The Wonders of Carbon Fiber

Far a long time, I have harbored the burning desire to build my own mountain biking helmet light.  Commercial lights are very expensive, and are always behind the curve when it comes to the best,  brightest, and most efficient LEDs and drivers.  They often have sub-optimal thermal management, which further reduces the efficiency of the LEDs, and their beam patterns don't always seem the most sensible (which may admittedly be a function of my eyes and riding style... can't please everyone).

Well, I am now embarking on my adventure!

The core of any light is, obviously, the light source.  I picked up some Cree XM-L LEDs that are rated at about 850 lumens at 2800 mA, which is very bright.  Advertised 1000 lm at 3A, but the response curve from the manufacturer doesn't match that claim.  Whatever, they're freakin' bright.  They're on copper stars, which is great because I can solder them to a copper heatsink to maximize heat dissipation while in operation.  Getting those LEDs was quite the ordeal, due entirely to my inattention the the address that I had logged with PayPal... my old Houston one.  So, the order shipped to Houston.  I mailed postage to the address in the hopes that the kind person living there would ship it to me rather than toss it.  It seems that it could not be delivered, so was returned to the shipper and I got another chance to get my address right.  Which I did, and finally (2 months?) got my LEDs.  They went from Hong Kong to Houston to Hong Kong to Tucson.  Fun times.

I decided that I wanted to make the body structure as light and strong as possible, mainly to offset the added weight of the copper heatsink.  Copper is awesome at distributing heat, but is a lot denser than aluminum... it's a trade-off.  I decided on ensuring proper heat dissipation over weight savings.  So, that means that I need something really lightweight and strong for the body.

Enter:  Carbon Fiber!  Sweet!  It's strong, lightweight, and stiff.  The downside is that it is generally sold in flat sheets, which are (due to the stiffness) not real bendable.  That means that I have to learn how to form carbon fiber fabric into carbon fiber composite by the judicious use of scissors, a form,  and epoxy.

After a lot of research online, and on particular site where the guy made his road bike frame himself, I settled on the following materials:  5.5oz 3x4 harness weave satin carbon fiber fabric (it's a neat pattern of cross-bundles of carbon fiber (called tows) that's looser and more conformable to complex shapes than a standard 2x2 twill), West Systems 105 resin and 207 hardener (makes a clear rather than a slightly translucent epoxy), and a kick-ass high tech toughening agent.

My first layup was 3 layers wrapped around a wooden form.  I then wrapped that in sticky-side out electrical tape, as it was mentioned that the epoxy doesn't stick to the vinyl.  This served to compress the carbon fibers together and squeeze out excess resin.  The optimal ratio is about 50:50 by weight, which is about what you get if you squeeze out all the resin except what fits between the fibers in the fabric.  I let it set up for ten hours, and discovered that my clever plan to simply slide the composite off the mold was thwarted by mold irregularities and the extreme stiffness of the piece.  I ended up cutting the mold/composite into sections and knocking the shorter bits of wood out of the composite hoops.

At the top right you can see two hoops epoxied together.  The top has been coated with more epoxy for a glossy coat, and the insides are the texture of saran wrap.  The outer fabric weave looks wavy because... it is.  Thus, a lesson in how important it is to be careful of the weave to avoid distortion.

Bottom right shows the cross-section, and the consequence of more pressure at the curve than the flat, which is exactly what you'd expect if you wrapped elastic tape around a solid wood block.  It is not, however, what I wanted.

So, a little more research lead me to the concept of vacuum-bagging, wherein a composite piece is sealed into an elastic bag and then subjected to vacuum throughout the cure process.  This has the effect of applying even pressure on all surfaces of the piece and avoiding the elastic tape effect.

Vacuum-bagging will be the subject of my next post.