Hi Gary,
I wrote up something awhile back and have it on www.amaglider.vze.com under carbon fiber facts. I hope this might help you some.
Carbon tow is basically carbon fiber thread or rope. It is made up for many little short filaments which are about 7 microns in diameter. The 12K means it has per cross section about 12,000 filaments. The fiber area cross section of 12K tow is .000070 in^2. Carbon Tow also comes in 1K, 2K, 3K, 6K.
Now I never had great success working with carbon fiber tow which I try to split apart like you describe. I know some other modelers do this and when I talked with them they told me they are just very careful about separating some fibers out.
What I do is a little different and I have some logic behind it. First try to get the 1K carbon fiber, it is hard to find but I have seen it from time to time being sold. Next I saturate the carbon fiber tow with glue then I place it between two flat plates and apply as much pressure as I can using C-clamps. The logic behind this is that carbon fiber is a composite and its properties are a function of its components. Typically we are interested in the stiffness of carbon fiber laminate per its weight. We don't want to have any non-glued filaments or voids so we need to make sure the thread is saturated with glue.
Now the carbon fiber is pretty stiff. A number which describes this stiffness is called modulus. The "typical" carbon fiber has a modulus of 30,000,000 psi. It about the same as steel. Now the other component that going to determine the stiffness of the composite laminate is the glue. All glues have a poor modulus compared to carbon fiber, typically about 620,000 psi. So the composite stiffness really depends on the ratio of carbon fiber to glue. So for example something that has a lot of glue in it, say 25% glue 75% fiber will have a composite modulus of 5,460,000 psi. Now if I can get some of that glue out say 60% fiber and 40% glue then the modulus of the composite is 17,200,000 psi. So getting the fiber saturated with glue but also getting the glue out is very important. My experiment indicated that on average if you applied no pressure you get around 75% glue. If I put as much pressure as I can then I get the glue down
to about 40%.
So this is why I squeeze the thread or threads(layed side by side with tension to keep the threads straight) between two plates. Squeezing also helps to flatten the fiber out and make a very thin part. The glue spills out the sides and ends. Once it cures then I cut the width I want. Then I glue this part onto the wood spar.
That is what works for me, I hope this helps you.
Cheers,
Kurt
--- On Fri, 11/7/08, Warthodson_at_aol.com <Warthodson_at_aol.com> wrote:
From: Warthodson_at_aol.com <Warthodson_at_aol.com>
Subject: [Indoor_Construction] carbon tow
To: Indoor_Construction_at_yahoogroups.com, freeflightml_at_yahoogroups.com
Date: Friday, November 7, 2008, 2:41 PM
I have some carbon tow that is described as "12K" which I purchased from "CST". It is a 25' long ribbon that is Approx. 1/4" wide & Approx. 0.004" thick.
I cut a piece about 10" long & tried to separate out a few "strands" which I wanted to glue onto a spar to stiffen it. I could not get a 10" long strip, unless I had quite a few (I don't know how to be more specific) "strands" More "strands" than I wanted. I have never worked with carbon tow before. Is this normal? Am I using the correct stuff?
Gary Hodson
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Received on Sat Nov 08 2008 - 08:25:42 CET
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