Jowo #6 Mini Fountain Pen.

This is a tiny pen that'll only take Standard International Short cartridges. 
If you're looking for a full-size pen. Check here:
https://www.printables.com/model/747037-jowo-6-fountain-pen

This is a fountain pen that uses a Jowo #6 nib unit & Standard International Cartridge/converter. You will have to purchase those separately. I recommend the Opus 88 units available from JetPens, but any Jowo #6 housing should work.

I like to chase the threads with a M7.5x0.5 tap. It helps if you're printing with an Ender 3. With a printer like the Bambu, when I tested it, no tap was needed. 

Tough PLA works best as regular PLA can be brittle.

Print with a 0.4mm nozzle max. Adaptive layer height down to 0.1mm because there's fine detail on all of the parts. Your printer needs to be well-calibrated to print this accurately.

I like to run the calibration cube here before I run a set of pens to check my flow. https://www.printables.com/model/81314-flow-calibration-cube 
 

Print all parts with the largest flat face down and using a brim can help with bed adhesion. No support should be necessary. 
Print them one at a time for the best result to reduce stringing. You will need to deburr/chamfer the bottom edge with a sharp knife or sandpaper just to ensure a proper fit and removal of the brim if you used one. 

Because the print will still be somewhat porous and not seal as well as a traditional resin, I've found that Private Reserve Infinity Inks work best. Dipping the section to fill is not recommended since the section will absorb and stain. I prefer to syringe-fill converters or use cartridges.