Bridges and overhangs that sag or deform

Symptom

On bridges (sections printed in mid-air between two supports) the plastic droops, leaving strings or sagging lines underneath. On overhangs, the edge drops, comes out rough or curls up.

Likely causes, from most to least likely

1. Not enough part cooling: the fan doesn't work, runs too slowly or the air doesn't reach the nozzle properly.
2. Bridge too long: bridging only works well over short distances.
3. Bridge speed or flow too high.
4. Overhang steeper than the material can handle without supports.
5. Materials such as ABS or PC printed with no fan at all on overhangs or small parts.

Checks to tell which one it is

• With the printer off, check that the part cooling fan spins freely and its air outlet isn't blocked; in the slicer, check its speed for bridges and overhangs.
• Measure the length of the bridges in the model.
• Compare the overhang angles with the slicer's overhang threshold.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Make sure cooling works

   Check that the part cooling fan works and raise its speed for bridges and overhangs in the material profile.

   Check: Bridges stay taut without sagging.

   If that does not fix it: Lower bridge speed and flow.

2. Lower bridge speed and flow

   Reduce bridge speed and the bridge flow ratio: the strand is stretched behind the nozzle and sags less.

   Check: The bridge comes out straight underneath.

   If that does not fix it: Enable bridging perimeter detection.

3. Enable bridging perimeter detection

   Enable detect bridging perimeters in the slicer so overhangs use the bridge settings, with more fan.

   Check: Overhanging edges come out cleaner.

   If that does not fix it: Reorient or split the part.

4. Reorient or split the part

   Rotate the model to avoid bridges and overhangs, or cut it into parts that print well and glue them afterwards.

   Check: The part prints without problematic mid-air sections.

   If that does not fix it: Add supports where needed.

5. Add supports where needed

   Enable supports for overhangs beyond the threshold, or add a support halfway along a long bridge with a support enforcer.

   Check: The overhang or bridge comes out supported and clean.

   If that does not fix it: With ABS or PC, adjust the fan.

6. Low fan for high-temperature materials

   With ABS or PC, Prusa suggests trying the fan at a low speed (10-20%) if overhangs or small parts fail, without overdoing it so as not to cause warping or cracks.

   Check: Overhangs improve without the part cracking.

   If that does not fix it: If it doesn't improve, support them.

Prevention

• Design with printing in mind: avoid long bridges and very steep overhangs.
• Keep the part cooling fan clean and in good condition.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Poor bridging · Prusa Research
• Cooling · Prusa Research
• Support material · Prusa Research
• Warping · Prusa Research