Fill Density: There is a range of Hollow thin walls (1 wall), Hollow thick walls (2 walls), Low infill, Medium infill, and High infill. Changing the infill will change the estimated completion time of the print job, as well as determine how hollow or hard a printed object can be. For example, small, thin limbs my require more infill to prevent them from snapping into pieces, while large vases may not require any infill. Poor bridging happens for the top layers of hollow prints, because our software is not optimized for 90 degree overhang. In a perfect world, the extruder should slow down and the fan speed should turn up so that we can get an instantly cooled top layer. Try printing with low infill instead of hollow, and you should have much more success printing the top layer of the model.