Snap fit joints are one of the simplest ways to join two bodies together. They can be designed to undergo many assembly-disassembly cycles without any adverse effects to the body. They are easy to manufacture, and come in a wide variety of shapes and sizes. The one common design aspect that all snap fit assemblies have is the presence of a protruding part which deflects briefly upon the introduction of the incoming mating part, and which then snaps back into place.
Remember your indestructible Nokia 1200? It made use of a very well-designed snap mechanism to ensure the device’s safety through repeated falls. Or the Lego that you loved playing with as a child? — a snap-fit mechanism. The belt buckles? Food cans? Be it camera covers, production of cars, a simple battery case for a remote, or even a huge rocket, everywhere we look we are surrounded by them.
The purpose of this study is to validate the SimScale FEA solver for snap-fit simulations by comparing the results to the experimental data presented in this Plastic Snap Joints for Plastics by Bayer Material Science . This includes confirmation that the mating force and the force at the maximum deflection of the snap seen in the simulation matches the experimental values.
The CAD model was created in Onshape and is shown below. To reduce the size of the model for meshing and simulation, only half of the snap-fit was modeled.
Figure 1: Snap-Fit Geometry
The snap-fit geometry was meshed in SimScale using the tet-dominant meshing algorithm with quadrangular surface elements. Mesh refinements were added to the fillets and tapered surface of the snap fit. The refinements are done to areas of interest based on the results.
Figure 2: Snap-Fit Mesh
The simulation was performed as a Static Analysis with nonlinearity set to True. Nonlinearity is introduced into the problem via physical contact between the block and the snap.
Figure 3: Fixed Boundary Condition
For the simulation setup, a fixed boundary condition was applied to the small box against which the body deforms.
The snap was given a displacement boundary condition prescribed in the x-direction as 0.016*t. Under this condition, the snap body will move a total of 16mm towards the fixed body. To ramp the displacement over a series of steps, the displacement (0.016) is multiplied with time (t). This distributes the displacement over the complete simulation “time” (as it is a static simulation, the time is a pseudo time by itself) without having to input values in a table.
Figure 4: Displacement Boundary Condition
A symmetry boundary was added to one face of the model. As a best practice, symmetry can be used to reduce the size of the finite element model, which, in turn, reduces the time and cost of the analysis.
Figure 5: Symmetry Boundary Condition
The physical contact condition between the block and the snap was done using the Frictionless Penalty Contact algorithm. In a penalty approach it is possible that the faces in contact penetrate each other slightly depending on the defined contact stiffness that couples the interpenetration with the consequential reaction forces. Increasing the Penalty Coefficient helps to reduce excessive interpenetration.
Figure 5: Physical Contact: Master (blue) and Slave (pink)
Results and Conclusions
As shown in Figure 6, the snap deforms and snaps into place. This snap-fit is already well designed with the snap end being tapered, hence the strain on the final body is not excessive. But without this taper, the strain on the snap would be much higher, causing an earlier breakdown and hence limiting the life of the snap.
Figure 6: Snap-Fit animation
For comparison, the initial (points) and the final (solid) position of the snap body can be seen below:
Figure 7: 16 mm displacement
The maximum mating force is defined as the reaction force in the x-direction. Because only half of the model was simulated, the result is multiplied by a factor of 2.
Figure 8: Mating Force
The force at the maximum deflection of the snap is defined as the reaction force in the z-direction. Again due to symmetry, the result is multiplied by a factor of 2.
Figure 9: Deflection Force
The simulation results closely match the experimental results. The little difference can be attributed to the meshing grade which could be fine-tuned to improve the results.
Figure 10: Comparison of results with experimental data
 Plastic Snap Fit Design, http://fab.cba.mit.edu/classes/S62.12/people/vernelle.noel/Plastic_Snap_fit_design.pdf
 Smarter Snap-Fit Design using FEA, Webinar recording, https://www.simscale.com/snap-fit-design-using-fea/