I used to make knives back in the day and kinda miss it and I don't have space and tools to make steel ones so I designed this little keychain knife from a scratch to be easily 3d printable in 3 parts that easily click together creating a little slip joint (non-locking) knife.
For the practicality, I have made this as a keychain that I can put on my keys and show people what you can 3D print, open letters and cut the tape on boxes once in a while. Blade retention and durability depend on what materials you use. If you by any chance have a machine that 3d prints metal you can have a fully functional knife and you can get some fairly nice results with carbon fibre or metal filled filaments but it will be nowhere near the same as if it were hardened and made of metal. If you use PLA it's pretty much just a letter opener.
As you can see in the background of my pictures I made a lot of them making sure that it is as good as it can be and it is a fairly small thing so if your printer isn't set up that well I would suggest you scale it up a bit first.
I will try to add in a short video with timelapse of it being printed and final assembly just in case.
Disclaimer if you decide to purchase this file you are the one responsible for a physical product you create from it. As of me posting this, I am not aware of any place where it would be illegal to print a knife but I don't know everything. From what I know I can tell you that although it is just an innocent keychain/toy that can hardly cause any damage there are countries with strict rules in which this item could be classed as a knife or a weapon and therefore illegal to carry in public places and I would strictly advise anyone to NOT take this on a plane.