Tips for Designing Clips for Plastic Parts

Whether you’re looking for ways to connect parts without using external fasteners or you’re creating a closure that needs to be opened and closed hundreds of times, clips are a crucial part of many mold designs. They are also a bit complicated to design and require many special considerations.

Here are the top tips for designing clips for plastic parts.

Basic Functional Requirements for Snap Locks

Also known as a clip, a snap lock:

• Should be designed to compensate for tolerances
• Must be designed to work within the plastic’s strength limits
• Should be designed to withstand opposing separation forces
• Can be designed for either repeated or one-time use
• Should ideally be engaged with little to no residual stress
• Can be designed for on/off bidirectional applications
• Should be designed to account for tool design
• Can be designed to apply a constant residual force
• Should ideally only interlock two parts by constraining them in a single axis
• That is designed to be repeatedly used should be designed to limit the deflection within working stress levels

Types of Snap Locks

There are three main snap locks: cantilever, annular, and torsional.

Cantilever

The most common type of snap lock and the easiest to design is the cantilever snap lock. It’s based on a simple beam designed to deflect a certain amount based on the height of the snap hook.

Right-angle profile cantilever snap locks provide very secure interlocks. In contrast, equilateral or half-round profiles allow snapping two parts on or off by pushing them together or pulling them apart.

Annular

Annular snap locks have a protruding locking feature attached to a contiguous edge or wall that must deform to allow the locking protrusion to snap over the mating locking feature. The forces applied to deform and snap two parts together are difficult to predict or calculate, making annular snap locks the most difficult to design, optimize, and prototype. These snap locks are typically seen in pen caps, snap-on bottle caps, plastic containers, and some electronic housings.

A variety of factors influence the performance of annular snap locks, including:

• The amount of interference
• Wall thicknesses
• The materials of the mating parts
• Molding tolerances
• Part size and geometry
• Location on a surface
• Flatness

Torsional

Torsional snap locks are ideal for applications requiring a radial lock like a push-release lock, threaded-bottle-cap safety lock, or a ratchet lock. The snap locks are more difficult to predict than cantilever snap locks but are easier to predict than annular locks.

The stressed part of a torsional snap lock must flex within the working stress of the material while also inducing enough forces to perform its primary function. Additionally, finger pressures to engage or disengage the snap must be comfortable for the average person.

Fastening Designs

Clips and other fasteners come in a variety of shapes. Some fastening designs include:

• Tree insertion
• Butterfly tab
• Latch
• Hand-grip
• Rib cage grip
• Rabbit ear
• Wedge clip
• Wedge clip with TPE insert
• TPE grip

Design Considerations

While you should always consider these tips for designing molded parts, clips for plastic parts require additional focus on a few specific design considerations.

Deflect Within the Strength Limits of the Plastic

While snap locks must include temporarily deforming materials with interlocking features like mating detents and hooks, they must also be designed to restrict deflections within the strength limits of the material’s tensile strength to prevent permanent deformations.

Snap locks designed for fewer than five repeated flexures may be designed with stresses up to the material’s elastic limit, but snap locks intended for repeated use should not exceed the material’s maximum working stress level – typically about 50% of its elastic limit. However, it varies depending on the temperature.

Stress and Flexibility

Several factors affect the stress caused by flexing a clip.

• Longer flexing arms create less stress for deflection at the end. You can get creative to lengthen the arm: loop or coil the arm to allow more length in less space, notch the wall through which the clip is attached, or design the wall to flex slightly without being notched.
• Smaller hooks create less stress than larger hooks, so keep hooks as small as possible while still functional.
• Avoid sharp corners and other features that can concentrate stress over small areas. Use rounded corners and fillets at the base of the clip.

Draft

While the draft is always an essential part of mold design, it’s crucial when designing clips since they are typically long and narrow. Not only will incorporating a minimum of 3 degrees of draft help the part release from the mold, but it strengthens the clip at its base where it needs reinforcement.

Additionally, you should ensure the through-hole at the base of the clip is significantly larger than the clip-head to allow clearance for the core in the mold that forms the underside of the clip-head.

Testing and Validation

Testing and validating the strength and flexibility of a clip while it’s still a CAD model can help save a lot of time and money compared to fixing problems at the prototype level. Consider requesting a sophisticated finite element analysis (FEA) program to help you iron out issues before milling a mold.

Get a Quote or Learn More Today

For more than 50 years, companies across the United States have chosen Universal Plastic Mold (UPM) as their large part injection molding manufacturer of choice. We have the experience to help you create the perfect clips for your parts while increasing your overall part quality, reducing overhead, and speeding up your time to market.

To receive a quote or learn more, click here or call 1-888-893-1587 today.