A snap-fit that feels loose, or one that cannot be pressed home, is almost never a design mistake in the shape sense. It is a tolerance-direction mistake. Three parameters decide the outcome: undercut depth, wall thickness of the beam, and insertion angle. Each has a working band, and the bands only work if the tolerances are assigned in the right direction.
Undercut depth sits between 0.5 and 1.5 mm. The important part is not the nominal value but the sign of the tolerance: only positive. A snap-fit is not a screw. If a screw comes loose you turn it another half-turn. If the undercut drifts negative in production, the interference is gone the moment the parts click together, and no downstream process can rescue it. Nominal values should therefore lean conservative, and the tolerance should be one-sided. A tight fit can be polished out of the mold. A loose one is scrap.
Beam ratio, not just thickness
The beam is a one-shot elastic element. PC and ABS allow roughly four to six percent strain. The longer and thinner the beam, the higher the strain at the root for the same undercut. The practical control is the ratio between root-to-load-point length and thickness: ten to one is safe, twelve to one is the edge, beyond that the part will whiten on the second disassembly and crack on the third.
Disassembly count belongs in the design input, not in a later conversation. A product that needs a serviceable battery compartment, designed with a single-use snap, cracked on the third inspection. The fix was not a thicker hook, which would have doubled the interference, but a two-stage snap: a long arm for travel and a short thick arm for locking. The mold only needed two inserts changed.
Insertion angle is a trade with the assembly line
A shallow insertion angle assembles easily but feels soft under a thumb. A steep one feels crisp but pushes assembly force up, sometimes past what a line operator or a fixture can deliver. The comfortable band is 25 to 35 degrees. Inside that band the choice is contextual: automated assembly takes the low end, user-serviceable parts take the high end.
A crisp click needs a defined impact point. Give the undercut face a 0.5 to 1 degree over-centre angle so the elastic return has something to snap against. On the mold this is a shut-off insert, so the machining cost barely changes, but it has to be called out on the drawing or it will not exist.
The five numbers
Undercut 0.5 to 1.5 mm, one-sided positive. Assembly interference 0.1 to 0.3 mm. Beam ratio at or under ten to one. Insertion angle 25 to 35 degrees. Draft at least one degree, including the inside of the arm. When all five hold, the snap is designed. When any one is missing, the failure shows up in the field.
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