Nylon Absorbtion / Desorbtion (Help!!!!) 2

[ctowerh]

I am having problems with some parts that are Nylon 66 7% glass filled. After a period of time in a relatively low humidity environment, my parts are drying out and becoming very brittle. I am looking for two things:

a) Is there a way to predict how long it will take the parts to beome brittle (1.5mm wall thickness) at a given humidity level?

b) Any suggestions for either treating Nylon 66 7% glass filled or chosing another nylon (or another material) to prevent this problem

Thanks in advance,

Craig

 

[patprimmer]

a) Yes there is. Most traditional nylon polymer producers have data in the form of graphs for time vs humidity vs thickness for the uptake of water. It can be reversed with a little thought and calculation. The rate is dependent on the temperature and concentration gradient.

The rate for desorption depends on the rate the water travels through the nylon. The reaction to the surface is virtually instant, but the diffusion through the nylon is very slow.

The climate would need to be VERY dry if the previously conditioned parts were to lose enough water to become very brittle.

7% glass is probably a fairly poor choice for tough parts. Why is it 7% and what do you mean by brittle, and what is the mode of failure.

Any number of materials might be less brittle, it all depends on why you chose the material in the first place and what other properties are really important to maintain.

Regards
pat pprimmer@acay.com.au
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]

My suggestion would be to go to the material supplier and get him to solve a problem his product created. If he does not respond, Talk to Dupont about their Super tough nylon with glass in it. it has a rubber mod. which does away with the dry problem.

 

[rnd2]

Why do you have to use nylon?
What do you mean by relatively low humidity?

 

[ctowerh]

Update:

Glass filled Nylon was selected for this part before I came on board. I am in the process of changing the material to Xenoy (1760E 11% glass filled) for it's flexural strength, resistance to chemicals and resistance to temperature fluctuations. I am trying to figure out what to do with the existing Nylon parts that I have in stock.

My current Nylon parts have been stored in a temperature controlled warehouse in New England. With the heat on all winter the humidity is low, but certainly no where near 0%. At this point, with minimal force the parts will break and shatter. When we received the parts (3-6 months ago) we did a 100% inspection and found no issues with the parts breaking.

I have a feeling it is a material processing issue on the molders part, but I am hesitant to do a bunch of processing tests (in China) due to the fact that I am changing materials anyway. They claim that they maintained 6-8% humidity in the material as a drying condition.

If I re-hydrate the parts by soaking them in water for ~48 hours they repond well. I am trying to figure out how long they will hold up for.

Thanks for all the input.

 

[patprimmer]

They will respond to immersion in water. If the water is hot, the uptake will be quicker. If the water is over 60 deg C, the parts will be annealed and may change dimension compared to parts conditioned for a longer time at less than 60 deg C.

A real down and dirty test for moisture conditioning is to rattle the parts together. If they clink or tinkle, they are dry, if they have a dull sound, they are conditioned.

For a comparison, leave some parts in an oven at 120 deg C foe 24 hours, and other parts in hot water for 24 hours, then rattle both and listen to the difference.

By keeping the warehouse warm, when outside is very cold, you are effectively creating a dehumidifier, which will dry the mouldings a lot faster than if they were left outside. It is relative humidity that causes the drying, and as you heat the air, you reduce the relative humidity.

Before changing from nylon, I would very carefully consider why it was originally chosen, and whether or not there have been problems in the past.

The 6% to 8% humidity in the material as a drying condition has me a bit perplexed and concerned.

Nylon should be dried to below 0.3% moisture content in the nylon before moulding, and it should be conditioned to about 3% moisture content in the nylon to obtain normal properties in a normal open air environment.

If treated right, nylon is a very tough polymer. That's why it is used a lot in things like chair bases, high quality plastic hammer faces, high quality screwdriver handles, high quality power tool housings, fuel filters, carburetor parts, fuel pump parts, spray gun parts, all sorts of sail boat fittings, car engine parts like inlet manifold, radiator header tank etc. Xenoy would not survive many of these applications, no matter what the guy from GE says.

GE does have a reputation for having very enthusiastic salesmen. To get a balanced view, I would also seek advice from a few of the likes of DuPont, BASF, Ticona, RTP company, DSM and whoever Elf Atochem are now.


An alternative to 7% GF nylon 6.6 might be acetal, or unfilled nylon 6.6 or 25% GF 6, but it depends on what chemicals, what load, what temperatures, what mould shrinkage you now need as the mould is already made for 7% GF nylon 6.6


Regards
pat pprimmer@acay.com.au
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.

 

[rnd2]

I agree with Pat Primmers gut feeling that all is not well with the material itself.
Save a lot of time and naval gazing and cull them out of the system. Suspect parts, unless destroyed have a habit of returning and now, its your call, not the previous guy.

 

[ctowerh]

Thanks for all the info. The reaction to Xenoy seems mixed so I'll try and describe my situation and see what you all think is the appropriate material.

- I am designing an electrical connector where two mating pieces flex to snap into the connection so higher flexural yield strength is desirable.

- Due to the way the electrical connection is made, it is also desirable to have a relatively stiff material to keep the contacts in place.

- The parts will be handled often so they need to have some chemical resistance. (perfumes, hairspray, etc.)

 

[ctowerh]

One more question:

When we originally received the parts they appeared hydrated and performed well. Am I correct that due to poor processing the parts de-hydrated much faster and to a greater degree than they should have? Is there any way to test the new parts that appear hydrated to see if they are processed correctly?

 

[rnd2]

No. Normal regular nylon parts stored in normal regular storage do not become brittle the way your parts appear to have. The only exception might be that they were exposed to sunlight for lengthy periods or were subjected to some sort of intense irradiation. As they arrived in your companies care apparently in good condition surely they were not subsequently left out exposed to sunlight or other radiation for a long time?
WAG here but if these parts came from a then new supplier and previous to this batch the parts were acceptable I would be rushing back to the old supplier of nylon.

 

[patprimmer]

My best guess is that the nylon resin is of poor quality, or the resin was wet when it was moulded.

The nylon pellets need to be dry for the moulding process, but the mouldings need to be conditioned before high impact or high elongation is applied to the part.

A poor surface finish is the most obvious indicator of the material being wet when moulded. Light weight parts is also an indicator of voids, or insufficient pack during moulding.

Regards
pat pprimmer@acay.com.au
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.

 

[mysundance2000]

I have seen Noryl used for connectors with flexing fingers.

Mike