A question for someone who works with carbon fiber...

"BaconSandwich is tasty." (baconsandwich) 03/31/2014 at 10:47 • Filed to: None

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I've been reading a bunch lately about how to work with carbon fiber and fiberglass. Working with it (especially carbon fiber) seems like quite an involved process, but still something that I'd eventually like to try out. I understand the general process, but there's something I'm still curious about.

For doing cosmetic pieces, it seems like people often don't bother with the vacuum bagging. From my limited understanding, the vacuum bagging helps draw out any bubbles that may have been trapped in the resin. Bubbles in the resin makes a part weaker where the bubbles are. For this reason it makes perfect sense to use a vacuum to draw out any bubbles. I've also seen it done where they use the vacuum to draw the resin through the piece (like this:

). This seems a bit more complicated, and it seems like there's a lot more potential for things to go wrong (i.e.: resin not being drawn through all the parts).

Wouldn't it make more sense just to layer the resin and cloth by hand, then throw the whole part into a vacuum chamber? I mean, you'd need one big enough to fit the part (which could be a problem if you are doing things the size of car hoods), but it seems like there'd be a lot less messing around, and less risk of a botched job. I'm afraid that when I do eventually try it, that I'll get a leak in the vacuum bag, like these guys did:

In talking this over with my wife, she did have one valid point: perhaps the vacuum bag helps pull the different layers of fabric together tighter. If you were to use just a vacuum chamber, perhaps it would result in the same tight package?

Anyone have any thoughts here?

Grindintosecond > BaconSandwich is tasty. 03/31/2014 at 11:17

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The only thing a vaccuum chamber does is make a vaccum. You need vacuum on one side of a material and pressure on the other side. in the vacuum bag there's absence of pressure inside and atmospheric pressure on the outside of it squishing the material against the mold. In a bag process you only have about 30psi acting on the layers (that is the standard air pressure you breathe). In a chamber you have the chance to increase pressure inside above 30psi and extract the vacuum out of the bag so you can have as much pressure as you want as long as the chamber can support it...that gets expensive and thats how they make race cars and plane parts...in the "autoclave"

Bicycles have tiny small aluminum bricks that have the shape of the junctions inside and a laytex baloon in the shape of that piece they stick pre-cut patterned carbon shapes from and layer it all up and then bolt it together inside the brick...then inflate that baloon to 250psi and heat the brick to 250 deg.....you get super compressed layers and zero bubbles from the heat making the resin really runny...like hotglue.

Axel-Ripper > BaconSandwich is tasty. 03/31/2014 at 11:24

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Vacuum bags will pull excess air and resin out quite well. To use an Autoclave, you first vacuum bag it, then use that to put more pressure on the outside of your vacuum bag, basically creating more of a vacuum inside the bag. This pulls even more excess resin out and keeps the layers very tight together, which is great for structural use, but usually overkill for cosmetic applications. Cosmetic stuff doesn't care about strength most of the time, so a good layer of prepreg or degassed resin is fine for the outer layer.

Aaron M - MasoFiST > BaconSandwich is tasty. 03/31/2014 at 11:24

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The big difference between vacuum bagging and not is weight. You need a certain amount of resin to cure the entire piece, and what the bag does is not only distribute the resin more evenly, it also pulls excess resin out and away from the piece.

I did a lot of carbon fiber work in college for nerdy extracurriculars ( http://www.andrew.cmu.edu/user/buggy/Swe… ), and stiffness and weight were our key considerations. It's also worth keeping in mind that for cosmetic pieces, loose fabric and resin will probably look ugly, especially if there are complex curves. Most cosmetic pieces (actually most high-volume pieces period) are made with pre-impregnated fiber (or pre-preg) which keeps the weave consistent during handling and curing.

BaconSandwich is tasty. > Grindintosecond 03/31/2014 at 11:30

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So the reason for using the vacuum bag is then to apply pressure to the layers of fabric, not just to draw out bubbles in the resin? Interesting. I wonder how much of a difference it makes.

That's a pretty nifty video. Carbon fiber bikes still scare me a bit, given that when things fail, they fail catastrophically.

I find it quite interesting that Konigseig uses pre-preg fabric.

Racescort666 > Grindintosecond 03/31/2014 at 11:44

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You're on the right track. One small correction: atmospheric pressure is 14.7 psia at sea level.

BaconSandwich is tasty. > Axel-Ripper 03/31/2014 at 11:46

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Is the structural difference between vacuum bagged parts and non-vacuum bagged parts really that significant, or is it mostly for saving weight by drawing off excess resin?

BaconSandwich is tasty. > Aaron M - MasoFiST 03/31/2014 at 11:46

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Good to know! I suppose vacuum bagging really makes sense when you start working with honeycomb, as I imagine doing a hand-layup of a honeycomb piece without vacuum bagging would result in a lot of resin in the honeycomb layer.

I figured to start with, I'd like to try making a fiberglass roof-top carrier for my car. If that goes well, then maybe I'll go on to try something a bit larger and more complicated, like a canoe. Eventually I'd like to try something with carbon fiber, but the principles between it and fiberglass seem fairly similar, so I might as well get my feet figuratively wet with fiberglass first.

Axel-Ripper > BaconSandwich is tasty. 03/31/2014 at 11:59

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If you don't vacuum bag it (Just plain wet layup) you can have some issues with the carbon/resin not settling into all of the corners, especially if there are tight ones, and you can have some issues with bubbles appearing on the surface. Structurally they will be heavier because of the excess resin, and I do believe that the strength can be effected by excess resin as well. I suggest reading Competition Car Composites. Its a good intro to composite layup.

BaconSandwich is tasty. > Axel-Ripper 03/31/2014 at 12:13

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Good to know, and thanks for the book reference. I still like the idea of vacuuming a piece, I'd think that tossing it in a vacuum chamber would be a lot easier than messing around with bags.

I've got another book ordered from the library as well. The more information the better!

Aaron M - MasoFiST > BaconSandwich is tasty. 03/31/2014 at 12:21

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We worked a lot with honeycomb and it was a royal pain...honeycomb is much more resistant to curving in more than one direction than fabric is, so getting the structure we wanted required a lot of planning.

We started with fiberglass as well, as it is cheaper and similarly easy to work with. As far as complicated pieces, it's important to plan your layup, and figure out how to get the smoothest coverage with the least amount of material. You're almost certainly going to want to paint the end result anyway (both because it won't be as pretty as professional auto pieces and because of UV damage). In the end, though, as long as you can make a good mold you should be able to lay up most anything.

Axel-Ripper > BaconSandwich is tasty. 03/31/2014 at 13:37

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Well putting it in a vacuum chamber really wouldn't do a whole lot other than pull the air bubbles out. With bagging, the force of the bag due to the vacuum is what pulls everything tighter.

Newsboy > BaconSandwich is tasty. 03/31/2014 at 15:57

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A vacuum alone won't remove the air bubbles from the resin, or from between the layers of carbon weave. As a result, you'll have voids in the structure that will reduce strength pretty dramatically. Voids are where fractures start, the more voids, the more likely a part is to fracture. Vacuum bagging or using a mold and a compression bladder compacts the layers, reducing the number of voids, even moreso in an autoclave.

From an academic paper on the subject:

"The fatigue test results and the proposed theory indicate that voids have a strong detrimental effect on the fatigue life of composite structures if the void content is above a critical value."

http://www.ewp.rpi.edu/hartford/~mcph…

BaconSandwich is tasty. > Newsboy 03/31/2014 at 16:24

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I'm a bit surprised that a vacuum alone isn't enough to remove voids. I suppose it depends partially on the viscosity of the resin being used, the number of layers, and how tightly the layers were sandwiched during manufacturing. Nice link to the paper. I was digging around the other day trying to find academic papers on using just vacuum chambers, but couldn't find much.

Newsboy > BaconSandwich is tasty. 03/31/2014 at 16:39

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As someone else mentioned, the differential pressure created by using an autoclave and a vacuum bag is more effective than a vacuum alone.

Related: if you follow bicycles at all you'll know that Trek calls their carbon frame construction technique OCLV: Optimum Compaction Low Void.

I.E.: Stronger and lighter.

Grindintosecond > Racescort666 03/31/2014 at 23:09

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or 29.92 in.Hg.....close enough to 30" manifold pressure. got my descriptors wrong

samssun > Grindintosecond 04/04/2014 at 21:12

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1 atmosphere is more like 14.7 psi