Snap-Fits

For high-volume production, molded-in snap-fit designs provide economical and rapid assembly.  In many products, snap-fits are designed for one-time assembly without any provision for nondestructive disassembly.  Where servicing is anticipated, design allows for release of the assembly with an appropriate tool.  Other snap-fit designs, such as those used in battery compartment covers for calculators and radios, are designed for easy release and re-assembly over hundreds or even thousands of cycles.

Use of snap-fit assemblies can deliver many benefits:

• An integral element of the plastic part – no other components
• Can replace screws, nuts, and washers
• Easy automation can reduce assembly costs
• No other fasteners, adhesives, solvents, welding, or special equipment
• Design can minimize risk of improper assembly
• Can be designed to engage and disengage

Things To Be Aware of When Using Snap-Fits:

• Some designs may require more complex or expensive tooling
• Snap-fits that are assembled under stress will creep
• It is difficult to design snap-fits with hermetic seals. If the beam or ledge relaxes, it could decrease the effectiveness of the seal.
• Can be damaged by mishandling and abuse prior to assembly

In all snap-fit designs, some portion of the molded part must flex like a spring, usually past a designed-in interference, and quickly return (or nearly return) to its original position to create an assembly between two or more parts.  The key to successful snap-fit design is to have adequate holding power without exceeding the elastic or fatigue limits of the material.  Since the snap-fit generally requires an undercut, a mold with side action is frequently required.  In certain cases however, the undercut can simply be popped off the mold.

Additional Information:
“Snap-Fits For Assembly and Disassembly”
“Design calculations for snap-fit joints in plastic parts”
Chapter 9, page 66 of Designing With Plastic - The Fundamentals