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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.

Written by James Low

How to design Snap-fit Joints for 3D Printing

Snap-fit joints are a quick, easy way of connecting two 3D-printed components by using interlocking features. Not only are they a low-cost and time-saving connection method, but they can also reduce the number of necessary parts in an assembly. Plus, they offer the possibility of rapid assembly and disassembly.

In this article, you'll learn which snap-fit connectors are the most appropriate for 3D printing , best practices to consider when designing snap-fits with plastics, and which 3D printing process is the most suitable for manufacturing your snap-fit connectors.

What are the advantages of snap-fitting?

3D-printed snap-fits don't have injection molding design limitations (e.g. draft angles, separation lines, wall thickness, undercuts) and can be easily designed and altered. This makes them ideal for rapid prototyping, where clearance and fit are critical. This is why they're often applied in the design and testing of enclosures .

3D printed enclosures with a snap-fit construction
Snap-fit construction for a DIY loudspeaker

What are common types of snap-fit connectors?

Two main types of snap-fit connectors are appropriate for 3D printing: cantilever and annular.

Cantilever snap-fits

Illustration of a cantilever snap-fit joint

The most common snap-fit joint is the cantilever, consisting of a protrusion (some type of bead or hook) at one end of the beam and a structural support at the other end. This protrusion is inserted into a cut-out or slot and deflects upon insertion. Once fully inserted, the protrusion bends back to lock the connection into place.

3D printed part with a cantilever snap-fit
A cantilever snap-fit on a 3D-printed enclosure

Cantilever snap-fits are easy to design and prove intuitive for the user during assembly and disassembly. In many cases, this is the cheapest way to join two parts together.

Annular snap-fits

The annular snap-fit utilizes a hoop strain to hold a pressed part in place. Common examples of annular snap-fits are bottle and pen caps. With annular snap-fits, it is possible to achieve a waterproof seal around the joint.

Schematic of an annular snap-fit

The rest of this article focuses on cantilever snap-fits.

Snap-fit design

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

What are best practices for designing snap-fits? 

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

Taper the design:

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

Fillet the base of the cantilever:

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

Increase width:

Increasing the width of the clip adds strength to the design, if this is feasible given the design constraints. Trial and error may be required to achieve the correct stiffness. However, the clip should be at least 5mm wide.

Deflect during assembly:

To help reduce stress and improve the connection strength, ensure that the snap-fit cantilever is only deflected during the assembly rather than when the components are connected together.

Consider build direction:

If possible, 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.

Add lugs:

Consider adding lugs to 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

If you’re looking 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

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

For fused deposition modeling ( FDM ), a tolerance of 0.5mm is recommended. For all other 3D printing processes, a tolerance of 0.3mm is best.

Which materials are used for snap-fit joints?


The table below describes the main 3D-printing technologies and whether they are appropriate for printing snap-fit joints.

Process Description
FDM This is a cheap, effective way of manufacturing snap-fit connectors but it has lower accuracy than other printing methods. When printing with FDM, the use of strain-resistant materials, such as ABS, Nylon and TPU, is recommended.
SLA SLA resins can be used for snap-fits, but are relatively brittle, increasing the likelihood of breaking after repeated use. Durable SLA resin is recommended.
SLS SLS is suitable for printing functional snap-fit prototypes or end-use parts that will be opened and closed many times. For maximum tear resistance, consider using an SLS Nylon.

Best practices for designing snap-fit joints

  • Implement good design practices that reduce stress (fillets, build direction and locating lugs) and strain (tapered profiles, cantilever width).

  • Use a 0.5 mm tolerance for FDM snap-fit connectors and 0.3 mm for all other 3D-printing processes.

  • Prototyping plastics are ideal for design confirmation, but they are typically lower in strength than SLS or material jetting materials. Thus, these are not suited to applications where the connection will be opened and closed repeatedly.

Want to learn more about 3D printing? Read our full guide: What is 3D printing?

 

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