What’s the main reason not to use 3mm filament in a Bowden setup? I’ve seen lots of people unequivocally state that they think it’s a bad idea, including Richrap in his excellent writeup of his Rostock build, but what is the actual problem here? Is it simply not having enough Esteps per volume of extruded filament, the speed of the average extruder, or slippage of the hobbed bolt on the filament itself, or what?
Back-pressure. 3mm down to even a 0.5mm nozzle has somewhere close to 9x the back pressure associated with extrusion. It’s a lot of stress to put on the motor, the bowden tube, it causes a large amount of hysteresis, etc. You need a short travel zone for 3mm filament, so that you can quickly overcome pressure issues, get proper retraction, etc.
1.75mm in a bowden tube overcomes this because there’s not nearly as much back pressure, so there’s not as much leaking/oozing, it’s easier to drive directly with the motor, so the motor can retract faster, etc.