Why carbon fiber thread counts don't matter but weight does

We already discussed how carbon fiber fabric is woven from tows (or yarns) made up of thousands of filaments to a level of density giving a weight per square meter in the previous post.  Well I don't know about you, but by the end of that my head was spinning with yet more questions and I needed to go and test out my 410 thread count sheets.

The question I most puzzled over was why the yarns were 1K, 3K, 6K and 12K?  Why just those and why not a linear progression by weight?  Well the truth is I don't know and haven't been able to find out and to be quite honest for our purposes I don't need to know.  All I can guess is that somewhere along the line these became de facto standard and various manufacturers in the chain tooled up to make and cope with these.  If anyone knows a better answer I'd be pleased to hear it.

Most of the rolls of fabric we buy in are 1250 cm wide.  Our freezers are this length, our cutting benches are sized accordingly, our cutting templates are all calculated on this size, etc, etc. We would be pretty hacked off if suddenly the whole industry moved to 2000 cm wide fabrics.  Why are they 1250 wide?  Well I don't know, I could make some wild guesses but it doesn't matter, they just are.  See what I mean about raising more questions? 

So back to filament counts.  A 12K tow will be 6 times thicker than a 2K tow but also 6 times heavier.  It will also be stronger but we'll look at that aspect later.  Fabrics made using these thicker tows are useful but we don't usually use them on their own.

Generally the higher the filament count the coarser the material.  It's a bit extreme but try imagining the difference if you stroked a piece of fabric woven from silk embroidery thread versus one woven from silk rope.  The rope cloth will have a bumpier surface, be thicker, heavier and more difficult to tuck into a corner.   OK, you could make the silk rope into a less bumpy, thick and heavy material by weaving it slightly less densely but then it wouldn't be as dense and there may be gaps showing.  If you hung them both on a wall and tried to paint them which one would need most paint? 

It is the weight of the carbon fibre fabric that determines the thickness, (this is a massive generalisation but it will work for now).  A rough approximation is 200gsm = 0.2mm.  So if you want the finished article to be 1mm thick then a 1000gsm fabric should do it, right?  Well no, not quite.  It might work if the finished article was completely flat and the same thickness throughout and you could get hold of a 1000gsm fabric and you didn't mind a bumpy surface. 

Most items made from carbon fiber fabric are made in moulds (2nd wild generalisation but keep working with me) so you need to be able to drape it into the mould, build up layers to the required thickness, with an appropriate surface layer for the finish you want. 

We make door cards for Subaru and Mitsubishi rally cars, FIA regulations say they have to be 1mm thick.  We use two layers of 200gsm 3K 2x2 twill and a layer of 600gsm 12K 2x2 twill.  The 200gsm gives us a fine surface finish and the 600gsm gives the bulk.  This is the most cost effective combination for this application as the 200gsm costs almost as much as the 600gsm and we are saving labour in only having to cut and layup 3 layers rather than the potential 5 layers. 

So those pesky generalisations?  I've not said anything about resins, pre-preg versus dry materials, composite mixes or manufacturing methods yet have I?

So what about thread counts?  Well if you recall from the previous post the thread count is the number of ends (weft) and picks (warp) in a square inch so that gives you the density of the fabric.  We already know that a certain weight is achieved by using an appropriate tow to the right density.  If you know the weight, tow size and weave you know the thickness and the surface appearance so who cares about thread count, it's not as if we are buying cotton sheets after all.
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Why carbon fiber thread counts matter

You'll often see carbon fibre fabric described as a weight, tow size and weave.  Such as 600gsm 12K Twill.

Don't be misled into the notion that tow size is the same as the thread count.and, like cotton sheets, the higher the better.  We'll get to the thread counts a bit farther on. 

We use different weights with different tow sizes and different weaves.  They do different jobs so we choose the right combination of materials for the job in hand.

Carbon fiber fabrics are made from incredibly thin carbon fiber filaments that are much finer than a human hair. 

These filaments are bundled into tows (or yarns) used for weaving the fabric.  These tows will commonly have 1K, 3K, 6K or 12K filaments.  Different weaves are created by changing the number of vertical tows (warp) that the horizontal tow (weft) cross. 

So in a plain weave the weft crosses one warp at a time whereas in a 2x2 twill the weft crosses 2 warps at a time with an offset on each line to create the diamond pattern.  Clear?  OK, I think we need pictures.

This is showing the pattern for plain weave. Obviously this would be a ridiculously narrow piece of fabric and you would need many more warp tows to make something usable but you get the picture.

This is what plain weave carbon fiber fabric looks like.

 

 

Now we come to 2x2 twill.  This is the 2x2 twill weave pattern.

 

This is what 2x2 twill carbon fiber fabric looks like.

Now the filament count together with the density of the weave (how tightly the warp and weft are bound together) is what determines the weight of the fabric.  And we all know that the whole point of carbon fiber is light weight, strength and how gorgeous it looks on finished products such as iPhone cases. 

So a 200gsm 3K 2x2 carbon fiber fabric has 3 thousand filaments per tow, has been woven with a twill pattern to a density that means it weighs 200 grams per square metre.  A 600gsm 12K Plain weave, well you can do it now.  You also know what a 400gsm 12K 2x2 carbon fiber fabric is.

Where do thread counts come in?  The thread count is a definition of how tight the weave is.  In weaving terms it is the number of ends (wefts) plus the number of picks in a square inch.  Thus a 400 thread cotton sheet has 200 horizontal threads and 200 vertical threads in each square inch so it feels very smooth and luxurious.  Carbon fiber fabrics will typically have thread counts between 8 and 20 but that doesn't mean they are not luxurious, you just wouldn't make bed sheets out of them.

Now the next obvious question is why isn't it a simple progression in filament count to weight?  Why doesn't the 400gsm have 6K tows and the 600gsm have 9K tows?  Well give me chance, I did say it was a big topic. 

What is the point of this blog?

I've been working with carbon fiber and other composites for six years now and quite honestly I find it all fascinating.

I was an international IT project manager, my husband was a product designer with a background in composites production, we both had a history and love of motorsport so why not start our own composites business? 

I can give you so many reasons why not, but that is with the benefit of hindsight and thank goodness we didn't know the challenges that would face us before we started. 

Like a lot of things in life, if you knew how difficult it would be you might not start, once you start you've made an investment you have to keep going, once you succeed you're glad you did.  We're still working on the succeed bit but getting there.

We have our own range of Group N rally parts for Mitsubishis and Subarus.  We also make parts for other specialist motorsport companies (I'm sworn to secrecy about most of these so please do not ask me for details).  We've now applied our carbon and design knowledge to making iPhone cases. 

See our range of motorsport parts at www.m-techcomposites.co.uk and our range of cases at www.m-techdesigns.com.

I'd like to share my steep learning curve (warts and all) about:-

* carbon fiber; fact and fiction
* how it's made; different materials and methods of manufacture
* carbon fiber in action; applications and interviews with customers and suppliers

So a big subject, where would you like me to start?

First tentative post, what's the water like?

Feet pointed at blogging pool, paddle for a moment, feet out.  Well that wasn't too painful was it?