Why Does the Curling Occurs?
Curling during extrusion happens when temperature is too low/high. It can be avoided by keeping the outer surface of nozzle clean, if there is previous molten plastic on top of the nozzle this happens.
What is the best extrusion temperature?
PLA best extrusion temperature is between 220-235, The extrusion temperature also depends on the hotend and extruder you use, play around with the temperature and find the best temperature that works for you.
What is the printing speed?
For Good quality print use : 45 mm/s, medium quality print at 60 mm/s, for fasting low quality printing run at 120 mm/s. It depends on the many factor software, hardware, and build, etc.,