Prototype tooling - part came back under strength and brittle 2

[Marc_Bloke]

Hi there,

I recently received a batch of injection moulded parts from an aluminium prototype mould tool. The material used was Zytel 70G30HSLR.

Upon receipt of the parts I noticed how brittle they were in comparison to other glass filled PA products - in particular a 50% glass filled part I have lying around on my desk. We carried out testing on the parts and they failed quite catastrophically.

Having spoken to a couple of people I'm aware of the hygroscopic nature of PA and the need for conditioning in order to reduce the brittleness. I'm also aware that the moulder runs the moulds cold which has an effect on the material properties - how much so I'm unsure.

What I am conscious of now is redesigning the part and when it gets moulded in production it being over strength.

Does anyone have thoughts on the above?

Thanks in advance

Marc

 

[ornerynorsk]

We do this also for proof-of-concept parts and certain very low volume parts, and put waterlines in where we can because some nylons are going to need mold temps as high as 140 to 150 deg F for an optimum part. I think your material is around 100 deg F, which presumably is still much higher than ambient. You can sit there with a temp gun and keep shooting parts until the mold temp rises, or try to use a heat gun to try and preheat a little bit. Parameters are important to the properties of the piece coming out of the tool. We are very insistent to our molder that just because it's a temporary tool doesn't mean we can just cheat on everything else. We try to have them duplicate correct parameters as close to real life production conditions as is humanly possible, temporary tool or not.

Another thing we have done is put cartridge heaters in a temporary tool where water lines are not possible. Anything to get the temp up within your processing range.

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.

 

[rickfischer51]

Running a cold mold may reduce degree of crystalinity. It has been said that the two most important parameters that control mechanical properties of a polymer are molecular weight and degree of crystalinity. Not sure how that would affect ductility. I would think that higher crystalinity would reduce ductility, but I'm not sure. Rapid cooling can increase ductility because it locks in free volume. Yes, nylons are hydroscopic, but increased water content post molding usually reduces brittleness because it acts like a plasticizer. If the resin is not sufficiently dry during molding, then hydrolysis might occur, which reduces molecular weight and therefore ductility. I think your resin is stabilized against hydrolysis. In my experience, brittleness in glass filled resins can result when the fibers get chopped up during processing, like maybe screw speed too fast or residence time too long. IF this were mine I would run TGA, and when the organics are burned off take a look at your fibers. Do this for molded parts and raw pellets. Also, run melt flow on pellets, runners and parts and see if you are getting degradation due to processing. Also, take a look at the molding parameters and compare them to the recommendations from DuPont. Also, is failure initiating at a weld?

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[Marc_Bloke]

Thanks for your responses.

Unfortunately I don't think the moulders will let us go there in person and play around with the tool. The point that just because it's a temporary tool doesn't mean corners can be cut is a good one. Indeed, the recommendations stipulated by the resin manufacturer surely must be followed by the moulder - they were the ones who provided me with material data sheet - with which data I designed the part to be strong enough (in theory).

Points about screw speed being too fast and residence time too long are taken on board and will check with moulder.

Failure didn't occur at weld - it failed where I was expecting, just at a lower load. The load was applied rapidly, and where I thought the ductility of the material (or so I thought) to allow the load to be taken, instead failed almost instantly.

A plastics salesman I spoke to said the brittleness was because the part wasn't conditioned and recommended boiling the part for an hour per mm of thickness. If I'm manufacturing upwards of 500,000 parts per year - surely this kind of post processing just isn't feasible?

 

[btrueblood]

Read the following threads, esp. the replies from patprimmer and Demon3:

https://www.eng-tips.com/viewthread.cfm?qid=89698

https://www.eng-tips.com/viewthread.cfm?qid=182700

I'd not boil, but just soak parts in room temp. water for a few days before testing.

 

[ornerynorsk]

I'm no expert guru in injection molding, but I've never heard of "conditioning" parts in boiling water on a production level. The salesman is either being purposefully misleading, or needs further training on the product that he sells. It's not uncommon for a molder to take a misting bottle and give a few squirts of water inside of a bag when packaging nylon parts during dry seasonal weather, but "cooking" your parts? Nope!

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.

 

[btrueblood]

I've boiled nylon to dye it before, works well (another tip from good old Pat, MHRIP). But you are annealing the nylon in doing so, and losing some tensile strength, also it will hydrolyze a bit, further weakening the stuff.

 

[ornerynorsk]

Yes, RIT textile dye works great! I wonder if 212 F is enough for any meaningful annealing? A number of years ago we had a very difficult part in Zytel 70G33L that wasn't doing what we needed it to. Moldflow said one thing, the part did the opposite. We had to end up performing dry fixtured annealing in order to get a feature back within design intent. After running a series of empirical tests, we wrote the process specs at 250 F with a 3 hour soak and a closed-oven return to ambient of about 6 hours. Funny thing, the overall properties improved as to actually be stronger in break tests. Must have had some nasty stresses that were concentrating in a critical spot. We still run about 5000 pcs a year and verification is still coming out at nearly 15% improvement of load-until-break over the non-annealed ones.


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.

 

[Pud]

Min tool surface temp for PA66 is 70C.
I would suspect material was not dry before moulding.
If it's from China verify it's actually what you asked for and not 'black plastic'
Conditioning by boiling can be done without saturating the polymer by boilng in a solution of (iirc!) potassium acetate. Recipe in a Du Pont guide somewhere.

www.tynevalleyplastics.co.uk

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[Marc_Bloke]

If I switched to a glass filled PolyPropylene do you think I would still encounter similar issues with conditioning?

 

[Demon3]

PP does not need conditioning because it doesn't take up water. PP properties will change somewhat in the hours after molding as crystallization finishes. What are the property requirements for the part? Why was nylon chosen?

Chris DeArmitt PhD FRSC
President

Plastic materials consultant to the Fortune 100
Creating New Materials - Problem Solving - Innovation Keynotes - Expert Witness

http://www.phantomplastics.com

 

[rickfischer51]

High strain rate will induce brittle behavior if the rate is high enough. Try soaking parts in water and retesting with a slow strain rate. Soaking may not be practical in production, but it might at least have some diagnostic value. Also ask them to mold some in unfilled nylon and test those. If they passed, it might point the finger at fiber damage as mentioned above.

I'm curious about your material choice and your suggestion of trying PP. For half a million parts, material choice could have a major cost impact. PP is significantly cheaper than nylon, and if your employer is anything like my past employers, the first thing purchasing will say is hey, can we use 75% regrind PP? You might want to take a look at your material selection process and make sure you understand the ramifications of your material choice on part function and cost.



Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[Marc_Bloke]

Nylon was chosen for it's high strength. Given certain other factors considered we decided that a high strength material, although more costly, would be best for the design as appose to a lower strength, cheaper material that was thicker at stress concentration areas.

This is the main requirement for the part - in its application the part will be loaded rapidly and will sustain this load for around an hour and then after this the part doesn't carry any load for the remainder of its life.

There is no requirement for the part to resist high temperatures and the part will not come into contact with chemicals. UV resistance would be favourable but not a necessity.

Alkali resistance is a factor however, so PP would be favourable over in Nylon in this respect.