Overhangs and bridges: how far the air holds
What angle holds without support, how far a bridge can span without drooping, and how to read a part before printing it.
An overhang and a bridge look like the same problem and are not. An overhang is supported on one side and free on the other: the new layer grips the one below and sticks out a little. A bridge is supported at both ends and touches nothing in between: the plastic crosses the gap stretched in mid-air and solidifies in flight. They tolerate different things and they fail differently.
The angle: the rule and its bands
The angle is measured from vertical. Zero degrees is a straight wall; ninety is a flat roof in mid-air. The 45° rule is real, but it is not a switch: there is a middle band where the part still comes out, only with a worse surface.
| Angle from vertical | What happens | Support |
|---|---|---|
| Up to 45° | Comes out clean. The underside is nearly as good as the rest. | No |
| 45° to 60° | Prints, with a slightly rough underside and visible layer stepping. | No, if the face is hidden |
| 60° to 75° | Starts to droop. Edges curl downwards. | Advisable |
| Over 75° | The plastic falls into the void and tangles on the nozzle. | Essential |
The bridge: how far it can span
A straight bridge between two solid anchors spans up to roughly 30 mm in PLA without trouble, and rather less in PETG, which cools more slowly and therefore sags more. Beyond that the first strand starts to dip in the middle, and although later layers correct it, the underside comes out rippled. Past 50 mm it needs supporting or redesigning.
How to read your own part
- 1Place it mentally on the bed in the orientation you will print it and look at it from below. Anything you can see from there that does not touch the bed is an overhang.
- 2Of those, separate the ones anchored on both sides. Those are bridges and will probably print unaided.
- 3The ones anchored on one side only: measure the angle. Below 45°, do nothing.
- 4For those over 45°, decide whether that face is visible. If it is not, add support and move on. If it is, redesign: a chamfer nearly always solves it without changing the function.
If you would rather not do this analysis, upload it to the configurator and leave supports switched on: it tells you the extra cost before you pay anything. But five minutes of redesign is usually cheaper than supports across a whole run.