Overhangs and bridges that sag or curl: cooling, speed and supports

How to recognise it

An overhang is a sloping wall that rests only partly on the layer below; a bridge is a section printed in mid-air between two supports. When something is wrong, overhangs show edges curling upwards, rough surfaces or layers drooping downwards, while bridges have strands that hang or come loose. A curled edge can also be hit by the nozzle on the following layers.

Bambu Lab measures overhang as a percentage: 0% means a fully supported line, 100% a line with no support, i.e. a bridge. The higher the percentage, the faster the material has to solidify so it does not sag.

The most common causes

Insufficient cooling: Prusa explains that on overhangs and bridges the plastic has to solidify immediately, otherwise the filament sags; that is why slicers increase the fan on these sections. With ABS, ASA, PC and some flexible materials, however, too much fan can cause warping and layer separation: these materials need a compromise.

Speed too high: Bambu Lab notes that in high-speed printing, walls that are too fast make overhangs much worse even when the layer time would be enough to cool them; that is why Bambu Studio slows down only the overhanging parts. A temperature that is too high, thick layers and small parts with very short layer times make things worse, because the material stays soft for longer.

Geometry: beyond a certain angle, which depends on material, nozzle and printer, the wall does not have enough support. You then need supports or a change to the model, for example a chamfer instead of a horizontal overhang.

Checks to do, in order

Use a small overhang test with increasing angles and change one setting at a time.

1. Check that the part cooling fan spins freely and that the air duct is not blocked or bent; check that the material and printer profiles are the right ones.
2. Increase the fan on bridges and overhangs: in PrusaSlicer this is “Bridges fan speed” in the filament cooling settings, while “Dynamic fan speeds” raise the fan according to how much the line overhangs.
3. Keep overhang slowdown enabled (“Slow down for overhangs” in Bambu Studio, on by default) or reduce the outer wall speed.
4. Lower the nozzle temperature 5 °C at a time, staying within the filament's range.
5. Reduce the layer height, at least where the overhangs are, using variable layer height.
6. On small parts, print several copies together or set a minimum layer time so that each layer has time to cool.
7. For angles that remain defective, enable supports or rotate the part on the bed to reduce overhangs.

Notes from manufacturer documentation

Prusa (PrusaSlicer): “Bridges fan speed” overrides the fan speed on bridges and overhangs and is best set higher than for normal printing. With “Dynamic fan speeds” the fan changes according to the overlap with the layer below: 100% is a fully supported line, 0% a bridge. The “Keep fan always on” option is normally disabled for ABS, ASA, PC and some flexible materials. Prusa also warns that printing very slowly for long periods to respect the minimum layer time can cause heat creep.

Bambu Lab (Bambu Studio): “Slow down for overhangs” divides wall lines into overhang ranges and assigns a speed to each; 0 mm/s means no slowdown. Since version 1.9.1 the speed changes gradually from one range to the next, but only with the “Classic” wall generator; with very short layers the actual speed also depends on the cooling slowdown.

What users report in the forums

In a thread on the Bambu Lab forum (X series printer, PLA), overhangs were curling even at 45° despite trying different speeds, temperatures and fans. The author got acceptable overhangs up to about 60° by increasing the overhang speed instead of reducing it; another user solved a similar case by switching to the “Classic” wall generator with the same speed for all ranges. So the result depends on printer and material: it is worth trying both directions.

On the Prusa forum, a user with an enclosed MK3 printing ABS could only manage bridges of 2 mm. They reported improvements by reducing the layer height where the overhangs are, raising the temperature to within about 10 °C of the recommended maximum and keeping the fan moderate, never above 50%. These are individual experiences, not official guidance.

When replacing a part is justified

Only if the part cooling fan is noisy, slow or stopped, or if the air duct is damaged or fitted incorrectly: in that case replace the faulty component. Otherwise overhangs improve with settings, part orientation and supports; a more powerful auxiliary fan is worth considering only on printers designed for one.

How to verify the result

Record the model and variant, material, slicer profile and change made. Repeat the same test and compare the result with a photo taken before the adjustment. Keep the change only if it improves the defect without introducing others. Use your model’s instructions for temperatures, calibration and disassembly; this guide is not based on our own hands-on tests.