Snap Fits

 

p/n 31-D108 Rev A

 

Snap Fits

 

Designing components with snap fits can save time and money in production by reducing material costs and part quantities as well as improving ease of assembly. Traditional thermoplastic materials are ideal because of their high flexibility and ability to be easily and inexpensively molded into complex geometries. Therefore, it is advantageous to have a good understanding of snap fit design and how the design may interact with 3D Systems MultiJet printing capabilities and material properties so that the right parts can be prepared for any given design intent. If done correctly, 3D Systems MultiJet Printers (MJP) are ideal for prototyping using snap fits.

 

Snap Fit Overview

In its simplest form, a snap fit is a small protrusion (hook, stud, or bead) which is deflected during assembly to catch in a depression on the mating part. A protruding part of one component is deflected during the joining process and catches a feature in the mating component. 

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Simple cantilever snap fit assembly

The shapes of the male and female parts determine the forces required for assembly and disassembly or whether the joint can be separated after assembly. 

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Separable or inseparable designs are possible

After joining, the snap-fit features should return to a stress-free condition to avoid long term creep and/or chemical/physical stress fracture sensitivities. There are many types of snap fits, the most important and common types of snap fits are:

  • Cantilever snap joints (simple beam, U-shaped or L-shaped)
  • Annular snap joints
  • Torsion snap joints
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Cantilever Annular       

Torsion

 

Snap Fit Design Principles

The cantilever joint is typically representative of other snap joints and exposes many of the design principles and best practices. The cantilever joint is modeled by a fixed-free beam with a point-applied end load. 

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This design is common and can be easily approximated using classical beam bending theory. Most bending beam equations are algebraic in nature and are well documented. Material properties are also commonly available for MJP printers and traditional materials. Therefore, even the relatively unskilled can perform simple calculations and explore snap fits in their design. 

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Cantilever Beam: Deflection-Strain Formula Examples

E: Flecural Modulus (material stiffness)

P = Force, Y = Deflection, b = Width of Beam

As the exact forces are not known when designing a model, it’s common to use deflection and strain rather than force and stress values to set initial dimensions. The deflections can be calculated from the proposed geometric design. The strains can then be solved using algebraic beam equations as shown above. Stress in a beam is dependent on its geometry and maximum deflection during assembly (typically the deflection required for assembly and accounting for any possible over bending by the user). 

The force to assemble and disassemble snap-fit assemblies is highly dependent on the overhang entrance and retraction angles.

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Upper – Simple cantilever beam and mating snap insert (y is the approx. overhang depth)

Lower – Beam tension and compressive stress due to bending

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 (E) or by increasing the cross-sectional moment of inertia (I) of the beam (i.e., making the beam thicker, especially in the direction of bending makes it stiffer). The product of these two parameters (EI) will determine the total rigidity of a given beam length. The integrity of the snap fit assembly is improved by increasing the overhang depth (dimension y in the figure above).  More overhang results in more material overlap and high holding integrity. As a result, the beam must deflect further and, therefore, requires a greater effort to clear the overhang from the interlocking hook. However, as the beam deflection increases, the beam stress also increases. This will result in a failure if the beam stress is above the yield or ultimate strength of the material. Thus, the deflection must be optimized with respect to the yield strength or strain of the material. This is achieved by optimizing the beam section geometry to ensure that the desired deflection can be reached without exceeding the strength or strain limits of the material. 

If the simplest geometry will not work sufficiently, often the best solution is to change the cantilever cross section. The worst-case stress and strain is found at the root of the cantilever. Therefore, the snap fit will likely fail at the root of the feature and care should be taken in the geometry at this point. The most common changes include tapering the width or thickness or both.

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Tapered cantilever snap fit in both width and thickness

As is good engineering practice, a fillet should be added to the root of the cantilever to minimize the effect of stress concentrations. It is common for the designer to go through several iterations (changing length, thickness, deflection dimensions, etc.) to design a snap-fit that meets all the design requirements. 

Designing Snap Fit with 3D Printed MJP Technology

One of the great strengths of 3D Systems MultiJet Printing (MJP) technology is in its broad material set with simple, automatic material changeover capability. This includes a completely waste free changeover option and generally allows for quick, hands-free swapping of the materials based on daily job requirements. Many materials are offered for the products, and each different material is optimized for specific application uses cases based on customer needs. For example, all the Rigid and Engineering materials can be used for drilling/taping/machining/pressing and achieve the good surface finish, sharp corners, fine features, and create high fidelity true-to-CAD parts that is well known for MJP technology.  The rigid white, clear, and gray materials (M2R-CL/WT/GRY) are optimized for general-purpose properties. They are fairly rigid with good heat deflection temperatures, and yet maintain adequate elongation for many use cases. They are good for concept modeling and light end-use prototype part needs. The Engineering materials M2G-CL (Armor) and M2G-DUR (ProFlex) were designed for the most aggressive engineering applications. Their stiffness is sufficient for most needs, have high elongation and impact resistance. Proflex is not as stiff as Armor, but has the highest toughness and elongation. Both materials can be used for most practical engineering applications including injection molded part designs and end-use parts for functional prototypes and jigs and fixtures. These materials were specifically designed for the best performance in snap fit applications and/or when elongation, plastic deformation, and/or toughness is required. Therefore, if you are planning on 3D printing a complex snap fit design, it is recommended to focus on either of the engineering materials. Armor is stiffer and should be tried first. For extreme designs with sufficient thickness, ProFlex may be the better material. It is commonly known and accepted that objects printed on 3D printers can be more susceptible to failure along the z axis due to layer line bonding issues in that axis. This is particularly true for extrusion type filament printing technology. For example, this diagram shows how the layer lines are printed in the different axis.

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Layer lines related to 3D printed snap fits

 

Snap fits tend to have thin cross-section and so users typically will try and optimally place the parts for the best snap fit performance. When printed upright (pictured at left), the forces that deflect the snap fit also put tension between the layers, making it significantly more likely to shear and break. Printed on its back (pictured at center), the layer lines are printed continuously in the direction of the bending and the part will be more robust. The best orientation is to print the snap fits with the layering on its side on its side (pictured at right) which creates the most robust structure with the best part performance. Generally speaking, these types of issues are not a problem for 3D Systems MJP technology except for the most aggressive and thin snap fit designs. The technology is designed such that the curing of the materials does not take place until it is buried under other layers of material. The different layers coalesce together in all directions and result in very good isotropic material properties, i.e., uniform in all directions. If there are problems with the integrity of a design and the correct materials are being used and all best practices are followed, one should always try and print the snap in a different orientation and there may be a difference in performance.  This same rule applies for gear teeth, ratcheting levers and/or other parts that undergo stress and deflection or repeated cyclic bending or stress. 

 

Common Problems and Solutions in Snap Fit Design

Stress Concentrations

Sharp corners concentrate stress at the root of the cantilever causing it to be more susceptible to shearing off. Make sure there are no sharp corners to act as stress concentrators, especially on the tensile side of the cantilever. It is common practice and is also advantageous in a 3D printed design to add a fillet in the base of the cantilever to reduce any possible stress concentrations due to the sharp corner.

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Adding a fillet to the root of the bend can greatly improve performance and fatigue life.

 

Many use case applications can be solved with a cantilever type snap-fit. For special cases and/or tight packaging, the U or L shaped snap may be advantageous. Both the U and L shaped design are easily designed with smooth large radius features which typically have reduced strain during assembly.

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                                            Cantilever                             U-Shaped Cantilever                        L-Shaped Cantilever

 

The U-Shaped design has the additional benefit of constraining the maximum amount of deflecting that a user can apply as the moving beam contacts the backup surface of the part. This eliminates the possibility of the user over deflecting the cantilever causing failure. The total deflection can also be quite large allowing for easily removable parts.

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U-Shaped Cantilever Beam:  Limits over bending and allows for removable parts

 

 Leveraging the capability of 3D printing during prototyping

Traditional injection molding design rules dictate constant wall thickness to avoid plastic sinks and/or voids. An MJP printer has no requirement for constant wall thickness or draft, etc…  This allows one to experiment with increasing the root fillet diameter or other features to improve the properties of a 3D printed design or as a way to harden an injection molded design for functional prototyping needs. Finally, for very small snap fit designs which may introduce very thin sections, one might consider an alternative compressive type annular snap fit design. Compressive type snap fit designs are common in industry for high cyclic applications and/or can be used successfully with brittle materials when other types of designs prove difficult.

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Left – Annular compressive ball or cylinder type snap in

Right – compressive square snap example

 

Creep/Stress Relaxation

Many traditional and 3D printed plastics are susceptible to creep – the gradual, permanent deformation of the material under stress. Over time this creep can compromise the connection between the male and female parts, or even render it useless. To avoid possible issues with creep, design the parts in such a way that deflection only happens during assembly. After the snap fit is assembled the parts should not be subject to prolonged bending or tensile stress. If possible, use a chamfer on the tip of both the beam and the mating snap insert to allow the mating surfaces to hold, but also slid apart when pulled in tension. Add a backup support feature for the tip of the beam to hold the snap into place. This design will be less sensitive to creep and stress relaxation of the long beam and will hold in place even if the beam does creep to some extent due to cyclic use. This can be easily implemented with a U-shaped cantilever design but other design types are possible. 

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Backup snap fit design that holds the fitting into place.

 

Fatigue or Repetitive Loading Failure

Repeated assembling and disassembling of snap joints can cause failure at stresses much lower than the rated stress of your material. Fatigue failure typically happens at high loading frequencies (hundreds if not thousands of cycles). If you anticipate high cycle frequencies for your component, careful selection of a fatigue-resistant material using S-N curves is essential. Also, it is very likely that the 3D printed prototype used for your design iterations may likely not achieve similar cycles.

 

Conclusion

Simple snap fit design can be done easily using algebraic equations and published material properties with some iterative trials. 3D printing is an excellent way to iteratively test a snap fit design. However, more complex snap fit design with high cyclic requirements can be a difficult matter depending on the exact design requirements. Stress concentrations, stress relaxation, and fatigue are all difficult design considerations and constraints for snap fit design. Snap fits offers cost and performance utility, but can be an iterative process. 3D Systems MJP ProJet 2500 Engineering materials M2G-CL (Armor) and M2G-DUR (ProFlex) were specifically designed to created robust snap fits simulating the properties of traditional injection molded plastics. It is highly recommended that all engineering best practices are followed to improve the function of a snap fit and improve fatigue and reduce prototyping life cycles.

 

MJP Snap Fit Design Examples 

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Diagnostic printed assembly part that has two printed hinges and two printed snap fits.  One snap fit holds the top onto the bottom and the other allows the bottom to split.  Both the snap fits are simple cantilever style with uniform geometry.  These features work well in the rigid and engineering materials

 

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This is a simple cantilever beam type of snap fit on a plastic carabineer.  A small gusset is used on the outside tensile side of the cantilever to reduce stress concentrations.  A large radius could have also been used.  This part was made with the rigid clear M2R-CL and it will finally break if bent all the way to the flat on the bottom.  For the engineering materials M2G-CL and M2G-DUR the cantilever part can be pressed all the way down to the maximum extension and can even be twisted around many times without breaking.  

 

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Simple functional snap buckle printed with M2R-WT.  Note the center beam feature that constrains the maximum deflection of the two cantilevers and avoids over extension failure.  

 

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This is a diagnostic that contains 12 different U-shaped cantilever snap fits.  They each have different cantilever thickness and different cantilever length and width. There is a printed hinge on all 12 of the cantilever snaps. In M2R-CL and WT, all 12 printed assemblies worked correctly. 

Note that each U-shaped cantilever has a constrained maximum deflection and avoids over extension failures.

 

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Simple cantilever type snap fit that is commonly found on a nylon electrical connector.  This in fact is a very difficult part to print and achieve correct functional performance.  This type of snap fit will work with both the engineering materials M2G-CL and M2G-DUR