There are multiple processes available for producing carbon fiber parts. Some processes, such as injection molding, are used with a range of materials, while some are more specific to carbon fiber composites. Which process you choose depends on the specific composite materials involved, details of part design, and the application. The available budget and production volume should also factor into the decision.
Most carbon fiber fabrication involves some type of molding to form the carbon fiber composite into the required shape needed for the final application. The processes for molding can vary in both tooling and complexity.
Open molding is a common process often used for fiberglass composite fabrication. Open molding can be done with hand layup, or alternatively by spray-up, a semi-automated alternative. The spray-up process involves spraying catalyzed resin and fiber into the mold, by blowing chopped fibers directly into the sprayed resin stream, so that the materials are applied to the mold simultaneously.
Hand Layup and Vacuum Bagging
Hand layup involves placing layers, or plies, of either dry carbon fiber fabric or prepreg sheets by hand onto a mold to form a laminate stack. Resin is applied between each ply for dry fabric and then the layers are debulked with hand rollers or by using a vacuum-bagging technique. Debulking consolidates the layup and removes air trapped between the layers which can cause voids and weaken the part.
Resin-Infused Carbon Fiber Fabrication
While steps are taken in the spray-up process to reduce VOCs, increasing regulations in the US and EU limiting worker exposure to hazardous air pollutants have created a need for improvement in this area. Additionally, the demand for faster production has driven the development of more automated carbon fiber fabrication processes. There are several types of resin-infused carbon fabrication processes available.
Resin-transfer molding (RTM) Involves using a two-part, matched closed mold, typically made of either metal or composite material. Dry reinforcement is placed into the mold and the mold is closed. Low-viscosity resin is measured and mixed, then pumped into the mold under low or moderate pressure through injection ports.
Vacuum-assisted resin transfer molding (VARTM) involves drawing the resin into the mold using only a vacuum, as opposed to being pumped under pressure. Neither high pressure nor heat are needed for this carbon fiber fabrication process. Larger, more complex parts can be created less expensively this way due to the lower cost of the tooling needed for the VARTM process.
High-Volume Molding Methods
For high production quantities, a high-volume thermoset molding process called compression molding has often been used. This process typically uses expensive but durable metal tools. Sheet molding compound (SMC), a composite material typically made of chopped fibers sandwiched between sheets of thick resin paste, is placed on a set of steel dies. Once the SMC is ready for molding, it is assembled on a heated mold, which is then closed and clamped before high pressure is applied. As the viscosity of the material lowers, the SMC flows to fill the mold cavity.
Perhaps the most commonly known molding process, injection molding, is a quick, high-pressure, low-volume, closed process. It is most often used with thermoplastics, but automated injection molding of bulk molding compounds (BMC) has become more common over the decades. The BMC injection molding process involves a ram or screw-type plunger forcing a measured amount of material through a heated barrel to inject it into a heated closed mold.
There are a number of other high-volume molding methods for carbon fabrication that involve combinations of these two processes or other, similar processes. Some examples of other high-volume molding methods include tube rolling, filament winding, pultrusion, automated fiber placement (AFP), and automated tape laying (ATL).
Additive manufacturing, or 3D printing, is an automated process that creates a 3D object from a series of 2D, cross-sectional layers. Additive manufacturing techniques always begin with computer-aided drafting (CAD) solid models. Proprietary software is then used to slice the model into thin 2D cross-sectional patterns which tell the 3D printer how to stack the 2D slices to create a physical 3D part.
3D printing makes use of a number of different materials these days, based on the specifications of the 3D printer being used. Additive manufacturing methods commonly used today are stereolithography (SLA), fused deposition modeling (FDM), selective laser sintering (SLS), digital light processing (DLP), and PolyJet printing. FDM is the method most commonly used for fiber-reinforced plastics, making it the best choice for carbon fiber fabrication. Regardless of the method used, 3D printing is a method usually reserved for prototype tooling, allowing a part to be quickly available for the evaluation of form, fit, and occasionally testing.
The best method for carbon fiber fabrication for your project depends heavily on the volume of parts to be fabricated, your budget, and what the final application for your parts will be. Some carbon fiber fabrication methods lend themselves to DIY, while others require hiring a professional carbon fiber fabricator. Knowing the variety of carbon fabrication methods available makes it easier to determine the best method for your application.