Printing Bridges: When Bridges Sag
are explained briefly when you hover over or tap them.
A bridge is a straight stretch that the printer prints freely in mid-air between two points of support, for example over a door, a window or a recess. This works surprisingly well, but not always: the lines sag, break off or form a wavy, stringy surface on the underside.
How do you recognise bad bridges?
- Sagging lines in the middle of the bridge, the underside is curved instead of straight.
- Broken or loose strands dangling downwards.
- Gaps between the individual bridge lines.
- Uneven surface above the bridge, because the next layers sit on a poor base.
Why do bridges sag?
The plastic comes out of the nozzle liquid and has nothing beneath it. It has to solidify quickly enough to carry its own weight. If it does not, it sags. Three things decide this: how fast it cools, how much material is in the line and how strongly the line is stretched in the process.
The bridge settings in the slicer
All common slicers detect bridges automatically and print them with their own values. The default profiles are usually a good start. If bridges still turn out badly, these are the adjustments to try.
Fan
Cooling is the most important factor. The line has to solidify before it sags.
- PLA: part cooling fan at 100 % for bridges. The Prusa profiles do it this way.
- PETG: somewhat less, the Prusa profiles use 50 %. Too much cooling can weaken layer adhesion.
- ABS, ASA: much less, 25 % in the Prusa profiles. Too much air here leads to cracks and warping.
If cooling is poor, Bambu Lab also advises lowering the nozzle temperature slightly and, where fitted, running an auxiliary fan harder. More on this in Cooling.
Speed
Slower is not automatically better. Printed too slowly, the line has no support for too long and sags. Too fast, and it breaks. Depending on printer and material, the Prusa profiles mostly print bridges at 20 to 40 mm/s. Bambu Lab names both directions: if the bridge sags, print a little faster. If it breaks, slower.
Flow
Less material makes the line thinner and lighter, so it cools faster. Many slicers have a separate bridge flow for this. In the Prusa profiles it is mostly between 0.95 and 1.0, and lower for some materials.
Solution for sagging: reduce the bridge flow in small steps, for example by 0.05. Too little flow, however, leaves gaps between the lines.
Thick bridges
OrcaSlicer, Bambu Studio and PrusaSlicer offer an option for thick bridges. The lines are then printed as high as the nozzle is wide. This makes long bridges more stable and tear-resistant, but the underside becomes somewhat rougher. Bambu Lab explicitly recommends the option for bridges that break.
Temperature
A slightly cooler nozzle means the material solidifies faster. 5 to 10 degrees less than in the default profile can help, as long as the layers still adhere well to each other. More in Setting the print temperature.
How long can a bridge be?
That depends heavily on material, cooling and printer. Prusa writes that the best results are only achieved over short distances, and gives under 30 mm as a guide value. With well-tuned PLA and strong cooling, longer bridges are also possible, but the underside gets rougher with every centimetre.
| Length | Expectation |
|---|---|
| up to about 10 mm | works with almost any profile |
| 10 to 30 mm | usually clean with good cooling |
| over 30 mm | possible, but sagging likely, consider supports |
Filled materials, for example with carbon fibre, often bridge worse than plain PLA or PETG.
Orient the model correctly
Before you turn any settings, it is worth looking at the orientation:
- Bridge the shortest direction: Rotate the model so that the bridge spans the shortest possible distance.
- Avoid the bridge: A part can often be laid so that the opening is at the side and no bridge arises at all.
- Round holes on the side: A horizontal hole has a flat arch at the top that acts like a bridge. A teardrop or diamond shape instead of a circle avoids this if you design the part yourself.
Supports as a way out
If the bridge is too long or the underside has to look clean, supports are the more reliable solution. Bambu Lab recommends exactly that for spans that are too long. How to place supports without them leaving marks is covered in Support structures.
Quick checklist
- Can the model be rotated so that the bridge gets shorter or disappears?
- Fan for bridges suited to the material, 100 % for PLA?
- Bridge sagging? Print a little faster or reduce the bridge flow.
- Bridge breaking? Print slower or enable thick bridges.
- Try the nozzle 5 to 10 degrees cooler.
- Over 30 mm and the underside matters? Use supports.
Continue with: Holes in the top surface, because the first top layer over the infill also behaves like a bridge.