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

 

[btrueblood]

DragonForge:

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[Pud]

Try again:

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

http://www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

 

[MikeHalloran]

I second sectioning a part or two to see what's inside, or not inside.

A Grit-Edge blade in a hacksaw frame is a very valuable and effective engineering tool.

Mike Halloran
Pembroke Pines, FL, USA

 

[DragonForge]

Sorry guys, I should have been a lot more clear on this. There are actually 3 gates along the length of the blade.

They can be seen here:

http://img338.imageshack.us/img338/6840/imagesix.jpg

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:

http://img191.imageshack.us/img191/8828/imagefour.jpg

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.

 

[DragonForge]

@ onerynorsk

"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.

 

[DragonForge]

@ Pud

"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.

 

[ornerynorsk]

Just a few thoughts. It is so hard to troubleshoot things precisely without seeing the tool and having a couple of parts in hand, but your pictures have helped clarify things.

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.

 

[patprimmer]

I have to disagree with ornerynorsk on this occasion.

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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