Yesterday I spent quite a while printing with great success, until the last print of the evening. By now the printer had been laying down filament for a few hours, and about 3/4 through printing a coaster, I heard the dreadful clicking sound that indicates a fouled nozzle. I immediately shut things down and called it a night.
Later on, while considering why the filament would jam after all the corrective actions I’d taken, it came to me.
My printer has one fan, which came as part of the extruder assembly I had purchased. Seeing there was a fan connector on the RAMPS board, I connected the fan there and found the slicer software would control that fan. The two settings were “always on” and “auto”. I had chosen auto.
Reading about auto mode indicated the fan speed was a function of print time per layer, determined during the encoding process. I thought this a bit odd, but figured it would be fine.
I asked my friend which setting he used, and indicated that his extruder fan was wired directly to 12v, and that the slicer setting was for a print cooling fan, not the extruder fan. Further reading of the RAMPS manual verified this. The parameters affecting the auto setting now made more sense to me, as you would want more cooling on a print where the layers are taking longer to print.
Further reflection also suggests why I was having problems. First, all my jams occurred after much printing, when things had been hot for some time. With the extruder fan not running all the time (auto setting), the heat from the extruder hot end would gradually travel up the metal feed tube. Eventually, the filament in the tube would become warm further from the nozzle. When this happened, it would be harder to feed the warm filament and it could resist and jam, much like trying to push a piece of hard spaghetti vs. cooked spaghetti. The purpose of the fan is thus to keep the feed tube cool so the filament remains cool and feeds properly.
Later today (after my dive) I will rewire the extruder fan to the 12V supply so that it’s running whenever the printer is on.