Conventional Carbon Fiber Prepreg Molding Method

In the sophisticated world of aerospace and high-end manufacturing, carbon fiber prepreg molding technology is like an "invisible artisan", shaping high-end materials with extreme craftsmanship that combine strength and lightweight. As a classic process in the field of composites, the traditional carbon fiber prepreg molding method has gone through decades of iterations and is still the "backbone" of high-end manufacturing. Today, we'll take a deep dive into dismantling the core logic of this technology to see how it morphs from a sheet of resin-impregnated carbon fiber cloth into a key component of a spacecraft and the body skeleton of a race car.

What is Carbon Fiber Prepreg?

To understand the molding method, we must first understand the basic concept of "prepreg". Simply put, carbon fiber prepreg is the "perfect combination" of carbon fiber tow and resin - in a strictly controlled temperature and pressure environment, epoxy resin, phenolic resin and other matrix materials are uniformly impregnated into the carbon fiber fabric to form a viscous composite material coil or sheet with a certain viscosity.
This "pre-impregnation" feature distinguishes it from the dry fiber molding process: the resin content and distribution are set in advance, and the subsequent molding only needs to focus on how to make the material fit accurately in the mold and fully cured, significantly reducing the complexity of the on-site operation. It is like pre-made dough before baking, the recipe has been adjusted, the only thing left is to master the heat and time of baking.

Conventional molding method

（1）Autoclave Molding
The gold standard process for manufacturing aerospace grade high performance composites. It involves sealing the carbon fiber prepreg layup and mold in a vacuum bag and placing it in a large, high-temperature, high-pressure vessel, the hot press tank. During the curing process, uniform high pressure (several atmospheres) and high temperature are simultaneously applied to the tank, allowing the resin to fully flow and compact the fibers, resulting in parts with extremely high fiber content and low porosity.
The advantage of this process is that it produces complex structural parts of unrivaled quality with excellent mechanical properties and consistency. However, the disadvantages are extremely obvious: the hot press equipment itself is extremely expensive, consumes a lot of energy, and has long and costly production cycles, so it is usually limited to aerospace and F1 racing cars, where extreme performance is required.
（2）Compression Molding
A highly efficient process for medium to high volume production. It puts a fixed amount of prepreg or molding compound (such as SMC) into a preheated metal mold, then closes the mold and applies high pressure and temperature, so that the material flows inside the mold cavity and fills the cavity, and then cures and molds after insulation and pressure preservation.
The advantages of this process are high degree of automation, fast production, high dimensional accuracy of the product and both sides are smooth. However, due to the need to withstand high pressure rigid mold, the initial investment cost is higher. It is very suitable for the manufacture of bulk structural parts that require good surface quality, such as automobile body panels, battery cases and high-performance sports equipment.
（3）Vacuum Bag Molding
A basic and commonly used process that utilizes atmospheric pressure to compact composite materials. A series of auxiliary materials are covered with a hand-glued or prepreg layer and the entire system is sealed with a vacuum bag, which is continuously pumped by a vacuum pump to create a negative pressure, so that the atmospheric pressure is uniformly applied to the surface of the product, thereby removing air and compacting the structure.
This method can significantly increase fiber content, reduce porosity, and improve the uniformity of resin distribution, and its cost is much lower than the hot press tank. However, it can only provide a maximum pressure of about 0.1 MPa, and the upper limit of performance is not as good as that of the high-pressure process, which is widely used in shipbuilding, prototyping, and composite parts with medium performance requirements.
（4）Roll Wrapping
A directional process specializing in the production of high-performance thin-walled tubes. Carbon fiber prepreg is cut at a specific angle and then precisely wound under tension on a mandrel, usually followed by wrapping of a compression band to apply compaction pressure, and then heated and cured in an oven and demolded to obtain the pipe.
This process allows precise control of fiber orientation (e.g., 0°, ±45° combinations), achieving excellent axial reciprocating mechanical properties with good dimensional accuracy and surface quality. However, it relies on prepregs and mandrels, has limited production efficiency, and is mainly used in high-end sporting goods such as golf club shafts, fishing rods, and bicycle fork legs.
（5）Filament Winding
An automated process for manufacturing rotary high strength members. The continuous carbon fiber tow is impregnated in a resin bath and then precisely laid by a computer-controlled winding head onto a rotating mandrel in preset profiles and angles until it reaches the designed thickness, and then cured to shape.
The greatest advantage of this process is that the fibers are continuous and uniformly tensioned, allowing for extremely high fiber volume content and strength utilization, especially for containers subjected to internal pressure. However, it is limited to convex rotomolded shapes and expensive equipment. Typical products include high-pressure gas cylinders (CNG/Hydrogen), piping and rocket motor cases.
（6）Pultrusion
A highly efficient process for the continuous production of constant cross-section composite profiles. Like "spaghetti", it pulls continuous carbon fiber tows or fabrics through a resin bath for impregnation, then passes them through a heated precision steel mold where they are preformed, compacted, and continuously cured, and then finally pulled out by a tractor and cut to a constant length.
The process is extremely productive, allowing for automated continuous production, high raw material utilization and significant cost effectiveness. However, its products are limited to linear constant cross-section profiles, and the longitudinal strength is much higher than the transverse. Common products include truss bridge members, cable bridges, ladder poles, and various rods and profiles.
（7）Pressure Bag Molding
It can be regarded as an enhanced version of vacuum bag molding. Based on the vacuum bag system, the entire encapsulated mold is placed in a sealable pressure tank, which not only evacuates the interior, but also passes compressed air into the tank, applying a higher positive pressure (usually 0.4-0.6MPa) to the exterior of the vacuum bag, thus providing greater molding pressures than pure vacuum.
This method significantly improves the fiber content and densification of the part without the need for a large investment in a hot press tank, and outperforms the vacuum-only bag process. It is ideally suited for the manufacture of large parts that cannot fit into a hot press tank, such as small to medium sized ship hulls, train car parts and large radomes.

Comparison of Carbon Fiber Prepreg Forming Methods

Future: Innovation in Heritage

Tradition doesn't mean stagnation. Today, the industry is breaking through bottlenecks through "micro-innovations": for example, the development of low-temperature, fast-curing resins that shorten curing time from 2 hours to 30 minutes; the adoption of automated laminating equipment that replaces manual stacking to improve efficiency; and even the introduction of AI algorithms into mold design to optimize pressure distribution to reduce defects.
These improvements have allowed traditional processes to maintain their performance advantages while gradually transitioning to "high efficiency and low cost," and continue to play an important role in the composites field.
From spacecraft to sports equipment, the traditional carbon fiber prepreg molding method interprets the manufacturing philosophy of "slow and steady work" with solid craftsmanship. It may not be the most advanced, but in scenarios that require extreme reliability, this "tradition" is precisely the most precious quality assurance.