tpu case enlargement.

[trxt]

We are producing cell phone cases completely made of TPU.(Bayer desmopan) Shore A 75.

But we started to get some customer complaints on because cases are enlarging in all directions almost %6-10 scale ratio.

We made lots of lab tests including thermal shock , chemicals etc.. but i didnt encounter such that enlargement problem. Bayer couldnt comment anything about this issue.

Really , i couldnt find the root cause of this enlargement issue.

What are your comments?

Thanks..

 

[Demon3]

My best guess is that it's swelling due to contact with a liquid. That could be an oil, hand cream, cleaning agent or similar.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[trxt]

Hello Chris,
My best guess was also contacting with chemicals because the weight of the cases had been increased after they come from customer(%1-3 weight increase , %6-10 volume increase) but i tried every chemicals i found in lab. I have even tried airplane fuel, interesting but i didnt encounter any enlargement in lab..

By the way ether based tpu is the right material for that kind of application? if not , what do u recommend?

 

[patprimmer]

TPU should be good, but there are a great number of grades with very different properties, including chemical resistance.

You need to look at the manufacturers chemical resistance charts for chemicals that result in swelling and a list of potential chemicals in the environment and find matches.

It is most likely some cosmetics or personal care product.

Regards
Pat
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[Pud]

It may be frozen-in stresses relaxing if they are ok off the tool.

As it's an elastomer, it is possible to compress the part when moulding - try reducing the holding pressure or time.

H

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Harry

My limited experience with PU tells me if you over pack it, it expands from the mould and the parts are to big, but it is apparent on ejection.

Maybe if it's run at around 0 deg C mould temperature in a dehumidified enclosure?

Regards
Pat
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[patprimmer]

Opps, bumped a button.

The stress can be moulded in enough so it needs time or heat to relieve itself. Maybe different grades react different. I only ever moulded a few.

Regards
Pat
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[trxt]

Hello Pud,

All parts are molded with the same injection parameters

including holding pressure and time.

But , Only %1-3 parts has come as customer complaint.

If the reason of swelling had been high holding pressure , would swelling appear for all molded parts that has the same injection parameters ?

Mold has a needle gate and cold runner is used.

 

[patprimmer]

Same machine settings does not necessarily mean same in cavity settings.

If the cushion is set to small for instance, it will be inconsistent pack.

If the check valve is worn, it will be inconsistent.

If there is small pieces of contamination obstructing the gate occasionally, it will be inconsistent.

If you have a very narrow processing window but a fairly poor control on the machine, it will be inconsistent.

If you run tower water, it changes temperature which might affect the machine oil temperature or the mould temperature.

Regards
Pat
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[trxt]

bayer desmopan document says that , contact of acetone , benzene , MEK , etit alcohol etc.. chemicals cause swelling.
Today , i made a trial with these chemicals at lab. .
Yes , sweeling is occured around %1-3 ratio. But it was reversible. When sweelled parts heated around 55 C degree. Parts come to their first dimensions.
but the sweeling of parts that came from customers are all permanent. I also heated up them all today.
Any comments?


Pat and Harry,

Tomorrow , i will make the trials by using extreme holding pressures and timings in order to observe the difference and their affects.I will let you know the results.

Do you advice me to try such another things else in order to diagnose the reason of swelling?

 

[patprimmer]

Effective mould temperature might have an impact as PU is actually semi crystalline.

Regards
Pat
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[Pud]

Pat, you're quite correct re overpacking make it difficult to eject - but the amount will depend on the part. We make some small TPU "gears" for want of a better description - about 30 dia. I can alter the size by at least 0.3 before ejection difficulties. Great things stripper plates!!

Another off the wag chart: How long is the material in the hopper (if not using a hopper-dryer)? We find that a couple of hours is max before it's too wet to process. This is a Polyester base - no idea how hygroscopic PE based stuff is.

We have also found (a long time ago) that it is very shear sensitive - i.e. back pressure. Too much shear=degradation.

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Harry

I did not mean it was difficult to eject although it can be sticky.

I meant that size variations can be apparent on ejection as you might get expansion rather than shrinkage from the mould under some circumstances.

Regards
Pat
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[Pud]

eject although it can be sticky

Aaahh - indeed so! Re-reading the posts, your (Pat) idea re variation in gating might well be the problem - overpacking some cavities preferentially might be implicated - gate freeze off times and all that. Trials with short shots and an accurate weigh balance might be in order.

H

http://www.tynevalleyplastics.co.uk

 

[trxt]

Hello Harry,

today , i have tried to molding by using extreme holding pressure and times. I tried minimum and max. pressures and times. I didnt observe any major dimensional difference or swelling.

Pat,

Mold temperature is almost 23 C degree which is recommended by Bayer.

I think that chemical reasons of swelling is more reasonable for this case.

if requested , I can send swelled and standart parts to your adresses for your valuable evaluations. Just let me know.

 

[MacGyverS2000]

Fingernail polish remover is usually acetone (except the acetone-free stuff)... any chance the expanded units belonged to women? No idea what kind of crap is in hand lotion (again, used more often by women). Just a WAG...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[trxt]

You are right Macgyver,

This is what i first thought when i encountered with this problem. So i requested the gender list of customer complaint from customer service. Unfortunately almost half of complainers are woman only.

But , what i tried in lab is that acetone , solvent , benzine and paint thinner cause swelling.
Benzine swelling is reversible.
But others are permanent.

I dont know if sudation for some people can be a reason of swelling?
Has anyone experienced this?

 

[MacGyverS2000]

Can you send one of these cases to a lab and let them to a spectroscopic analysis on the offgas byproducts as they heat it up? It should tell you at least what chemicals, if any, are stuck in there.

Dan - Owner

http://www.Hi-TecDesigns.com