Here are some more of our experiences with a little different PTFE extruder. This is what we are using for the heater/nozzle on our makerbot. It takes 4 bolts through the washer to attempt hold it together.It uses two extra bolts and an extra washer to restrain the PTFE melt chamber.

Here you can see my current melt zone on the left, heater and nozzle on right. The aluminum plate actually sits at the top (E above), it combined with retaining plate it is fitted against act as a heatsink. The filament melts in the PTFE and enters the heater mostly liquid allowing extrusion with low input pressures when it works. It has experienced the following failures: the PTFE hole being smashed in by excessive clamping force,  leakage if the PTFE doesn't get clamped hard enough, after time it creeps in on the filament clamping it to a stall..

It is easier to rebuild, after the PTFE is deformed I have been able to reuse it after filing a little length off the PTFE and redrill the filament hole. Still far from optimum as I have had to rebuild it several times. We have used this extruder to print Toby's Mendel and another half so far.

This is our first attempt at Mendel barrel. We turned the PTFE barrier to slide inside the heater a little on hope the pressure help with the seal.  The machining is overly complicated. We have done a slight amount of extrusion, but don't expect the PTFE to resist deformation or leaking for long, but will update you with the failure.

We have only been printing with PLA. We are still playing with finding the bombproof extruder, but it is looking like we will have to fully imitate Nophead or BfB to get there. I really like the extremely low friction of PTFE and imagine it has to be easier to push plastic through than stainless steel, but it is so soft. Current plans are to try glass filled PTFE.