In general you want a small melting zone. That is often a problem with PLA which melts so easy. So going to high makes it creep up the extruder increasing friction until motor does not have enough force. Seen this on several extruders. Often you can fix it by cooling down to 80-100°C and pulling out the filament. With correct temp. you get a big part of the widened part and can even see how far it got wider. Solution is
a) printing with lower temperatures so it does not creep up that much
b) cooling of extruder part above heating zone (not heating zone!)
Don't know RF2000 - some extruders have a heat barrier to prevent this and some have cooling ribs and a fan on it to even better cool the upper part.
The problem also varies between PLA vendors. Polymaker PLA for example claims to have reduced this effect so it prints like ABS. One printer that often has this problem prints at least fine with that filament, but it is not really cheap.
