This is a Twist Open Ballpoint Pen in which all parts can be printed easily by 3D printing(besides refill), you can open it by twisting the cap.The spring is also combined with the parts, so it's no need to buy additional springs.

To ensure the 3D printing spring structure works well, ESUN PLA+ filaments, 0.1 layer height, and 40-60% infill density are recommended. the part with spring needs supports, but don't add supports in the nib hole. The partA and partB can be printed with no support.

The refill is the Zebra JF series, you can buy it on:https://www.amazon.com/s?k=Zebra+JF+refill&crid=3754XNSW3FWT2&sprefix=zebra+jf+refill%2Caps%2C501&ref=nb_sb_noss_1See the assembly Introduction on ASSEMBLY CARD, there is also a assembly video for reference:https://www.youtube.com/watch?v=Pd1lhACxuKs

There is a free version which is bigger than this one can download at: https://www.thingiverse.com/thing:5433906see my other designs at:https://www.cgtrader.com/sailboat-fan