Planks from carbon tow.

[sigurd]

I am building unidirectional 30 x 3 mm planks from carbon tow. These can be put to many uses, such as daggerboards, masts and beams. I saw it done first at Rob Denney's website. He uses a wetout machine, which wets and squeegees the tow as it is pulled off the roll. I use a different technique. Firstly because I didn't bother making a wetout machine, secondly because I find it difficult to stretch the tows in the mold without having them fixed at one end.

Mold
The mold is made from 3mm plywood. This is probably not airtight so I coat the base plank with epoxy first, sand a bit, then glue two more 3mm strips to it, with a slip angle sanded in. I use 3m 90 spraycan, it's reasonably fast PU like glue.
The mold gets a thin layer of thickened epoxy, spread with gloves then sanded. Then some layers of mold wax, and then a layer of PVA release agent each time it is used.

Cutting tows
The tows are cut, counted and one end of them is slided into a little clamp I made from plywood, with a 3 x 30 mm gap in it. When full, it gets screwed down onto the end of the mold. The other ends I keep in a bucket.

Wetout
I pour some epoxy into the mold, take one tow at the time and stretch them out and squeegee them down into the epoxy with a small piece of towel. From time to time I pour a new strip of epoxy. There are just barely too many tows to fit into the mold, so at the end I can use something flat to squeegee out some epoxy.

The rest
A layer of peel ply, a couple layers grid fabric to absorb excess resin, and the vacuum film. Refigerator pump sits in a tray with water and has a wet cloth over it, otherwise it will surely overheat.
I have a thin surgical hose that is laid over the vacuum film, under towels, and hot water goes through it from the tap. It kicks off the epoxy in short order, otherwise I would be babysitting the pump for 12 hours or so. I don't know whether I am ruining the planks by getting the wet resin too hot so it starts gassing. Input welcome.
I can't find my scales but a 2m plank ate just under 135 milliliters of epoxy. A tad is squeegeed out, the peelply is soaked and there are some wet patches on the grid cloth.

Finally
I have tried some planks without fixing the tow ends, and it just is not very confidence inspiring to feel the tows wiggle about in the mold as one massages them back and forth into the resin. I have doubts that I could produce a dry but soaked plank with reasonably straight fibers this way.
A hand held tow wetout machine would have been nice.
I took some pictures that I will upload shortly.

The end.

[Herman]

I am curious. Not only for the system you developed, but also as one of my customers is a pultrusion factory, which is hard to beat in terms of economy and properties.

[sigurd]

Hi Herman, I have not found as cheap rods as I can make these, I think - except maybe for the fact that the epoxy was shipped to me as dangerous goods, so I ended up paying more for the freight than the stuff itself - the carrier seems to have been a blood sucker.
No doubt will pultruded stuff be even more stretched and probably optimal resin ratio. Perhaps you have an opinion about whether kicking off epoxy (the 285/285 from hexion/r-g) with heat is good or bad?

[Herman]

If the epoxy was packed in more then 5 liter* per container, then indeed it falls under the dangerous goods laws. Below that, and the SENDER should have sent it under "limited quantities". The shipper just invoices based on the info he gets.

* for other chemicals the amounts may vary.

As for curing: postcuring is neccesary with 285/285. Curing with heat is possible, but beware that airbubbles can grow. If you have hardly any air, add some heat to speed up the process. If you suspect some air, let cure at RT, then postcure.

Pultrusion works at temperatures around 180 degrees C.

[sigurd]

Thanks. Does it have to postcure within a specific time, or can it still post cure after months?

[Herman]

It can postcure after months, although a tighter window is recommended. In theory, strength is somewhat better when postcured asap due to longer molecule chains. I would not worry though.

[sigurd]

Ok. Here are the pictures.

[sigurd]

Do you think 100C water ok to use for post cure?
Thinking some sort of natural circulation boiler feeding a long tray or tube.

[Herman]

You want to postcure it submerged? I have no idea of the size of the strip, but put it in an oven! Perhaps even an isolated metal can, with some lightbulbs in them.

[sigurd]

I elongated the mold to 3.6m. I should be able to do what you say.

[sigurd]

How do I test them? I should like to know the compression strength so I can use a reasonable amount in beams and such.
I was a bit vexed when I read that Graphlite rods are 6.5 times stronger in compression (1900 MPa) than wet layup tow. They are expensive, but if it is true, they are cheaper than diy planks per strength.
But can I ask what number you guys plug into your calculations for vacuumed uni carbon in epoxy? Maybe you have a reason to use that number too?

[groper]

Uni directional carbon / epoxy @ 60% Vf =;

Tensile strength = 2000mpa, Compressive Strength = 1300mpa.
Modulus of Elasticity, tension = 130Gpa, Compression = 115Gpa
Poission`s ratio = 0.25u

Source = Hexcel composites.

[sigurd]

That's good numbers then. Thanks.

[dinoa]

It's hard to beat Graphlite using the methods you describe. I see little advantage
in rolling your own seeing Graphlites very high tensile and modulus will allow you to decrease amount used.

Dino

[sigurd]

The Graphlite at Aircraftspruce is 5 times more expensive than the home made planks per Kg.
The pultruded rod of T300 at R-G.de is 4 times weaker in compression than the Graphlite. But the hexcel stuff is not so far off.
I don't know how the Graphlite is tested, and the numbers vary a bit.
The R-G test is DIN EN ISO 604:2003-12
The Hexcel test is ASTM MOD. D695
Are those comparable? If it is, why are the T300 rods so poor compared to the Hexcel tows and Graphlite rods.