I would call your test a success. The good news is that, knowing the rest of the process works with the 0.4 nozzle, you have isolated the problem considerably. It now comes down to determining what the 0.6 HF nozzle/PPA-CF combination will reliably tolerate.
I had suspected this might be the outcome based on my own experience. As I may have mentioned earlier, Bambu Lab, and frankly much of the rest of the 3D-printing industry, appears to put the majority of its profile-development effort into 0.4 mm nozzles. If you search this forum over the last few years, there are plenty of unhappy stories involving 0.2, 0.6, 0.8, etc., particularly once you move beyond basic materials such as PLA.
You may want to start with the following order:
-
Temperature tower - Often this does not yield a single obvious winner, but it is a good starting point and should give you a useful operating range. If you want help interpreting the tower, upload a photo and I can point to the likely best temperature.
-
Maximum volumetric flow - Based on what you just proved, this is where I would focus first. The 0.6 HF nozzle may simply be trying to push PPA-CF faster than that particular hotend/material combination can melt and move reliably. Set the maximum volumetric speed conservatively and see whether the wall degradation disappears.
-
Flow rate - Once you know you are operating below the volumetric limit, calibrate the flow ratio. That will correct systematic over- or under-extrusion, but it will not compensate for exceeding the hotend’s sustainable melt rate.
I would also be cautious about concluding that the problem is simply the 0.6 mm nozzle size. Your test really isolates it to the 0.6 HF configuration, not nozzle diameter alone. The HF internal geometry may itself be part of the problem with CF-filled materials, which would also be consistent with the clogging and filament-break behavior you described.