Carbon Fiber Tow Sizes: 1K, 3K and 12K
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The K number is the count of filaments in each bundle, or tow, in thousands. A 3K tow has 3,000 filaments, each about 7 microns across, roughly a tenth the width of a human hair. A 12K tow has 12,000. The K number says nothing about the grade of the fiber: a 3K fabric and a 12K fabric can use the same fiber and have the same strength per gram.
What tow size changes
- The look. Smaller tows make a tighter, finer weave. 3K 2x2 twill is the classic carbon look. 12K weaves have big, bold squares.
- Fabric weight. A 3K twill is commonly about 200 g/m². 12K fabrics are usually heavier per ply, often 400 g/m² and up. 1K fabrics are light, often around 100 g/m², for thin skins.
- Ply count. Heavier plies build thickness faster, so a thick part needs fewer of them, which saves layup time.
- Cost. Large tows are cheaper to make per kilogram, so 12K and bigger cost less than 3K, and 1K costs the most.
- Drape. Fine fabrics follow tight curves more easily. Heavy 12K fabrics tend to bridge small radii.
Does tow size change strength?
Not much, for the same fiber and weave. What matters more is crimp, the waviness of tows as they go over and under each other. Thicker tows in a woven fabric make larger undulations, which can cost some compression strength. Unidirectional material avoids crimp entirely, which is why structural laminates are often unidirectional plies with a woven ply on the outside for looks and handling.
Spread tow
Spread tow fabric starts with a large tow, often 12K, spread flat into a wide, thin tape before weaving. The result is thin plies with very little crimp and a large checkerboard pattern. It's lighter per ply and stiffer for its weight than a conventional weave, and it costs more. We compare the two in 2x2 twill vs spread tow.
Picking one
- Visible cosmetic surface: 3K twill or spread tow, depending on the look you want.
- Thick structural laminate: 12K or unidirectional plies inside, a cosmetic ply on top.
- Thin, light skins and small parts: 1K or thin spread tow.
Material and layup selection is part of our carbon fiber work.