Bridge Sag
Unsupported horizontal spans (bridges) droop in the middle, creating a V or U-shaped sag between support points.
What It Is
Unsupported horizontal spans (bridges) droop in the middle, creating a V or U-shaped sag between support points.
How It Forms
A bridge deposits filament across open air between two solid points. The filament stretches between them like a clothesline. Gravity pulls the middle down.
Counterintuitively, faster bridging is better. Slow bridging gives the molten filament more time to sag under gravity before reaching the other side. Fast bridging stretches the filament taut before it can droop.
Cooling is critical — the filament must solidify mid-span. Without cooling, the solidification front can’t keep up and the bridge sags even at high speed.
Bridge flow should also be reduced — less material means less weight pulling down, and the thinner strand cools faster.
Visual Signature
- V-shaped or U-shaped dip in horizontal spans
- Wavy, uneven bottom surface on bridges
- First bridge layer sags, subsequent layers improve (they have something to print on)
- Very long bridges fail completely → spaghetti strand
Root Causes
| Cause | Calibration Variable | Direction |
|---|---|---|
| Bridge speed too slow | Speed Profile (bridge) | ↓ |
| Cooling too low | Cooling / Fan Speed | ↓ |
| Temperature too hot | Extrusion Temp | ↑ |
| Bridge flow too high | Flow Rate (bridge) | ↑ |
How the Auto-Tuner Detects It
- Camera: Bridge test print — measure sag distance at center of bridges of varying lengths. Compare to acceptable threshold.
How the Auto-Tuner Fixes It
- Calibration: Test bridges at multiple speeds and fan settings. Camera evaluates sag. Find optimal bridge speed + cooling + flow for each filament.