tpu case enlargement.
tpu case enlargement.
(OP)
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..
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..






RE: tpu case enlargement.
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: tpu case enlargement.
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?
RE: tpu case enlargement.
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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RE: tpu case enlargement.
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
www.tynevalleyplastics.co.uk
RE: tpu case enlargement.
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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RE: tpu case enlargement.
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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RE: tpu case enlargement.
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.
RE: tpu case enlargement.
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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RE: tpu case enlargement.
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?
RE: tpu case enlargement.
Regards
Pat
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RE: tpu case enlargement.
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
www.tynevalleyplastics.co.uk
RE: tpu case enlargement.
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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RE: tpu case enlargement.
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
www.tynevalleyplastics.co.uk
RE: tpu case enlargement.
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.
RE: tpu case enlargement.
Dan - Owner

http://www.Hi-TecDesigns.com
RE: tpu case enlargement.
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?
RE: tpu case enlargement.
Dan - Owner

http://www.Hi-TecDesigns.com