3d Print Design Sliding Joint

How to design snap-fit joints for 3D printing

This guide discusses the benefits of 3D printing snap-fits and gives design and material recommendations.

Designing snap-fit connections

In general, snap-fits will encounter the most stress during attachment and should return to their neutral position once the joining process is completed.

Once the parts are mated, an undercut holds them together. Depending on the shape of the undercut, snap-fit assemblies can also be designed to be permanent. A well designed snap-fit with suitable material can be used many times without any noticeable fatigue.

Good design practice

Several design features can be implemented to reduce stress and strain on the snap-fit assembly. These include:

Taper the design

A snap-fit cantilever with constant cross section has an uneven distribution of strain (top image). A good design tip is to diminish the cross section of cantilever beam over its length (bottom image). This uses less material and results in a more even distribution of strain in the cantilever.

Filleting the base of the cantilever

The addition of a fillet at the base of the cantilever can help to distribute the stress over a broader area making the connection stronger. The radius of the fillet should be at least of 0.5x the thickness of the base of the cantilever.

Increase width

Increasing the width of the clip will add strength to the design (if this is feasible in the design constraints). This may require trial and error to achieve the correct stiffness however the clip should be at least 5mm wide.

Final position

To help reduce stress and improve the strength of the connection ensure that the snap-fit cantilever is only deflected during the assembly and not when the components are connected together.

Build direction

It's important to consider build direction. If possible, try to avoid designing snap-fit cantilevers that are built up from the bed vertically (in the z direction) as these are inherently weaker due to the anisotropic nature of 3D printing. A snap fit built in the direction shown in the image to the left will be much stronger.

Add lugs

Consider adding lugs into your assembly to assist with alignment of components and to transfer some of the shear loads that the clips may be subjected to.

Advanced design guides

Advanced design of snap-fit connections is out of the scope of this article. For more detailed information on the design of cantilever snap-fits the MIT snap-fit design guide and snap-fit design for plastics offer excellent technical background and design specifications.

#### Tolerances

Variations in 3D printing properties including printer calibration, print materials and printer technology mean there are no strict tolerancing rules when printing snap-fit connections.

For FDM a tolerance of 0.5 mm is recommended, and a tolerance of 0.3 mm best for all other 3D printing processes.

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