Poor bridging in 3D printing — 5 causes of sagging strands

Symptom: polymer thrown across an empty space does not hold a straight line. The strands sag, twist, snap halfway across the span or gather into a tangle that the hotend then drags across the whole layer. The underside of the bridge looks like shreds, and the layers laid on such a base inherit its waviness.

Bridging is the one moment in a print when the material has no support underneath it at all. The freshly extruded strand hangs between two walls, held only by its own tension and by how quickly it manages to solidify. If it stays soft for that fraction of a second, gravity wins. The whole trick therefore comes down to three things at once: cooling, which hardens the strand in flight; tension, which straightens it; and the amount of material, which must be neither excessive nor too small.

Most common causes and fixes

1. Part cooling is too weak

This is cause number one, and with most materials removing it is enough to get clean bridges. Over a void there is no bed and no layer below to take the heat away, so the airflow is the only heat sink. A fan with too little output, a damaged cooling duct, or cooling limited to 30% in the profile all give exactly the same result.

How to check: pause the print on a bridge layer and listen to the part cooling fan. Check as well that the airflow really hits the nozzle and not somewhere beside it — a printed but badly fitted cooling duct is a common problem on cheap printers.

What to do: on bridge layers raise cooling to the maximum permitted for the material. With PLA and PETG that is comfortably 100%. With ABS and ASA you only have 0–40% and 0–30% respectively at your disposal, because above those values the layers stop bonding — which is why bridges in these polymers will always be worse and are better designed shorter.

Porima material	Recommended cooling	Nozzle range	Chamber
PLA	100%	200–230 °C	open
Tough PLA	100% (fan required)	210–240 °C	open
Hyper PLA+	100%	220–250 °C	open
Silk PLA	80–100%	230–260 °C	open
PLA/CF	60–100%	220–250 °C	open
PETG	100%	240–260 °C	open
PETG Transparent	80–100%	240–260 °C	open
TPU Flex 98A	40–70%	230–260 °C	open
ABS	0–40%	250–280 °C	enclosed, 50 °C
ASA	0–30%	250–280 °C	enclosed, 50 °C
PA (nylon)	30–100%, no cooling for the first 5 layers	260–290 °C	enclosed, heated
HT PETG	0–10%	270–300 °C	open or enclosed, bed adhesive recommended

Source: the official FDM technical data table from the manufacturer, Porima. The complete parameters for all 25 materials are in the print temperature table.

2. The bridging speed is badly chosen

There is no single right answer here, because the error works in both directions. Too fast: the strand is stretched more than it can take and snaps halfway across the span. Too slow: the material hangs in the air for a long time while soft, so it simply droops. On top of that comes the printer itself — a light, wobbly frame will start to vibrate on a fast bridge, and vibration also breaks the strand.

How to check: look at the bridge from the side. Strands that are straight but broken mean the speed is too high. Strands that are continuous but bulging downwards mean the speed is too low or the cooling too weak.

What to do: set a separate bridge speed in the slicer (Bridge speed in PrusaSlicer and OrcaSlicer, Bridge Wall Speed in Cura) and change it in 10 mm/s steps, each time printing the same test model with several spans. The optimum usually lies clearly below the perimeter speed.

3. The nozzle temperature is too high

Hotter polymer is thinner, sets more slowly and holds less tension. On normal layers you will not notice it, because the material is lying on a substrate. Over a void it becomes visible immediately. If bridges fail while the rest of the print looks good, it is often a sign that you are working at the upper limit of the range.

What to do: lower the temperature by 5 °C and repeat the bridge test. Keep going down until the strands stop bending, but not below the manufacturer's lower limit — that is where layer separation begins. With PETG, which prints at 240–260 °C, moving from around 255 °C to 245 °C is usually enough.

4. The filament has absorbed moisture

A bridge is the most sensitive test of filament dryness anyone could devise. Water vapour released in the nozzle tears the strand into small, foamed sections that have no tensile strength at all — and tensile strength is the only strength that counts over a void. The print may look perfectly fine everywhere else and fall apart only on the bridges.

How to check: crackling and hissing at the nozzle, and small bubbles in the extruded strand. PETG, TPU and nylon pick up moisture within a few days of opening the packaging.

What to do: dry the spool before printing and keep it in a dry box during long jobs.

Material	Drying temperature	Time
PLA and PLA variants	30–40 °C	2–4 h
PETG and PETG Transparent	30–40 °C	2–4 h
HT PETG	50–60 °C	6–12 h
ABS / ASA	40–60 °C	6–12 h
TPU Flex 98A	40–60 °C	2–4 h
PLA/CF	55 °C	4 h
PA (nylon)	65–80 °C	8–12 h — mandatory

Details and drying methods: filament drying — temperature and time for every material.

5. There is no flow compensation on bridges

Over a void the slicer cannot calculate flow the same way as on a layer lying on a substrate. A strand suspended in air naturally narrows under its own weight, and if we feed it as much material as usual, the excess hangs down and gathers into droplets. That is why slicers have a separate flow multiplier for bridges, set below unity by default. It does happen, however, that in a reworked or imported profile this parameter has been reset to the value of an ordinary layer.

How to check: open the bridge settings in your slicer and compare the flow multiplier with the rest of the profile. If both read 100%, there really is no compensation.

What to do: enable and slightly reduce the Bridge flow ratio, changing it in small steps. Pay attention as well to the direction in which the bridge is filled — the Bridge infill direction option lets you set the strands parallel to the shortest span, which on rectangular openings can do more than all the other settings put together.

When the problem is in the model, not the settings

No FDM printer will span a bridge of arbitrary length, and it is not worth losing a week tuning a profile to a geometry that is simply unsuited to it. If the span runs to several centimetres and the bridge has to carry further layers on top, the right solution is to change the model's orientation, add supports, or replace a sharp transition with a chamfer at an angle the printer will handle as an ordinary overhang. The material matters too: PLA bridges better than anything else in our range, because it sets quickly and tolerates full cooling. ABS and ASA are at the opposite pole, because they work in an enclosed chamber with cooling limited to a few tens of percent, and HT PETG runs with practically no cooling at all. With these materials you design bridges short and support them without hesitation.

Related problems

• Pillowing / holes in the top layer — a failed bridge over the infill is exactly the same mechanism.
• Support scarring — the alternative to bridging and what it costs the surface.
• Wet filament — the most common hidden cause of strands snapping.
• Under-extrusion — too little material means a broken bridge on the very first attempt.
• Weak infill — a bridge over sparse infill has nothing to rest on.
• Layer separation (delamination) — the risk after lowering the temperature too far.
• Ghosting / vibration — on fast bridging, vibration breaks the strand.