Moulded in Clips

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Every Friday we’ll post a new video – each one giving you a deeper Insight into how to design better parts. We’ll cover specific topics such as choosing the right 3D printing material, optimising your design for CNC machining, surface finishes for moulded parts, and much more besides.

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Insight: Moulded in Clips


Hello and welcome to this week’s Insight.

This time around we’re going to be talking about something rather important in injection moulding – stress. Specifically, how stress applies to plastic clips.

Every time a clip gets used, the material it’s made from suffers a little bit of stress as it bends and then snaps back into place. 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 snap.

There are a few things that contribute to the effectiveness of a clip, but perhaps the most obvious of these is the material you make it from.

Obviously, a spring clip requires a fair bit of flexibility. The more it can stretch and move and then snap back into place the better.

However, sometimes – for whatever reason – you find yourself needing to use a resin that isn’t quite as flexible as you’d like. This is annoying, but it isn’t the end of the world quite yet, as there are several ways to design clips that minimise the amount of stress caused when they operate, and therefore minimise the chance of any damage or deformation.

Now, there are three factors affecting the stress caused by flexing a clip.

The first is the length of the flexing arm. A longer arm creates less stress for a given deflection of the end – think about how much easier it is to slightly flex a long piece of plastic than a short one.

Even if your design limits the length of the clip’s flexing arm, you still can increase the arm’s effective length in several ways. One is to loop or coil the arm, allowing you to squeeze in more length in less space. Another is to notch the wall to which the clip is attached, making the notched part of the wall, effectively, part of the clip arm. Similarly, you can design the wall itself to flex slightly without being notched. This accomplishes the same result.

The second factor we need to think about is the size of the hook at the end of the arm. Obviously, the hook must be large enough to do its job, but the smaller it is, the less the arm moves as the hook engages, and therefore the less stress it’s under.

Finally, we need to be aware of features such as sharp corners that can concentrate stress over small areas. This means paying special attention to the base of the clip, where right-angles can cause lots of stress in just one part. Instead, rounded corners and fillets can be used to distribute stress just as they are used to strengthen other features in a moulded part.

There is one other big issue that should always be considered in designing a clip, and that’s 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, which is the location that has the most bending.

Just don’t 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 visualize this core, try to give it a minimum of 3 degrees of draft and make its length no more than 8 times its thickness.

This doesn’t sound too complex, but in practise it can get quite fiddly. There are some CAD packages out there that include simple finite element analysis programs, but if you use a lot of spring clips, consider buying a separate, more sophisticated one. It could save you a fair bit of time and money in the long run.

Hopefully that’s helped you out with making the most of your clips, and keeping their stress levels to a minimum.

That’s it for this week. I look forward to seeing you again next Friday.


With special thanks to Natalie Constable

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