Screw hider or "end cap" for a 15mm hole. The diameter of the inner circle is actually 16mm, in order to put pressure on the plastic when inserted into the hole, to hold it in place.Printed this in ABS on a Kossel Clear with a layer of 0.1mm, speed set to 20mm/s, and added a brim (I'm using Cura as a slicer). I first reduced the speed to 42mm/s, but that gave an uneven inner circle, so I further lowered it to 20mm/s, and that prints the piece very nicely on my Kossel Clear delta printer. It takes around 8 minutes to print, so try playing around with different settings, or leave it as it is.