Design Guidelines for Snap Fit Joints

Projects & Prints
1

I want to print parts that can be assembled. I have no mechanical engineering background so I didn’t even know the relevant terminology. After a bit of research I learned that one way to do this is with snap-fit joints. I looked for design guidelines online on Google and amazon, figuring there must be well-known rules for how to design joints that are strong and reliable.

I found these great free PDFs, all by chemical companies:

These books also seem to contain a lot of useful information (according to their amazon previews), but they probably go well beyond what I need, and they’re not cheap:

Feel free to recommend others if you know any.

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2

Yes, follow these guidelines and your prints should come out more predictably as well.

Good luck!

3

The challenge with snap fit joints is that the current generation of resin gets very brittle as it cures.  When it’s still “fresh” a well designed joint can and will work (I’ve done plenty), but after a week or two of continued curing, the joints will typically break.  A “snap fit joint” requires the material to flex, just a little, but flex all the same, and well cured Form Labs resin does not like to flex at all.

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4

I know this is an old thread, but …
Is this also true of Durable resin - or is that resin too flexible for effective snap-fit joints?

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5

Durable resin is probably the easiest resin to design and print snap-fit parts in, but it can be possible for a lot of different Formlabs resin, including the general-purpose resins, Draft v2, Tough 1500 and Tough 2000, and even Rigid 4K. It might be possible to do snap-fits of some kind in Rigid 10K, but it probably wouldn’t be practical. Similarly, snap fits in Flexible and Elastic resins wouldn’t be very meaningful because of how relatively soft and stretchy those resins are.

Snap fit parts require a design where the deformation of the snapping features stays far enough below the materials’ tensile and/or flexural elongation at break, while also not requiring too much force. Durable has relatively high elongation and lower stiffness than other resins, enlarging the window for snap-fit parts to function as designed.

The practical details also depend on the scale and particular nature of the snap fit, like whether it’s a 5mm ball joint for a figurine, or a 10mm ball joint for a tripod, or a bunch of latches for a clamshell case, and so forth.

6

Protolabs may have some design guidelines for snap fit joints on their web site.

7

Instead of snapfit try making tabs and slots where one or the other is slightly larger or smaller. Then connect, add a bit of resin to the joint and use a 405mn handheld laser to fuse.