Poor Bridging

Bridging occurs when the printer has to extrude filament between two supported points with nothing underneath. A small amount of sagging is normal, but well-tuned printers can bridge surprisingly large gaps. Part cooling is particularly important for PLA because the extruded strand needs to solidify quickly. Make sure the part-cooling fan is operating correctly. Reducing bridge speed can sometimes help, although excessively slow printing gives the filament more time to sag. Some slicers have separate bridge speed, flow and cooling settings. Too much bridge flow produces a heavy strand that may sag. Too little flow produces weak or broken bridges. Model orientation can make a major difference. If possible, rotate the part so the shortest distance is being bridged. For extremely long bridges, supports may still be necessary. Common causes: insufficient cooling, poor bridge speed, incorrect bridge flow, excessive bridge distance.