Bridging is when the Ultimaker must print a flat, horizontal part of the model mid air. Although using a support structure for such a part could be a solution, it often works better to just bridge the gap without using support. The Ultimaker will have to drag lines of plastic between already printed parts, in a way that the plastic won’t fall down when being printed. This page explains which settings are recommended for the best bridging results.
Orientation of the model
First of all, consider the orientation of the model on the build plate. In some cases you’re better of rotating the model, so that there are less/smaller overhanging parts.
To ensure the lines of plastic on the bottom layer of the bridge stick well to the pillars, it’s important to not print too fast. A high print speed will lead to bad adhesion of the plastic to the pillar (the nozzle might drag it around); thus the plastic will fall down. By using a lower print speed the plastic will have more time to properly stick to the pillars, resulting in a good adhesion. For PLA you could for example try decreasing the print speed to 20-30 mm/s, instead of using the default 50 mm/s.
Besides print speed, temperature is also an essential factor for bridging. In general we can say that a lower temperature works better. The reason for this is that the material is less liquid and doesn’t string that easily at a lower temperature. Don’t decrease the temperature too much though; this could lead to under extrusion. A temperature between 180 C and 200 C should work well for PLA (as long as you decrease the speed as well).
Additionally, cooling of the material is also important. Ensure that the print head fans are running at full speed while bridging, to harden the plastic as soon as possible.
If the bridge is positioned close to the build plate, it could also help to decrease the bed temperature a little bit. Due to the heat of the heated bed, the plastic could still be a bit soft, so by lowering the bed temperature you can minimize this effect.
If you feel confident after reading these tips, feel free to put your newly gained knowledge to the test and do the bridge-test yourself.