Hi, I am also a starter with an UM2+, just a few weeks busy.
Because I mostly want small technical parts I use (the better, I believe) Polymax PLA from Polymaker.
Then the nozzle 0.25mm, a 100% infill, 220deg. nozzle temp. and, however given as optinal, a bed temp of 60 deg. Fanspeed 50%
I also recommend the use of (Dlac) hairspray, just a very little bit on the plate.
This gives me very fine, sharp edged, good removable parts.
General rule for me:
-Layer height should not exceed 2/3 the nozzle size. (I usually keep it under 1/2)
-If you use 0.4mm nozzle, 0.2 is fine, but 0.3 will start to give you bad layer bonding depending on material.
I haven't discovered a minimum yet, I've used 0.8mm nozzles to print at 0.09 layer height with no ill effects. So I can still cut down time and have a nice surface finish.
Flow should be dependent on your material rather than your nozzle size. The software will calculate the proper extrusion volume by changing your nozzle size appropriately.
For speed, you can go fast with the medium nozzles, 0.4 and 0.6, but you have to increase your temperature proportionally to your speed. For 0.25 and 0.8 I would slow it down for different reasons.
0.25 is a small hole, you physically can't squeeze that much material through so if you go too fast it will grind.
0.8 is large so you either need high temperatures to keep the filament hot while extruding a large amount or you have to slow it down.
You'll get a feel for it! but that's my 2 cents.
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