Solution for blistering PPO

[deenor]

Hi,
I'm producing a casing from a glass filled mPPO material. One of the requirement is to have surface conductivity. In order to fulfill this, we use carbon black as the conductive additive. The problem is I found the surface of the molded part tends to form blister or loose debris. The customer was horrified & is asking for improvement. Anybody has any suggestions or ideas? I've tried evaluating on process parameters but none of them ever showed any significant improvement.

regards

 

[patprimmer]

Is it a casting or an injection moulding. I was not aware that PPo could be cast.

Why is it PPO. There are quite a few other possibilities for injection mouldable conductive thermoplastics. Stainless steel fibre filled polycarbonate springs to mind, but there are quite a few possibilities depending on what other properties are required

Regards

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[Pud]

Processing mPPO:

Pre-dry @ 110 Deg C for 1-2 hrs - moisture content should be below 0.05% otherwise you will get streaking.

Melt temp (i.e. actual melt!, not barrel settings) should be below 270 Deg C.

Mould should be at 70-110 Deg C.

A photo of your defect might be useful....


Cheers


Harry

 

[deenor]

Hi,
The product is casing/small container & the process is injection molding. PPO is used due to its ability to withstand moderate baking temperature. Carbon black is used because it is very cost effective. In another word, they're all given factors.

Can the surface finish of the insert affect this? Glossy, matte or rough. Because I've seen other products (not produced by us, but using same material/filler) did not have this problem. The difference is the molded surface looks very shiny & smooth. I don't have any photo to share at the moment.

regards

 

[dwightdixon]

You may find that you can buy a precompounded product from people in MN that would do the job. It is likely you are not properly wetting out the carbon by simply adding it to the injection molding machine.

 

[patprimmer]

Sorry, I read casting not casing. I obviously suffer from a mild form of dyslexia.

I guess you are using Ketjenblack high surface area carbon black. Are you adding it or is it included in the compound when you buy it. If it is overloaded or poorly dispersed you might get the symptoms you describe.

When calculating how much to use you should consider the resin content of your glass filled compound. For instance, if it is 50% GF, you only need half as much additive.


Regards

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[Demon3]

You may find that a wetting agent (surfactant) in low amounts (<1 weight %) helps to wet and disperse your carbon black, thereby solving your problem.

Pat makes a good point that if there is glass fibre then there is less resin and the carbon black dosing should be relative to the amount of resin excluding the glass fibre. However, 50 weight % of glass fibre only replaces about 25 volume % of resin so you need to reduce the carbon black by 25% (not 50%).

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[patprimmer]

Demon3 is correct. The amount of carbon should be proportional to the volume not weight of resin.

Also, the surfactant he mentions helps a lot, but be careful the one you use does not cause solvent stress cracking in PPO.

Regards

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[Pud]

Have you tried moulding the parts from the PPO without the carbon black?

This will show whether the problem actually lies with the additive or is more basic.

Cheers


Harry

 

[patprimmer]

Have you checked the moulding conditions vs the manufacturers recommendations.

Is the mould temperature hot enough.

Does it improve with drying. Even if the base resin does not normally require drying, sometimes the additives pick up water either directly or via a carrier or coating. Sometimes drying removes volatiles other than water.



Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[deenor]

Carbon black is added to achieve conductivity. The percentage is already at the threshold value where anything lower will make it fail to meet the requirement. The alternatives are carbon fiber, stainless steel fiber or other IDP additive. The issues with these alternatives are higher cost & non-permanent conductivity for IDP. The glass fiber is added to achieve the desire mold shrinkage & strength.
The good news is I've managed to significantly reduce the blister/loose debris by adding certain percentage of PS. The downside is it has slightly affect the mold shrinkage but that can be fix. Thank you very much for all of your input. Thanks.

Regards

 

[Demon3]

There is an Asahi patent for molding PPO without any need to add PS. They use a special additive that improves the flow of the PPO. Maybe they can help.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell