Composite Manufacturing Processes
A carbon fiber part is two materials working together: fibers that carry the load, and a resin that holds them in place and gives the part its shape. How the two are combined decides how strong the part is, how it looks, what the tooling costs and how many you can make in a day. The sections below cover the main methods, starting with hand layup and ending with the automated tape laying used for aircraft wings.
What carbon fiber is
Each carbon filament is about 5 to 7 µm across, roughly a tenth the width of a human hair. Most are made from a polymer fiber called polyacrylonitrile (PAN). The PAN fiber is stretched, heated in air to about 200 to 300°C to stabilize it, then heated to 1,000°C or more without oxygen, which drives off nearly everything but the carbon. Higher final temperatures give stiffer, high-modulus fiber.
Thousands of filaments are bundled into a tow, which is where names like 3K (3,000 filaments) and 12K come from. Tows are woven into fabric, laid flat as unidirectional tape, or chopped. See what tow size means and carbon fiber grades.
On its own the fiber is strong only along its length and can't hold a shape. Set in resin, usually epoxy, it makes parts that are stiffer and stronger than aluminum for their weight, but only in the directions the fibers run. Glass and aramid fibers are used the same way, so everything below applies to fiberglass too.
Layup, fiber direction and fiber content
A laminate is a stack of layers called plies. In a unidirectional ply every fiber points the same way; in a woven ply the fibers run in two directions. The stack, with the angle of each ply, is called the layup or ply schedule.
- Plies at 0° carry tension, compression and bending along the part.
- Plies at ±45° carry shear and twisting.
- A quasi-isotropic layup (0°, 45°, 90° and -45°, repeated) behaves about the same in every direction in the plane, like a metal sheet. It is easy to design with, but it gives up much of the advantage: its stiffness is around 45 to 55 GPa, against 130 GPa or more for unidirectional plies along the load, and 69 GPa for aluminum.
Two numbers describe how well a laminate was made. Fiber volume fraction is the share of the laminate that is fiber rather than resin; more fiber means a stiffer, stronger, lighter part. Void content is the amount of trapped air, which weakens the laminate in compression and between plies. Every method below is a different trade between pushing fiber content up, keeping voids down, and what that costs.
Wet layup
Dry fabric is laid into a one-sided mold and wet out with resin by brush or roller, one ply at a time, then left to cure, often at room temperature. The tooling can be cheap: a fiberglass mold, a machined tooling board pattern or a 3D printed mold.
The trade-off is resin content. Hand-laid parts are often 30 to 40 percent fiber by volume, so they're heavier and less consistent than prepreg parts, and the quality depends on the person doing the work. Only the mold side comes out finished. Wet layup suits prototypes, one-offs, large parts made in small numbers and a lot of fiberglass work.
Vacuum bagging
A wet layup is covered with release film and a breather cloth, sealed under a plastic bag and pumped down to vacuum. Atmospheric pressure, about 1 bar, presses the plies together and pushes excess resin and air out into the breather. Fiber content rises to roughly 40 to 50 percent, with fewer voids. It's the cheapest upgrade to a hand-laid part.
Vacuum infusion
In infusion, the dry fabric goes into the mold and is sealed under the bag first. Vacuum compacts it, then pulls resin in through feed lines until the whole stack is wet out. Because the fabric is compressed before any resin arrives, fiber content is higher than in wet layup, often 50 to 55 percent, and the process is clean because the resin stays inside the bag.
Infusion is how many boat hulls, wind turbine blades and large panels are made. The skill is in planning the resin flow: where it enters, where the vacuum is pulled, and how to keep any area from being left dry. It's also called VARTM (vacuum-assisted resin transfer molding). We compare it with prepreg and wet layup in prepreg vs wet layup vs vacuum infusion.
Resin transfer molding (RTM)
RTM uses a closed, two-part mold. A dry fiber preform is placed inside, the mold is clamped shut and resin is injected under pressure. Both faces come out finished, the thickness is set by the mold, and fiber content is typically 50 to 60 percent.
High-pressure RTM (HP-RTM) injects fast-curing resin into heated steel molds and can cycle in a few minutes, which is why it's used for structural car parts at production volumes. The molds are expensive, and cutting and shaping the dry preform is a large part of the labor.
Prepreg
Prepreg is fabric or unidirectional tape that comes with the resin already in it at a controlled ratio. It's stored frozen, around -18°C, and has a limited working life once thawed. Plies are cut, laid into the mold and cured with heat, so there's no mixing and no wetting out, and the resin content is the same in every part.
The material costs more and needs cold storage and an oven, autoclave or heated press. In return it gives the most consistent laminates, which is why it's the standard for aerospace, motorsport and high-end parts.
Autoclave curing
An autoclave is a heated pressure vessel. A bagged prepreg layup goes inside under vacuum, and the autoclave adds pressure, typically about 6 to 7 bar (85 to 100 psi), while it heats the part, commonly to 120°C or 180°C depending on the resin. The pressure squeezes voids out of the laminate, so autoclave parts reach fiber contents around 55 to 65 percent with very low void content.
It's the highest quality route, and the slowest: a cycle with heat-up and cool-down often takes several hours, and autoclaves are expensive to buy and run. Aircraft structure and race car chassis are made this way.
Out-of-autoclave prepreg
Out-of-autoclave (OOA) prepregs are made to cure in an oven under vacuum pressure alone. They're only partly impregnated when they're laid up, which leaves dry paths that let trapped air escape before the resin flows in and fills them. With a good vacuum and careful debulking, they get close to autoclave quality for many parts at a fraction of the equipment cost.
Compression molding
In compression molding, prepreg or a chopped-fiber charge is placed in a heated, matched metal mold and pressed shut in a hydraulic press. The pressure is far higher than in an autoclave, and fast-curing resins can cure in minutes. Both faces come out finished, the thickness is set by the mold, and every part comes out the same.
The matched mold is the expensive part, so compression molding pays off from the hundreds of parts upward. It's how our SSD Enclosure 1 shell is made. Forged carbon is compression molded from chopped fiber (see forged carbon vs woven), and sheet molding compound (SMC), a paste of chopped glass fiber and resin, is pressed the same way for car body panels at high volume. More on the tools in compression molds for carbon fiber.
Bladder molding
For hollow parts with a complex shape, prepreg is laid around an inflatable bladder or a removable core and closed inside a heated mold. During the cure the bladder is pressurized from inside, pushing the laminate out against the mold wall. The outside comes out finished and there's no seam to bond. Carbon bike frames and hockey sticks are typical bladder molded parts.
Filament winding
A winding machine feeds continuous fiber, either wet with resin or as preimpregnated tow, onto a rotating mandrel while a carriage travels back and forth to set the wind angle. Hoop winds near 90° resist internal pressure; low-angle helical winds add bending stiffness. Fiber content is high and the process repeats closely from part to part.
It's the standard way to make pressure vessels, including carbon tanks for hydrogen and natural gas, as well as pipes, drive shafts and tubes. The limits: the shape needs to be roughly convex, since fiber bridges over hollows; the mandrel has to come out; and the outer surface is rough unless it's wrapped or ground.
Roll wrapping
Pre-cut prepreg sheets are rolled around a steel mandrel on a rolling table, wrapped tight in shrink tape and cured in an oven, then the mandrel is pulled out. Each sheet can have its own fiber angle, and tapered tubes are easy. Most high-performance carbon tubes are made this way, including bike tubes, fishing rods and golf shafts. See pultruded vs roll-wrapped tubes.
Pultrusion
Continuous fiber is pulled through a resin bath and then a heated die that shapes and cures it, making constant-section rods, tubes, flat bar and profiles in any length. It's the cheapest way to make long composite sections, and fiberglass rebar is made this way. Almost all the fiber runs lengthwise, so pultruded parts are very stiff along their length but weak across it, and they can split when clamped or loaded sideways.
Braiding
A braiding machine interlaces fiber over a mandrel or core at plus and minus angles, forming a continuous sleeve that follows changes in diameter. Braided preforms are then infused, RTM molded or bladder molded. They handle twisting and impact well and are common in tubes, shafts and sporting goods.
Automated fiber placement and tape laying
Robots and gantry machines lay prepreg onto a tool automatically. Automated tape laying (ATL) puts down wide tape for large, gently curved skins. Automated fiber placement (AFP) lays bands of narrow tows that can steer around curves and be cut and restarted on the fly. Both are used for aircraft fuselage sections and wing skins, where part size and volume justify machines that cost millions.
Tailored fiber placement
Tailored fiber placement (TFP) stitches tows onto a base fabric along curved paths, so the fiber can follow the load around holes and into attachment points instead of being cut from a straight sheet. The result is a near-net-shape preform that wastes little material, which is then infused or RTM molded.
Thermoplastic composites
Most composites use thermoset resins like epoxy, which cure once and can't be remelted. Thermoplastic composites use matrices such as nylon (PA6), PPS or PEEK instead. Pre-consolidated sheets, often called organosheets, are heated and stamped in a press in about a minute, and can be overmolded with injection molded plastic or welded together. They're tougher and recyclable, but need higher processing temperatures and are harder to form into complex shapes.
Continuous fiber 3D printing
Some 3D printers lay a continuous carbon, glass or aramid strand inside a nylon print. The parts are far stiffer than plain plastic prints and work well for fixtures and brackets, but fiber content and strength stay well below a molded laminate. See the seven categories of 3D printing.
Side by side
Typical figures. Fiber content depends on the material, the layup and the shop, so treat these as a guide, not a specification.
| Method | Typical fiber content | Tooling cost | Typical quantities | Good for |
|---|---|---|---|---|
| Wet layup | 30 to 40% | Low | One-offs to dozens | Prototypes, large one-off parts |
| Vacuum bagging | 40 to 50% | Low | One-offs to dozens | Better hand-laid parts |
| Vacuum infusion | 50 to 55% | Low to medium | One-offs to hundreds | Large panels and hulls |
| RTM and HP-RTM | 50 to 60% | High | Hundreds to tens of thousands | Two finished faces, structural car parts |
| Prepreg, oven cure | 55 to 60% | Medium | One-offs to hundreds | Low void content without an autoclave |
| Prepreg, autoclave | 55 to 65% | Medium to high | One-offs to hundreds | Aerospace and motorsport structure |
| Compression molding | 50 to 60% (prepreg) | High | Hundreds and up | Repeatable small and medium parts |
| Filament winding | 55 to 70% | Medium | Any | Tubes, shafts, pressure vessels |
| Roll wrapping | 55 to 65% | Low to medium | Dozens and up | Round and tapered tubes |
| Pultrusion | 50 to 70% | Medium | Continuous lengths | Rods, profiles, rebar |
Choosing a method
- How many parts? For one to a few, wet layup, vacuum bagging or prepreg on an inexpensive mold. For hundreds a year, oven-cured prepreg or compression molding. For thousands, compression molding, HP-RTM, SMC or thermoplastic stamping.
- Which faces show? Open molds finish one side. If both sides need a finish or the thickness has to be exact, use a matched mold: RTM or compression molding.
- How hard is the part loaded? Primary structure calls for high fiber content and low voids: prepreg cured in an autoclave, an oven or a press.
- What shape is it? Tubes: roll wrapping or filament winding. Constant profiles: pultrusion. Hollow, complex parts: bladder molding. Large panels: infusion.
- What can you spend on tooling? At low volumes the tooling often decides the method. See prototyping a carbon fiber part without a production mold and what drives the price of a carbon fiber part.
Send a drawing, the quantity and how the part is loaded, and we'll suggest a process and a tooling route. See carbon fiber and composites manufacturing.
Related reading
- When to use carbon fiber
- Prepreg vs wet layup vs vacuum infusion
- Compression molds for carbon fiber parts
- Forged carbon vs woven carbon fiber
- Carbon fiber vs aluminum
- Carbon fiber part cost factors
Terms are defined in the glossary, and every guide is listed in the knowledge base.