This is just a snap ring (i know i called it two different things so far, but i don't really know what to call this) that goes on the axle of the fan, and is absurdly easy to lose, and very annoying in general. Do to the ease in which it can get lost, i won't be able to upload a pic unless i find one, or i need to repair my fan. Here's hoping that i just find one... although i suppose i could print another one too....
1.8 (Might be better at lower or higher resolution, need to test)
100% (It's basically solid anyway, so this may not matter.)
Use bright colors, print multiple, be happy that you only lost half of them by looking at them, store EXTREMELY carefully. I printed mine in pla and abs, both worked, the abs maybe slightly better.
If possible, find a lab with a clean room and a robot that can interact with very small things to remove these from the print bed. If you don't have access to that, be prepared to lose a few. And than lose a few more. With the remaining one (and it will probably be just one), carefully put one on the axle as you normally would, and after you're sure that its on, test your fan (also carefully). If it's being stubborn, one drop of mineral oil should do the trick. If it still isn't spinning and you're sure it's not your fault, use a different ring. If that doesn't work, find a goat to sacrifice to the fan gods, or just replace the fan.
The story of how one little ring can be a extraordinary annoyance.
After countless seconds mourning the loss of the previous ring, I decided to fight back against the injustice of the fan world... by seeing if anyone else had made one of these for people who are cursed to lose things. That failed, and so, after heroically whining to the wise and mighty maker of the fan blades (which work quite well after a tweak or two) i decided to extremely heroically make my own. The journey to tinkercad was difficult and fraught with danger, but against some slight odds i made it, Once there, i did my best to not procrastinate. 3 hours and and a millenium falcon with a benchy launcher later, i finally completed my epic journey, and brought forth a new ring. After printing one, losing it, printing three, losing three, printing eight, losing a few and scrapping a few, i finally had one that was... well, it was serviceable, and it was late, and i was tired, so i didn't feel like printing more, even if they only take like 5 seconds to print. And so on the axle it went, and fixed my printer was! Until i discovered that tweaking the fan blades is REALLY important. Than i had to print more. but at least everything works. Now if only i could find the allen wrench....