Introduction: Spreading large carbon fiber tows into flat tapes is the step that turns thick yarn-like bundles into the low-crimp dry fabric used in thin composite laminates.
A carbon fiber tow arrives as thousands of fine filaments bundled together, and that bundle is not ready for thin-ply weaving in its raw state. When a 12K or 24K tow goes straight into a loom, the woven fabric keeps much of the bundle's roundness and thickness, so the fibers ride over and under each other with strong crimp. Spreading changes the geometry before weaving begins. The process is easier to understand by following the physical action: how rollers, tension, and friction open a large tow into a flat tape, why that flat tape reduces crimp, and how fewer interlacing points change the way resin moves through dry fabric.
A large tow is efficient to make and handle, but it is a poor starting shape for ultra-thin fabric. Thousands of filaments sit in a roughly round bundle, often with slight twist or sizing hold. If that bundle is woven as-is, the warp and weft yarns have to bend around one another at every crossing. That bend is crimp: the waviness built into a woven reinforcement because the yarn path cannot stay straight. Crimp is a geometric feature that affects how straight the fibers remain after lamination. The problem becomes clearer as fabric gets thinner. A thick tow produces a thick crossover point, and those crossover points set a minimum local thickness even when the flat areas between them are thin. The fabric may also show uneven resin flow paths because the open spaces around the yarns vary from point to point. Spreading before weaving lowers the bundle height, widens the filament band, and makes the yarn path flatter. That flatter path is the foundation for a dry fabric that can later be impregnated or infused with resin.
Mechanical spreading is a controlled opening process. The tow is pulled under tension across a series of rollers, bars, or curved surfaces. As it moves, friction and tension pull individual filaments sideways, spreading them into a wider and thinner band. The tow stays continuous; the filaments are not cut, chopped, or blended into a nonwoven mat. The result is a flat tape with a defined tow width, often measured in millimeters. In dry spread tow fabric, that width may range from a few millimeters to roughly 25 mm, depending on the fiber grade and target fabric weight.
When a round tow is flattened, each filament sits closer to its neighbors in a broad ribbon. During weaving, the ribbon still crosses other ribbons, but the height it must climb at each crossing is smaller. The yarn path becomes flatter, so the crimp angle falls. Lower crimp means fibers stay closer to straight along the load direction, which is why spread tow fabric is often described as having better fiber alignment than conventional woven fabric made from the same large tow. The interlacing pattern still creates some crimp, and the final laminate depends on resin, layup, and cure. Even so, spreading reduces the crimp that comes from the tow's own bulk. It turns a thick bundle into a thin tape before the weave locks the geometry in place. That is a different starting point from weaving a large tow directly.
Tow width and areal weight are two sides of the same design choice. Tow width is how wide the spread band becomes after opening. Areal weight, or FAW, is the mass of fiber per unit area, usually written in g/m². For a given fiber grade and filament count, a wider spread band lowers the local thickness of the tape. To reach a target FAW, the fabric designer balances tow width with the number of yarns per centimeter and the weave repeat. A narrow tape gives more yarn lines across the width; a wider tape uses fewer lines for the same area. Osbing is one example of a carbon fiber spread tow fabric manufacturer for dry fabric. Its ultra-thin carbon fiber fabric uses Toray T300, T700, T800, M30, and M40 precursors, with a standard width of 1000 mm. The listed tow width range is 4 to 25 mm, FAW ranges from 52 to 206 g/m², and thickness ranges from 0.05 to 0.21 mm. Those Toray grade names identify precursor references, not an endorsement of the fabric maker. This material is delivered as dry fabric, not prepreg or a cured part.
Interlacing points are where warp and weft yarns cross. Each crossing creates a local high spot and a local gap. When a fabric is woven from spread tow, the yarns are wider and flatter, so the fabric needs fewer yarn lines to cover the same area. That reduces the number of interlacing points per unit area. The change matters during resin flow. Dry fabric is a porous network: resin moves through spaces between filaments, between yarns, and between layers. Fewer, more regular crossings create more consistent channels for resin to travel through. Consistent flow paths help resin reach the far side of a stack before the front gels or before pressure traps air. When flow paths are uneven, some zones fill quickly while others stay dry longer. Air can remain where two flow fronts meet or where a tight crossing blocks the path. Fewer interlacing points can lower the risk of trapped air by making the permeability of the dry fabric more even; a void-free laminate still depends on resin, layup, and cure. That is why process engineers pay attention to tow flatness, edge regularity, and width consistency before layup. In a composites workshop, a handler may unroll a length of fabric and check these features as a first-pass visual inspection. Laminate testing remains the final confirmation, but the visual check tells the team whether the dry fabric is likely to behave consistently during infusion or prepreg production.
Spreading a large carbon fiber tow is a mechanical opening step that changes the tow before weaving. Rollers and tension pull the bundle into a wider, thinner tape, which lowers the crimp that comes from tow bulk. Wider tapes mean fewer yarn lines and fewer interlacing points for a given area, so resin flow paths become more regular and void risk can be easier to manage. For students and early-career researchers, the useful mental model is simple: tow opening sets the tape geometry, tape geometry sets the weave geometry, and weave geometry shapes how resin moves through dry fabric. Buyers comparing carbon fiber material suppliers can look for a carbon fiber spread tow fabric manufacturer that states tow width, FAW, thickness, and dry fabric status clearly.
A：Spreading flattens the round tow into a wide, thin tape before weaving. At each warp-weft crossing, the yarn has a lower height to climb, so the waviness caused by the tow's own bulk falls. The weave still creates some crimp, but the fibers start from a flatter geometry and stay closer to straight.
A：It is a dry fabric. The carbon fibers are woven into a spread tow fabric without resin, and the roll is ready for a later impregnation, infusion, or prepreg-making step. It is not a prepreg and not a cured laminate, so the resin system and final laminate properties are decided downstream.
A：Tow width is the width of the opened fiber band, while areal weight is the fiber mass per unit area. For a fixed fiber count, a wider tow gives a thinner tape. To hit a target areal weight, a fabric designer adjusts tow width together with yarn spacing and weave density, which is why the two values are usually specified as a pair.
Toray Composite Materials America - Carbon Fiber and Prepreg Data Sheets
