Tougher than Cycoloy? (polycarb + abs)

[Pevac]

I am on a mission to find a replacement injection moulding polymer to replace the current Cycoloy (PC +ABS) blend.

We have parts that are failing due to an impact force and we have changed the geometry as much as it allows to get rid of stress concentrators and to better distribute the forces, however we would like to see if we can also improve the properties by using a different material.

Does anyone know of a polymer which would have similar Flexural modulus to Cycoloy (2.6GPa) (between 2 and 3Gpa), Flexural Yield strength of more than 80Mpa and greater toughness than Cycoloy. Izod Notched (ASTM D 256) for Cycoloy is 5.3J/cm or 10ft-lb/in.

I have found some Polycarbonate/PET and Polycarbonate/Acrylic blends which look like they would do the job, but I wanted to see if someone could recommend something else?

Thanks in advance!

 

[patprimmer]

What is wrong with straight up PC.

If you have a mould, it's pretty easy to try out the candidates.

Using material with higher data sheet impact strength does not necessarily result in parts with better resistance to breaking in a real world environment. It really depends on the nature of the break. Often an increase in elongation at break is the property that stops things failing, or a better chemical resistance or better fatigue resistance.

Also, actual mouldings often fall well short of data sheet properties due to moulded in stress, orientation of properties and polymer degradation which is sometimes not visible.

Regards

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[Pevac]

Thanks for the answer!

1. Apparently straight up PC does not repond well to chemicals (besides, the PC/Acrylic and PC/PET blends seem to offer much higher toughness).

2. We will try the candidates. Trying to determine ALL the possible candidates at this stage.

3. As the mode of failure is very much similar to a notched izod test (a great force at very short period of time - impulse) - therefore it is not failure under tension, or chemical attack or fatigue. (It happens on new products too)

4. Yes this is not an ideal world, but I am trying to tackle this by varying the known parameters before we start fiddling with moulded in stress and thinking about orientation and polymer degradation - Not sure how I can detect or influence these factors from a design office?

 

[patprimmer]

Your polymer supplier will be able to give you a test method for testing in moulded in strength.

You can test the melt flow index to determine if there has been any degradation of the polymer during processing.

With PC/ABS orientation will not be an issue, but it might become an issue with other materials, and you should look at along vs across the flow properties to determine this.

Then to determine flow direction, study short shots, starting at say 10% shot weight and increase in increments of say 10% until you have the complete progressive collection. If the flow is obvious, you might only use 20 or even 25% increments to get the picture.

I still doubt that izod value will accurately predict product failure as an izod test holds the part in a vice and strikes the part with a hammer very close to and parallel to the top of the vice jaws. This measures energy to break in rapid application of concentrated shear. Real world rarely isolates the force into such a small area and simple shape. In the real world, mostly the blow causes some distortion which is spread over an irregular surface such as a change in thickness or to the intersection of a rib. The load then is unevenly applied. Extra elongation allows for more load sharing before the crack starts.

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.

 

[dwightdixon]

I have used the Zytel ST high impact grades and also their old 400 series with good results.

 

[patprimmer]

Impact modified nylon will be substantially down on flex mod, especially after moisture conditioning. It will also have substantially higher shrinkage.

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.