CFRP is a composite material composed of carbon fiber as the reinforcing phase and resin as the matrix. How to transform individual carbon fibers with a diameter of 7 μm into parts of various shapes and specifications? Below, we will introduce the forming methods for CFRP. Commonly used forming methods include hand lay-up, spray-up, autoclave curing, resin transfer molding, compression molding, winding, continuous pultrusion, and injection molding and so on.
Hand Lay-up Molding
Hand lay-up molding is one of the earliest molding methods used and serves as the foundation for all other methods. This method does not require specialized equipment, has low investment costs, offers high flexibility in processing parts of various sizes, and features a simple process. During the manufacturing process, a release agent and gel coat are applied to the mold. Pre-cut prepregs are then laid over the mold and resin is brushed onto them. This process is repeated until the desired thickness is achieved, after which the part is cured and demolded. Parts produced using this method have high porosity, a loose texture, low density, and relatively low strength. The quality of the parts depends on the skill of the workers, resulting in inconsistent quality and low production efficiency. This method is primarily used for small-batch production of parts.
Spray-up Molding
Spray-up molding involves applying a mixture of chopped carbon fiber and resin to the mold surface using a spray gun until the desired thickness is achieved. Air is then manually pressed out using a rubber roller, followed by curing to form the final product. While this method offers some improvement in processing efficiency, it still cannot meet the demands of mass production. Due to the use of chopped carbon fiber, the resulting parts have limited load-bearing capacity and cannot be applied to high-performance fields such as aerospace. Therefore, this process is primarily used for the production of ship hulls, bathtubs, and transition layers of storage tanks.
Autoclave Molding
To address the shortcomings of the hand lay-up molding method, such as poor surface quality and high porosity, the autoclave molding method was developed. This method involves stacking pre-impregnated fabric cut to the desired shape in the specified direction and position on the mold. Once the desired thickness is achieved, a film is applied to the surface to create a sealed chamber, which is then evacuated. The mold, with the unformed part attached, is placed in the autoclave and heated and pressurized under negative pressure, causing the part to cure in a high-temperature, high-pressure environment.
Parts produced using this process exhibit excellent surface finish, low porosity, strong material design flexibility, and superior mechanical properties, making it the most effective forming process for maximizing the functionality and performance of CFRP. It is suitable for manufacturing complex curved parts such as aircraft brackets, wings, fairings, and doors. However, this process has high equipment investment costs, high production costs, high energy consumption, and part dimensions are limited by the size of the autoclave.
Resin Transfer Molding (RTM)
Resin Transfer Molding (RTM) is a molding process that uses carbon fiber woven fabric as the intermediate substrate, with resin injected and cured within the mold. RTM consists of two steps: First, the carbon fiber woven fabric is laid out according to performance and structural requirements, placed in the mold, heated, and pressurized to form a preform; The second step involves placing the preform into the mold and injecting the resin-based curing agent under pressure to fill the pores of the preform, thereby achieving resin-carbon fiber curing.
RTM is a low-cost molding method that does not use prepreg fabric or autoclaves. The technical advantages of RTM include high processing quality, high precision, low porosity, and high fiber content in the final product. This method can achieve a smooth surface without the use of gel coat, has high production efficiency, and is widely used in the production of sports equipment and secondary structural components for aircraft. Derivatives of RTM include molded process, flexible auxiliary RTM, co-injection RTM, and high-pressure RTM (HPRTM).
Compression Molding
Compression molding is a forming method that involves placing intermediate materials such as prepreg fabric, sheet molding compound (SMC), and bulk molding compound (BMC) into a heated mold, then using a press to apply pressure and cure the material. Since molds are used in the forming process, this method can produce parts with high precision and good quality in a single operation. Additionally, this method offers advantages such as high efficiency and suitability for large-scale production of high-strength parts. However, the mold manufacturing process is complex and involves high upfront costs. SMC and BMC are intermediate materials made from chopped carbon fibers mixed with resin, which exhibit a certain degree of flowability before curing. Therefore, SMC and BMC can be molded into complex-shaped parts using compression molding, such as screws and brackets with ribs and protrusions. However, since the reinforcing phase in the parts consists of chopped carbon fibers, the strength and stiffness of the parts are influenced by the fiber morphology and cannot fully leverage the advantages of carbon fibers.
Winding Forming
Cylindrical, spherical, and tank-shaped containers and compartments are common component shapes in the aerospace industry. However, using short fibers to manufacture pressure vessels does not fully leverage the advantages of carbon fiber. In such cases, a forming process is needed to create cylindrical or spherical composite components using continuous fibers. Winding molding effectively addresses this issue. During the process, fibers are wound along a predefined path around a continuously rotating rotary mandrel, then cured and demolded.
During fiber winding, a CNC system controls the core mold’s rotation angle, rotation speed, nozzle position, angle, and fiber tension, ensuring the fibers are laid according to the planned route. Fiber winding is the most efficient and effective forming process in composite material manufacturing, minimizing human interference and producing components with uniform and stable performance. Fiber winding methods can be categorized into dry winding molding, wet winding molding, and semi-dry winding molding based on the type of intermediate substrate used.
Dry winding molding is a process where resin-impregnated prepregs are wound around a mandrel along a predetermined path and then cured and demolded. Dry winding offers advantages such as environmental friendliness, high efficiency, and excellent finished product performance. However, winding equipment is expensive, and prepreg costs are several times higher than those of conventional fibers, so this method is primarily used in the aerospace industry, where product performance requirements are stringent.
The wet winding process involves immersing continuous carbon fibers in resin from a resin tank and guiding them through a nozzle to wind around a mandrel along a planned path. This method has lower requirements for base materials and is suitable for most rotary objects. However, processing quality is influenced by multiple factors, making it difficult to ensure consistent product quality.
The semi-dry winding process lies between the two methods. The winding process has high automation, high production efficiency, and good product quality, but its application scope is limited and requires specialized equipment.
Continuous Pultrusion
Continuous pultrusion involves pulling carbon fiber bundles out at a relatively low speed using a traction device, impregnating them with resin in a resin tank, and then passing them through a heated mold with a fixed shape. The mold cures the resin as it passes through, achieving continuous curing. In theory, there is no limit to the length of products produced by continuous pultrusion, making it particularly suitable for the manufacture of pipes, rods, channel sections, I-beams, and square sections.
Injection Molding
Injection molding is a forming process developed based on the principles of metal die casting. The injection molding process uses short-cut carbon fiber particles as the intermediate substrate. Under heated conditions, the material is extruded and pushed by a screw into the mold cavity, where it cools, solidifies, and is demolded to form the part. This process enables the production of complex-shaped parts, offering significantly improved strength and stiffness compared to conventional resin monomer injection-molded parts. However, its performance lags considerably behind that of parts manufactured using long-fiber molding processes. This method is suitable for the mass production of small-sized parts.
The processing of CFRP parts can be tailored to specific requirements based on factors such as the type of matrix, intermediate substrate, part geometry, processing costs, production volume, and quality specifications. Most forming methods utilize thermosetting resins as adhesives, with thermoplastic resins being used less frequently. Most CFRP forming methods, such as hand lay-up forming, autoclave forming, compression molding, dry winding forming, resin transfer molding, and various derivative forming methods, all use layered carbon fiber materials as the intermediate substrate, which are bonded together using resin. As such, CFRP components with a layered structure and a thermosetting resin matrix are the most common.