Please explain more. The question is too basic. The direct answer is "you change the nozzle whenever you want to".
Are you asking about what to do in cura when you change a nozzle? Or are you asking which prints come out better with which nozzle? .6mm prints about twice as fast as .4mm and .8mm prints about 4X as fast a .4mm but you get lower resolution on sharp corners. If you are printing a solid cylinder there is no loss in resolution by using a larger nozzle. But a cube will have rounded corners and will be more and more rounded the larger the nozzle.Edited by Guest
I am not sure what you mean by plotting? Here is how I use my selection of nozzles:
.25mm - Smallish models where I often have details smaller than the extrusion width of the standard .4mm nozzle. I do models in 1:100 and 1:160 scale and the nozzle makes a big difference
.6mm - Larger pieces, or things without a lot of detail, will have this nozzle on the printer. Pieces where there are a lot of vertical walls or where I do not care about layering look. I also use it for draft prints to check general fit
.8mm - Usually just for draft prints where speed is everything.
Here are a couple of articles on the topic:
Impact of Nozzle Size
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