Annular Snap Fit

How it works

A ring or lip on one part expands (or contracts) to pass over a ridge on a mating cylinder, then snaps back to lock. Think bottle caps, pen caps, or pipe fittings.

Cross section:

  ┌───┐   ┌───┐
  │   │   │   │  ← outer ring expands over ridge
  │   └─┬─┘   │
  │     │     │  ← ridge
  │   ┌─┴─┐   │
  │   │   │   │
  └───┘   └───┘

Key design parameters

Parameter	Guideline
Interference (ridge height)	1-3% of ring diameter
Wall thickness	Uniform around the circumference
Lead-in angle	30-45° for easy assembly
Retention angle	45-90° depending on permanence

Design tips

• Uniform wall thickness around the ring — uneven thickness causes uneven expansion and stress concentration.
• Add relief slots if the ring can’t expand enough — 2-4 axial slots in the ring reduce the required hoop strain.
• Keep interference low — annular snap-fits generate much higher stress than cantilevers because the entire circumference must deflect simultaneously.

FDM considerations

• Print with the cylinder axis vertical — layers form concentric rings that resist hoop stress well.
• Avoid printing horizontally — the ring cross-section would be built layer-by-layer with poor interlayer adhesion, and it will split along the layer line when expanded.
• Relief slots help with FDM — they reduce the force needed and avoid cracking.

When to use

• Caps and lids on cylindrical containers
• Lens holders
• Pipe/tube connections
• Sealed enclosures

Common failures

• Ring cracking — strain too high, no relief slots. Especially common in PLA.
• Uneven engagement — ring doesn’t snap evenly around the circumference. Cause: non-uniform wall thickness or out-of-round printing.