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Dec 07, 2019, 06:53 AM
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Carbon fiber vs Glass Fiber


If I use the same gsm glass fiber and carbon fiber for similar foam cores, which will weight more, GF reinforced or CF reinforced?

What I want to understand is which holds more epoxy and what are the advantages offered by both kinds in terms of strength, weight, finish etc..
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Dec 07, 2019, 07:13 AM
supreme being of leisure
ZAGNUT's Avatar
for the same areal weight carbon will need more resin and weigh more in the end but be stiffer and stronger....partly because of the greater strength of carbon and partly because of the greater laminate thickness.

that's the simple answer. it also depends on weave type, especially with glass. there are weaves like 7500 that suck up huge amounts of resin to build thickness quickly and then weaves like 7781 which is very close in weight but designed to use the minimum amount of resin. 7781 is 300g/9oz but because of the weave will print through less than some other cloths a third its weight.....7500 will give an ugly ass finish.
Last edited by ZAGNUT; Dec 07, 2019 at 07:21 AM.
Dec 07, 2019, 02:43 PM
Registered User
Hi ZAGNUT, do you have any information about different fibers requiring different resin ratios? My understanding is that the 60%/40% is a typical layup ratio and down to 35% resin content for prepegs. I am unaware of differing ratios for glass, carbon, or Kevlar. Taking this into consideration they would weigh the same.

You have peaked my curiosity.

Regards

Rick
Dec 07, 2019, 07:48 PM
supreme being of leisure
ZAGNUT's Avatar
resin ratio depends on the weave. non-crimp fabrics like spread tow and stitched fabrics use less resin, regular weaves use more, weaves with twisted yarns use even more. the 7781 i mentioned is a very tight satin weave which falls somewhere between a regular weave and the non crimps. the 7500 has fat twisted yarns with big giant gaps between them...in a hand layup you need really thin resin with the 7781 or it will be really difficult or even impossible to wet out while with the 7500 too thin of resin won't stay in the big gaps of the weave.

and then there is the difference in density: kevlar is lightest, carbon a bit heavier and glass the heaviest by a big margin. so for identical weaves and weights of fabric kevlar will need the most resin and be heaviest, carbon a bit lighter and glass quite a bit lighter than both
Dec 08, 2019, 11:20 AM
Registered User
So your talking about filling the weave with excess epoxy which only adds weight not strength. I guess I'm looking at the question differently. If you have a 6 ounce glass, carbon, or Kevlar fabric with a 40% resin ratio they will all weigh 10 ounces.
I'm trying to understand how this ratio for example with prepegs seems to be 35% or so and I'm not aware that different materials require different ratios.

I'm asking if the different materials require a different ratio from a structural perspective, since we're discussing structural fabrics. I'm not asking about filling open weaves with excess resin to make them pretty. I do understand some weaves need more resin than others to make a nice finish for our models, but do the different types of fiber need a different ratio of resin because of their physical properties?

Zag, I edited my post and deleted some while you replied, your fast on the keys today
Last edited by Air Head; Dec 08, 2019 at 11:54 AM.
Dec 08, 2019, 11:47 AM
supreme being of leisure
ZAGNUT's Avatar
Quote:
Originally Posted by Air Head
So your talking about filling the weave with excess epoxy which only adds weight not strength. I guess I'm looking at the question differently. If you have a 6 ounce glass, carbon, or Kevlar fabric with a 40% resin ratio they will all weigh 10 ounces.
no...because of the different densities each fabric will have a different volume even though they all weigh the same per given area. so if you use the same resin ratio by volume glass will be lightest, carbon stiffest and kevlar toughest...

Quote:
I'm trying to understand how this ratio for example with prepegs seems to be 35% or so and I'm not aware that different materials require different ratios.
prepreg isn't magic. you can get the same ratio with really good bagging or infusion....and infusion isn't magic either, crappy technique can result in way too much resin

Quote:
I think it's fair to say that carbon is stronger and lighter than glass, not stronger but heavier, is it not?
its only lighter if you use less of it...simply replacing 6oz glass with 6oz carbon will add weight.
Quote:
If I got this wrong I'll throw away my 2.5 ounce 1k tightly woven carbon fiber fabric that I paid 50 bucks a yard for, and use 2.5 ounce glass that cost ten times less.
only if the 2.5oz glass will still give you the strength and stiffness you need...if you have to go up to a 4oz glass for that then the carbon starts making sense
Dec 08, 2019, 12:02 PM
Registered User
I erased some of my post while you replied.

So are you saying I can't use a 40% ratio for glass and carbon, the different materials require different ratios? I guess this is where I'm getting hung up because I did not understand this to be the case. Your mentioning volume and I only weigh it and mix up resin based on that.
Dec 08, 2019, 12:14 PM
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Thread OP
Thanks, Zagnut. Your information is really helpful and thanks to u too, Airhead for extending the discussion.

Cheers.
Dec 08, 2019, 12:23 PM
supreme being of leisure
ZAGNUT's Avatar
by volume all work with the same ratios....by weight its all screwy.

play around with the laminate caculator on rg composites site. its pretty close to reality and can give a good idea of how different reinforcements behave
Dec 08, 2019, 01:54 PM
Registered User
Thanks for that, I'm learning something
Dec 14, 2019, 02:29 PM
Dark Side of the Red Merle
Curtis Suter's Avatar
This is something Adam P. sent me in 2011:

Kevlar doesn't soak up any more resin than any other fiber does. People get confused because they think that 1.7 oz Kevlar and 1.7 ounce glass contains the same amount of fiber. The 1.7 ounce Kevlar when saturated will always be heavier than 1.7 ounce glass. Why? The specific density of Kevlar is 1.44 grams/cc while the density of glass is 2.55 g/cc. A 1.7 ounce Kevlar contains a higher volume of fiber than a 1.7 ounce glass. Those extra fibers require more resin to have the same saturation level as 1.7 ounce glass. The 1.7 ounce Kevlar will be thicker than the 1.7 ounce glass. More fiber=more thickness. Most people always think of fiber:resin in terms of weight, but it really helps to think of it in terms of volume of fiber and volume of resin. When laminates are tested for building specs. the fiber:resin ratio is typically communicated in terms of volume ratio. This tells you how well the fibers are saturated regardless of the fiber or resin used. The fiber:resin ratio by weight doesn't give you a good comparison between fibers. If you wanted a 50:50 ratio by volume (equal saturation) you would do glass at 70:30 by weight, carbon at 62:38 by weight, and kevlar at 57:43 by weight.
Dec 14, 2019, 08:39 PM
Balsa breaks better
Thermaler's Avatar
Quote:
Originally Posted by Curtis Suter
This is something Adam P. sent me in 2011:

Kevlar doesn't soak up any more resin than any other fiber does. People get confused because they think that 1.7 oz Kevlar and 1.7 ounce glass contains the same amount of fiber. The 1.7 ounce Kevlar when saturated will always be heavier than 1.7 ounce glass. Why? The specific density of Kevlar is 1.44 grams/cc while the density of glass is 2.55 g/cc. A 1.7 ounce Kevlar contains a higher volume of fiber than a 1.7 ounce glass. Those extra fibers require more resin to have the same saturation level as 1.7 ounce glass. The 1.7 ounce Kevlar will be thicker than the 1.7 ounce glass. More fiber=more thickness. Most people always think of fiber:resin in terms of weight, but it really helps to think of it in terms of volume of fiber and volume of resin. When laminates are tested for building specs. the fiber:resin ratio is typically communicated in terms of volume ratio. This tells you how well the fibers are saturated regardless of the fiber or resin used. The fiber:resin ratio by weight doesn't give you a good comparison between fibers. If you wanted a 50:50 ratio by volume (equal saturation) you would do glass at 70:30 by weight, carbon at 62:38 by weight, and kevlar at 57:43 by weight.
Thank You Curtis, the info I had misplaced somewhere in the 20 + terabytes of info I have collected over the years.


Been searching for it about a week now . . .


Joe


Balsa Breaks Better
Woodies Forever
Dec 15, 2019, 08:42 AM
Everything is broken
JimZinVT's Avatar
Quote:
Originally Posted by Curtis Suter
(quoting Adam)....If you wanted a 50:50 ratio by volume (equal saturation) you would do glass at 70:30 by weight, carbon at 62:38 by weight, and kevlar at 57:43 by weight.
I'm assuming those ratios are fabric : resin?
I don't think I could get a glass layup fully wetted out with only 30% resin. My usual practice is to weigh all the fabric and mix an equal weight of epoxy. There is always a little leftover when done wetting out, and a little more pulled out by vac. bagging. So I'm somewhere south of 50:50, but definitely not close to 70:30.
Dec 15, 2019, 09:10 AM
Dark Side of the Red Merle
Curtis Suter's Avatar
Me either Jim, I've never try to get such an exacting amount of mix. When I have if it's too light of a mixture I have a failed part but I've never had a failed part being slightly too wet and debulking.

Curtis


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