Composite materials are made by combining a reinforcement (usually fibers) with a matrix (usually a resin or metal) so that the two work together to create something stronger or lighter than either material alone. The specific manufacturing method varies widely depending on the application, but the core idea is always the same: position the reinforcement where you want strength, surround it with matrix material, then consolidate and cure the part into its final shape.
The Two Core Components
Every composite has two essential parts. The reinforcement, typically fibers made of glass, carbon, or aramid, carries most of the structural load. The matrix, often a plastic resin, holds those fibers in place, distributes stress between them, and protects them from moisture and damage. Think of it like rebar in concrete: the steel bars resist pulling forces, while the concrete fills in around them and handles compression.
The direction fibers are oriented matters enormously. Fibers resist force best along their length. A sheet of carbon fiber is extremely strong in the direction the fibers run but relatively weak perpendicular to them. That’s why most composite parts are built up from multiple layers (called plies) with fibers oriented in different directions, so the finished part can handle loads from multiple angles. Manufacturers aim for a fiber volume fraction around 65% in high-performance parts, meaning roughly two-thirds of the finished material is fiber by volume, with matrix filling the rest.
Thermoset vs. Thermoplastic Matrices
The choice of matrix resin shapes how a composite is processed. Thermoset resins (like epoxy or polyester) start as liquids, then undergo a chemical reaction called cross-linking that permanently hardens them. This reaction can happen at room temperature or with added heat, but once it’s done, the material can never be melted or reshaped.
Thermoplastic matrices work differently. They require heat to soften the plastic enough to flow around the fibers, and the temperature needed depends on the grade. Commodity thermoplastics process at temperatures as low as 350°F. Engineered thermoplastics used in sporting goods and recreation products need 400 to 600°F. High-performance thermoplastics used in aerospace demand temperatures above 600°F. The advantage is that thermoplastic composites can, in theory, be reheated and reformed, making them more recyclable.
Hand Layup: The Simplest Method
Hand layup is the oldest and most straightforward way to make a composite part. A worker places sheets of dry fiber or pre-impregnated fiber (called “prepreg”) into an open mold one layer at a time, brushing or rolling liquid resin into each layer if using dry fiber. It requires minimal equipment and works well for prototypes, custom parts, or low-volume production. The tradeoff is inconsistency: fiber alignment and resin distribution depend heavily on the skill of the person doing the work, and the parts tend to have more trapped air than those made with more controlled methods.
Vacuum Bagging and Autoclave Curing
To improve on hand layup, manufacturers seal the part inside a flexible plastic bag and pull a vacuum. This compresses the layers together, squeezes out excess resin, and draws trapped air from between the plies. Increasing the vacuum level decreases the void content (tiny air pockets inside the part) in a predictable way.
For the highest-quality parts, the vacuum-bagged layup goes into an autoclave, which is essentially a large pressurized oven. The autoclave adds both heat (to trigger or accelerate curing) and external pressure on top of the vacuum. Increasing autoclave pressure drives void content down even further. The recommended approach is to establish vacuum first, then apply a minimum of about 4 bars of autoclave pressure, with 12 bars being optimal for the best results. Autoclave-cured composites have excellent structural homogeneity and minimal defects like porosity or areas where the resin separates from the fiber. This is the standard process for aerospace-grade carbon fiber parts, from aircraft fuselage panels to helicopter blades.
Resin Transfer Molding
Resin transfer molding, or RTM, takes a different approach. Dry reinforcement fibers are placed into a rigid, closed mold. The mold is sealed shut, and liquid resin is injected under pressure, flowing through the fiber layers until the entire mold is filled. The resin then cures inside the mold. Three parameters are closely monitored throughout the process: mold temperature, injection pressure, and the flow front of the resin as it moves through the fibers.
RTM produces parts with smooth surfaces on both sides (since both sides contact the mold) and achieves more consistent fiber-to-resin ratios than open-mold methods. It’s cost-competitive for medium-volume production and is widely used in the automotive industry for body panels and structural components.
Automated Fiber Placement
For large, complex parts, manufacturers use robotic systems called automated fiber placement (AFP) machines. A robotic arm or gantry lays down narrow strips of pre-impregnated fiber tow onto a mold surface, compacting and heating each strip as it’s placed. The machine can lay material at speeds up to 2 meters per second. AFP tows can be as narrow as 3.2 mm, giving the machine precise control over fiber orientation and allowing it to follow complex curved surfaces that would be difficult to cover with wider sheets.
A related process, automated tape laying (ATL), uses wider tapes ranging from 150 mm to 300 mm and works best on flatter or gently curved surfaces. Both methods are standard in aerospace manufacturing, where they build fuselage sections, wing skins, and other large structures that demand precise fiber placement over expansive areas.
Common Manufacturing Defects
Even with controlled processes, several things can go wrong during composite manufacturing. The most common defects include voids (trapped air or gas pockets), resin-rich zones where fibers are sparse, misaligned fibers that don’t follow the intended orientation, and areas where the resin fails to fully wet the fibers. Each of these weakens the finished part.
Delamination, where layers separate from each other, can occur during production or later in service. Misaligned fibers are particularly damaging because they create uneven stress distribution, concentrating loads in small areas and leading to premature failure. Voids can sometimes be managed by pre-treating the reinforcement at high temperatures (around 565°C for several hours) before layup to drive off moisture and contaminants that would otherwise create gas bubbles during curing.
Trimming, Drilling, and Assembly
Once a composite part is cured, it rarely comes out of the mold ready to install. Most parts need trimming, drilling, and sometimes surface finishing before they can be assembled into a larger structure.
Drilling is the single most common machining operation for composites, because mechanical fasteners like bolts and rivets require holes. Routing is used to trim parts to their final shape, and band saws or circular saws with specialized blades handle straight cuts. The tooling matters: glass and carbon fiber composites require cutting tools with multiple edges made from cemented carbide or polycrystalline diamond, materials hard enough to withstand the abrasive fibers. Aramid fiber composites present a different challenge because the fibers are tough and tend to fuzz rather than cut cleanly. Specialized opposed helical cutters that shear the outermost fibers inward produce the cleanest edges on aramid-reinforced parts.
After machining, parts are bonded to other components using structural adhesives, fastened mechanically, or joined through a combination of both. Bonded joints distribute stress more evenly than drilled fasteners, but many aerospace applications use both methods together for redundancy.