impact resistance vs degree of crystallinity

[Rojan]

Hi,

I have been thinking recently which material is more impact resistant - the one with higher or lower degree of crystallinity? For example PA6 at room temperature. What does your commom sense tell you? Do you know any sources where I can dive into the subject?

Thanks for your comments!

Robert

[Suhas]

Hi Robert,

Good question. Crystallinity does not have much to do with impact resistance. For example:
PP - crystalline - good impact
PPS - crystalline - bad impact
ABS - amorphous - good impact
PS - amorphous - bad impact

It is more to do with the chemistry, molecular attraction and so on. Additives play a big role too. In case of Nylons, it is the water that makes it flexible - the reason why you have two data sheets for nylons - 50% RH and Dry as molded.

Regards,
Suhas

[Rojan]

Suhas, thanks for the comment.

I am thinking of best processing parameters to make parts with best impact resistance (the material is PA6). If I set mold temperature at let's say 20 deg C then the degree of crystallinity would be lower than processed with mold temp set at 80 deg C. The glass transition temperature of PA6 is around 65 deg C so at 20 deg C the amorphous portion of the part will be at glassy state. The question is this: should I process with mold temperature below or above Tg to get parts with best impact resistance... Hmmm What do you think? I know water comes into play but I would like not to take H2O into consideration now.

Robert

[chrisprocess]

My belief was that cooling slowly will remove internal stresses and toughen your material. In this case I'd run at 80C!

[MTUHusky]

Suhas, thanks for the comment.

The question is this: should I process with mold temperature below or above Tg to get parts with best impact resistance... Hmmm What do you think? I know water comes into play but I would like not to take H2O into consideration now.

Robert

The short answer is below the Tg to gain impact resistance.

The question you need to answer is what is the operating temperature of the final part/assembly? If your part/assembly operates at higher temperatures, then all of the crystals that you prevented from forming, will try to form again and will warp and distort the part, possibly to failure. Not only that but you need to know what kind of forces this part/assembly will see in the field because by stopping the formation of crystals you have locked in a lot of stress. I would definitly do some testing on these parts before you release them. These internal stresses will become more of an issue if you are operating at colder temperatures.

The best thing you could do (if the customer will allow it) is to contact the material supplier and see what they offer in terms of impact modifiers. What your trying to do is totally possible, but it is typically reserved for extreme situations where your part is designed, your mold is built and you cannot change/modify the material.

Husky

[Joel JS]

Hi Robert,

Suhas has alluded to this already. But adding to water to parts as they cool will dramatically improve impact resistance with Nylon.

Hope that helps,
Joel

[Rojan]

The operating temperature would be room temperature so way below the Tg. That was a theoretical question rather than a real life application.
My thinking now is that if the degree of crystallinity is high the macromolecules in the crystals are packed tightly and are so close that intermolecular forces are also high which would give lower impact resistance. The opposite situation, amorphous region does not have macromolecules packed tightly together, the distances are higher like in the situation when plastificators put macromolecules apart enhancing impact resistance. Thinking that way amorphous portion has better impact resistance? What do you think? Is my thinking correct?

[Suhas]

Hi Robert,
Molecules can be flexible both ways. Think PP or PE.
So here is where the whole DOE comes into play. All DOE is structured is experiments with data analysis. Even if you do only 2 experiments - Low Mold Temp and High Mold Temp and look at your results - that is a DOE. Lot of people are 'scared' about DOEs.
Having said that, I suggest you do your experiments and record the data and look at it to see what improves the impact strength. No big analysis or statistics required. (if you want that, let me know).
Another point - Good and Bad impact strength is a relative term. You have to test your parts to what you need. Some parts molded from materials with low impact strengths can be 'overdesigned' to achieve the required properties.
Let us know what you find.
Regards,
Suhas

[JayDub]

Physical properties are anisotropic. If you crush a cheap styrene cup it will split parallel to the direction of flow, but will be flexible in the perpendicular direction. So if you need impact resistance in a particular part orientation, you may be able to improve it by controlling material flow in the cavity. That would generally involve gate location, which suggests taking part property requirements into account before building the mold.

[chrisprocess]

Regarding your question, my common sense tells me a higher crystallinity would be preferred but I don't have any data to support it..

Now for my $0.02

Sure, the supplier may tell you the Tg is around 65C but you need to look at the method in which they used to determine it.

For example:

from ASTM D-680

4. Summary of Test Method
4.1 A bar of rectangular cross section is tested in the
edgewise position as a simple beam with the load applied at its
center to give maximum fiber stresses of 0.455 MPa [66 psi] or
1.82 MPa [264 psi] (Note 3). The specimen is immersed under
load in a heat-transfer medium provided with a means of
raising the temperature at 2 6 0.2°C/min. The temperature of
the medium is measured when the test bar has deflected 0.25
mm [0.010 in.]. This temperature is recorded as the deflection
temperature under flexural load of the test specimen..

..


What you should note is that they only use 66psi or 264 psi of pressure. And it's only measured when the test bar deflects to a distance of 0.010" - which is arguably dependent on the cross sectional area of the test bar.

There are clearly some issues with this. It may be okay when comparing different materials, but that's about it.

Search PVT graphs (pressure, volume, temperature) - there was a discussion on this forum some time ago ago.http://www.injectionmoldingonline.com/forum/showthread.php?t=874

I think the end goal would be to have the same temperature and pressure throughout your plastic-melt of your part geometery. So everything crystallizes at the same rate.

Just for fun, if I were to take a stab at high-strength parts it would be cooler melt temp (not too cold obviously) high mold temp, high pack pressure, and long cooling time

Run a DOE to determine it - you could probably do it in a day..