Look at the first strand across the gap in the slicer preview. A bridge needs something to land on at both ends. If one end starts in midair, adjusting bridge speed cannot give it an anchor. Rotate the part, change the geometry or add support at that end before tuning the profile.
Read the first layer over the opening
Consider a cable clip with a rectangular window 8 mm wide and 32 mm long. If the roof strands cross the short dimension, each strand spans 8 mm. Across the long dimension each spans 32 mm, four times as far. These are drawing dimensions for this example. The calculation tells you which direction to try first without assuming your printer can bridge a particular distance.
| What the slice or underside shows | Next change |
|---|---|
| The lines run the long way across a rectangular window | Inspect the bridge angle or choose another orientation. Confirm the strands now meet solid material at both ends. |
| Parallel strands attach at both ends but hang in the middle | Keep the geometry fixed and compare a bridge-specific cooling, speed or flow change. Change one at a time. |
| The slicer labels the roof as ordinary extrusion | Check the previous layer’s geometry and the preview’s feature colors. Verify the bridge settings apply to this region. |
| The first line is a loop around a hole with no supporting ledge | Add local support, alter the roof shape or reorient the opening. A speed test cannot repair the missing endpoint. |
Prusa identifies cooling, bridge speed and bridge flow as adjustments to investigate. Simplify3D recommends checking the preview to see where bridge settings apply and distinguishes the bridge interior from its perimeter. Each slicer implements these controls differently. Prusa poor bridging · Simplify3D bridge diagnostics
Test the opening you intend to print
Use a short section containing the real window, with the same filament, orientation, wall geometry and material profile as the final part. Record the span, line direction and single setting changed. Inspect the underside against its intended use: a little sag inside a decorative cover may be fine, while the same sag in a sliding cable clip can obstruct the cable.
If acceptable clearance still requires support, paint it only where the preview shows the span failing, then check how you will remove it. Prusa’s support guide covers manual placement and changing orientation to reduce supports. Consider a replacement cooling fan only after checking the stock one and confirming the problem follows the bridge rather than the geometry. Support material · Part-cooling fan fit