An experiment to make a plastic "coil" spring printed in a single piece. The design has concentric cylinders that restrict the movement to mainly vertical and an outer springy part that provides resilience. The more vertical bars, the stiffer the spring. The more horizontal rings, the greater the extent of movement. It's resistant to twisting, but would do very badly in tension I think.Also note that the spring part need not be on the outside of the concentric cylinders: it could be embedded between them, so protected from damage.Print with at least two extra shells, use 0.2 layer thickness, and print in ABS. These details are important because it won't work otherwise.The three shells are needed because the extruded plastic has to bridge the verticals to make the springy parts and it is this material that makes it into a spring. The 0.2 layer thickness ensures that the horizontal rings in the model align with the slicing and they get printed. PLA is probably too stiff to make this a useful object.The Blender model uses booleans to add the verticals and the horizontals to the basic cylinder objects, so be sure to look on the other layers for those.