Ok, but the same problem happens with PLA as well. I suspected the skipping too but I looked closely at the video and the motor is actually turning backwards, and it pulls the filament out of the bowden tube with it. This is probably why the problem causes serious under extrusion. The filament is simply pulled out of the mechanism every so often by the intake gears. It still could be a hot end problem though because it virtually disappears at much lower speeds.
Once there is heat damage to the bowden tube it doesn't go away by itself and doesn't care what material is run.
BUT...Looking at the video again, it looks like a normal retraction. The motor is turning so slow it's hard to get a feel for what's going on.
After the motor starts back up does it start from that retracted position? It's supposed to go back to the previous position. You might check the "Travel / Retraction Prime Speed" setting in Cura. I think the default is 45mm/sec.
No, the motor continues turning forward immediately after the movement without moving the filament back. I believe this is why it causes such significant under extrusion. I will check the retraction settings in cura. I’m planning to check exactly how far the filament is pulled out each time to determine if it is just a normal retraction movement at the wrong time. It’s pretty confusing to me though that the problem disappears when I raise the temperature by a few degrees and slow the speed down to 15 or 20mm/s.
That's the extruder skipping steps when it can't provide enough torque to push the filament. You are clogged in either the nozzle or hot end. I'll put my money on the hot end. The higher temps required by PETG put heat stress on the bowden tube material that is within the hot end. Over time that changes the shape of the inside diameter and causes jams. The good stuff (Capricorn PTFE tubing) holds up better but it will still give out. I've had to replace the bowden tube twice.
If you carefully remove the bowden tube and put it in backwards (extruder end into the hot end) maybe you can bring it back to life for a while. Make sure the ends are cut perfectly square. It needs to provide a seal against the back of the nozzle.
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