Inconsistent warping in Nylon
Inconsistent warping in Nylon
(OP)
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
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






RE: Inconsistent warping in Nylon
RE: Inconsistent warping in Nylon
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.
RE: Inconsistent warping in Nylon
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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RE: Inconsistent warping in Nylon
jerzy
RE: Inconsistent warping in Nylon
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.
RE: Inconsistent warping in Nylon
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!
RE: Inconsistent warping in Nylon
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.
RE: Inconsistent warping in Nylon
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.
RE: Inconsistent warping in Nylon
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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RE: Inconsistent warping in Nylon
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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RE: Inconsistent warping in Nylon
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!
RE: Inconsistent warping in Nylon
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!
RE: Inconsistent warping in Nylon
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.
RE: Inconsistent warping in Nylon
htt
RE: Inconsistent warping in Nylon
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Inconsistent warping in Nylon
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.
RE: Inconsistent warping in Nylon
RE: Inconsistent warping in Nylon
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.
RE: Inconsistent warping in Nylon
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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RE: Inconsistent warping in Nylon
"Thanks for all the help so far with this!"
There's a really nifty way to thank these guys - click the link next to the little pink/purple star under any/each post you found helpful, where it says "Thank <name> for this valuable post!"
It's a way for them to be recognized by all users on this site as being a good person to keep around.
RE: Inconsistent warping in Nylon
imho,
As stated above, jetting. Gate too small, (should be min. 75% of max part thickness) wrong position (best max section of part), ideally gated against an obstruction.
The weld (cold shut) lines would suggest too cold a tool as well. 80C min.
Try cutting one up to see if there are voids in the parts. A sure indication of gate freeze off before packing completed. (I have had to train toolmakers to put "proper size" and "proper position" gates in tools!)
Cheers
H
www.tynevalleyplastics.co.uk
Why be happy when you can be normal?
RE: Inconsistent warping in Nylon
A Grit-Edge blade in a hacksaw frame is a very valuable and effective engineering tool.
Mike Halloran
Pembroke Pines, FL, USA
RE: Inconsistent warping in Nylon
They can be seen here: ht
I am not sure how that affects what has been said so far? We are able to fill the mould with only the bottom 2 gates (at the thciker end) but are unable to fill with only 1 gate. We hoped that by not using the end gate may reduce the warp but it didn't.
The warp in the blade can be seen here:
htt
and this is after spending 24HRs in a straight jig!
Some from the batch were perfectly straight, others ended up like the one in the picture. No real pattern to it at all.
RE: Inconsistent warping in Nylon
"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?"
The 3 pictures are at the 3 gates - I should have made this more clear. The only marks are around the vents. What throws me is I have seen jetting before on other products wheras here there are 1mm fissures in the plastic around these areas.
The middle picture shows a 5omm long, 1mm deep random pattern on the surface (to the right of the jetting). I have no idea what this is.
The sprues gate directly into the mould.
RE: Inconsistent warping in Nylon
"As stated above, jetting. Gate too small, (should be min. 75% of max part thickness) wrong position (best max section of part), ideally gated against an obstruction."
Does the fact that there are 3 gates have a bearing on this?
The weld (cold shut) lines would suggest too cold a tool as well. 80C min.
The mould was designed as a hot tool. As soon as the blades were taken off this tool they warped to a great extent as soon as they hit the air. The tool is now cooled to stop this happening.
"Try cutting one up to see if there are voids in the parts. A sure indication of gate freeze off before packing completed. (I have had to train toolmakers to put "proper size" and "proper position" gates in tools!)"
I have just chopped up some blades from different batches. There are no voids apparent.
RE: Inconsistent warping in Nylon
One thought, more gate and more venting, if you can live with the subsequent trimming operations.
Second, nylon is (in)famous for drooling. While shutoff nozzles do restrict flow somewhat, this could be an issue if you're getting drool between shots, especially if the tool is being run cooler than optimum.
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.
RE: Inconsistent warping in Nylon
Nylons have very easy flow in general and rately need more than one gate in a thickish section.
How thick is this and how long is the blade asnd is the handle moulded in.
I certainly agree nylon is prone to drool, but a shut off or valve gate is a poor fix for bad practices in my opinion.
To control drool
1) Ensure the pellets are truly dry, all the way through the pellet.
2) Ensure no regrind is being used at least until problems are solved, then reintroduce in a controlled manner and test the effects.
3) Use a voltage control on the nozzle, not an on off type controller unless it is a PID type that cycles on and off in microseconds.
4) Have a powerful nozzle heater VERY close to the tip of the nozzle. Ensure it is a good fit with a smooth even round surface on the nozzle and the element so the element has very good uniform surface contact.
5) Have the temperature controller sensor deep into the nozzle.
6) Have a taper in the nozzle so it is slightly bigger on the outside end than the inside end so it is self clearing of a frozen slug.
7) Some insulation between the spru bush and the nozzle can help.
8) Carefully set and regularly reset melt decompress.
9) Use about 10% back pressure as a maximum. Screw back slowly so you only just get back in time for the next shot.
Regards
Pat
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