Design of a constant-force snap-fit mechanism for minimal mating uncertainty

For assembling injection-molded plastic parts having complicated geometry, snap-fit mechanisms are preferable to screws and chemical adhesives. The use of snap-fits relies on accurate calculation of interference and the associated mating force. The mating force is proportional to the interference, which is very sensitive to plastic part dimensional error. Insufficient interference would result in loose assembly, whereas excessive interference would impede assembly and possibly damage the thin-walled plastic parts. The possibility of unqualified interference impairs the high-value application of snap-fits to electronic appliances and automobiles. To alleviate the requirement for precise interference and to improve the assembly's robustness, this paper presents a constant-force snap-fit mechanism that maintains a regular mating force against a range of interference uncertainty. We propose a design formulation to find mechanism configurations that produce the most constant mating force. Illustrated simulations and experiments show that the mating force of the constant-force snap-fit is less sensitive to interference uncertainty than are typical snap-fits. Since uncertain mating force is minimized without demanding precise interference, we expect this mechanism to provide a ready alternative to existing snap-fit assembly applications.

► The mating forces of typical snap-fits are sensitive to interference.
► Insufficient or excessive mating forces would impair assembly quality.
► The new snap-fits provide constant mating forces against interference uncertainty.
► Computational models to facilitate the contact analysis are provided.
► The new snap-fit shows constant mating force for ± 0.3 mm of interference deviation.