I wanted to experiment with 3D printing an object with a built-in spring. This clip is designed so that when the clip is closed there is still some force from the spring holding it shut (as you would expect from a spring clip).

This is only a partial success: it works as expected, but if printed in ABS, the plastic will yield slightly when you open the clip, rather than being completely springy.

ABS is simply not a good material for making functional springs, as the yield strength is too low. However, with a little more care in designing the spring, it should be possible to design this so that it stays within its range of elastic deformation all the time.

After printing, simply slide the two halves of the hinge together to assemble the clip and put some force on the spring.