Snap-fits 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 will discuss the most appropriate snap fit connections for 3D printing, present good design practice 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 will discuss each of the 3D printing processes and compare their suitability when applied to 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:
Diminishing the cross section of cantilever beam over its length. This uses less material and results in a more even distribution of strain in the cantilever
Filleting the base of the cantilever.
- Increasing the width of the clip to add strength (if this is feasible in the design constraints).
- To help reduce stress ensure that the snap-fit cantilever is only deflected during the assembly and not when the components are connected together.
- It is 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.
- 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.
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 0.5mm is recommended with 0.3mm accepted for all other printing methods.
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’s recommended to use strain resistant materials such as ABS, Polycarbonate and PEEK.|
|SLA||SLA Resins can also be used for snap-fits but tend to be quite brittle making them less ideal.|
|SLS||SLS 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 a SLS nylon.|
|Polyjet||Good strength and elasticity combined with high resolution details makes material jetting ideal for snap fit applications. Simulated polypropylene and simulated ABS are the most common Polyjet materials for snap-fits because of their toughness and flexibility.|
|Binder jetting||Not suited for snap-fit connections|
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 a 0.3mm for all other printing methods.
- The addition of support will allow FDM printers to print wall angles greater than 45 degrees.
- SLS Nylon is best suited for snap-fit connections due to its tear resistance.
- General purpose plastics are ideal for low cost prototype confirmation but are typically lower in strength than SLS or Polyjet materials and are not suited for applications where the connection will be opened and closed repeatedly.