Snap Fit Design

[andyrivets]

How strong is snap-fit design plastics.

I am looking at designing a snap-fit case for DFM purposes on a product that is weighted at one end with approx 2kg of weight. The cantilever design will be semi-permanent, with the use of a tool to un-hook the clips. THe product will need to undergo the free fall test of 1m onto concrete.

Does anyone have any experience of these snap-fit features in plastic? Will the drop casue a failure of the snap hooks? I would be looking at 3-4 snaps on a hand held case size product.

Am i right to be concerned? Will my product hold together?

 

[SDETERS]

Check this site out it has all kinds of snap fit calculations

http://www.rpi.edu/dept/mane/deptweb/student/snapfit/index2.htm

 

[cpretty]

From your description I guess you are designing to UL standards for portable appliances, which are quite a challenge to pass mechanically with plastics.

From experience in a previous job, designing product from Polycarbonate for the agriculture industry:

We had historically created a case similar to what you describe. The clips were always a challenge as the impact energy is quite high on a 1m drop onto concrete. These clips required assembly with a hammer to knock into place, and followed all design guides etc. The end result was to add screws to backup the clips. Not a good afterthought though.

In the case I designed, we put serious thought into DFM etc, but weighed it up against simplicity of design, cost of tooling, dissassembly, and minimisation of risk.

Screws won hands down once you looked at the big picture for this project. A good screw boss design, and automated screwdrivers is still good manufacturing.

To get around dissassembly without specialised tools, a simple plug was added to one screw boss.

Your design may have different constraints, but it is worth spending time justifying the decision to use clips.

 

[andyrivets]

Thank you for your reply cpretty. I am actually from England so work to British Standards rather then UL. However i think the restrictions and rules are the same. We work to IEC61010, our design engineering bible.

My own thoughts have been expressed by you, the design idea requested by our marketing dept....(Those guys again!) was a great DFM idea, but i think your right that screws are the answer to overcome free fall testing.

I just need to find the space to achieve this....oooh and achieve CATIV 600V rating....which basically means haveing 14.3mm clesrance between outside world and any conductive internal parts...challenging...

 

[cpretty]

I was designing for a similar IEC standard for 14,500 volts, and understand your concern on screws. We worked on adding ribs from both sides of the case to increase the path distance for creepage and clearances.

Recessing screws into the screw bosses also helps, but depending on probe size for the test, this can be a problem.

Another option slightly outside the square is to use a custom made plastic quarter turn type fastener (bayonet). These don't rely on clip force, and can be made out of superior materials to the rest of the case to put cost into the important areas only.