Injected TPU abrasion resistance

[Guanoolic]

I'm having problems with abrasion resistance of a TPU flat part, with thickness 5 mm.

Even with the same parameters in the injection molding machine I can't have consistent value of abrasion resistance, I get values from 60 to 180 mm^3 and I need to have consistent values below 100 mm^3.

I'm trying to understand the way abrasion resistance is affected in the process, but hasn't been easy.

Hope someone can help me understand what I'm doing wrong.

Thanks for your time:

G

"Since light travels faster than sound, isn't that why some people appear bright until you hear them speak?"

 

[patprimmer]

My guesses would be variable moisture conditioning or variations in cooling rate either during or after moulding or variations in time after moulding that testing is done.

There can be considerable variations in mould surface temperature even if the cooling water temperature is the same.

Also the raw material might have batch to batch variations.

Also added colour might cause variations. A small amount of colour can cause a very large change in properties.

Abrasion resistance can be a very imprecise and variable scince.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[dwightdixon]

How are you measuring abrasion resistance? Many of these tests are subject to variation. The abrading media,if it is reused, such as sand paper or a grinding wheel, gets loaded up and prevents further abrasion. This gives you a low result. Temperature has a lot to due with it. Are you abrading too fast and heating the sample? That will give you too high a result. Are you allowing the Urethane sample to cool and the structure to develop fully? The usual recomendation for urethane is to wait at least 24 hours. I have found the falling sand to be reproducable but you OEM usually dictates how you are allowed to test. For urethanes, I have found the temperature of the mold and how long the clamp remains closed has an affect on abrasion resistance. Higher mold temperature and a slower cycle seems to help. Have you talked to your urethane supplier? They have the most experience with their material.

 

[Guanoolic]

All variables are the same, from heating and cooling temperatures and times, pressures, volumes, speeds, etc. and I still have the variation in the abrassion resistance. Moisture was tested by changing the temperature in the dryer and there was no significant change in the results.

About the cooling, the only thing that can be diferent is the cooling throw the mould surface, but that would reveal changes along the part, since I take 3 samples from each part to test the abrasion resistance the values would be diferent, but they are similar.

The raw material is from the same batch, so I don't except variations there.

The addition of colour (1%) was also tested, by using a masterbatch from 2 diferent suppliers and once again no significant variation in abrasion resistance.


I'm using ISO 4649 - Method A - Rubber abrasion resistance using a rotating cylindrical drum device, I know that the tests are far from 100% accurate, but values from 60 to 180 can't be the test variation. The tests are all made the same way, speed and temperature. All tests were made after at least 24hours of the moulding.

dwightdixon: "For urethanes, I have found the temperature of the mold and how long the clamp remains closed has an affect on abrasion resistance. Higher mold temperature and a slower cycle seems to help."

Do you have bibliography about this? I just can't find any infomation on TPU, can I compare it's behaviour with urethanes?

I talked to the TPU supplier, they made tests on the parts and found diferent values of abrasion resistance too, then they visited the company and all the tests were ok,after that no changes in the process and things went wrong again.

But what really confuses me is that without any changes in the process, now it's all wrong and maybe tomorrow is good, or the other way arround.

Is it possible that abrasion resistance in TPU can't be controlled at all?

Thanks for your prompt answers

G

"Since light travels faster than sound, isn't that why some people appear bright until you hear them speak?"

 

[patprimmer]

There is more to drying than simply temperature in the dryer

If you do a melt flow index of good and bad mouldings you might find a correlation between higher MFI and lower abrasion resistance. Uneven drying could result in uneven MFI after moulding.

Do you also have variation in hardness as tested. If so does it correlate to abrasion resistance.

Sometimes there are "secret" changes to the conditions as an operator makes unauthorised changes or their own reasons, like slowing a cycle down or reducing melt temperature.

How are you controlling the mould temperature.

How are you verifying cycle rate.

If you place a thermometer in the top granules in the dryer at various points, do you get the same temperature at all points. Variations will indicate channelling of air or granules. Channelling causes uneven drying. Also letting the dryer run low on material then topping up will cause a variation in dwel time in the dryer.

I am presuming that all cavities are being tested and cavity to cavity variation is noted.




Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Pud]

We mould TPU for an abrasive resistant application. The material is dried in a vacuum drier and due to TPU being seriously hygroscopic, we keep hopper residence time to 45mins max., whereupon the material remaining is exchanged for stuff straight out of the drier.

TPU is also very, very shear sensitive. Slowest screw speed possible, least back pressure etc. etc. Let the barrel put heat in, not the screw.

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

I know one guy who uses the Maguire partial vacuum dryers for PU with very good results.

http://www.maguire.com/

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Pud]

Pat: That's wot we got amongst others. Dries PC in 25 mins too!

Harry

http://www.tynevalleyplastics.co.uk

 

[Guanoolic]

Patprimmer:
There is no variation in hardness always between the defined values.
To avoid “secret”changes in the operation conditions, I’m writing down the parameters every day, more then once, they wouldn’t have time to change back to normal before I get there.
The mould temperature is controlled throw the water temperature, not very reliable but that’s what we have.

There are only two cavities and there are no significant changes between them.
Pud:
To avoid shear I made some tests, higher temperatures and lower speeds and back pressure, but I had bad results again.

I’ve been making some tests this last days, and had bad and good results, the good ones were obtained by reducing the volume injected to the minimum value, to avoid degradation of the excedent material from one cycle to the next, I took some samples today in order to confirm if the abrasion resistance as consistent values.


G

"Since light travels faster than sound, isn't that why some people appear bright until you hear them speak?"

 

[Guanoolic]

So far so good, guess reducing the volume of the injection helps a lot, lets wait for some more tests.

Thanks for the help

G

"Since light travels faster than sound, isn't that why some people appear bright until you hear them speak?"

 

[sizzer33]

Hi ,

This is really a good read. Its always a challenge to find out discrete problem with your parts. Seems like you're doing a very good job analyzing it.

Situations like this happened to me alot, where suppliers come over to help and it all went ok and by the time they leave, it all goes back to trouble again. If its so inconsistent now, imagine during production.

Have you thought of heating the parts after moulding? one of our customers use to do it to make the part stronger. i forgot the specific time & temp but it was around 90degC for about 12hours i think. This will allow more tolerance for you during injection moulding, but of course you have to invest in these oven cooker. We use to put it in the drier that drys parts after spray painting.

 

[patprimmer]

I don't think annealing will help PU abrasion resistance.

Every different polymer has it's on unique annealing conditions. Annealing can reduce moulded in stress and change degree and nature of crystalline structure in semi crystalline materials.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Guanoolic]

One of our suppliers, Huntsman, also tells that performs the tests after 12hours at 90ºC. I can make that for testing, but not for prodution because it's just impossible to store all that material in a heat dryer. Even for testings during production it's not good, because I need 24hours to aneal the material, then 24hours to let it rest and after 48hours I can do the tests... ok if there's any problem, 2 days of production it's a lot of product..

G

"Since light travels faster than sound, isn't that why some people appear bright until you hear them speak?"