Cantilever flex.
Engagement.
Snap design.
Snap fits use elastic plastic flexibility for engagement. Cantilever design parameters control engagement force, retention, fatigue life.
Key principles.
Flexible feature
Standard snap: cantilever beam with hooked tip. Tip engages mating undercut. Beam flexes during insertion.
30° typical
Engagement face 30-45° angle. Lower angle easier insertion, higher locks more permanently.
45-90°
Retention face 45-90°. 90° = permanent (cannot disengage); 45-60° = removable.
L >> t
Beam length >> thickness for elastic flex. L/t > 5 typical. Strain in beam < 1% for nylon.
Plastic strain
Strain at maximum flex < material yield strain. Nylon 1%, ABS 1.5%, PC 1.5%.
For repeated use
Repeated snap engagements fatigue beam. For 1000+ cycles, design conservatively.
FAQ
Calculating snap force?
F = (3 × E × I × δ) / L³, where E modulus, I moment of inertia, δ deflection, L length. Standard textbook formula.
Hook depth?
0.5-2 mm typical. Larger = more retention but harder to insert. Match to mating part tolerance.
Material selection?
Polypropylene flexible (good for living hinges). Nylon strong+tough. ABS general-purpose. PC stiffer.
Wall thickness?
Beam thickness 0.6-2 mm typical. Thinner more flexible but weaker.
Living hinge alternative?
Living hinge: thin section flexes repeatedly. PP standard material. Different design from snaps.
Production tooling?
Most snaps moldable in standard injection. Some undercuts require side actions in tool.
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