3D Printed Snap-Fits

This instructable aims to help you design your own 3D printed snap-fits. This instructable can be used in combination with any of the other strategy instructables.

The only required tools are a 3D printer and any CAD modelling program.

Different cases require different types of snap-fits. This website details, with diagrams, snap-fit design principles explaining how they work. Alternative examples are shown in the first figure above.

Most snap-fits work in the way as shown in the figure above. When inserting the snap-fit, it has to elastically deform to bend backwards. The snap-fit has to bend backwards just far enough for the snap-fit head to pass over the edge. Once the snap-fit head has passed over the edge it bends back and stays in place.

If you want a more detailed explanation of snap-fits click this link.

If you want a more detailed explanation of 3D printed snap-fits click this link.

Most snap-fits bend according to the formula and diagram shown in the figure above (for more information, check this link.):

• With F being the force applied;
• With L1, being the distance to the position where the force will be applied;
• With L2, being the distance to the position on which you want to know the deflection;
• With E, being the Young's Modulus, a material constant;
• And lastly Izz being the area moment of inertia.
• The area moment of inertia is geometry specific. How to calculate this value for two basic shapes is shown in the figure above. If you have another standard shape the following link will calculate the area moment of inertia for you. If you have more complex shape you will probably need your CAD program to calculate it for you, how to do this in SolidWorks is explained here.

But don't worry you won't necessarily need to understand these formulas and variables. In the next step a few useful relations have been derived to help you adapt the geometry to your case. You can simply use these relations.

To determine the right geometry and material of your snap-fits, there are two approaches:

• Approach 1, find and calculate all of these values (for 3D printing) and use the formula.
• Approach 2, make a test print and use the relations derived in the next step to tweak the snap-fits until they are satisfactory.

This instructable will follow approach 2 because if you are able to find all of the values used in the formula, this guide is most likely unnecessary.

The following relations assume that a rectangular geometry is used, meaning that Izz = b*h^3.

It is also important to note that the Force is positioned at L1, but the deflection is measured at L2.

Now you've probably still got no real idea on the right dimensions of your snap-fits, but that does not matter since you'll be using the relations from the previous step to iterate on the dimensions anyway. If you want you could check this link for more of an idea of the dimensions though.

When printing snap-fits it is important to keep the print direction in mind. Generally you want to print them in the orientation of layers shown in green in the figure above. This way the snap-fit will be less likely to break while it's bending. But in some cases printing the snap-fits in that direction is not possible or will require an immense amount of support. So whenever that is the case you may need to print them in the 'wrong' direction. This will be fine as long as the deflection of the snap-fit is relatively small compared to its length and is wide enough.

The deflection to length ratio we used was always around 1 to 8.

Also it might be worth taking into account how to remove the snap-fits again once they're in position. In many products snap-fits are used in assemblies that are almost impossible to remove once they are in position, this might be because the design itself hasn't accounted for removing the part, they are impossible to reach or because they are attached with 3+ snap-fits that need to bend backwards simultaneously.

In the figures are two methods we used to make the snap-fits more easily removable.

• By not making them use 3+ snap-fits (general).
• By making them easily reachable.
• Or by allowing the part with snap-fits to be pushed through.

It is important to print your snap-fits with 100% infill, otherwise they will break. It is understandable that you wouldn't want to print your whole model with 100% infill. In that case you can use the support blocker tool in Cura. By applying the support blocker to your model, you are able to edit the print settings for small parts of your model.

If you want further detail on how to work with support blockers, you can watch this video. If you want to edit the shape of your support blocker you can download the Mesh Tools plugin to be able replace the support blocker cube with any .stl or .obj file.

Look at this forum post to find out how you can use this tool.

Once you have printed your test snap-fit, test it! Is the force required to get it to the right deflection satisfactory? Should the snap-fit be stronger or weaker?

Use the previously shown relations to iterate on your snap-fit until it works for your purpose.

Once you have a working snap-fit you can implement it in your part.

Good luck!