Bridging vs Layer Thickness (for PETG, anyway)  

Bridging vs Layer Thickness (for PETG, anyway)

I was trying to print an object with a fairly large bridged area, approx 50 X 50mm. I was also trying to print this object with 0.15 layer height. The filaments creating the first layer of the bridge came out OK, but the next layer did not stick to the bridging filaments. It looked absolutely horrible, as if I had an intermittent extruder jam, but I know I didn't because other parts of the print (at a higher elevation than the bridge, parts that were supported "from the ground up") printed just fine.
So I did several experiments to see what was causing this terrible result. I played with fan speed and the speed of the printhead during the first layer of the bridge. But what solved the problem was going to 0.25mm layer height. My theory is that this made the strings on the first layer of the bridge stiffer, so they pushed up against the traces on the next layer with just a bit more force. Also the next layer up on the bridge had more mass per unit length, meaning it had more residual heat energy after extruding from the nozzle. These two factors in combination meant that the first layer could heat up and "weld" properly to the second layer. Without this "welding" action, it's like printing on a bed where there's no adhesion - a mess!