I designed this pen after seeing the atrocity that was my old pen design, which I designed two years ago when I first started 3d printing.

*The ink cartridge is supposed to be snug in the body piece. However, it is recommended to use some superglue (or equivalent) to stick the cartridge in place and use the cap to press the cartridge all the way down.

Link to the pen cartridges I used (non-affiliate): 

https://www.amazon.com/gp/product/B0769JVJWN/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1