Hi Victor,
Thanks for joining the forum.
We use E3D-V6 compatible nozzles on the Nupbox printers. Here is a great article that goes into more detail about some different options:
https://www.lpomykal.cz/v6-v5-brass-nozzles/
If purchasing a nozzle for use with carbon or metal fill filaments, it must be abrasion resistant. Tungsten carbide nozzles are a great option and are available from PhaserFPV https://www.phaserfpv.com.au/collections/3d-printer-nozzles/products/phaetus-tungsten-carbide-175mm-nozzle-ps-m6-v6-nozzle
Our standard nozzle diameter is 0.4mm. If you’d like to experiment with other options, that will be fine however we will need to change the machine back to its standard configuration after each job.