Overhangs and bridging: the 45° rule, fan, speed and slicer settings against sag

A wall that leans out a little further each layer, and a gap the strand has to cross in open air: the two places where an FDM print has nothing underneath, and the cause of rough undersides, curled corners and drooping strands. This lesson separates the two, explains the 45° figure, then goes through the slicer settings and how to test your own printer.

The previous lesson, prints lifting off the bed, used less fan to reduce shrinkage. This one is the opposite: the faster it cools, the less it sags.

Overhang versus bridge

• Overhang: each new layer sits slightly past the edge of the one below, as on the underside of a sphere or raised lettering. Every layer still rests partly on the line beneath.
• Bridge: a strand leaves an anchor on one side, floats across a gap and lands on the other, as across the top of a window cut into a wall or a counterbore hole printed face down. Nothing supports the middle.

Slicers treat them separately: overhangs are about “slow and cold”, bridges about “pull the strand taut and freeze it before it droops”.

Where the 45° rule comes from

Take a 0.4 mm nozzle, 0.2 mm layers and a wall tilted θ from vertical. Each layer steps out past the one below by 0.2 × tan θ.

Angle from vertical	Step-out per layer	Still resting on the line below (0.4 mm line)
30°	0.12 mm	about 70%
45°	0.20 mm	about 50%
60°	0.35 mm	about 13%
70°	0.55 mm	the whole line floats (more than a line width)

At 45° the new line still has half its width on solid plastic, so it bonds to the layer below before it can sag. Beyond that more of it floats, and by 60–70° almost the whole line is in the air. That is why most slicers default “support overhang angle” to 45°, and why OrcaSlicer suggests 45–60° for Make overhang printable.

The figure depends on layer height: a 0.12 mm layer at 60° steps out only 0.21 mm, about the same as 45° at 0.2 mm. Thin layers print steep overhangs better, at the cost of time.

45° is a safe value for every printer, not a limit. A well-cooled printer, PLA and thin layers reach 60° cleanly. But you have to measure your own.

The three levers for overhangs

1. Fan. The floating part must solidify before its own weight pulls it down. Polymaker recommends 100% fan for PLA on overhangs and bridges, about 50–80% for PETG, and little or none for ABS/ASA (20–50% on small features only), since strong cooling splits layers and warps the part. Prusa adds that even ABS/PC benefit from 10–20% fan on steep overhangs. PrusaSlicer 2.6+ has Dynamic fan speeds (Filament settings > Cooling), raising the fan only on lines that float.

2. Speed. A slow line has time to cool before the next one presses on it. Modern slicers slow down automatically by how much the line floats.

• PrusaSlicer: Print settings > Speed > Dynamic overhang speed, speeds set at 0 / 25 / 50 / 75% overlap with the layer below.
• OrcaSlicer: Speed > Slow down for overhang, with four overhang levels in mm/s or as a percentage of wall speed (0 = no slowdown), and Slow down for curled perimeters, which slows corners likely to curl up. The wiki recommends leaving it on.
• Cura: no separate overhang-perimeter speed of this kind; use a lower Outer Wall Speed overall, or Minimum Layer Time so each layer has time to cool.

3. Temperature. Hotter plastic is runnier and sags more. If the fan is at full and it still curls, Polymaker suggests dropping the nozzle 5–10 °C, but not so far that layers stop bonding (Prusa: if layers crack after raising the fan, raise the temperature back).

Extra tools in OrcaSlicer: Extra perimeters on overhangs adds wall lines on steep sections, and Make overhang printable fills a cone of material under any overhang steeper than the set angle; use it only where the shape may change.

Bridges: pull the strand taut

A bridge rests on nothing. It relies on the moving nozzle dragging the hot strand taut from one side to the other, and the fan freezing it before it droops. The settings:

Setting	Where	What it does	Guidance
Bridge flow ratio	PrusaSlicer: Advanced · Orca: Quality > Bridging > Flow ratio	Plastic per bridge line	Prusa suggests below 1 so the strand is pulled taut and sags less; Simplify3D says not below 100% so the underside seals → test on your own printer
Bridge speed	PrusaSlicer: Speed > Bridges · Orca: Speed > Bridge	Speed of bridge lines	Prusa: lower, to give the fan time. Simplify3D: some printers do better fast. Test.
Bridge fan speed	Filament > Cooling	Fan during bridges only	High (PLA 100%); even ABS gets some
Thick bridges	PrusaSlicer: Layers and perimeters · Orca: Bridging	Nozzle-diameter-thick lines instead of normal layer height	Off = prettier but shorter spans; on = longer spans but rougher. Prusa defaults to off
Bridge density (Orca)	Quality > Bridging	Spacing of bridge lines	100% normal; lower cools better but may sag
Internal bridge speed (Orca)	Speed	Bridges hidden inside the part	Default 150% of external bridge speed; fast is fine since nobody sees it

Length: the Hubs design guide says bridges under 5 mm are trouble-free and longer ones may sag or leave marks. That is very conservative; a well-cooled printer with PLA goes several times further, but nobody can promise it for your machine, so you test.

If a bridge is longer than the printer can manage, Prusa offers two options before supporting the whole part: rotate the part so the gap need not be spanned, or add a support enforcer, a small box mid-gap, for a single pillar. For counterbore holes printed face down, OrcaSlicer’s Bridge counterbore hole can add a sacrificial layer that closes the hole for you to drill through afterwards.

Worked example: find your bridge flow in one print

The method is from the Prusa Knowledge Base and works in any slicer with per-object settings (PrusaSlicer, OrcaSlicer, Bambu Studio).

1. Load a bridge test model with stepped spans, say 10, 20, 30 and 40 mm (several are free to download; Prusa links one in its article).
2. Place four copies on the bed.
3. Right-click each > Add settings > Advanced > Bridge flow ratio, and set 0.8, 0.9, 1.0 and 1.1 in turn. Everything else identical: PLA at 205 °C, bridge speed 25 mm/s, bridge fan 100%, thick bridges off.
4. Print once and turn the parts over.

Look for straight, taut strands with none hanging down, and a fully closed underside with no gaps showing the infill above. If the 0.8 copy is taut but see-through, and the 1.1 copy is sealed but droops mid-way across the 40 mm span, this printer’s answer is 0.9–1.0, and its “clean span” is the longest gap that did not sag on the copy you chose. Write both numbers into the material profile; any gap longer than that gets split or supported.

Do the same for overhangs with a model that has 30/40/50/60/70° faces side by side: one print shows where the underside starts to roughen. OrcaSlicer ships both tests in its Calibration menu.

Safety

• A curled-up overhang corner stands above the current layer; the nozzle hitting it can knock the part loose and make belts skip. A repeated “tick” means stop and look.
• 100% fan on ABS/ASA splits layers and warps parts. Keep their bridge/overhang fan settings separate from PLA; do not share a profile.
• Lowering nozzle temperature for overhangs is fine, but if layers separate when you flex the part by hand, you went too far; a load-bearing part will snap along the layers.

Common mistakes

• Adjusting bridge flow to fix a problem caused by Thick bridges. If bridges come out unusually thick (0.4 mm at a 0.2 mm layer), check whether Thick bridges is on first.
• Treating 45° as a hard rule and adding support everywhere past it when the printer manages 60°. Wasted peeling and needless marks.
• Fighting a long bridge with fan alone. Past what the printer can span, no setting helps. Rotate the part or prop the middle with a single support enforcer.
• Forgetting that bridges land on infill. Fewer than 3 top solid layers sag into the infill grooves, because each of those is a small bridge too.

The next lesson, supports: when and how, covers what lies past both rules: parts that need holding up, and how to do it without scars.

Sources

• Prusa Knowledge Base: Poor bridging
• Prusa Knowledge Base: Layers and perimeters (Thick bridges, Detect bridging perimeters)
• Prusa Knowledge Base: Warping (fan 10–20% on steep overhangs for ABS/PC)
• OrcaSlicer Wiki: Overhang speed, Bridging, Overhangs
• Polymaker Wiki: Cooling
• Hubs: How to design parts for FDM 3D printing
• Simplify3D: Poor bridging