Cantilever Snap Fit — Eurisco Labs

The most common snap-fit type — a flexible beam with a hook that deflects during assembly and locks behind a mating surface.

How it works

A beam anchored at one end with a hook/nub at the free end. During insertion, the hook deflects over a ledge, then springs back to lock behind it. The most widely used snap-fit in plastic part design.

Rest position:        During insertion:      Locked:

  ┌──────┐             ┌──────┐             ┌──────┐
  │      ├─┐           │      ├─┐           │      ├─┐
  │      │ │ hook      │      │╱  deflected │      │ │ engaged
  │      ├─┘           │      ├─┘           │      ├─┘
  │      │             │      │             │      │
  ████████             ████████             ████████
  (root)               (root)              (root)

Key design parameters

Parameter	Guideline
Length-to-thickness ratio	Minimum 5:1 (longer = less strain)
Max strain (PLA)	~1.5%
Max strain (PETG)	~2-3%
Max strain (Nylon)	~4-6%
Taper	0.5x thickness at tip vs root — distributes stress
Hook angle	30-45° for easy assembly, 90° for permanent

Design tips

Taper the beam — thicker at the root, thinner at the tip. Distributes bending stress instead of concentrating it at the root.
Add a lead-in chamfer on the hook for smooth insertion.
Round the root transition — a sharp corner at the beam root is a stress concentrator and the most common failure point. Even a small fillet helps.
Calculate deflection — beam deflection = (strain × length²) / (1.5 × thickness). Use this to check if your design stays within material strain limits.

FDM considerations

Print orientation matters — the beam should flex perpendicular to layer lines. If layers are horizontal, the beam should flex up/down. Flexing along layer boundaries causes delamination.
Avoid thin beams under 1.5mm — FDM layer adhesion is weak, and thin cantilevers snap at layer boundaries.
Test first — print a test piece before committing. FDM tolerances vary between printers and materials.

When to use

Removable covers and panels
Height-adjustable shelves and inlays
Battery compartments
Cases with snap-on lids

Common failures

Root fracture — beam breaks at the anchor point. Cause: no fillet, beam too short (high strain), or wrong print orientation.
Creep — beam slowly deforms under constant deflection. Cause: beam is held in a partially deflected state permanently. Solution: design for zero deflection in the locked position.
Hook wear — hook rounds off after repeated cycles. Cause: hook too small, material too soft. Solution: increase hook depth or use a harder material.