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Discussion Starter · #1 ·
Made my first carbon fiber brake lever today. I made a fibreglass mold from the original lever and then laid up the carbon fiber in the mold. Vacuum bagged and cured at about 50C. The lever blade has 9 layers of alternating woven and unidirectional cloth. It seems extremely strong and only exhibits a tenth of the deflection of the original aluminum lever under a 5kg load. Process was not difficult or particularly involved. I'll probably knock out a few more spare levers. It's got me fired up to make some more things out of carbon fiber.

On the tech side, I used West Systems 105 epoxy resin and the 205 fast hardener. I used both multiple coats of mold release wax and then a final coat of PVA which I allowed to dry before doing the lay-up.

I'd encourage anyone who hasn't tried CF yet to give it a go.

Fresh from the mold:


Test assembly:


Some more of my dribblings on the subject here
 

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Discussion Starter · #3 ·
febikes said:
Make sure you wear all the recommended protection.
An excellent point. I'd highly recommend a proper face mask with the appropriate chemical filter cartridges for working with the epoxy fumes and against any stray fibers. Wet sanding under running water is also a good idea to get rid of the dust.
 

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Discussion Starter · #5 ·
Thanks guys. After doing some more reading up I decided to finish the lever in a 2 part gloss polyurethane protective coat:



It certainly makes the CF weave pattern stand out. It also shows every slight little imperfection. You can see on the lever tip that the fibres of the first weave layer look "disorganised" where they were forced into the tighter shape of the mold.
 

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Very nice.

I see in your linked "dribblings" that the weight came out about the same as the original Al lever. Did you weigh the epoxy and cloth going in to the mould? Do you know what the resin/fibre ratio was?

Oldbikerider.
 

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Discussion Starter · #7 ·
Good question. I didn't think to do that, but sorting of working backwards:

The 25mm carbon weave ribbon weighs 6.92g/m. The unidirectional carbon ribbon weighs 8.66g/m. In total I used 5 x 14cm of weave and 4 x 14cm of uni ribbon. So a total of 9.7g of carbon fibre. There was not too much excess trimmed, but I didn't weigh the off-cuts. Assuming 30% was trimmed then there is about 6-7g of carbon fibre in the lever and 1-2g of resin.

I don't know if that ratio of material is good or bad, but the vacuum bagging certainly forces almost every last bit of resin out. No air bubbles or pockets of resin are visible.
 

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If those weights are real, then you've got a pretty dry laminate, to the point where I'd be worried about dry spots. It's usually hard to achieve even a 1:1 resin to fibre ratio with a wet layup like that, and the best strength to weight occurs somewhere near 1:1. Maybe you trimmed off more than you think?

Oldbikerider.
 

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Discussion Starter · #9 ·
I really worked the resin through manually when laying the fibres so I'm pretty sure it was wet-out properly. My math on the CF weight is probably out. Trimming involved a 3-5mm border all around the lever and a fair amount of internal material was removed at the pivot and actuator bolt.

I'll definitely aim to make some more accurate measurements for the next set.

What is the best way to do that? Weigh things at each step of the process?
 

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Pre weighing everything and precisely wetting the fabric out is one approach, but with wet layups, particularly with small parts like that it's better to use excess epoxy and screed or blot it back to near 1:1 (weighing to confirm).

When doing components like that I lay all the dry fabric out on a clean HDPE lined bench top, apply the resin with a plastic scraper, work it in to the fabric, then place layers of paper towels on top and roll over with a hard roller to blot up all the excess resin. Then lift up the pieces using the end of a small hobby knife or tweezers and place them in the mold one at a time. Sometimes it helps to paint the mold surface with a thin layer of epoxy first to make sure it's wetted out all over the surface - this helps to avoid air bubbles or overly dry spots against the mold.

For larger sheets of fabric, I use the same procedure, but place the over wet fabric in the vacuum bag with layers of paper towels for the drying out step.

Unlike glass, it's actually really hard to remove too much epoxy from carbon cloth, so the above techniques are pretty safe in terms of getting down to optimum resin ratio and not going too far, but if you're not chasing every last gram a bit of excess epoxy will make the layers a bit easier to get in the mold and consolidate against each other, so don't overblot the fabrics.

If the part has sharp external corners it helps to make up a "gelcoat" using colloidal silica and black tinter in some epoxy. This is filleted (using a gloved finger) into the sharp internal corners in the mold, allowed to harden slightly, then a new batch of resin is mixed for wetting out the fabrics. When you get this right, it's very hard to see where you've cheated with the gelcoat, and it also nicely hides areas like you pictured with the weave breaking apart. Many production frames use this technique around difficult areas.

Oldbikerider.
 
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