Carbon fiber is a high-performance material known for its exceptional strength and light weight. It consists of thin filaments composed of carbon atoms, which provide remarkable stiffness, high tensile strength, and an impressive strength-to-weight ratio. These properties make it a sought-after material in industries like aerospace, automotive, and sports equipment. Manufacturing carbon fiber parts involves specialized techniques, which this article will explore.
Essential Building Blocks
Creating a carbon fiber part relies on two main ingredients: the carbon fiber itself and a resin matrix. Carbon fibers originate from precursor materials, most commonly polyacrylonitrile (PAN) or petroleum pitch. These precursors undergo a heating process in an oxygen-free environment, transforming them into thin, strong carbon filaments. Once formed, these filaments are bundled into “tows” and can be woven into fabrics or arranged into unidirectional tapes.
The resin matrix binds the carbon fibers together. This resin, often epoxy, polyester, or vinyl ester, encapsulates the fibers, transfers loads between them, and protects them from environmental factors. Resins are categorized as thermosets or thermoplastics. Thermoset resins, like epoxy, cure irreversibly through a chemical reaction, becoming rigid solids that cannot be re-melted. Thermoplastic resins, conversely, can be melted and reshaped multiple times, offering flexibility in manufacturing and potential for recycling.
Primary Manufacturing Techniques
The creation of carbon fiber parts involves several distinct manufacturing techniques, each tailored to specific requirements for part geometry, performance, and production scale. Pre-preg layup combined with autoclave curing is a common method. Pre-preg refers to carbon fiber reinforcement pre-impregnated with a precisely controlled amount of thermosetting resin, typically epoxy. These sheets are stored in refrigerated conditions to prevent premature curing.
During manufacturing, plies are cut and layered onto a mold surface, oriented to achieve desired strength and stiffness. The layered assembly is then placed in a vacuum bag to remove trapped air and compact the laminate. This bagged mold is transferred into an autoclave, where it undergoes a controlled heating and pressurization cycle. Heat cures the resin, while pressure ensures full consolidation of the plies, minimizing voids and resulting in a dense composite part.
Another category of processes involves infusing liquid resin into dry carbon fiber. Resin Transfer Molding (RTM) is a closed-mold process where dry carbon fiber fabric, known as a preform, is placed inside a rigid, matched mold. The mold is closed, and a low-viscosity thermosetting resin is injected under pressure, filling the cavity and wetting out the carbon fibers. This method offers excellent control over fiber volume, yields parts with smooth surfaces on both sides, and can produce complex geometries with integrated features.
Vacuum Infusion (VI) is a related technique. Dry carbon fiber layers are laid onto a mold, and a flexible vacuum bag creates a sealed cavity. A vacuum pump draws air out, and atmospheric pressure forces the resin into the mold, impregnating the fibers. This vacuum-driven process helps to ensure complete wet-out of the fibers and reduces voids, making it suitable for larger or intricate components.
For higher volume production, compression molding is used. This technique begins with a pre-formed charge of carbon fiber, often in chopped or sheet form, combined with a thermosetting resin, such as sheet molding compound (SMC) or bulk molding compound (BMC). This composite material is placed into a heated, matched metal mold. The mold halves are brought together under significant pressure, causing the material to flow and fill the cavity. Heat from the mold accelerates the resin’s curing, resulting in a solid, precisely shaped part. This method is efficient for producing parts with complex shapes and offers short cycle times, making it suitable for automotive or industrial applications requiring moderate to high production rates.
Filament winding is a specialized automated process for creating hollow, rotationally symmetrical carbon fiber structures. Continuous carbon fiber filaments are pulled from spools and guided through a resin bath, becoming fully impregnated with thermosetting resin. These resin-saturated filaments are then precisely wound onto a rotating mandrel. The winding machine controls the tension, speed, and angle at which the fibers are applied, allowing for specific fiber orientations to optimize the part’s strength.
Once winding is complete, the mandrel with the wound fibers is cured in an oven. After curing, the mandrel is removed. This technique is used for components requiring high strength-to-weight ratios and internal pressure resistance.
Finishing Touches
After molding and curing, carbon fiber parts undergo several finishing steps to achieve their final form and appearance. The first stage involves trimming and machining, where excess material is removed. Precision machinery is then used to cut specific features, drill holes, or shape edges with accuracy.
Following machining, surface preparation ensures optimal adhesion for subsequent coatings. This involves sanding the part to create a smooth surface. Finally, coating and painting processes are applied. Many carbon fiber parts receive a clear coat, which provides protection against ultraviolet (UV) radiation, preventing yellowing and degradation of the resin over time. For parts requiring specific colors or additional protection, paint layers can be applied, enhancing both aesthetics and durability.