I have spent a long time messing about with the extruder design trying to get over these problems.
1. The PTFE and nozzle seperate easily under pressure
2. The heater is awkward to make from nichrome wire
3. Nozzles are difficult to make especially the 0.5mm holes which break expensive drills.
In the process I have made a lot of junk.
My new design is based on NOPHEADS. It uses 22mm brass bar with 3 holes. One small hole to hold the thermistor held in with fire cement.
The next hole is 6.5mm and holds the ceramic resistor to heat the nozzle.
The third has a 5mm long 8mm tapped hole for the PEEK insulator and then is tapped all the way at 5mm and has the bottom drilled to 5mmm to insert a 0.5mm drilled out welding tip. The reason for the 5mm drilling out at the bottom is to get the welding tip as far in as possible to maximise the heat transfer.
I have tried this out and have found that I do not need the support that Nophead uses as the Peek/brass threaded section holds well under high temperature (300 degrees) and with it extruding as fast as possible. I have also tried to pull it out and failed. Only a long run at extrusion will proove if it works.
The drive design in the picture above does not yet work reliably as the threaded rod/ drive pulley has too much play and slips. I will stiffen this up with another support pulley when I get time.