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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