I know that the main concern of this type of systems, are the retractions, but from what I have been able to find out, the polymers have certain characteristics, it may be the same polymer but for different types of technologies their characteristics change, for example when they melt they flow as a liquid, but rather as a hot dough, and in theory that prevents material from dripping down the nozzle, but in case these drips occur, I plan to build a pneumatic system that stops and opens the nozzle very fast for extruder displacements .

Here you can find a system that closes and opens the nozzle.

https://wiki.utopiamaker.com/index.php/Extrusor_de_Pellets_para_Impresión_3D