Need high fatigue life alternative to ABS 1

[montyw]

Hello,
I am looking for recommendations for an alternative to our current ABS material that is failing due to fatigue.

Our current part is failing in the field after 3-6 months of use. The failure is a fatigue failure near the pivot pin of a trigger. The surface shows typical clamshell patern failure and it is slightly polished where the break starts and whit/grey where it the break ends.

We used a PC/ABS blend on a prior version of the part and by going to the ABS (Entec-Resins 756) almost doubled the life using our stress test jig.

Shrink rate may not be totally critical so I am open to going away from ABS altogether.

What are my options?
Glass filled ABS
mineral filled Nylon
PP

Thanks!

 

[dwightdixon]

I would think an acetel would be my answer. Celcon or Delrin. If moisture exposure is controlled, nylon might also work.

 

[Pud]

ABS and PC are probably the worst choice possible for a bearing of any sort...

As dwightdixon has suggested, Acetal would be the best choice - moulded correctly (i.e. high tool temperature) it will give good fatigue performance. It is the only polymer used as a 'spring'.

You might look at filled versions (probably mineral to reduce warpage), to get the shrinkage value down as it is quite high as an unfilled material compared to ABS. (Although it may only require a core pin change from your op).

Cheers & beers

Harry

 

[montyw]

Thanks for the recs - I have used Delrin in an older design and we did have some breakage, which is why I stayed away from it.

Looking back the breakage was more from impact, and if you hit anything hard enough I suppose you can break it. The fatigue failures I am seeing now are driving me crazy.

The choices in filled material are endless, what will a mineral filled acetel's impact strenght be (comparable to the current ABS is fine)

Thanks again!

 

[Demon3]

Acetal / POM is famous for fatigue resistance so the other posters made a good recommendation. You asked about strength. You can find the properties of ABS, ABS/PC, POM and all other polymers for free at

http://www.matweb.com

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[Pud]

Impact modified Acetal/POM is readily available.


Harry

 

[patprimmer]

For really good fatigue life you have quite a few choices. Which one you choose will depend on price and other properties required.

You give no indication of the application nor the environment.

Nylons are very good to fatigue as are acetals, polyesters, polyolefins and polyurethane.

Acetal is chosen over nylon for springs because of it's better and more consistent flex mod and it's somewhat better creep resistance.

Polypropylene is used in live hinge applications that need to go through millions of flex cycles in applications like automotive throttle pedals.

Nylon is used in mechanical components and housings for power tools like jig saws, vibro sanders and hammer drills.

Large concentric radii vs sharp transitions will at critical junctions of parts will dramatically improve the fatigue life of any plastic part.

Regards

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[patprimmer]

Also, so long as the fibres are oriented across the fail point, glass fibre reinforcement will dramatically improve fatigue life of any part that is tested to application of cyclic load rather than cyclic application of constant displacement.

Nylons and polyesters respond particularly well to fibre glass reinforcement.

Regards

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