Cooling carbon tooling

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#1

Has anyone any experience of direct cooling of carbon fibre tooling?

I am looking to design a tool to aid the cooling process of a composite part. I know we can use the likes of copper piping with water/oil attached to the rear of the tool, however, due to the probable poor thermal conductivity of carbon, im not sure how effective this would be. I could put the tubes within the mould but again, copper inside a carbon tool……possible problems, particularly as it’s going to 190/200 deg C. One possibility is to mould the tool at a lower temperature with released, straight copper tubing (as opposed to the traditional u-shaped tubing for cooling) through the mould and remove the copper tubing after initial cure. Then post-cure to 200 or whatever with no copper tube internally. With this method however, Im worried about moisture penetration into the carbon tool. I would appreciate anyone’s thought on this technique or any other techniques.

Thanks v much

#2

What cooling rate are you trying to achieve? Carbon will heat and cool much faster than fiberglass. How big is your tool? How thick will you make it?

#3

Sorry I should have stated that hojo thanks, looking a cooling rate of about 3 deg C per minute: Its not a definitive value but Im looking to do is use this as a development tool for future projects on improving throughput. Total surface area of the tool is approx 1.3m2 so not overly large but again this is a proof of concept. Thickness is non uniform but about 50mm in certain places, allowing for internal channels of about 20mm diameter also.

#4

Carbon has a much better heat transfer than copper… Pitch based carbon can be twices as good. Pan based (the stuff most of us use) is still very good. So I would not worry about that.

Also when cooling you are working with a medium to transport the heat (water) and dump the heat to the air using a readiator.

The size of the radiator has a lot of influence, and water speed has a bit of influence as well. 3 degrees per minute seems very doable. Find out about radiators and their capacity, and make some calculations.

For keeping 200 watts of processor heat under control (less than 40 degrees C) a small radiator from a cars heating system is plenty. Most have aluminium radiators, but try and find copper ones. Nissans usually have copper ones, but there are others.

#5

Be careful you aren’t cooling just one part of the mold. Uneven heating/cooling is a major cause for warped parts in oven/autoclave cured parts. Carbon/Epoxy has almost 0 CTE, but only in the direction of the fibers. Unless your mold has perfect balance and orthotropy, you will have some warpage. Many tools in aerospace are made from Invar because it “has the same CTE as carbon”, but I’ve seen many warped carbon parts come from such tooling if there is uneven temperatures or unbalanced layups.

#6

Thanks for the advice. Thanks for that Herman, the heat exchanger side of things should be relatively straight forward, just the tooling design itself is the unknown! but thats very interesting about the pitch and pan fibre properties - didnt know that. Thanks also Coriolis; after speaking with another friend on here, we may use a lined tube to prevent moisture ingress into the carbon tool with a possible. We undoubtedly will have uneven cooling, hopefully it can be controlled well enough. Ill post up pics with progress in the next few months.
thanks

#7

We used to cool our composite molds with copper tubing imbedded in an aluminum filled epoxy on the back of the molds. I think it was from Ptm&w. The filled epoxy was used to spread the heat around more evenly.

#8

While there are one or two carbon fibers that have higher thermal conductivity that copper, standard PAN fibers have much lower conductivity. And epoxy resins like all plastics have very poor thermal conductivity. Using a blower to blow cold air works very well. Cooling any faster can crack or warp your part or mold.

#9

Refrigerated room or box??

#10

I’m sorry, but I’m not able to see the reason for forced cooling of a carbon mould. The thickest carbon mould is no more than 8 mm. thick, and it cools down by itself in room temperature air that sometimes the problem could be just the opposite, to avoid eccessive cooling ramps. If it is used into an autoclave, the autoclave itself will provide the necessary cooling ramp, if it is used in an oven, just put it outside it as soon the safety temp for the resin has been reached (80° for a standard prepreg).
Please keep in mind that backing the carbon mould with anything having a different CTE will bring a LOT of warping issues. This is the main reason why, when a metal supporting frame is requested under the tool, it has to be ingeneered so to let it expand freely without affecting the tool. (and that’s the reason why in the aerospace industry, all the backing structures are made in carbon fiber too). Beware.