POOR BRIDGING (SAGGING SPANS)

A bridge line is a tiny rope of plastic deposited across open air with nothing supporting its underside. For it to hold its shape, the plastic must cool and solidify fast enough to resist gravity before it droops. Two physical realities determine whether it can: how much heat the line is carrying when deposited, and how fast that heat can be removed. Hotter plastic flows further before solidifying. Cooler plastic solidifies sooner but bonds less to the supporting walls at each end. The sweet spot is the temperature that's just barely cool enough to solidify in mid-air but still hot enough to fuse at the endpoints. For PLA at 100% fan, this works well across a wide bridge range. For PETG at 30-50% fan, the temperature window is much narrower because cooling is limited. Flow rate matters because lighter lines weigh less and sag less. A bridge at 90% flow has the same stiffness contribution but 10% less mass pulling it downward. Counter-intuitively, reducing flow improves bridging because the loss in line cross-section matters less than the gain from reduced sag. This breaks down past 80% flow - lines too thin can't span at all.