Polycarbonate parts breaking

[jonalawson]

Hey Guys,

Just after some info about an injection molded poly carbonate part we are having trouble with.

The part is a clear Polycarbonate price ticket housing for a retail store. It has been injection molded in clear poly carbonate.

The design has 2 plastic clips on the rear which clip onto a wire basket in the store. We chose poly carbonate because it is strong and durable and these items are used rigorously in store everyday.

During prototyping the parts performed well(3d printed parts), however during the 1000 part store trial, we have had some issues with these parts breaking.

We are finding that the clips on the rear are starting to become brittle and snap off when used. The reason why this is very confusing is that the parts are extremely strong, if you try to break the clips it is pretty much impossible. Yet out in the field we are finding that they are becoming brittle somehow and snapping. The break is always in the same point. So we are figuring that it is a design fault, but was wondering if anyone had information about how a poly-carbonate part could become brittle?

tks

-jon

 

[cowski]

Does it come into contact with any chemicals?
Does it sit out in the sun all day?

www.nxjournaling.com

 

[jonalawson]

It gets cleaned with water only, or in some stores maybe soapy water.

It does not sit out in the sun. And in most case breakages occurred on parts which had only been trialed for a few days. But again...we took parts that had one clip broken(there are 2 clips per part) and tried to break the second clip and couldn't break it!

 

[Demon3]

Polycarbonate is famously bad for environmental stress cracking meaning exposure to chemicals makes it get extremely weak. This can happen in ways not immediately obvious. For example traces of oil on a drill bit. A wipe with a cleaning cloth. A spray of furniture polish. A sticker placed on it. That's most probably the reason. Would recommend transparent ABS instead or impact modified PMMA.

Chris DeArmitt

Expert consulting & training

http://www.phantomplastics.com

 

[jonalawson]

Can anything be done or added to PC to make it more resilient to environmental stress cracking?

 

[MikeHalloran]

Anyone who comes up with an additive to defeat PC environmental stress cracking will shortly become a billionaire.

So far, the problem has defeated some pretty smart people.

The worst part is that environmental exposure does not immediately craze PC or cause any other detectable change in it. ... but subsequent loading will cause immediate failure at low load, even if substantially displaced in time.

If you can't find who/what/how the stressor is being applied, and probably even if you can, you'll need to change to a different resin.



Mike Halloran
Pembroke Pines, FL, USA

 

[Demon3]

I agree with Mike. A different resin can be used and should work in your existing molds if the right one is picked.

Chris DeArmitt

Expert consulting & training

http://www.phantomplastics.com

 

[jonalawson]

What do you think about replacing polycarbonate with clear PETG?

 

[Demon3]

Candidates would include transparent ABS (Terlux from BASF for example), impact modified PMMA, PETG or, if you don't need impact resistance then use SAN and save a lot of money.

Chris DeArmitt

Expert consulting & training

http://www.phantomplastics.com

 

[Pud]

I would also look as Styrolux (SBC* by Styrolution - others make it). It doesn't require pre-drying prior to moulding and it's high impact. No tool modifications will be required - same shrinkage as PC.

* Styrene Block Copolymer

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[ornerynorsk]

I would first check to see what kind of mold release the molder is using. It's something your molder should be familiar with if they mold PC, but you never know . . .

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]

Good point there onerynorsk...

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[berkshire]

Ouch,
I just had a mental picture of mold release with aromatic hydrocarbons in it.
B.E.

You are judged not by what you know, but by what you can do.

 

[Paulkuklych]

Where is the injection point (gate) located?

Is the gate the same distance to each clip?

During molding, does one clip fill before the other? Check short shot.



Paul Kuklych

http://www.improve-your-injection-molding.com