About this video
Why 3D Printed Bridges Sag and How Bridge Flow Fixes Them
Bridges are one of the trickiest features in FDM 3D printing. When your nozzle travels across an open gap, the extruded filament has nothing directly underneath it, so it sags, droops, or strings unless the printer is told exactly how much plastic to lay down. That "how much" is controlled by the bridge flow ratio, and getting it wrong is one of the most common causes of ugly top surfaces on prints.
How to Calibrate Bridge Flow with PrintPal
The fastest way to dial in this setting is with a purpose-built bridge test model. PrintPal hosts a free calibration model that prints a fan of bridges at incrementally increasing flow ratios, so you can compare them side by side in a single print.
Step-by-Step
- Open PrintPal and navigate to the bridge calibration model.
- Print the model at your usual speed, temperature, and cooling settings.
- Once finished, flip the model over and inspect each bridge segment.
- Identify the bridge with the cleanest underside, the least sag, and the most consistent extrusion.
- Read the flow ratio printed next to that winning bridge (for example, 1.6).
- Enter that number into your slicer's bridge flow or bridge extrusion multiplier setting.
For most direct-drive printers the ideal value falls between 0.85 and 1.05, while Bowden setups and printers that benefit from over-extrusion on bridges often land closer to 1.2–1.8, which is why personal calibration beats guesswork.
Pro Tips for Better Bridge Prints
- Cooling matters most. A part-cooling fan at 100% is non-negotiable for long bridges.
- Lower bridge speed. Slowing travel-driven extrusion to 20–40 mm/s gives molten plastic time to set.
- Disable combing and z-hop over bridge spans to avoid stringing across the gap.
- Re-test after filament changes, since different PLA, PETG, and ABS blends behave very differently.
Frequently Asked Questions
What is a good bridge flow ratio?
Most printers perform best between 0.9 and 1.1, but Bowden and high-flow setups often need 1.2–1.6.
Do I need to recalibrate for every filament?
Yes. Color, brand, and material type all change melt behavior, so re-run the test whenever you switch spools.
Does bridge flow affect top-layer quality?
Absolutely. A calibrated bridge flow directly improves the underside of overhangs and the integrity of top surfaces built over internal voids.
With one quick calibration print, you can say goodbye to droopy bridges and hello to crisp, clean undersides on every model.
