Brittle PBT problem

[BNicolas]

Hi,

I hope i'll be understood though my english has to be worked.

The problem concerns injected electrical connectors made of PBT Duranex(c). There are little elastic "clips" which let the conector pass at first and block it (hoping i'm clear). And so, the problem is that about 5% of the parts have the clips brittle and then break during the resistance test (A needle enter the hole, push the clip to simulate the connector before a camera checks that all clips are here).
I'm about to do some tests on parameters and i'd like to have you're thinks.

An observation was that clips whick break are mainly at a position which partially filled in first time and fully at last (know through tests whith not enough material)

I have 2 ideas for solution:
-Not hot enough at nozzle so that the fluidity is not good enough (at the moment about 250°C)
Mold is to cold so that there is a too fast cooling of the material inside which causes layers apparition and decreased pass size to fill the clips. (clips enter is not more than 1mm)
Both are linked

-A study carried out by the european plasturgy center (PEP) showed that ther eis an amorphous area at the base of some clips, could it be more brittle than the crystalline one? And so the material should be kept cooling longer. (we saw that a longer time for cooling without any pressure remaining decreased broken parts to 0.2%, but after some time it goes up again)

Thank you for reading and for the help you may bring.

Nicolas

 

[patprimmer]

Inconsistent impact with PBT can normally be atributed to one or more of three causes.

1) Uneven material progress or uneven air distribution in the dryer so some pellets are not fully dried.

2) Uneven shrinkage within the moulding creating stresses. This includes post moulding shrinkage which is quite common in PBT.

3) Really 2 part 2. Uneven crystallisation and post crystallisation which continues for considerable time after moulding.

The last 2 can be cured by annealing.



Regards
Pat
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[BNicolas]

Patprimmer,

Thank you for your help,

I was discussing about making an annealing on a very bad serie to see if there is a difference. So that, if I don't see any i could considere my problem isn't 2) or 3)?
For the annealing, which temperature and time should I try?

If the annealing solves my problem, is there any parameters I can try to change instead to anneal all of the products?

Nicolas

 

[patprimmer]

I dont know off the top of my head, but Ticona will have recommendations in their literature. You can visit their web site and look it up as easily as I can.

Regards
Pat
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[BNicolas]

Hello,

Thank you for your advice, I got that.

Best regards, Nicolas

 

[Pud]

In my experience (and my wife's first response when asked) PBT brittleness is invariably due to "damp" material.

The stuff moulds great when damp (high gloss etc) but the parts are about as tough as dog biscuits.

A common problem with small shot weights is the time the material (hopefully dried to less than 0.04% moisture?) in the hopper if it is unheated. I (we) allow 2hrs max in unheated hopper.

PBT will reach the maximum recommended
moisture level for processing within approximately
2 hours at ambient conditions of 23°C and 50% RH

mold

temperatures in the range of 30-130°C. Mold temperature
should be increased as the wall thickness of molded
parts is reduced.

Note the comment re wall thickness. We always use a mould surface temp of 100°C min, to make sure properties are retained.

If all else fails - Read the Instructions!.

Q: Why is is it that Du Pont find difficulty spelling "mould" and it's derivatives?

Cheers

H



www.tynevalleyplastics.co.uk

Beauty is in the eye of the beer holder.

 

[btrueblood]

Maybe cuz they're French?

 

[BNicolas]

Hi guys,

thx for answers.

Tests have been done and now here is a result which offers a 0% break rate.
Surprisingly it has been obteined by reducing the mold temperature from 60°C to 40°C.( In fact these parameters gave us 0.8% against 9.6%.
Our theory was:
60°C--> mold is filled via large vein firstly and at the end the center where veins are smaller(and where are the breaking clips)
40°C--> The cooling system which rounds the parts slows the filling by large veins and then the center is filled earlier than before. Which has been proved by unfilled part tests.

To go further we added 2sec after part ejection during which the mold is open and cooled longer. It showed effective because we reached the 0%.

If we can understand the injection upgrade, I'm not sure to understand why it works, by matier side. Why my PBT is no more breattle? :/

After your help those pieces of imformation I hope we can go further for understanding since the problem is solved.


NB: Everywhere I look mold is used and not mould. Is there a problem whith frenches?

Nicolas

 

[BNicolas]

Sry for 2nd post, I didnt find out how to edit.

Misstaken farther and further.

We are drying the PBT for 5 hours @120°C following Duranex technical sheet.

Nicolas

 

[patprimmer]

If a colder mould fixed it, my guess would be it reduced the level of crystallinity. Did the mould shrinkage also decrease.

Regards
Pat
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for site rules

 

[BNicolas]

I don't think there was shrinkage, the surface stat was good and the clips seemed to be fully filled.
My first idea was about grain size lowering since brittleness could comes through large grains. And with less time, growth is shortened. But since parts core is filled earlier and is hotter that's not possible, is'nt it?

If the solution is to lower cristallinity, is it possible to lower even more the mold temperature and shorten the cooling time (when there is no more pressure, just parts kept in close mold, which is at the moment 7sec).