How to design Snap-fit Joints for 3D Printing
Written by James Low
Snap-fit joints are a quick and easy way of connecting two 3D printed components together using interlocking features. Not only are they a low cost and time saving connection method but they can also reduce the number of parts in an assembly and offer the possibility of rapid assembly and disassembly.
This article discusses the most appropriate snap-fit connections for 3D Printing, presents good design practices to consider when designing snap-fits with plastics and briefly introduce advanced design specifications redirecting the reader to more detailed information on this topic.
Finally this article examines which 3D printing processare more suitabile to manufacture your snap-fit connections.
3D printed snap-fits also do not have injection moulding design limitations (draft angles, separation lines, wall thickness, undercuts etc) and can be easily designed and altered. This makes them ideal features for rapid prototype generation where clearance and fit are critical and because of this they are often applied in the design and testing of enclosures.
Common snap-fit connections
There are 2 main types of snap-fit connections that are appropriate for 3D printing:
The cantilever is the most common snap-fit joint and consists 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 locking the connection into place.
Cantilever snap-fits are easy to design and intuitive for the user during assembly and disassembly. In many cases, it is the cheapest method of joining two parts together.
The annular snap-fit utilizes 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.
For this article the focus will be on cantilever snap-fits.
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:
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.
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.
Technologies and materials
The table below discuss the main 3D printing technologies and whether they are appropriate for printing snap-fit joints.
|FDM||Cheap and effective way of manufacturing snap-fit connections but lower accuracy than other printing methods. When printing with FDM it is recommended to use strain resistant materials such as ABS, Nylon and TPU.|
|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.|
|Material Jetting||Good strength and elasticity combined with high resolution details makes Material Jetting good for snap-fit applications. Durable resin (PP-like) and Digital ABS are the most common materials for snap-fits because of their toughness and flexibility.|
|Binder Jetting||Not suited for snap-fit connections|
Curious about the cost of prototyping with 3D printing?
Rules of thumb
- Implement good design practices that reduce stress (fillets, build direction and locating lugs) and strain (tapered profiles, cantilever width)
- Use a 0.5mm tolerance for FDM snap-fit connections and 0.3mm for all other 3D printing processes.
- SLS Nylonis best suited for snap-fit connections due to its tear resistance.
- Prototyping plastics are ideal for design confirmation but are typically lower in strength than SLS or Material Jetting materials and are not suited for applications where the connection will be opened and closed repeatedly.