Carbon filament for electrical mold heating 1

[nash0427]

Hi All,

Just wondering, has anyone tried heating of molds by wiring carbon filaments that are already laid into the mold as structural reinforcements of the mold. If so, what are the gotcha's? What are parameters that one would need? Would this save energy and give me a Greener part ;-) ?


Would really love some discussion on this.

Thx,
N

 

[Compositepro]

It can be done and I've tried it to some extent. Direct electric resistance heating of molds is challenging using standard heaters. Using carbon fiber requires a good understanding of electricity as well as carbon fiber which few people have.

The first fundamental problem is that composite molds are poor conductors of heat so temperature uniformity is poor if you use a constant power heat source like resistance heaters rather than a constant temperature heat source like an air circulating oven.

Then there are electrical safety issues. You have to control the current path and insulate so that people can't get hurt. Carbon is not a great electrical conductor so voltages can be high.

If you look at standard electric cartridge heaters the resistance wire may be at 1000F or 1500F when the surface of the cartridge is 500F which heats an aluminum mold at 300F. Carbon will oxidize in air above 600F. Epoxy resins cannot be exposed to more than 400F for long without degrading.

In vacuum furnaces carbon-carbon heating elements have been used for some time and will operate up to 3000F.

 

[nash0427]

Thanks for your feedback.

So if I understand correctly, its better to soak the entire mold in hot circulating air, w.r.t. even heating locally? How about flat heating elements with aluminum sheets on them, attached on the mold (conforming to the mold)?

I would like to do away with recirculating hot air heating. But from what you seem to experience its the way to go for uniform heating. Am I reading you correctly?

 

[Compositepro]

You do understand correctly. Temperature control is a matter of balancing heat energy in with heat energy lost. Without mixing or high thermal conductivity material, each point in an object will have a different balance and therefore temperature. During heat-up the heat capacity of thick versus thin areas is important. Even in standard ovens thick areas heat more slowly and lag more behind oven temperature. This is one reason the rate of temperature change (ramp rate) is also controlled when curing composites.

 

[allcomposites]

There is a patent from Airbus on mold heating using dielectric conductivity of carbon fibers. I remember an article on JEC Composites magazine few years ago.

You can increase thermal and electrical conductivity of your laminate by adding carbon black particles or better carbon nanotubes. This will help on evenly heating the mold.

 

[NCSUMAN]

Alot depends on the temperatures you require and the size of your mold. Maintaining uniform temperature across the mold is key. Ovens or heat tents work best. I wouldn't advise using carbon fiber unless you really need to. CompositePro is right, there are issues with getting power safely to the tool, not creating a hazard, and getting uniform heat. Flexible heating pads work OK for flats, contours and lower temperatures. I have also formed in cal rods.

 

[berkshire]

I tried this in the 1980s and had the same problems with uniform heat distribution.
I though it would be a great way to heat a mold without heating the whole shop.
I went back to using heat tents, after finding under cured areas in my parts away from the carbon filaments.
B.E.

 

[RPstress]

You can use carbon foam tooling and electrically heat it.

http://www.cfoam.com/pdf/Composite_Tooling_CFOAM.pdf

Not sure how well the .pdf covers electrical heating (if it does), but that's the stuff.