Design More Effective Clips on Plastic Injection-Molded Parts

Make sure your clips are well designed to handle stress and recover from use

You have seen the routine on TV—good cop, bad cop. Bad Cop is brittle, scary, and liable to snap at any moment. You would not want to add to his stress level by not telling him what he wants to know. Good Cop, on the other hand, is flexible and easy‑going.

Moulded‑in clips on plastic parts cover much the same range. Their job subjects them to a certain amount of deflection as they move out of position and then back again. Often, they face the same deflection over and over again. Good clips—that is, well‑designed clips—handle the stress with ease, recovering fully after each deflection and remaining none the worse for wear. Bad ones that are poorly designed often break immediately or fail to fully recover, become weakened and eventually break. Unlike TV cops, however, good and bad clips do not work in teams. Each works alone (or fails to work, as the case may be).

One Crucial Decision: Material Selection

One crucial decision that contributes to the effectiveness of a clip is the choice of material. Obviously, a spring clip requires some flexibility. If, for some reason, the resin in which a part is moulded cannot be as flexible as you would like, other factors must compensate for that inflexibility.

For applications requiring high flexibility and fatigue resistance, materials such as acetal (POM) or nylon are often preferred. Understanding how a polymer’s glass transition temperature (Tg) affects its mechanical behaviour is critical here—materials used above their Tg exhibit rubber-like flexibility, while those below Tg remain glassy and brittle. For a deeper dive into how thermal transitions affect material performance, see our guide on glass transition temperature of polymers .

Stress, Flexibility, and Fatigue Resistance

There are three factors affecting the stress caused by flexing a clip:

1. Length of the flexing arm – A longer arm creates less stress for a given deflection of the end. If your design limits the length of the clip’s flexing arm, you can increase the arm’s effective length by looping or coiling the arm, notching the wall to which the clip is attached, or designing the wall itself to flex slightly.

2. Size of the hook – The hook must be large enough to do its job, but the smaller it is, the less the arm has to move as the hook engages.

3. Stress concentration features – Sharp corners concentrate stress over small areas. Pay special attention to the base of the clip where rounded corners and fillets can be used to distribute stress.

When a clip is repeatedly stressed beyond its elastic limit, it may develop stress whitening, crazing, or micro-cracks—early warning signs of eventual fatigue failure. For more on how stress manifests on part surfaces and how to identify potential failure points during design, see our guide on cosmetic defects in injection moulding .

The Role of Fillers and Additives

If your application requires a material with higher stiffness but you cannot sacrifice too much flexibility, it is worth understanding how fillers and additives alter mechanical properties. Glass fibres, for example, increase strength and stiffness but reduce elongation at break—making the material more prone to brittle failure in clip applications. For a detailed breakdown of how different fillers affect flexibility, strength, and impact resistance, see our guide on moulding materials: polymer fillers and additives .

Considering Draft

One other issue that should always be considered in designing a clip is draft . Because clips are long and narrow, it is particularly important that they be properly drafted along their lengths. In addition to easing ejection of the part from the mould, this also strengthens the clip at its base—the location of the maximum bending moment.

And do not forget to make the through‑hole at the base of the clip generously larger than the clip‑head. This allows clearance for the core in the mould which forms the underside of the clip‑head. If you can visualise this core, try to give it a minimum of 3 degrees of draft and make its length no more than 8 times its thickness.

For comprehensive guidance on applying draft to your parts, including how draft angles affect ejection quality and part strength in tall, narrow features like clips, see our draft angle guidelines .

Design Validation

Some CAD packages include simple finite element analysis (FEA) programs. If you use a lot of spring clips, consider buying a separate, more sophisticated FEA package—it could save you lots of time and money. Validating your clip design before tooling begins is one of the most effective ways to avoid costly mould modifications later.

Summary Checklist for Clip Design

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