Bridge Test Generator: Find How Far Your Printer Can Bridge
Set the shortest and longest span and the step, and the generator builds a row of pillar pairs with a flat bridge between each, the span written on the base. Print it without supports: the longest bridge that is still flat is your limit. Then tune the fan, speed and flow to push it further.
How to test and tune bridging
- 1
Choose the spans
Start with 10 to 100 mm in steps of 10. A second test across the range where the bridges start to fail, in steps of 5, shows the exact limit.
- 2
Print without support
Turn supports off and use your normal layer height, speed and temperature, so the test shows what real prints do.
- 3
Read the bridges
Look under each bridge. Flat and tight is good; sagging, stringy or broken is past the limit. The labelled span tells you how far it went.
- 4
Tune and print again
Raise the part cooling, slow down the bridge, lower the temperature or flow a little, and print the test again. Change one setting at a time.
How to read a bridge
Turn the print over or look from the side. The first layer of a bridge is printed in the air, so what you see underneath is how well that layer held.
| Flat and tight | The underside is flat, the lines are straight and touch each other. The printer handles this span without support. |
|---|---|
| Slight sag | The middle is a little lower than the ends, but the lines are continuous. Fine for hidden faces. More cooling or a slower bridge will help. |
| Stringy | The lines are thin or separated, like strings. The plastic stayed hot for too long: raise the fan and lower the temperature. |
| Drooping | The lines hang down in the middle or touch the bed. The span is past the limit at these settings: cool more, slow down and try again, or use support. |
| Broken | The lines have snapped or are missing in the middle. This span cannot be bridged without support. Use it as the upper limit for design. |
| Your limit | The longest span that is flat or has only a slight sag. Design bridges in your models to be shorter than this. |
What affects bridging
A bridge is plastic printed with nothing under it, so it has to cool and set before it sags. Three things matter most: how fast the plastic cools, how fast the nozzle moves, and how much plastic comes out. Cool fast, move at a steady speed and put out a little less plastic, and the lines stay tight.
Part cooling is the biggest. Most printers bridge well with the part fan at or near full speed on the bridge layers, even for materials that normally need little cooling. Temperature is next: a few degrees lower makes the plastic set sooner. Speed is the third: bridges often work well at a moderate speed, because moving too slowly lets the plastic heat the line it is laid on, while moving too fast can leave thin lines.
Flow is the last: slightly reduced flow on the bridge layers gives a thinner line that sags less. Because every printer and filament is different, the test is the way to find your own numbers.
Bridge settings in common slicers
Cura has a group of bridge settings (turn on "Enable Bridge Settings") for the wall and skin speed, the flow and the fan speed. PrusaSlicer and OrcaSlicer have a bridge speed, a bridge flow ratio and a bridge fan speed in the cooling settings, and an option for "thick bridges" that prints bridges as round lines rather than flat ones. Names change a little between versions, so look for "bridge" in the search box of your slicer.
A sensible way to start is to set the fan to 100% for bridges, the bridge speed to 20 to 40 mm/s and the flow to about 90 to 100%, then run the test. If the bridges are stringy, lower the nozzle temperature by 5 degrees. If they droop, slow them down a little more. Change only one value between prints, so you know which one helped.
Remember that the longest span you can bridge cleanly is a limit for your design as well. If a model needs a longer bridge, add a support under the span, or split it with a small pillar or arch, or turn the model so the span prints as an angled wall.
Bridges sorted? Make a new model
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More calibration tools
Bridge test FAQ
The STL holds the exact spans in the table. Results depend on your printer, filament and settings, so treat the notes as a guide and trust what your own print shows.