliably printable and 50 mm as the point where you should add supports; well-tuned cooling and speed can push some setups to 100 mm or more, but that varies by filament and printer.
Why do my bridges sag in the middle but the edges look fine?
The middle of a long bridge cools last and has the most unsupported filament pulling down on it, so it’s the first place sagging shows up; shortening the span or slowing the bridge speed usually fixes it before the edges are affected.
Should I use a different fan speed for ABS bridges than PLA?
Yes. PLA bridges best at 100% fan speed, but ABS and ASA need reduced cooling (roughly 50-70%) to avoid warping and layer separation elsewhere in the print, so bridging on those materials relies more on temperature and speed tuning than on max airflow.
Does bridge flow ratio matter if my bridges already look solid?
Not much — bridge flow ratio mainly helps when lines aren’t bonding to each other (increase it) or when bridges look over-extruded and saggy despite good cooling (decrease it). If your bridges already print flat and solid, leave it alone.