Inconsistent warping in Nylon 2

[DragonForge]

Hello Chaps

I posted here back in 2010 and got some really helpful advice. I am hoping somebody here can help with my latest problem.

We produce injection moulded sword blades for the martial arts industry. We are now using an Impact Modified nylon with fantastic results. It is much tougher/heavier than the Impact Modified PP that we were using and flexes like tempered steel. It also does not need to be conditioned before using it. All in all it is the right plastic for the job.

However, as soon as the blades come out of the mould they start to warp. The go straight into jigs where they are left to cool. However, when removed from the jigs some are straight and others are not. Maybe 40% are straight enough. The rest have warped in one direction or another.

All the jigs are the same and are all kept in the same room (and therefore temperature).

We have tried 5 different syles of jigs now including spring loaded jigs that keep the blades compressed as they shrink and have tried cooling them in water whilst in the jigs. Still VERY inconsistent results.

The moulders and material supplier have basically said that this is the problem you get with moulding long lengths of nylon and have left it at that. Not really what I want to hear. If all of the blades were warped then I would take this as the case but given that 40% of them are straight, 40% less than straight and 20% very warped there has to be "something" affecting them other than "nylon does this".

Does anybody have any suggestions about why we have such inconsistent results?

Thanks in advance

 

[cloa]

Maybe you should be specifying your moulding conditions as that may be the real issue. How thick are these blades? Is the geometry simple (does it change between blades)?

 

[DragonForge]

The blades are produced 1 at a time and are all coming from the same mould.

The blades taper along both planes 54mm wide x 16mm thick to 20mm x 8mm along a 1200mm length. As such they do not cool uniformly (the narrow end cooling much more quickly).

The jigs do hold them straight until they are removed and then it is obvious they have warped. But they warp in different directions along both planes.

 

[patprimmer]

My tip is you have slight variations in degree of crystalisation, probably from mould temperature variations from side to side from shot to shot.

You could try rethinking cooling channel plumbing stratergies to get more uniform temperature.

You could try significantly longer cooling time.

You could try significantly hotter and significantly colder mould temperatures.

Make sure every shot is fully packed and the screw is holding a consistent small cushion.

If you are adding colour, try no colour and see if that fixes it. If it does, consider a different master batch and better control of MB addition and mixing.

Regards
Pat
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[jerzy]

If you could determine the time sequence of the parts by marking with label or scribing a serial number, you could see if any pattern exists and this would be a big help in ascertaining the cause. If the warped parts occur in consecutive parts followed by consecutive streams of good parts, you might be able to correlate with changes in the process.

jerzy

 

[ornerynorsk]

We dealt with a similar situation years ago on molded-in-stresses on a glass-nylon clamp.

We ended up fixturing the part, as you are doing, but after the initial cool to ambient, we re-heated, in the fixture, to something like 250 F and soaked for 3 hours, and then returned slowly (3 to 4 hours) to ambient. Our results were very good and we saw incredible consistency in the finished parts. We also gained in regard to breaking strength!

One note, we ran an extensive series of tests at varying times and temperatures to arrive at our soak temperature and time, inlcuding mechanical cycle and break testing. I would suspect your application is not as critical, but you would still benefit from optimising the parameters.

Just a thought, wanted to share what worked for us.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[DragonForge]

Thanks for the suggestions so far chaps! Greatly appreciated.

Ornerynorsk, I like the idea that it may be moulded in stress causing this - I will set up an experiment along the lines of your methods and see what comes of it.

Just so I am clear on this, the pat was heated to around 250 F, it was then soaked for 3 hours (in hot/boiling water?) and then left to cool (in the water) for 3-4 hours?

Thanks for the input!

 

[ornerynorsk]

Actually, the soak and cool was in an ambient circulating-air oven. Good luck with your experiments!

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[ornerynorsk]

Actually Dragonforge, there are a couple of Pat's suggestions that would be very simple to try as a first attempt, to see if significant changes in processing parameters might remedy the warping for you. I believe he is spot-on regarding the variation in crystallisation temp, otherwise you would see more uniformity.

Two other things that have not been touched on, is your material being dried thoroughly and are you using any regrind?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[patprimmer]

I think from memory, the annealing temperature for nylon 6 in air is 180 deg C. Your material supplier should be able to provide data. Time to anneal depends entirely on section thickness. The required time and temperature may cause yellowing.

Nylons can also be annealed at lower temperatures (60 deg C being the critical number) in water, but this also takes considerable time and corrosion of jigs and staining or spotting on parts can be an issue. That is what was behind my mention of playing with mould temperatures and cooling times suggestions. If annealing in water it is essential to circulate the water very well.

With unfilled nylon, any warping is much more likely to be due to differential shrinkage due to different degrees of crystallisation.

Regards
Pat
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[patprimmer]

Oooppps

I just remembered IMPACT MODIFIED nylon 6. Impact modifiers also substantially alter annealing temperatures and degree and rate of crystallisation. They also alter melt viscosity which may induce flow related moulded in stress.

Regards
Pat
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[DragonForge]

"I believe he is spot-on regarding the variation in crystallisation temp, otherwise you would see more uniformity.
Two other things that have not been touched on, is your material being dried thoroughly and are you using any regrind?"

Ah, I shall have this looked in to as well. Thanks!

We are not using regrind and I "assume" the factory are drying the material thoroughly.

Situation is, we own the moulds but have the parts manufactured by injection moulders on the same industrial estate as us. As such, I am very much in their hands.

To date the best suggestions they can offer is "nylon does this" and "nylon is a pain to mould". This is why I am here - and I am glad I am as you guys have come up with some fantastic suggestions!

Thanks!

 

[DragonForge]

"My tip is you have slight variations in degree of crystalisation, probably from mould temperature variations from side to side from shot to shot."

This will certainly be looked into further - Thanks!

"You could try rethinking cooling channel plumbing stratergies to get more uniform temperature."

Another thing to look into.

"You could try significantly longer cooling time."

Tried this one. It does make me think that insulating the jigs to allow for much slower cooling may help as cooling them rapidly just makes them warp faster.

"You could try significantly hotter and significantly colder mould temperatures."

Make sure every shot is fully packed and the screw is holding a consistent small cushion.

I will make sure this is the case

If you are adding colour, try no colour and see if that fixes it. If it does, consider a different master batch and better control of MB addition and mixing.

Tried this one.

Thanks for all the suggestions!

 

[DragonForge]

Thanks for all the help so far with this!

Just one more question if I may? I have attached an image of where the material is going through the gates. The blade has fissures at these points and other blemishes. I am informed that this is just part of the moulding process when using this material (this did not occur with PP).

Is this correct? Should nylon mould like this or is this indicatve of any problems with the material?

Thanks again for any suggestions - they are greatly received.

 

[DragonForge]

Link doesn't seem to work but image can be found here:

http://imageshack.us/photo/my-images/195/bladeyt.jpg/

 

[MacGyverS2000]

I am not a plastics guy by any stretch of the word, but don't those types of marks mean the charge is slowing down and cooling before being packed tightly into the mold?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[ornerynorsk]

"We are not using regrind and I "assume" the factory are drying the material thoroughly."

Yes, the factory will usually dry material to a degree. HOWEVER, nylons MUST be dried pre-press. No exception that I am aware of.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[DragonForge]

I do know that the material is dried before moulding. However, I do not know if this has been done thoroughly enough. Perhaps this is where the problem "may" lie. The factory have been moulding for over 20 years so I would suspect they are drying it correctly.

 

[ornerynorsk]

Regarding what looks like burn marks near the ejector pin line in the middle picture, where is it in relation to the whole part? You may need more venting. Are there other areas with similar marks?

Does the sprue gate directly into the mold or do you have a runner with cold wells at the end to capture cooled material, and do you have only one injection point? (bottom pic looks suspiciously like nozzle drool, but hard to tell)

Sorry, when I made the comment regarding drying, I was referring to the resin/raw material factory. Any competent molder will definitely dry pre-press. My misunderstanding.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[patprimmer]

ornerynorsk

I have to disagree re reliability of moulders and drying nylon.

When employed as a tech support troubleshooter at a major nylon supplier, I was of the opinion that 90% of my work could be eliminated if moulders knew how to properly dry nylon, how important it was to do so and then how to moisture condition correctly after moulding.

For one, a large number will not believe that you cannot dry nylon by blowing hot but wet air over it.

OP

That said, I don't believe that wet material is causing this specific problem.

From what I saw in the picture, although it was far from clear, the marks are jetting marks and the gate is VERY small and the gate is on the side of the part.

The gate should be at least 1/3 the thickest section and up to 2/3 the thickest section for optimum properties and full pack.

The gate should be very close to the end of the moulding and should have the taper from the runner designed to direct material toward the top opposite corner of the moulding as it squirts from the gate. This is so it hits a wall, then fills as a progressive flow front from that corner rather than a jet squirting a longer distance and cooling enough for the surface to skin or jell before some packing pressure is applied to hold the skin in place as the molten material flows below the surface and progressively rolls out to meet the surface as the mould fills.

Also, as mentioned by ornerynosk, good vents are required as poor vents restrict fill speed and trap air causing burns as the trapped air is compressed. For a vent to work, it must stay clean and vent to air, not just to another cavity. This means a short thin land to avoid flash, then a deep vent to atmosphere so the deep part does not get blocked by mould deposits like waxes or monomers or very low MW impurities of the polymer.

If the vent is poor and that limits fill speed, that may restrict the final pack as the material is already to cold.

Regards
Pat
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