Cross-section of plastic cantlever plug

Designing Injection Moulded Snap Fits

Moe Aung Tips and Tricks

Two of the most important questions to ask when developing a new product are: can I make it and will it work?

Developing a new product, especially those involving applications such as plastic parts that contain snap fits, can be difficult to manufacture and may fail in service if not designed properly. That is why it is extremely important to use engineering simulation tools to validate that your designs are manufacturable and will be fit for purpose.

In this blog post, focused on the “will it work?” question, we specifically look at a multi-component, injection-molded desk grommet cable manager that contains two snap fits and is made out of ABS plastic. But why use snap fits in the first place?

Injection moulded cantilever plugs

Because there are numerous advantages to using snap fits in your designs, including:

  • Snap fits are the simplest, quickest, and most cost-effective method of assembling two parts.
  • They can be assembled and disassembled numerous times.
  • Snap fits are environmentally friendly. Thanks to their ease of disassembly, components of different materials are easy to recycle.
  • When using materials such as thermoplastics, snap fits can be highly flexible and are easily and inexpensively molded into complex geometries.

Plastics cantilever snaps

When it comes to types of snap fits, there are a number of versatile options available for use. In most engineering material applications, including this cable manager, snap fits use a cantilever design. Cantilevers can be optimized for length, thickness, deflection and dimension, which can help you discover a snap fit with a lower allowable strain for a given material. For applications with tight packaging requirements, “U” or “L” shaped cantilever snaps are your best bet.  Our cable manager makes use of the classic cantilever style snap fit.

Regular CantileverL Shaped CantileverU shaped Cantilever

Now let’s investigate the math behind snap fit design. It turns out that you can apply classical beam theory to most snap fit assemblies. Here is how beam theory can be applied to snap fits:

  • A typical snap-fit assembly consists of a cantilever beam with an overhang at the end of the beam.
  • The main design consideration of a snap-fit is integrity of the assembly and strength of the beam.
  • The integrity of the assembly is controlled by the stiffness of the beam and the amount of deflection required for assembly or disassembly.
  • Rigidity can be increased either by using a higher modulus material or by increasing the cross sectional moment of inertia of the beam.

Cantilever forces

Finally, material properties need to be taken into account. Strain on your snap fit can change depending on the amount of filler. And the coefficient of friction dictates that if that force is too high, there will be too much force on the snap fit causing it to fail (lessened by two materials that connect easily). Fortunately, there are general guidelines to help you avoid this type of stress failure. In our cable manager, we’ve used ABS because it has both good structural integrity due to the acrylonitrile and styrene content and flexibility due to the butadiene rubber content, which helps avoid snap fit failure.

From a design standpoint, one solution to the most common failure is to incorporate a fillet radius at the juncture between the beam and the wall that reaches a ratio of radius to wall thickness (R/t) of at least 50 percent. Note that going beyond 50 percent results in a marginal increase in strength and may cause other problems, such as internal voids and sink marks. In the case of sink mark issues, a smaller radius can be used, but it may increase stress in this area. A second option is to add the radius only on the tensile side of the beam.

Correct cantilever mould examples

Remember a design is fit for purpose if it can continue to function under all of its working conditions and be economically manufactured with acceptable quality. Now that you understand the basics of snap fits, why not call us about our SolidWorks Essentials training?

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