One of the greatest disadvantages of plastic materials is their flexibility and relatively low stiffness. One of the greatest advantages of plastic materials is their flexibility and low stiffness. Your perception of which of these statements is true depends on how you, as a designer, optimize the inherent properties of plastic materials. I personally prefer the latter statement, especially when I am designing products to be aesthetically appealing and easily assembled with minimal hardware.

At one time, all products required hundreds of screws to be assembled, which demanded extensive amounts of labor and parts. The aesthetics of the final design were often compromised by numerous exposed screws and fasteners. Today’s industrial designers don’t want exposed fasteners compromising the aesthetics of their product designs, and manufacturing engineers are being pressured to produce high-quality goods more efficiently. The solution to this apparent paradox is utilizing snap fits as a means of assembling parts that is ideally suited for plastic materials. The remainder of this article will be dedicated to discussing all the considerations associated with properly designing snap fits.

Before I discuss the types of snap fit designs and their associated design parameters, I’d like to focus on some basic functional requirements for snap locks, which are listed below.

• A snap lock must be designed to work within the strength limits of the plastic.
• A snap lock can be designed for single one-time use.
• A snap lock can be designed for repeated use.
• A repeatedly used snap lock should be designed to limit the deflection within working stress levels.
• Ideally, a snap lock should only interlock two parts by constraining them in a single axis.
• A snap lock can be designed for on/off bidirectional applications.
• A snap lock ideally should be engaged with little to no residual stress.
• A snap lock can be designed to apply a constant residual force.
• A snap lock should be designed to account for tool design.
• A snap lock should be designed to compensate for tolerances.
• A snap lock should be designed to withstand opposing separation forces.

Although there are three basic types of snap locks—annular, cantilever and torsional—they all share the design considerations listed above.

Annular snap lock

The distinguishing attribute of an annular snap lock is the attachment of the protruding locking feature to a contiguous wall or edge that must deform to enable the locking protrusion to snap over the mating locking feature. In my opinion, these snap locks are the most difficult to design, prototype and optimize because the forces applied to deform and snap two parts together are very difficult to calculate or predict. Annular snap locks are often seen in snap-on bottle caps, pen caps, plastic containers and low-cost consumer electronic housings. The performance of an annular snap lock is highly dependent on the materials of both mating parts, the wall thicknesses and the amount of interference. Other critical considerations include part size and geometry, molding tolerances, flatness and location on a surface.

Torsional snap lock

Torsional snap locks are ideally suited for any application requiring a radial lock, such as a ratchet lock, threaded-bottle-cap safety lock or push-release lock. Designing a torsional snap lock is much less complicated to predict than an annular lock but more difficult than a simple cantilever snap. The stressed torsional portion of the lock must be designed to flex within the elastic working stress of the material while inducing enough forces to perform its desired function. Finger pressures to engage or disengage the snap should also be comfortable for the average person. These pressures will be a function of surface area of the release button and the force required to deflect the snap. In addition, the snap must be designed for ease of molding, tolerances, material properties and product life.

Cantilever snap lock

Cantilever snap locks are the most commonly specified snap locks and the easiest to design. They are based on a simple beam, which is designed to deflect a specified amount based on the height of the snap hook. The snap-hook profile is typically designed with a profile of a right triangle with a tapered leading edge, an equilateral triangle or a half-round configuration. A right-angle profile will provide a very secure interlock that can only be disassembled under normal conditions by manually releasing the snap. The equilateral and half-round profiles enable snapping on or off two parts simply by pressing them together or pulling them apart. We will examine the design considerations associated with each of these snap options based on the previous list of parameters.