Snap-Fit Cantilever Designer
Snap-fits are designed to a strain, not a stress. At the deflections a real snap needs, linear bending theory predicts a stress well above the material's yield strength and the part survives anyway — the event is brief and the plastic is non-linear. So the limit is permissible strain, and the modulus you use to turn that into a force is the secant modulus at that strain, not Young's modulus.
Beam
Material & permissible strain
Not the Young's modulus from a materials table — at the strain a snap fit actually works at, plastics are well off the linear part of the curve. Read Es off the resin's stress-strain chart at your design strain.
Angles
Lead angles of 15–30°; above about 45° with high friction the parts may not go together at all. For a joint meant to come apart, a return angle of 30–45°.
Live beam
The beam is drawn to scale and shaded by strain utilisation — green well inside the limit, amber approaching it, red over. Bending strain concentrates at the root, which is why the root fillet matters and why a tapered beam does better: it spreads the strain along the length instead.
Elastic cantilever theory with a permissible-strain limit, after the Ticona and Bayer snap-fit design guides. Does not cover creep under sustained load, fatigue over many cycles, weld lines, or the roll-off failure mode described below. A helper, not a substitute for engineering judgment.