Snap-fit design

Theory

A snap-fit relies on controlled elastic deflection of a cantilever beam. The hook tip travels past the retaining edge and the beam recovers. Critical parameter is strain at the arm root: ≤2% for brittle materials (PLA, standard resins) and ≤4% for tough materials (PETG, Nylon, TPU).

Application

Orient the snap arm to deflect in XY (highest-strength plane for FDM). Chamfer the leading face 30–45° and the locking face 90° for permanent or 60° for releasable locks. Minimum arm length 8× arm thickness to keep strain in the elastic zone. SLA excels for intricate snap geometry at sub-0.2 mm detail.

Common mistakes

FDM snap-fits along Z are brittle: layer bonds fail at ~60% of XY strength. Avoid PLA for repeated cycles; use PETG or ABS. Over-deflection during first assembly cracks the root: add a 0.3 mm fillet. SLS snap-fits tend to be stiffer due to higher crystallinity of PA12.

Related terms: Clearance for moving / mating parts, Living hinge, Interference vs clearance fit selection