Assembly Considerations
Snap-fitting is a simple, economical, and rapid way of joining two components. During assembly, one of the components that has a protrusion is momentarily deflected to clear a depression in the mating component. Assembly is completed when the deflecting component catches in the depression and relaxes to the original stress-free state. Because the snap-fit joints exist in a stress-free state after the completion of the assembly, they do not lose strength over time due to stress relaxation effects as observed in press fits. Also, because the snap-fit features are molded into the components, additional parts or bonding agents are not needed. Snap-fit components only need to be snapped in place, which does not require elaborate equipment. Because of these advantages, snap-fit joints are preferred over other joining techniques in applications where they are deemed suitable. In some cases, if a lifter or side action is needed in the mold for molding the undercut, the cost of the mold may increase slightly. The force needed for the disassembly of a snap joint can be changed by simply altering the geometry of the snap features. The features can be designed such that the assembly becomes non-separable, if desired. Elastic modulus of most engineering thermoplastics lies in a range that makes them ideal candidates to be used in snap-fit applications. Properly designed components can be easily deflected during the assembly without the fear of permanent deflection. When designing the joints, calculate the force needed to deflect the component and ensure that the strain on it during deflection is within the permissible limits of the material. Other important criteria are the forces needed for assembly and disassembly. Always provide appropriate radius at the internal corners. The checklist highlights only the cantilever snap joints, which are the most commonly used snap joints. For design assistance with U-shaped cantilever, annular, and torsion snap joints, consult Bayer's "Snap-fit Joints for Plastics" manual. |