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Plastic snap fit design

amccaffrey-2
Participant

Plastic snap fit design

I have always successfully designed snap fit details in plastic mouldings using the BASF design guide, obtainable online in PDF format. The design normally features a straight snap fit but I want to incorporate a curve in the snap fit design.

Currently, there is a formula to determine the strain factor and I usually work to this (differences occur depending upon the plastic used of course) and not exceed the percentage figure (6% in most plastics).

However, my design has to incorporate a curve in the snap fit. Does anyone know if this affects the strain value? For example, I have a 90 degree section where the radius is 10.85 mm and the curve extent is 90 degrees. This gives me a flattened out distance of 17 mm. with the formula being 3 x th x C/2 x L². The L² would thus be 17². However, I think that this is incorrect and the 17 mm should be less as the material thickness is greater where the bending will occur and thus more strain.

Does anyone have any ideas on this? Currently, I have estimated the distance to be 50% less meaning less strain on that part of my clip. I have shown the clip in a rough draft of the design.

I'll be designing this fully in Creo 2.


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3 REPLIES 3

Re: Plastic snap fit design

use FEA.

and first do a simple (basf) model so that you are sure that you got your boundary conditions right.

Re: Plastic snap fit design

The curved portion of your design will not behave the same way as if it was flatten out.

If you don't want or cannot use FEA to validate your design, you shouldn't use the rule-of-thumb evaluation for a straight horizontal clip for that curved design.

If you know your material specifications, the clip maximum displacement, and you have the clip 3D model, this FEA analysis is not that complicated of a job.....

Re: Plastic snap fit design

Sorry guys, I don't appear to have the FEA module as part of my CAD system. I'm sure there must be a mechanical formula out there somewhere and I wonder how it was calculated before FEA and electronic systems were implemented? BASF don't appear to be much help on this issue although they do have a formula online for straight clips. I knew I should have become a lawyer instead of an engineer. So much more money and so much easier a job using lingo that nobody can understand!

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