Since the 1970s, carbon fiber has been used as a comparatively inexpensive replacement for metals like titanium, due to its high strength-to-weight ratio and stiffness. For that reason, finding a way to 3D print with carbon fiber and other reinforcement materials has become the goal of a number of startups and established manufacturers in the additive manufacturing space.
We’ll take a look at this wonder material in a brief series that tracks the roots and lifecycle of carbon fiber, how it is used in manufacturing and the ways that it has woven its way into 3D printing. To understand how and why carbon fiber reinforcement is used in 3D printing, it helps to know how it is used in traditional manufacturing and the way carbon fiber parts are made.
Carbon Fiber in the World
Whether you know it or not, carbon fiber has already wound its way into your world. Though it is predominantly used in aerospace, it has also become increasingly incorporated into the automotive, civil engineering, electronics and sporting goods spaces.
One of the primary goals in the aerospace sector has been to reduce a plane’s weight and, thus, reduce the amount of expensive fuel needed to propel that massive vehicle through the skies. The Boeing 787 Dreamliner became the first aircraft to be made from 50 percent composite materials, mostly carbon fiber, but was recently surpassed by the Airbus A350 XWB, which contains 52 percent carbon fiber-reinforced polymer (CFRP) parts.
In the automotive industry, CFRP components are more likely to be found in specialty vehicles, such as race cars and supercars, due to the price of carbon fiber. BMW developed an expertise with the material in its mass-produced i3 and i8 electric vehicles, which featured chassis made with a large amount of CFRP parts. By using carbon fiber, the auto company was able to reduce the weight of the vehicle, allowing the electric battery to push the cars further. The newer iNext, however, will feature fewer CFRP items in part because increased battery capacity and greater demand for EVs, rendering carbon fiber less cost-competitive.
If you’re an avid tennis player or cyclist, you may have invested in a carbon-fiber racket or bike frame. Otherwise, you may have driven across a bridge that has been retrofitted with carbon fiber reinforced concrete.
How Carbon Fiber is Made
Carbon fiber was invented in the latter half of the 19th century, to serve as the filaments within lightbulbs. It wasn’t until 1958 that Union Carbide began manufacturing carbon fiber for manufacturing, an objective simultaneously being pursued by researchers around the world.
Today, about 90 percent of carbon fibers are manufactured by heating a petro-derived polymer called polyacrylonitrile (PAN). Due to its near ubiquity, we will stay focused on the production of PAN-based carbon fiber. PAN is first spooled into filament yarns, before it is heated to 300°C as a means of stabilizing it in anticipation of the subsequent step: carbonization. During carbonization, the precursor material is drawn out in long strands and heated up to 2000 °C in an inert (oxygen-less) chamber. Without oxygen, the material doesn’t burn, but instead sheds all but the carbon atoms. The result are sheets of carbon layered as a filament only five to 10 microns thick. The carbon fiber is then immersed in gas (air, carbon dioxide, or ozone) or liquid (sodium hypochlorite or nitric acid) so that it can bond with other materials more easily.
Carbon fiber can be wound into a reel, known as a “tow”, and used in that form; however, it is more common to see it woven into sheets. These sheets are then combined with a polymer resin matrix to create CFRP parts. How this occurs depends largely on what type of component is being made.
A carbon fiber sheet might be placed into a mold, which is then filled with resin and heated or exposed to air until it is hardened. Or a mold might be lined with the reinforcement fiber cloth and then put into a vacuum bag, which is flooded with resin. These processes are usually labor-intensive, which is why, for mass production, metal molds are used to stamp the matrix material and carbon fiber material together.
Speeding these processes up is the use of pre-impregnated materials. Prepregs involve the use of reinforcement fiber that have already been soaked with a polymer matrix and can be readily deployed.
For large-scale operations, such as building aircraft, more advanced and expensive methods of laying carbon fiber have been developed. Automated tape laying sees prepreg tape, three- to 12-inches wide, rolled onto a part, heated and compressed, using a large mechanized gantry system. Though it doesn’t have the same throughput, automated fiber placement (AFP) technology performs a similar operation with carbon fiber tow, allowing for greater precision and geometric complexity.
Worth knowing about carbon fiber is that it exhibits anisotropic (directionally-dependent) properties. It’s helpful to think of a layer of fiber reinforcement material like a plank of wood: stiffest along the grain. For this reason, carbon fiber is often woven in a crisscross fashion when reinforcement cloth is manufactured. This characteristic will come into play as we learn about the various methods used to incorporate carbon fiber into 3D printing, such as the strength of chopped carbon fiber as compared to continuous carbon fiber.
Carbon Fiber in 3D Printing
Much of the above terminology, technology and applications are found throughout the world of carbon fiber 3D printing. Some emerging methods for 3D printing with fiber reinforcement rely heavily on prepregs, while others use tow. In part two, we’ll look specifically at the variety of techniques that currently exist for 3D printing with carbon fiber.
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