Snap fits: closing a part without screws

A snap fit is the most elegant joint 3D printing allows: no screws, no inserts, and the part closes with a click. It is also the one that breaks most often, and almost always for the same reason — not the design of the clip, but the way it was printed.

Why they break

A clip works by bending. An FDM part is a stack of layers glued one on top of another, and its weak point is precisely the bond between them. If the clip prints upright, the layers sit perpendicular to the bending: flexing it does not bend plastic, it peels two layers apart. It snaps first or third time, cleanly, at the base. Laid flat, with the layers running along the clip, the same geometry in the same material survives dozens of cycles.

The design rules

1. 1Long and thin beats short and thick. A short clip concentrates all the deformation in one tiny spot; a long one spreads it. Doubling the length dramatically reduces the stress at the root.
2. 2Always fillet the root. A sharp corner where the clip meets the wall is a stress concentrator and is, literally, the line it will break along. A generous radius there is worth more than any other change.
3. 3Taper towards the tip. A constant-section clip works only at its root; one that thins out spreads the bending along its whole length.
4. 4Gentle lead-in ramp, firm retention angle. The face it enters on should be shallow so it closes with little force; the face that retains should be much steeper. Make both the same and it either will not close or will not open.
5. 5Leave clearance between the two parts. Without it the assembly will not close however well the clip works: allow a few tenths of a millimetre on every mating face.
6. 6Add a stop. A face that limits travel prevents somebody flexing the clip past what it can take, which is how well-designed ones break.

Material matters as much as shape

PLA is stiff and brittle: it bends little before snapping, and for a clip that is exactly wrong. PETG bends considerably further and returns, and it is the default choice for anything with snap fits. Carbon fibre PLA is the worst possible option here, however technical the name sounds: the fibre makes it stiffer and more brittle still. If the clip will be opened and closed daily for years, consider ABS, or a TPU hinge outright.