Thursday, November 08, 2007

Checking back in with the world

OK, I've been gone for a while. Lots going on, much of which will provide fodder for the blog.

I'll have finished bike pictures of the green and blue bikes very soon, I promise. For today, we'll have something different to see.

What does that look like? If
you said a GAST DOA-V722-AA, you were right. And in that case you must be much smarter than me.

So what you say? Well, I came to the sad realization that my current vacuum pump was too small to do adequate work making wet wrapped vacuum-bagged joints on a bicycle frame.

There are a couple of ways around that obstacle. It's possible to buy a little device which creates a vacuum when hooked up to the hose of a compressor for about $100. Or you can buy a good vacuum pump. In my experience these tend to run from about $500 and up. Or, you can go to trusty eBay and find someone selling a nice industrial quality vacuum pump, used, for reasonable prices.

It took three or for tries, but I purchased this one for $47. It'll draw over 25" of mercury, which should be plenty of pressure for the joints I'm going to do. In fact, for a small object, where the carbon is being formed over Styrofoam forms, this is probably too much pressure. It would tend to distort the form during the cure.

The basics of the process go like this. I'll miter carbon tubes to fit together correctly. Then I'll glue them in place with an epoxy, using the frame jig. This will create a straight carbon frame, but no where near enough strength in the joints to be used as a bicycle.

From here, I will wet out layers of carbon fiber with a high quality epoxy. These layers will be wrapped in various patterns around the joint. Along the way, the carbon fiber will be oriented to use its strength (pulling or stretching) not its weakness (compression or pushing). This could then be allowed to harden - but it won't.

First of all, there is no way to wet out the carbon fiber thoroughly without giving it too much epoxy. This adds excess weight, which isn't what we want to do with carbon fiber. Also, epoxy isn't all that strong on its own. If there are thick gobs and goobers of epoxy between the layers of carbon fiber, its likely to weaken the joint. Finally, as much as we try, there can be voids between the strands of CF where we didn't penetrate with epoxy during the wetting out - these areas will be weak.

This is where the vacuum pump comes in. We'll layer the wrapped joint with a material that will release from the epoxy after everything is set up. There are a variety of alternatives for this. A very smooth material, say like mylar, can give a very finished finish to the CF. That's nice, but no ideal for our case. I'm going to start with Teflon coated polyester cloth.

It just occurs to me that Teflon isn't a great material for the world, so maybe I'm going to have to look for an alternative in the long run.

Getting back on track, this cloth breaths. Behind the cloth will be a layer of batting. It looks sort of like the stuff people put on table tops to look like snow, before they set up a winter scene with figurines. Anyhow, this joint, wrapped in polyester cloth, with batting over that, gets sealed into a large plastic bag. At the back side of the batting (and potentially in more than one spot), a hose (or hoses) are attached to the bag and then to a vacuum pump (this is where the GAST DOA-V722-AA comes in).

Turning the pump on creates a (partial) vacuum inside the bag. Outside the bag, where there is no vacuum, we have the weight of our earth's atmosphere pushing on every side of everything it touches. If our skins were hollow (implying a vacuum inside), they would be squished down to a nasty little lump.

In the case of our bike frame, the carbon tubing provides a strong stiff platform for the vacuum to push against. So, the bag is pushed against the batting, which is pushed against the Teflon fabric, which is pushed against the set joint. In a process not unlike squeezing a little packet of ketchup, the wet epoxy wants to get out of the way of all this pushing.

It gets squeezed up through the layers of carbon fiber, and through the Teflon fabric, into the batting. Generally, the epoxy is too stiff to push through the batting and into the pump - which is a good thing. And, because the batting has enough strength or loft to keep from flattening out, it is home to a vacuum that the epoxy wants to fill.

In this way, the layers of CF are pressed closely together where the epoxy can effectively bind them and allow the CF to be the strength of the joint. Meanwhile, excess epoxy is removed from the joint, helping keep its weight to a minimum. And, with a little luck, most voids will be located somewhere with epoxy behind them. As the epoxy is sucked out to the batting, it will go through these voids, leaving enough epoxy to make sure that these areas of the CF are at full strength.

I seem to have a machinist lined up now to make my lug spigots for the bottom bracket. Hopefully before Christmas, I'll have my first prototype carbon frame with wet-wrapped vacuum-bagged joints. And while the GAST may look mundane, this is why I'm excited that it arrived. And that's all for today.

1 comment:

Chauncey Matthews said...

Hey Rick, I'm not into the carbon thing, but this is fascinating reading- the first time I've seen any info on carbon framebuilding. I'm looking forward to seeing some pics!
btw-there's a guy who posts on the builder's page that says he makes lugs for the carbon frames he does/did. I think he called them "Exxon Gratek" frames- he might be worth talking to.
Thanks for the advice on the prize frames too, the event didn't pan out, but it's good to have your points in ming the next time the opportunity comes around.