# Snap-fit design

I'm designing 2 objects that should be attached to each other and removed when it's necessary. So I though about snap-fit as a solution. There is a lot of types of snap-fit but I'm planing to use a circular one do you know what are the parameters that should be respected to make a good snap fit? I am using solidworks.

• Do you know what material you're planning to use yet? – Ethan48 May 25 '15 at 18:38
• I'll be using 3D printing to make a first prototype. – Lavender May 28 '15 at 7:33
• please also check the resolution of your 3d printer, and the strength properties of the 3d-material that you produce. Otherwise you may arrive at wrong conclusions (it may be removed with more force than you design for when built by other methods).. – Gürkan Çetin Jun 26 '15 at 18:47

Below is simple table for designing snap-fit joints from Bayer Snap-Fit Joints for Plastics a Design Guide, please consult the pdf for more info. As I understand your joint is C cross section with 1st type. You should also check for spherical joint clearance should be y/2.

y = (permissible) deflection (=undercut), E = (permissible) strain in the outer fiber at the root; in formulae: E as absolute value = percentage/100, l = length of arm, h = thickness at root, b = width at root, c = distance between outer fiber and neutral fiber (center of gravity), Z = section modulus Z = I c, where I = axial moment of inertia, Es = secant modulus, P = (permissible) deflection force, K = geometric factor.

In order to use this formulas you need to consult material specific allowable strain limits (E) and secant modulus for appropriate temperature. If your material has distinct yield point, I guess it will have if it's not reinforced with fiber $0.7\epsilon_{y}$ can be taken. Even though please check design guidelines. Below are some more resources for your design.

BASF Snap-Fit Design Manual

Sabic online tool (Needs Registration)

The main questions here are:

• What material are you using?

• How much load is there on the joint?

• How many times does it need to be removable?

Snap fit may be a good idea, but it's difficult to elaborate further when I don't know the usage case:

1. There has to be a compromise between being able to connect the pieces and the strength of a the joint. A stiff joint is strong but could be impossible to push into place or break it if excess force is applied.
2. I would try to design not to carry any ordinary structural load on the snap fit and rather unload the connected members independently. Since the joints have a bit of weakness they are more difficult to predict the strength of and will be easier to break. The snap-fit in my eyes should ideally only hold the object from sliding apart in that sense.
• What material are you using? I'll be using 3D printer to generate a first prototype . How much load is there on the joint? it's not heavy at all it's about 20 grams. How many times does it need to be removable? frequently it's about a type of little objective that will be attached to a smartphone case, it's not about a big system. – Lavender May 27 '15 at 9:37