The heating and print head needs a small redesign to stop the filament jammin inside the PTFE.
Currently the plastic filament as it melts in the heating chamber, is swelling to a wider diameter than the original diameter of the PTFE, and is getting stuck in the feeder tube when the filament retracts.
I have even tried the low voscousity blue PTFE which is supposed to allow the filament to pass through more easily. It does not work
The entire mechanism from the internal diameter of the heating chamber through to heating tube should be one diameter. Only when it reaches the tip of the head should the size change as it gets smaller rather than larger, and the head is hot so the plastic ther is melted.
This will mean that as the filament retracts and cools down slightly (thus hardens again), it doe not get jammed in the small diameter PTFE.
I have also tried - as in one you tube video advises, to insert a small tool - a hex keyy from the kit in my case - down the end of the feefer PTFE tube to widen the inside of it. This also makes the outer diameter wider and harder to insert into the heating head.
As I mentioned previously, you could make the PTFE inner diameter wider to allow the filament to pass thorugh it more easily. By the time it passes through the feeder cog and guide it gains knurl teeth marks which increased the size of the filament and also makes it more prone to breaking if anything goes wrong.
This would also mean the filament passes through a wider feeding system, and hopefully no more jams at the end of the PTFE tube.
I hope my problems and the possible solution help in the design of filament printers, which ATM are very good and improving all the time. There are always some fault, but with this kind of feedbacl and hopefully product redesign, hte errors become less and less.