Lets talk about Carbon!

General Composites Discussions Products Talk
#1

Howdy Y’all, Ive been lurking around in the shadows for awhile now. I’d like to thank you all for all the information on this forum. Hopefully one day I can be as helpful to someone else. With that being said I figure it’s time to get out there and ask a few questions.

So Carbon Fiber composites are only as strong as the sum of is materials. Starting with Carbon Fiber filaments what separates the average carbon from quality carbon or high strength Carbon? Anything else other than the amount of carbon contained in that particular filament? How to tell the difference between good and poor quality? Who makes the highest quality?
Any help is greatly appreciated.

6K6K6

#2

Don’t know all the answers to your questions, but one characteristic to measure carbon quality (strength) is its tensile strength. Cheap ones can start at 300-400 Kpsi (1000 pounds per square inch), aerospace probably starts at around 600 Kpsi, and I know Toray makes a ‘T800’ line that is an 800 Kpsi fiber. I’m sure they go even higher than that as well in some exotic types.

#3

Starting with the carbon filaments, that is where it all is. There are several types, which as Craj said, indicate the strength per unit. It’s a normal standard that companies go by. The most common modulus is 200, 300, 700, and 800. Incicated by some companies by T300, T700, or IM3, IM7. They are higher and lower as well. Low modulus like 100, or high modulus like 1000.

There is no way to check to see what is good or bad, until you design your part, and know what strength it needs to be. That is where many people get stuck on when designing parts. Composites is a fully engineer designed material, meaning you need to know BEFORE hand what you need. Or, you can build many parts, many ways and test each one.

Now, depending on where you purchase your fibers, might be an issue. Most places will have certificates on the fabric, stating what it is. It’s their business to give you what you want. There are many companies that don’t care, and will sell you crap made IM3 and call it IM8. Or parts, where you can never find out what fiber it really is. They are out to make money only. Go with known distributers, and not cheap over-seas discount houses.

#4

I am overseas, from your point of view…

Anyhow, a supplier of woven or knitted material should be able to tell you the exact fiber that is used. For 5.7oz/ft2 this usually is a T300 or equivalent, for multiaxials usually a T700, or a T600.

-know before you buy
-define what you need
-define what is available

#5

Ok tensile strength of the filament is its main attribute.
Ultimately the stronger each individual filament is the thinner and stronger the finished product can be.
Does color play a small role in being able to distinguish between quality?

Alright so Im gonna be at the mercy of the carbon distributor,
They grade the product by the modulus of carbon contained in each filament. But what is it that distinguishes low modulus 100kpsi carbon between high modulus 1000kpsi carbon? Cook time, chemical wash, pre cursor used to create the carbon its self?
Absolutely no way to tell quality? Color, texture, other then tensile strength? Any reputable distributors or manufacturers here in Texas or else where in U.S?

Any difference between woven or knitted fabrics?

Thank Y’all
6K6K6

#6

What about the number of filaments? Up to how many k`s can the fabric be used for infusion?

#7

What a coincidence - I was just talking about this subject (quality of media) the other day.

This is the item in question: http://www.aliexpress.com/store/product/Carbon-Fiber-Twill-Weave-Fabric-Free-Shipping/401202_444222239.html

The first thing that stood out to me is that the weave is not even! Looking at the horizontal sections in that photo you can see that it looks a bit random, thin sections, thicker sections… not really a good look? Which is the first problem, because most people want their CF to look good.

The other thing I wondered is if the physical properties (strength etc.) would still be good? Because the weave is not even, there may be different properties throughout the same piece of fabric (and also differences in your finished part)?

I have emailed them asking for a data sheet and asking about the weave, wait and see what (if) they respond…
Seems to me like it’s a batch of seconds/rejects?

What are your thoughts on a product like this? Probably suitable for use in non-structural items, as an intermediate layer (not as a surface layer, because of the looks)?

Then again, when I look at the photos I would almost prefer to buy from my local supplier who I know has good stuff. The price isn’t all that much unless I was buying a lot of it. (Even then my local supplier would probably give a volume discount.)

#8

I would not consider using the crap shown.I am sorry, but that really looks bad. Not good for structural work (what is in it) or visual stuff.

#9

HA…aliexpress. Anyway…

Carbon fiber is black, all carbon is black. Any inconsistencies in pictures will be due to the camera, and lighting. If it is a REALLY off color, chances are, it isn’t even carbon fiber, but a dyed fiberglass/kevlar.

As per how many K in tows are good for infusion, again, not easy to answer. It depends on teh weave. A 24k uni, or flat woven tape (ie: spreadtow), is hard to infuse due to the lack of channels between the tows. However, a 24k roving (thick as hell tow), can infuse easier due to the spacing between tows. However, then you get into issues like: Spread tow only has a very thin distance to get resin INTO the “tow”, while a thick roving tow needs a lot of time, or thin resin to get into the center of the tow to wet it out.

Manufactures that are good, are all over the globe. They produce the carbon in specfic sized tows/yarns/tapes, on rolls, and then that is sent off to someone to weave. Sometimes they weave as well. There are 1000s of weavers in the world.

The strength of carbon is what it is. Plain and simple. It is what makes carbon carbon, or E-glass E-glass, or S-2 S-2 etc etc. However, that modulus and strength numbers are broken down. Then it can be used to design specific parts mathematically. Say you have… 50kpsi strength for a given fiber can relate into the cross sectional part of your part. Now, change the part from a unidirectional cross section, to a woven fabric, same cross section. Your strength will more or less be cut in half, because not all your fibers are in the same direction. See, this is where is gets complicated.

Oh, then there is bending, and what resin you use. A VER is different than an epoxy, is different than a thermoplastic.

#10

Yeah herman that was pretty much my initial reaction as well!
Thought I would give them the benefit of the doubt though… looks like my initial instincts were correct.

I did get a fairly quick response - but no data sheet is available, in fact no info is available. You can get a sample though. Haha. Most random mystery carbon I ever saw…

#11



How do you think this could be achieved?
I can only assume that higher modulus Carbon is Darker and less glossy then low modulus carbon and or dyed fabrics.

When faced with the problems of not fully saturating the fabric whether it be by not enough channels throughout the uni or not penetrating through the tow thoroughly in high tow applications such as 12k or 24k,
Would higher vacuum pressure help solve this? Or
Just better off to redesign the part with resin channels towards the problem areas? Or
Redesign the part all together with proper materials?
For the sake of time and money.

Again another assumption but wouldn’t a company that manufactures its Carbon from pre cursor to fabric have a better sense of quality versus A company who just weaves the Carbon into a fabric?
But then again wouldn’t the weavers have a better control of how tight the fabric is for infusion or wet layup?

I take it its hard enough to find good fabric, but what about sleeves or socks?


Found this data sheet, no one else offers data like this though?

Vinyl esters for tooling and compatibility with epoxy?
Any recommendations on epoxy resins for automotive?
Industrial applications like I beams?
Commercial table top epoxies for restaurants?
Best matrix for bending as well?
There has to ba a company out there based on epoxies of various applications.

#12

There are multitudes of different types, tow sizes, and uses for carbon, but I will try and put everything into a few “short” paragraphs (used to work for Toray, now with a weaver/stitchbonder).

Carbon fiber (for use in composites) these days consist mainly of PAN (polyacrylonitrile) or pitch based fibers. There are others such as Rayon based and lignin based, but those are either outdated or in various stages of development. PAN based are the majority of fibers you see in the market and are generally classed as high strength/standard modulus, intermediate modulus, & high modulus. Pitch based fibers are more specialized with the low modulus versions mainly used in thermal management applications, and the ultra high modulus stuff is typically used in high end sporting goods and space applications.

Most of what is seen in the market place is the “high strength/standard modulus” carbon. This includes T300, AS4, T700S, STS40, Panex 35, etc. These fibers are around 95% carbon (the rest being remnence of the PAN precursor) and are generally a combination of cystalline and amorphous-like carbon molecular structure. The crystalline part generally gives the fiber its modulus while the amorphis part gives it its strength. All fibers in this category have filament sizes around 7 microns in diameter, but their cross section shape can vary (T300 is more kidney-bean shaped, T700S is round, some have striations, etc.). Quality for these fibers isn’t necessarily based on their properties; T300 was developed in the 1970’s and is used in many spec’ed in aerospace/defense type applications (i.e. the fiber manufacturer can’t change any of its properties or how its made). T700S was designed for better tensile strength and elongation, mainly for pressure vessel applications and other industrial apps. Both are high quality fibers, but have different strengths.

The intermediate modulus fibers are what’s typically used in primary structure aerospace applications (Boeing 787 fuselage) and include fiber such as IM7, T650, T800S/T800H. Unless you are in high end aerospace, military, or pressure vessel apps, you typically won’t run into these fibers as their cost is about double that of standard modulus (roughly). Filament diameter is reduced to 5 microns which makes them even harder to infuse than the standard modulus versions. Carbon content is again in the 95% range.

High modulus carbon is made on a low volume basis and is typically only used in very high end applications (space craft, high end bikes, etc.). These fibers are “graphitized” so their carbon content is around 99% and are sometimes called graphite fiber. Modului are high, but strength is reduced due to the higher degree of crystallization. Cost for these type fibers can run into the $1000/lb range…and that’s before its prepregged or put into fabric.

To address fiber quality again, the higher quality fibers have lower variation or properties (strength, modulus, yield, etc.) and are processed (woven, filament wound, etc.) with very little fuzzing/fiber breakage. Quality can also be address by how well the fiber pays off the package (bobbin), or how well the fiber bonds to the given resin (more dealing with the surface treament and sizing).

All carbon fiber is black…all of it is black no matter what grade it is. If you see colored carbon it either isn’t carbon, or its the resin that is colored. Epoxies work best (as that is typically what the sizings are based on), but you can use carbon with high-end full vinyl esters with some good results.

All in all you will most likely use standard modulus carbon. The small tow (3K, 6K) fabrics are great for cosmetics but can be expensive if you are actually using it for structural applications. 12K, 24K, and 48K tow sizes are typically used outside of aerospace apps when people want the performance and don’t care so much if its pretty.

So much for my nutshell…

#13

1: those pictures are either a dyed glass/PET/kevlar, OR can be a coated carbon fiber. If it is (I’ve seen metal coated holographic CF), it will be ULTRA expensive, or VERY hard to work with (painted fabric is like using an already resin infused part!!)
2: Vacuum pressure is a log scale. Most roughing pumps people use in composites that get 29"hg, are not that worse than a turbo-molecular pump, a chip manufacture might use, IN TERMS OF RESIN. The main driver of a better vacuum pump in composites, is just size, and reliability (when you are dealing with high-vac pumps anyway… 28-29.95")
3: on that note, you can use positive pressure on the vacuum bag, such as an autoclave. This compresses the fiber stack together more, and pushes the resin into the fibers, AND consolidates the layers together. Claves are expensive, and running them is expensive. There is work on making VARTM and prepregs better out of the autoclave, to get less voids in your parts.
4: For RTM and VARTM, you can design the resin flow channels to deal with your part geometry, and fabric layup. You can control it with using several feeds/vents, or specific flow media (distro media) patterns. This, along with resin input flow, can make a part as void free as possible, and allow it to penetrate all the tows, without causing issues. It just takes time.

It comes down to, what do you need. A company will produce several types of carbon, and in different sized tows. From that point, they can customize ANYTHING else…weave density, weave type, which size tow to use, etc. But as for quality, the company will want to produce the best thing they can, within the standard specs of that fiber type. Weaving just makes it useable in the real world, and quality isn’t much of an issue. Unless you suck at weaving, but you can see that. You can see any variations of the weave, or gaps, or breaks, or fuzzes. At this point, it falls onto the weaver to produce a product that people WANT to buy. Suck at weaving, your business will fail. As a business in china might not care, because they are just another pea in the bucket, they don’t care, because people buy it blindly because it is cheaper…then the end customer gets screwed.

Not good with resins, but any table top stuff should be FDA approved epoxy. There are tabletop resins out there…see who has it, and see if there are reviews.

I beams? Yes?

For resins that need to bend, the main ones are urethanes (thermo set, and plastic), thermo plastics, and I think some PER/VERs that are thin. Epoxy can bend, but only when VERY thin parts. I have made living hinges with ionomers, PP, PE, and poly-urea resins. Some are easy to use, some are hard. Fabric choices should include vectran, kevlar, thermoplastics. MAYBE carbon.

#14

Hi! New to these forums. Late to the conversation. I work with one of the major carbon fiber manufacturers and some weavers. Just wanted to comment that tgundberg is pretty spot on.

I wanted to add my two bits on the quality of the filament. Aerospace requires very tight range of variation in its materials. All manufacturers generate off-spec carbon fiber when the fiber is outside of this tolerance, whether it is lower or higher than spec. These fibers can still be “good” quality just outside aerospace spec. The fibers may actually have higher tensile strength than spec. As for fuzzing, while it’s true that fuzzing indicates fiber breakage, some of this is influenced by the amount of sizing on the fiber. Some applications call for higher amounts, some lower amounts. The same fiber with lower sizing may fuzz more. There are even applications that require no sizing.

For any fabric manufacturer, they should be able to provide the COA (or some variation) for the fiber they used to make the fabric. Whether or not they will is another question.

#15

Hey Chrisk! Welcome to Composites Central. It’s awesome if you can stick around for questions like this! If you have intimate details about manufacturing and weaving, I’m sure you will come in handy quite often!

#16

I concur. Tgundberg and Chrisk thank you for the manufacturer details. It really helps the education of which cloths to choose and why. There seems to be so much secrecy surrounding so much of that.

I glean what I can as I speak with my distributors and weaver contacts but often times I might still be slightly confused.

#17

Thanks, guys, for welcoming me. Glad to be here!

I am more involved with the production side of these materials but not terribly familiar with the hands-on uses. Though I get to see how these materials are manufactured and how they are put into use (SMC, layups, weaving, filament winding, pultrusions, etc.), my hands-on knowledge is limited. That is one of the reasons I wanted to join this forum to get a better understanding.

Unfortunately, I do not have a “shop” to work in to get familiar with composite work. I wish there were some sort of open shops that allow individuals to come in to work on projects. I would love to get the opportunity to get some hands-on experience.

#18

In our experience with customers the most important property for carbon fiber is cost. We would be more than happy to supply information on the carbon fiber that is used in the materials we sell but it is extremely rare that anyone cares about anything but the price.

This is a pet peeve of mine as fiber type can make a huge difference in parts from a structural and cosmetic perspective.

Unfortunately this results in fabric being produced with non-certified fiber to get the cheapest price.

#19

When the visual appearance is the driving factor, cost indeed is very important.

When carbon is used structurally, cost of course is important as well, but properties are equally important. Plenty of projects where properties are the driving factor, and for instance a M60J fiber is chosen (believe me, you do not select that fiber for its nice price)

#20

Even for cosmetics fiber type makes a difference. But as this thread demonstrates, some users do not realize that all carbon is black.
If you are Boeing, Lockheed, NASA etc. then you are absolutely correct. However we do have customers who buy thousands of yards of carbon fiber fabric with no fiber type specification.
We do not get questioned on fiber type in over 95% of sales. But rest assured they all care about the cost. :slight_smile: