I’ve recently upgraded my Prusa i3 MK2S 3D printer to MK2.5 and it worked fine first print, then I ran into this issue where one or multiple layers were almost completely missing as per below photo. I’ve tried a different slicer, a different profile on my Slic3r Prusa Edition, played with the settings including extruding, speed, different temperatures, and had no luck !
I’ve decided to watch the printer until it fails to see what it was actually happening at the time of the failure and this helped me a lot in diagnosing the problem.
I have noticed that when it was failing, it was making this noise every 2 seconds and the filament was vibrating, and not actually extruding. I’ve instantly thought this has to be an issue with my extruder. I’ve tried adjusting the tension of the 2 bolts controlling the pressure of the 2 gears onto the filament, but I had no luck, the issue didn’t go away.
I’ve decided to pull apart my extruder to see if there’s something visibly wrong and I could immediately find something wrong.
One of he newly printed part as part of the 2.5 upgrade as the “extruder-idler” which has a pin through it, the pin goes through 2 tiny ball bearings and the ball bearings are inside the gear.
The other gear is mounted on the extruder’s step motor. The pin is 20 mm long, however the extruder-idler where the pin is going into, is 23.2 mm wide from loop to loop, therefore the pin isn’t long enough.
It is clear what had happened, initially I had the pin placed roughly in the middle so it’s not near the end of either side, but after one print, the pin had moved to one end and there was a massive play at the other end causing the filament to not being fed to the extruder, regardless on how much pressure I put on the gears.
The fix I came up with was replacing the factory pin 20 mm long 3 mm diameter, with a custom one 23.2 mm long 3 mm diameter which is the size end to end of the extruder-idler. It had to be smooth and same material or similar, but strong, so I’ve used a 3 mm drill bit and used an angle grinder to cut it to size starting from the end of the drill.
As soon as I’ve put it all together and started a print, voila, the issue was GONE!
As a second improvement, I’ve reprinted the extruder-idler this time with a 80% infill black PET-G instead of the 30% infill of the provided part.
I can confirm the printer had not failed once since !