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Yarn/Tow nomenclature

TOW is a term used mostly for carbon, meaning an untwisted bundle of continuous filaments. On the other hand, YARN is used mostly for fiberglass, usually meaning a twisted bundle of filaments, not necessarily continuous. Filaments come in various qualities, and form tows and yarns of various sizes. Individual filaments of the same material groups have usually the same density. Quality and size, are the two most important factors we should consider when looking at a code #.
A fiberglass code # explanation example: EC 9-68 glass fiber
(Type of glass)
S=High Strength 
S=Staple Fiber
Filament diameter in microns
Mass of yarn in grams per 1000 linear meters or Tex
Number of filaments in a yarn = (4000 X Tex) / (3.14 X D2 X 2.6) = (4000 X 68) / (3.14 X 9 X 2.6) = 3702 filaments / yarn.
- Tex = grams / 1000 m of yarn
- D = filament diameter in microns
- 2.6 = E glass density in g/cm3
With carbon fiber tow we usually have the “K” number, or the thousands of filaments per tow. 1K means 1,000 filaments / tow, 3K 3,000, 6K 6,000, 12K 12,000, etc.
6K is twice as heavy tow as 3K. 1K is 3 times lighter than 3K. High strength (HS) is a general purpose, cost effective carbon fiber, designated for industrial and recreational applications; it is also used for non structural components of aircrafts.
T300 is carbon fiber with over 20 years history in aerospace applications and is known for balanced composite properties, high quality, consistency and reliability. It is available in 1K, 3K, 6K and 12K tow sizes.
T700 is a never twisted carbon fiber with higher strength and tensile modulus than T300, and outstanding processing characteristics. Applications include aircraft and high performance sporting goods, where demanding conditions require superior composite properties.