How to Improve Bridging in 3D Printing

Poor bridging looks like drooping lines, stringy strands across a gap, or a rough underside with holes between lines. A bridge is a stretch of filament printed across open space with no layer beneath it. The plastic has to cool and firm up fast enough to hold its shape between two anchor points, and anything that slows that down makes it sag.

This guide is research-based: it relays Prusa and Bambu Lab documentation. Slicer menus and good values differ by printer, filament and slicer, so use the numbers here as starting points.

Why bridges fail

Prusa's bridging guide gives the root cause as sagging of filament printed mid-air, and says the printer's fans need to cool the extruded plastic in mid-air to create a solid connection. Bambu Lab adds a second reason: unlike a normal layer, bridge filament is not pressed flat against a surface. It keeps a rounder cross-section, so with default settings the line width can be smaller than the spacing between lines, leaving gaps, and gravity pulls the layer down.

1. Make sure the part is cooled

Cooling is the main mechanism, so check the part cooling fan is spinning and the duct is not blocked. Bambu Lab's overhang guide says to increase the part cooling fan and auxiliary fan percentage as needed, and to check both fans are working during printing. A lower nozzle temperature also helps plastic firm up sooner, but go in small steps, and stay inside the range printed on the spool. Materials printed with little or no cooling, such as ABS, give the plastic less help to firm up, so expect bridging to be harder. See fixing sagging overhangs.

2. Adjust bridge speed

Prusa recommends a lower speed for bridges (in PrusaSlicer, under Speed, then Bridges). Bambu Lab explains that reducing bridge speed gives the filament more time to cool and stabilize before the next line, but warns that slower is not always better and that the best value depends on the machine and filament. Try a few speeds and compare. Bambu also notes speed changes apply only to the nozzle currently selected.

3. Test bridge flow, in both directions

Here the two manufacturers differ. Prusa recommends a lower bridge flow ratio on the first bridging layer (Object settings, Advanced, Bridge flow ratio). The idea is to pull the string of extruded filament behind the nozzle so it drops less. Bambu Lab describes the opposite problem: gaps between round lines, which an increased Bridge Flow can fix by letting adjacent lines expand and touch. Bambu also warns that blindly adjusting bridge flow rarely improves quality and that you should test with the same filament and process settings you plan to print with. The sensible reading is that the right direction depends on the slicer and the symptom: gaps suggest more flow, tearing or sag suggests testing less. Change one value at a time.

4. Check thick bridges

Bambu Studio has a Thick Bridges option, where the bridge line has the diameter of the nozzle. Bambu says with it on, bridges are more reliable and can span longer distances but may sag, while with it off the bridge looks thinner and only works for shorter spans. If the layer height is low, the bridge flow can be too small and the bridge may be pulled off. If you use another slicer, look for an equivalent setting.

5. Shorten the span

This is the most dependable fix. Bambu says shorter bridge spans are usually more stable and sag less, and that changing bridge direction can reduce the unsupported distance. Prusa's options are:

• Rotate the object until the bridges disappear. Prusa calls this the best solution when feasible.
• Add support islands halfway across long bridges using support enforcers.
• Change the bridging angle (Print settings, Infill, Advanced, Bridging Angle, in Advanced or Expert mode).

For help with support, see supports explained. Designing the part with an arch, chamfer or sacrificial layer is another way to remove the problem entirely.

6. Keep filament dry and the nozzle clean

Wet filament oozes and strings, and a partly clogged or dirty nozzle makes lines uneven. Dry the filament if you hear popping, and for string-like wisps across gaps see fixing stringing.

Confirm the fix

Print a test with spans of 10, 20 and 30 mm and look at the underside. A good result has parallel lines touching each other with only slight sag. If the shortest span works but the longest does not, you have reached the limit for that filament and printer, so add supports or redesign (and if those supports then fuse to the part, see fixing supports stuck to the print). Related reading: fixing under-extrusion if the lines are thin all over.