ABS' DTUL

[Vig16]

We have an ABS part that undergoes accelerated age testing. The test is being ran at 56 degrees C for four months. We're finding that about 10% of the parts are snapping after the test is done. The part is a lever within a handle that is used to actuate certain pieces within the handle when the trigger is squeezed. There is a spring attached to the piece of ABS in the area that is snapping. Is this something that is caused by the accelerated age test? If not, what could be the problem?

 

[patprimmer]

Ummm

Do parts not subjected to the test still fail at the same rate or is there a change of failure rate.

It's blatantly obvious that if you get no failures, then deliberately degrade something then get failures, that the degradation process caused the failures.

Regards

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[Vig16]

The parts that were never subjected to the test have a 0% failure rate. This product is for a medical device that we just acquired, and had to do all of this testing all over again. Is there something that can be done or is it what it is?

 

[patprimmer]

It is what it is. The parts are failing the test.

Things can be done, like deciding if the accelerated test indicates a real world problem, improving design to add strength, using a more heat stable material.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Vig16]

Thank you very much!

 

[Demon3]

Here are some comments.

Firstly, it could be that your failures are in part due to traces of oil. ABS is susceptible to envirinmental stress cracking whereby any oils in there would attack the polymer. So, if you can, make sure no oil is present.

On the other hand, the most likely explanation is that the ABS is degrading due to the heat applied over time. This would be oxidative degradation of the polybutadiene particles in the ABS (ABS is SAN filled with polybutadiene particles to improve the impact resistance).

There are a few possible solutions:

1. Add more antioxidant in the ABS, e.g. Irganox 1010 or similar, Ciba or Great Lakes can advise

2. Use ASA instead of ABS, it's more expensive but the impact modifier in ASA doesn't degrade so the part won't go brittle over time

3. Use another grade of ABS, there are many suppliers and types. Get one with more antioxidant in the as delivered material

4. Do you need the impact resistance given by ABS? If not use SAN instead.

Hope that helps!

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

 

[Compositepro]

It may not be an oxidation problem but stress caused by the spring causing creep at elevated temp. Any knit lines in the area would also cause a problem.

 

[Vig16]

We don't really need the impact resistance as much as it just has to withstand the pressure of the spring in the handle. I'll take a a look at the location of the knit lines and then I'll get that information back to you guys.

Thanks again!

 

[Demon3]

In that case definitely try SAN. The knit lines in ABS are weak due to the rubber particles (I have data on that) and the SAN doesn't have rubber particles so the weld lines will be much stronger.

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