3D-printing carbon fiber is now a mainstay of the additive manufacturing (AM) industry. While companies like Impossible Objects seek to make large-scale carbon fiber a reality, Markforged and a number of filament suppliers have made 3D printing with this tough material commonplace.
In this article, we will trace the carbon fiber that has been woven into the industry from its roots to its final applications and possible future.
The Forging of Carbon Fiber
About 90 percent of carbon fiber starts as a polymer called polyacrylonitrile (PAN), while the remaining 10 percent comes from rayon or petroleum pitch. PAN is derived via free radical polymerization of acrylonitrile, which is in turn a derivative of the hydrocarbon propylene, a byproduct of oil refining and the processing of natural gas.
To create the fiber, the initial material, known as a “precursor,” is heated in air to a temperature of about 300°C in order to stabilize it for the next step, a process known as carbonization.
In carbonization, the precursor material is drawn out into long strands and heated in an inert chamber, often filled with argon gas, to an immense heat of 2000 °C. The lack of oxygen prevents the material from burning and instead causes the non-carbon atoms to be expelled, leaving only sheets of carbon layered into a single strand of filament.
Once this process is complete, the carbon fiber is oxidized via immersion in gas such as air, carbon dioxide or ozone, or in a liquid like sodium hypochlorite or nitric acid. This surface treatment occurs so that the carbon fiber can bond with other materials more readily. Finally, the strands are coated in epoxy, polyester, nylon, urethane or other adhesive to protect it during the winding or weaving process.
Chopped vs. Continuous Carbon Fiber 3D Printing
Whereas Markforged, as well as Russian firm Anispro, uses continuous strands of carbon fiber filament in its 3D-printing technology, every carbon fiber 3D-printing filament on the market relies on chopped carbon fiber. The difference between continuous and chopped carbon fiber is night and day.
Filament made from chopped carbon fiber sees small shards of carbon fiber dispersed throughout a traditional 3D-printing polymer, such as nylon, ABS, PLA or PEEK. Continuous carbon fiber is tougher due to the fact that thousands of carbon fibers are bundled together in long strands rather than broken up and scattered throughout a predominantly plastic part.
According to one meta study, though plastic components made with continuous carbon fiber reinforcement actually have tensile and flexural strengths up to 6.3- and 5-times greater than non-reinforced parts, chopped carbon fiber components just have worse porosity than carbon-free parts.
Stronger still may be 3D-printing processes that use traditional carbon fiber sheets. Once upon a time, EnvisionTEC had promised the release of a large-scale system capable of laying down sheets of reinforcement material, like carbon fiber, between layers of plastic, but we haven’t heard a peep about it since it was unveiled in 2016.
Impossible Objects, however, has also promised a sheet-based 3D printing system, which binds stacks of reinforcement material together using polymer powders and then binds the layers together in an oven. Since the company received $6.4 million in Series A funding in 2017, the most recent news that’s come out about Impossible Objects is that Ford installed two of its systems in its operations in 2018.
Since news about both of these companies has been released, another new carbon fiber 3D-printing technology has emerged from an Idaho-based company called Continuous Composites. The process, dubbed “Continuous Fiber 3D Printing (CF3D),”sees continuous strands of carbon fiber impregnated with a rapid curing thermoset plastic within the printhead and pulled out, at which point it is instantly cured using an energy source. The printhead is attached to an industrial robotic arm, allowing for six-axis control. The firm’s proprietary software leverages this ability to print objects with the optimal fiber orientation, something Stratasys and Siemens unveiled in 2016 but may or may not have delivered to market.
Applications
Carbon fiber is most used in the aerospace industry but is widespread in the automotive, sporting goods, civil engineering and electronics fields, as well. Frequently, the material is used to replace metal parts, reducing the weight and fuel consumption of an aircraft, automobile or other vehicle.
For instance, the Airbus A350 is 52 percent carbon fiber-reinforced polymer (CFRP) and the BMW i3 has mostly CFRP chassis. Carbon fiber is also used in high-end bike frames, tennis rackets and surfboards. You may also find it reinforcing bridges and retrofitting old structures.
The aforementioned examples represent the use of traditional carbon fiber reinforcement, in which large swathes of carbon fiber fabric are laid into a mold, often manually, but, in the case of the aerospace industry, sometimes with mechanical assistance. This labor-intensive process makes the use of carbon fiber expensive. For that reason, 3D printing holds the potential for automating carbon fiber layup and lowering costs.
At the moment, however, carbon fiber 3D printing is small in scale. Continuous carbon fiber 3D printing from Markforged is probably the most widely adopted, but it offers the build volume of only a desktop machine. This makes it suitable to replace metal tooling for manufacturing operations or producing custom auto parts, such as a gear shifter.
Future
There are cases of large-scale carbon fiber 3D printing in chopped form, namely the Big Area Additive Manufacturing system from Cincinnati Incorporated. The system was developed in part by the U.S. Department of Energy’s (DoE) Oak Ridge National Laboratory (ORNL). The technology has been used to 3D print entire vehicles using chopped carbon fiber-polymer composites. ORNL is currently working on the ability to print CFRP with greater carbon fiber content, so there may be large-scale carbon fiber 3D printing in the future.
What ORNL’s technology doesn’t address is the environmental cost of carbon fiber. Given the amount of heat needed to form carbon fiber, the process is about 14 times as energy intensive as forging steel. However, the material can also cut fuel consumption in vehicles by reducing automobile weight by 30 percent and aircraft weight by 20 percent. This, of course, is a false choice that takes continued global vehicle usage for granted. Another way to reduce vehicle emissions would be to supplant private vehicle ownership with more public transit.
A small addition to the emissions from the production process is the fact that PAN is a derivative of oil refining and gas processing in the first place. Plastic manufacturing makes up 1 percent of U.S. greenhouse gas (GHG) emissions and 3 percent of the country’s primary energy use. Not only is the role that plastics play an ecologically harmful one, but the supply of oil and natural gas will be increasingly difficult to access, potentially limiting the availability of carbon fiber in the future.
According to one estimate, 30 percent of carbon fiber becomes waste. It is possible to recycle carbon fiber reinforced parts, with most scrap carbon fiber chopped or milled for reuse. Using a process called pyrolysis, CFRP components are heated up to 400 °C - 600 °C (adding more energy input to the lifespan of the material), burning off the polymer so that it is totally lost. The recovered fiber can be reused but not in structural applications.
In the case that carbon fiber continues to be an industrial necessity, however, researchers are working to develop more sustainable forms. One possibility is the replacement of PAN with suitable polymers derived from naturally derived sugars, including waste plant materials.
For the DoE, one team of researchers has been able to convert plant waste into 3-hydroxypropionic acid (3-HP), which is then turned into a bioplastic known as acrylonitrile, capable of being used to create carbon fiber.
This process has several benefits over traditional PAN-based carbon fiber in that the catalyst used is three times less expensive, no excess heat is generated and the only byproducts are non-toxic. Unlike petroleum-derived carbon fiber, which generates toxic hydrogen cyanide, acrylonitrile only produces water and alcohol as byproducts.
Other possibilities for carbon fiber precursors include bioplastics made from cellulose or lignin. Like all bioplastics, however, plastic products may ultimately receive competition from land required for producing food in a world strained by rising populations and climate chaos.