Platinum Silicone Getting Hot and Smoking

[AndrewKo]

Hi, I'm working a project that uses a platinum cure silicone impregnated with iron powder that is overmolded onto a steel plate. the material interacts with microwaves heating the silicone and metal plate in order to reach temperatures of 450°F (232°C). We've made some prototypes and are finding that the silicone is emitting some kind of smoke when seeing these temperatures for just a minute or so.

The part in question is pictured for better understanding. The silicone is only .09" Thick and about 8" round
We are compression molding this at 330°F(165°C) for 4 minutes with a 400°F (200°C) post cure for 4 hours.
With just a quick inspection the parts seems to have cured fully with no bubbling or any obvious problems.



I'm wondering what may be causing this smoke to occur.

The material cant handle these temps?
The cure cycle needs to be longer or hotter?
The post cure cycle needs to be longer or hotter?

We are very new at molding silicone and this seems like a odd application. Any help would be much appreciated.

 

[TugboatEng]

I know nothing about silicone moulding but do know that most oils including any oils used to lubricate the metal forming dies, on your hands, etc... will certainly start to flash by 450°F. I would take a sample of the material and hold it at that temperature for an extended period. If the smoking stops and there are no changes in required properties then everything is fine.

 

[Compositepro]

"the material interacts with microwaves heating the silicone and metal plate in order to reach temperatures of 450°F (232°C)."

An iron plate reflects microwaves due to eddy currents that occur in the plate. Iron powder is used as a susceptor (

https://en.wikipedia.org/wiki/Susceptor)

to absorb the microwave energy and convert it to heat. It is very likely that the iron particles are going several hundred degrees hotter than the temperature you are measuring, and decomposing the silicone. It is the same for any heater, in that the closer you operate to the temperature limit of the heater, the lower the allowable power output. At the temperature limit, the allowed output is zero, because the heater is always hotter than what it is heating. You also have to consider the non-uniformity of the microwave intensity leading to hot spots.

 

[btrueblood]

If you post cure at 400F, there may be volatiles left that will burn out at 450 F. Try post curing at 450 F.

But, what I know of silicone is that it is of limited use above 400 F...because you decompose it.. as CP notes.

 

[Compositepro]

The thickness of your coating may also be a major part of your problem. You are applying microwaves to one side of a 0.090" coating and the cooling is on the other side. Susceptors used in microwave cooking packaging are only a few mils thick.

 

[AndrewKo]

@ TugboatEng & btrueblood:
We wash the samples before and after overmolding which I would think would take care of most if not all residue that may be left on the surfaces. Additionally we have taken several samples and post cured them at 550°F (290°C) for 1.5 hours. This seemed to reduce the amount of smoke seen, but not eliminate it by any means.

@ Compositepro:
You make a good point. If it is this "heater" issue it sounds like the only way to combat it would be to use a material that can better handle temperature well above our goal temp.