The PTFE spray experiments suggest that using a better PTFE or other non-stick coating in the heatsink might fix the clogging problem. The way to check this is to insert a small piece of PTFE into the base screw but it has a few problems:

1. Using PTFE tubing will force the filament merge point to be moved away from the heater block. The design goal was for the merge to be in the heater block so this is not good.
2. The screw is M6, so using a 4mm OD PTFE tubing means the the heatsink resin wall will be only 1mm thick. Is this strong enough?
3. The hotend can only be used at temperatures lower than the high-temp resin maximum due to the limits of PTFE (which is a bit of a shame).

For problem 1 and 3, these are acceptable compromises during testing. For problem 2, in tests the 1mm resin was too thin and would break easily. However, switching to a 3mm OD/2mm ID tube increased the wall to 1.5mm; which was strong enough.

The above shows the new design with a 10mm PFTE insert. In testing, the new PFTE liner did prevent clogging. This design works ... even if it is a compromise.