Nylon Discoloration

[jerzy]

I am running some initial samples on three molds using a heat-stabilized grade of nylon 6/6 colored with a light grey color concentrate. I have never run this grade of nylon before, but for over fifteen years I have used the same concentrate with a heat-stabilized grade of nylon 6 to mold many thousands of parts from four older molds.

The older molds are for similar but not identical parts and have larger gates and thicker walls.

The nylon 6/6 seems susceptible to light brown discoloration at weld areas and last parts of mold to fill
on some of the molds. This does not seem to be dieseling (black charring from trapped compressed gasses) because it seems to diminish with faster injection rate. I have had the problem with resin used right from the sealed bag, so over-drying is not the cause.

I am going to sample some of the nylon 6/6 without the grey concentrate to see if the colorant is causing problems.

The brown I detect looks like the browning that appears on the outside of purgings soon after they hit the air when leaving the nozzle. I would think oxidation is occurring: this brown doesn't appear when purgings are quenched.


Have any of you experienced similar problems with nylon, either heat-stabilized or not?

A side note: I have been impressed by the technical acumen shown by many of the posters on this forum.

Jerzy

 

[Pud]

Jerzy,

If you are using masterbatch, it may be the masterbatch carrier that is causing the problem - remember you are at least 50°C more than nylon 6 - often more than most general purpose carriers. Try some dry colour or high temp masterbatch or polymer specific carrier to prove.

Smaller gates will also increase melt temp due to shear heating.

Cheers

Harry

ps: Get ready for a treatise on nylon heat stabilisers....

http://www.tynevalleyplastics.co.uk

 

[jerzy]

Hello Pud,

Thanks for the ideas. And actually I'm looking forward to the "treatise on nylon heat stabilizers" because I'm thinking that the root of the problem lies there.

Your postings were among those that prompted my remark in my initial posting about the technical competence of this forum's members.

Jerzy

 

[patprimmer]

I am inclined to think it may be to do with nylon 6.6 being even more sensitive to nozzle freeze/drool cycling than nylon 6. This makes it harder to set the melt decompress so that you get no drool, but also do not suck air into the nozzle.

Has it always been moulded in the same machine. It is not uncommon for burnt material to stick round in a dead spot in the barrel then start to bleed out into the new material when the temperatures are increased significantly. Sometimes it comes out quickly in clumps and sometimes it slowly and consistently bleeds out over quite some time.
Also for nylon 6.6 run virtually no back pressure and reduce screw speed so the barrel is charged shortly before the next shot is due.

Regards
Pat
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[jerzy]

Hello Pat,

Thanks for the reply. You, like Pud, were the posters I had in mind when I remarked about the quality of this forum's content.

I don't think the discoloration is coming from the barrel, at least not completely: one mold showed no discoloration and it was run between the two that did, and the discoloration is not random, but occurs at same spots near end of flow path. I'm looking into increasing venting.

I am abashed to say this, but the press in which we are running this resin is over 40 years old and has no melt decompression. The screw shank is not even pinned or clamped to the injection quill. Lack of decompression hasn't been a problem when molding nylon 6, but as you say, the nylon 6/6 is touchier when it comes to drool control.

I should point out that the nylon 6/6 is 13% glass fiber filled; the nylon 6 is 40 % glass and mineral filled.

Thanks again for the ideas.

Jerzy

 

[patprimmer]

A 40 year old press IN GOOD CONDITION can mould nylon 6.6 no problem. You will need however to ensure some specific procedures or process control.

These are:-

The material must be dry. Test at the machine by purging and observe if bubbles form in the purging.

The check valve must be in excellent condition. Check this by observing how well it holds a cushion for an extended period of time.

The nozzle temperature MUST be precisely controlled with no overheat, underheat due to response lag in controllers. To achieve this you need several things.
Make sure the nozzle element is a very good fit on the nozzle wit perfect surface to surface contact and uniform clamping.
Make sure the nozzle element is at the very front of the nozzle.
Make sure the nozzle element has plenty of power to compensate or heat sinking into the back of the mould.
Do not use regular on off controllers. Use a simerstat or a light dimmer or a PID controller that adjusts the resistance in the circuit or goes through on off cycles many times per second and predicts the rate of temperature rise vs the set point and slows the rate as it approaches the set point.

Make sure the vents are clean and clear and vent to atmosphere. To test vent take some shots with some thin tape or paper on the partin line near where the marks occur.



Regards
Pat
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[jerzy]

Hello Pat,

We are using a variable autotransformer to control power to nozzle. I agree 100% with your comments about on-off controllers on nozzles with their small thermal mass.
The nozzle is performing well: no drooling or freze-up problems. We are using reverse-taper tip after using general purpose tip that gave problems.

I am wondering what experiences anyone has had with discoloration problems with light-colored nylons.
I have read a paper delivered at the Society Of Plastics Engineers ANTEC 2007 that said that light colors in heat stabilized nylons are significantly affected by slight changes in temperatures and exposure times and that oxygen accelerates this effect.
(Paper by Bruce M. Mulholland, Ticona Corp.)
I am wondering if I'm seeing this at the end of flow paths.


Thanks for response.

Jerzy

 

[patprimmer]

Copper halide type heat stabilisers do cause discolouration in light colours, but they do not look like a burn. They normall have a light transprent olive green, butcan vary to a copper colour or salmon pink. Copper sulphate is of course blue. Organic stabilisers are normally clean of colour.

Regards
Pat
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[Demon3]

As Pat is saying, the color in polymers often comes from the stabilizers or other additives. Nylon is commonly stabilized by copper salts (which is strange because they destabilize all other polymers). They can also be stabilized with hindered phenols as the other polymers are.

Correct choice of process stabilizer could solve your color issues. Check with the supplier.

If you want information you can try the book Stabilization of Polymeric Materials, Springer publishers, by Hans Zweifel (Ciba's former stabilizers guru).

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[Pud]

Can you try the PA6 at "top end" of melt range (270-290C) with the colour?

A pint I'm right lads?

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[Demon3]

Great idea. If the color comes from the nylon then the PA6 will be fine and the PA6,6 not. If the color comes from the masterbatch ingredients then the PA6 will discolor at high temperatures too.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[jerzy]

Thank you for the ideas. The book by Zweifel was not currently available on Amazon (but from another seller at way over $300 US); I see a new edition of "Plastics Additives Handbook" by him is coming out in a couple of days. If I'm still struggling, I might buy a copy.

We have run the nylon 6 at 270 C on some molds in the past.

I'm going to run one of the new molds with uncolored nylon 6/6 tomorrow, and we are getting a new shipment of nylon 6 in a day or two with which I'll sample one of the new molds that show the discoloration.

The discoloration is not severe I should point out:
some marketing people, preparing some show samples, were displeased with the small areas that showed the discoloration.

Thanks again guys.

Jerzy

 

[Pud]

You could just give the "marketing people" some made in PA6 - they'd never know the difference...


Harry

http://www.tynevalleyplastics.co.uk

 

[dwightdixon]

The glass is being segregated by the flow in the mold and the last place to fill is resin rich. The glass in the other areas will yield a slightly different color. You might increase your color concentrate to try to hide the "problem". The PA 6/6 is slightly higher in viscosity and will not segregate as much. The discoloring from the antioxidant is likely present in all areas of the part, but is covered up by the glass.

 

[Demon3]

I have both books. I will be surprised if Hans Zweifel manages to write a new edition of the Plastics Additives Handbook from the grave. He died shortly after the 5th edition was published in 2000.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[jerzy]

I ran one of the molds that showed discoloring with the PA 6/6 without colorant: it still showed some discoloration at end of flow path. While I was running these samples, the shipment of PA 6 arrived. I ran out barrel and ran some PA 6 with colorant at identical settings. No signs of discoloration. I think the PA 6/6 just not as stable as the PA 6 we have been running. I note that the PA 6 purgings made when emptying the barrel darken much less when exposed to air than the PA 6/6 purgings.

Aside to Chris (Demon3): When you recommended "Stabilization of Polymeric Materials", I searched Amazon for it. The search also brought up the handbook, 6th edition, to be published April 2. Amazon listed it as "by Hans Zweifel". They obviously just copied info from 5th edition. The "Stabilization" book looks like it might be worth waiting the 1 to 2 months that Amazon gives as delivery time.

 

[patprimmer]

Although 6.6 has a highr melting temp, 6 is more stable to discolouration at the same temp.

I really dout it is to do with the glass separation.

Regards
Pat
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[jerzy]

Hello Pat,

Is your statement, "Although 6.6 has a highr melting temp, 6 is more stable to discolouration at the same temp" a general rule? Would it apply to most grades of PA 6 and PA 6/6, or does it just happen to describe what I observed with the two grades I have been using?

Jerzy

 

[patprimmer]

All other things being equal, it is a general rule.

Regards
Pat
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