A bridge within 3D printing means that the printer must extrude plastic between 2 sections of the model being printed, without having support on the underside. When the hot plastic is pulled between the 2 points, it must be cooled down quickly to prevent gravity from pulling it down, and thereby forming hanging arches on the underside. It's normal for a printer to make between 4 and 6mm bridges before it becomes apparent that the printer has been challenged. To improve this, try the following;
Lack of proper cooling of the part while it is printing is often the reason why your print ends up as in the picture. Turn on the fan from your control panel, check if your fan is running as it should and feel under the nozzle (while it is cold) if it is working properly. If not, you can check the wires and see if they are damaged or you can just replaced the fan with a new one.
There are a lot of printers that come with only one fan to cool the part. In many cases this is perfectly fine but it can happen that the print itself can block the air from reaching the point to be cooled. A lot of people design parts that are ready to be printed and replaced with the existing parts. They can help redirect the air around to the other side of the nozzle and cover more of the model. Models deisgned specificly for your printer can often be found on sites like Thingiverse and MyMiniFactory.
Support is a kind of scaffolding that helps the printer build the model in places that would otherwise be challenging or impossible. Support is built together with the model and is made to be easily removed and thrown out when the print is finished. The slicer automatically generates it under surfaces on your model, where the angle exceeds about 45 degrees. Support is enabled in the settings of your slicer.
There are many "overhang tests" that can be downloaded from the internet, so you can test the limitations of your printer.
Printing more slowly causes the plastic to cool longer before it can hold itself up. Many of the big slicers like Cura and Simplify3D have options to set a specific speed for bridges.
Print a little cooler
By lowering the print temperature you can shorten the time that elapses from the plastic leaves the nozzle till it solidifies. Try lowering the temperature by 10 degrees (but no lower than 190 degrees, or whatever the minimum temperature is for your filament) and see if that solves the problem. This can help with the sagging of the lines but can in turn result in under extrusion, so be aware of that.
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