Prepreg vs Wet-Lay vs Forged: How Real Carbon Fiber Is Made

The short answer: carbon fiber parts are made by combining carbon fibers with an epoxy resin and curing that combination under heat and pressure. There are three main routes. Prepreg lamination cures pre-impregnated woven sheets in an autoclave and produces the strongest, lightest, most consistent parts. Wet lay-up applies liquid resin to dry fabric by hand and cures at room temperature — cheaper, heavier, less consistent. Forged carbon compresses short chopped fiber tows in a heated mould, producing a marbled, non-woven texture and allowing complex three-dimensional shapes. The manufacturing method, far more than the visible weave, is what determines how a part performs and how long it lasts.

What carbon fiber actually is

Carbon fiber is not a metal and not a plastic. It is a composite: thousands of filaments of nearly pure carbon, each roughly five to ten microns thick, bound together in a cured epoxy resin matrix. The fibers carry the load; the resin holds them in alignment and transfers force between them. Neither element is useful on its own.

This matters because two parts can look identical and behave completely differently. Fiber orientation, fiber-to-resin ratio, void content and cure quality decide stiffness and impact resistance. A part with too much resin is heavy and brittle. A part with trapped air voids fails at the voids. The process is the product.

The three ways carbon fiber parts are made

1. Prepreg and autoclave curing

Prepreg — short for pre-impregnated — is woven carbon fabric that arrives already saturated with a precise, factory-controlled amount of epoxy resin. It is stored frozen because the resin begins curing at room temperature.

Laminators cut and lay the sheets into a mould by hand, orienting each ply to the loads the part will see. The mould is vacuum-bagged and moved into an autoclave: a pressure vessel that cures the laminate under both heat (typically 120–180 °C) and pressure (around 6 bar). The pressure is the point. It squeezes excess resin out and collapses air voids before the epoxy hardens, producing an exceptionally high fiber-to-resin ratio.

• Highest stiffness-to-weight ratio of the three methods
• Very low void content, so no soft spots or delamination points
• Repeatable: every part from the same mould and cure cycle is effectively identical
• Superior surface quality, so the weave stays crisp and even under clear coat
• Most expensive: tooling, frozen storage, autoclave time and skilled labour

This is the aerospace and motorsport standard, and it is what serious automotive carbon fiber parts are made from.

2. Wet lay-up

Wet lay-up is the oldest and simplest method. Dry carbon fabric is placed in a mould and liquid resin is brushed or rolled into it by hand. The part cures at ambient temperature, sometimes with a vacuum bag, usually without one. It requires no autoclave and no frozen materials, which is why it dominates the low-cost end of the market.

• Much cheaper tooling and no autoclave investment
• Higher resin content, therefore heavier for the same stiffness
• Void content depends entirely on the operator's hand
• Inconsistent part to part, even from the same mould
• Often thicker than intended, which causes fitment problems on body panels

Wet lay-up parts are real carbon fiber. They are simply the weakest and heaviest way to make it. A visible weave under clear coat tells you nothing about which of these two processes was used — which is exactly why so much of the market gets away with it.

3. Forged carbon

Forged carbon — also called chopped tow moulding — uses short, randomly oriented bundles of carbon fiber mixed with resin instead of a continuous woven cloth. The mixture is packed into a heated steel mould and compressed until it cures.

Because the fibers are short and unoriented, the material flows into shapes a woven sheet cannot follow: sharp radii, deep undercuts, complex three-dimensional geometry. The result is the marbled, non-repeating pattern that makes forged carbon parts instantly recognisable — no two are ever the same.

• Handles complex 3D geometry that woven prepreg cannot conform to
• Quasi-isotropic: strength is broadly equal in every direction
• Distinctive marbled finish, unique to each individual piece
• Slightly heavier than an equivalent prepreg part
• Lower directional peak strength than optimally oriented continuous fibers

Forged carbon is not a cheaper imitation of woven carbon. It is a different material with different strengths, and it is the correct choice for thick, sculptural components such as mirror caps, shift knobs and steering wheel trim.

Side-by-side comparison

How to tell them apart on a real part

You cannot judge a part from a product photo, but you can from the part in your hands.

• Weight. Pick it up. A wet lay-up panel of the same size feels noticeably denser than a prepreg one, because the extra mass is resin.
• Edges. Look at unfinished edges and the back face. Prepreg parts show tight, evenly compacted plies. Wet lay-up often shows resin pooling, fuzzy fiber ends or uneven thickness.
• Weave alignment. On a prepreg panel the weave runs straight and continuous across curves. Distortion or pinching suggests hand-applied resin and no pressure cure.
• Backside finish. Cheap parts frequently have a rough, unfinished, sometimes fiberglass-backed reverse side. A quality laminate is carbon front and back.
• Fitment. The most honest test. Thickness inconsistency from an uncontrolled cure shows up immediately as panel gaps that will not close.

How RSI c6 makes its parts

RSI c6 designs, produces and hand-finishes every component in Italy. Bodywork and aerodynamic components are laminated in prepreg and cured in autoclave; sculptural and three-dimensional components are produced in forged carbon. No fiberglass, ever — not as a core, not as a backing layer.

Every piece is made to order, which is why lead times exist and why the surface is inspected and finished by hand rather than pulled from a warehouse shelf. Whether the part is a full carbon fiber body kit, a rear spoiler or a set of mirror caps, the laminate underneath is built the same way, and we ship worldwide. Once fitted, correct care and UV protection is what keeps that finish looking new.

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FAQ

✅ How is carbon fiber made?

Carbon fabric or chopped carbon tows are combined with epoxy resin in a mould and cured under heat and, in the best processes, pressure. The three main automotive methods are prepreg with autoclave curing, wet lay-up, and forged carbon compression moulding.

✅ Is prepreg carbon fiber better than wet lay-up?

Yes, measurably. Prepreg has a controlled resin ratio and is cured under pressure, giving a lighter, stiffer part with far fewer voids and much better consistency. Wet lay-up is cheaper but heavier and less predictable.

✅ Is forged carbon weaker than woven carbon fiber?

Not weaker overall — different. Forged carbon has roughly equal strength in all directions, while woven carbon can be stronger along the fiber direction. For thick, complex 3D components forged carbon is often the better engineering choice.

✅ Why do autoclave-cured carbon parts cost more?

Prepreg material must be shipped and stored frozen, moulds must survive high heat and pressure, autoclave cycles take hours, and lay-up requires skilled hands. Those costs are unavoidable and they buy real performance.

✅ How can I tell if a carbon part is high quality before buying?

Ask how it is made, not just what it looks like. Confirm it is prepreg or forged rather than wet lay-up, confirm there is no fiberglass in the laminate, and check that the back face is finished carbon rather than rough resin.