Snap Fit Designs for Plastic Injection Molding 1

[kaneohe007]

Hi,

I'm designing a product that needs to have snap fit features in two mating components (think clam shell), but the mold needs to be a straight pull (i.e. no cams, sliders, side actions, etc). What kinds of features might I explore to achieve this?

Can anyone recommend a book that illustrates many different kinds of snap features? This is the only one I can find:

http://www.amazon.com/First-Snap-fit-Handbook-Creating-Attachments/dp/1569903883

Has anyone read this one? Would it provide the information I'm seeking?

Thanks,

John

 

[bvanhiel]

John,

That's an excellent book and will answer all of your questions. Other places to look for information are the various plastic design guides put out by the resin manufacturers (the old GE one was pretty good if you can find it).

Snaps with no action will probably require bypass shut-offs (holes) in the clamshells to form the undersides of the snaps. If that's a problem then you might consider crush pins for assembly. I've used them many times for small parts.

-b

 

[kaneohe007]

Thank you very much! I'll buy the book.

I haven't used crush pins before. Does the book discuss crush pins? Or is this not even an injection molding process?

Thanks,

John

 

[bvanhiel]

I don't recall if he covers crush pins in the book. It's essentially molding a close fitting pin and socket on the opposing parts. Small ribs are molded into the pins to create an interference. If done correctly the pins will break before they can be pulled from the sockets. Assembly of the parts usually requires a small press.

The parts we used this on were small, perhaps 2-4" in length. The pin diameters were about .060"-.080" with 4 pointed ribs (imagine a square section superimposed on the round pin) with .005" or so of interference. We generally tweaked rib height by a couple of thou once or twice to improve the fit.

-b

 

[insideman]

My experience with crush pins has been dismal! Go with a proper snap every time.

 

[kaneohe007]

Assuming the shut-off holes don't pose any aesthetic problems, that is probably what I'll do. Thanks for the input, guys!

 

[Pud]

Here's a bunch of useful links. Some may no longer be active - some you've got to register

http://www.ets-corp.com/tools/development.htm

H

http://www.tynevalleyplastics.co.uk

 

[MikeHalloran]

I once designed a molded collet to pull a plastic cylinder axially out of a deep hole, against the force provided by four tight o-rings, rotted in place. In use, it was pushed over the end of the cylinder until its internal teeth engaged a groove in the cylinder. A simple thin metal sleeve was installed axially to prevent expansion of the collet, and a jackscrew pulled on the collet to retrieve the cylinder.

Both axial faces of the collet's teeth were beveled, as were the ends of the groove in the cylinder, so without the limiting sleeve, the collet could be pulled off the cylinder as easily as it was pushed on.

Which was also the trick to molding the collet's internal teeth without a collapsible core. When the part was pulled hot from the mold, the collet teeth were splayed out plastically. The molding press operator wrapped a rubber band around the teeth to pull them back into alignment while the part cooled.

It worked great in glass filled nylon. Glass filled polyester became too rigid too quickly, and the teeth cracked during extraction.

Where I'm going with that story: It may be possible to mold your snap fit features in such a way that ejection deforms them, and the press operator immediately restores them to the proper position while the part is still hot. ... but success depends a lot on what material you're using and what sort of relationship you have with your molder, and what your application requirements actually are.




Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

If your snaps are not too deep, you might look at "up and away" ejectors.

Q: Why does the tooling need to be "straight pull"? If it's a cost thing, then (imho) you have the wrong toolmaker! Unless the volumes are not at least in double figures, the cost benefit should be better.

Mike: I hope this is not another "trade secret" like the location ring size optimization!

Cheers

H

http://www.tynevalleyplastics.co.uk

 

[MikeHalloran]

Pud,
"... another "trade secret" like the location ring size optimization!"

Huh?

Have you confused me with someone else?

Have I forgotten an instance in which I was more of a PITA than usual?



Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

Mike, Insulating tape:

Not for doing it.

For admitting it.

H

http://www.tynevalleyplastics.co.uk

 

[plasgears]

The classical tooling for snap fit blades requires side pulls in the die, but small undercut blades can be made to 'jump the die' during retract. This would not require side pulls.

 

[unclesyd]

Here are two sites that have the calculation format for snap fits.

http://engr.bd.psu.edu/pkoch/plasticdesign/snap_calc.htm

http://www.sabic-ip.com/webtools/toolsHomePage.faces

 

[kaneohe007]

bvanhiel, I have a few questions regarding crush pins...any way I can contact you directly?

Thanks,

John

 

[unclesyd]

Here is another good source of information on snap fits in plastic.

http://homepages.cae.wisc.edu/~me349/lecture_notes/joining_methods.pdf