Because I lose tubes of chapstick faster than I can buy them.

Update
People seemed to be having trouble printing this, so I've added "version 2" which should fix those issues. The entire cap is slightly larger, the keyring hole is much more solid, and the top of the cap is now flush with the keyring hole so it can be printed flat.

RepRap

Plastic Dragon

No

No

Version
"chapstick_keychain_cap_v2" will be much easier to print, and should be more durable. Use that one.

Extrusion Width
Set your extrusion width (in Cura this is called 'line width') to exactly .6mm. The walls of the cap should be one extrusion path, so use Slic3r or Cura 2.3 (older versions can't do single walls). You should also set your wall thickness to 1.2mm. If you don't set these correctly the print will come out like garbage. See the attached photo for what this should look like in Cura.

Print Orientation
You have two choices for printing this; if you print it right-side-up (the top of the cap pointed at the ceiling), it should come out looking the best. The top will have a nice finish and the keyring hole should have a nice, even curve. The downside is that it will require some bridging (unless you use supports, which I don't recommend), and it's more likely to pop off the bed.

The other option is to print it upside-down, with the top of the cap on the bed. It will stick to the bed better and you won't have to do any bridging, but the top of the cap will be a bit uglier.

Size
The cap should be very snug when you first put it on (almost hard to get on), but after a bit it will loosen up and fit well. Err on the side of too tight rather than too loose since the cap will loosen up over time and you don't want the chapstick falling out. Scale the print by a little if it doesn't fit quite right.