If the $7-billion 3D printing industry is going to grab a substantial share of the trillion-dollar manufacturing market, the technology will have to evolve both in terms of the processes themselves and the materials they use. While Carbon and HP are demonstrating that the processes are catching up to and may even supersede conventional manufacturing methods, 3D printing still has a lot of progress to make in terms of adapting materials found in traditional production for additive manufacturing (AM).
Materials are being adapted, however, as numerous chemical companies, from DuPont to Eastman, jump into the growing 3D printing materials market, estimated to reach $8.3 billion by 2025 according to market research company IDTechEx. Photopolymers currently occupy the largest market share of 3D printing materials, but, in order to compete with traditional manufacturing, composite materials will be essential to making 3D printing a viable technology for replacing conventional processes.
As it stands, however, there are only a handful of methods for introducing this lightweight yet strong material into existing 3D printing processes. Here, we relay all of the known efforts underway to bring carbon fiber composites to the layer-by-layer world of AM. Hold on to your seats! We’re about to enter the high-performance world of carbon fiber reinforcement.
How Carbon Fiber Lines Our Lives
Cotton may be the fabric of our lives, but since the 1970s, carbon fiber has started to become the fabric of our industrial manufacturing operations. Though it may be most used in the aerospace industry, the material has been increasingly leveraged in automotive, sporting goods, civil engineering and electronics spaces.
Numerous aerospace manufacturers have been implementing carbon fiber reinforcement in manufacturing due to the material’s high strength-to-weight ratio. While still strong, carbon fiber can be used to replace metal parts, reducing the weight of the aircraft and thus also reducing the fuel required to fly an aircraft. Until the construction of the new Airbus A350 XWB, which is made up of 52 percent carbon fiber–reinforced polymer (CFRP) components, the Boeing 787 Dreamliner had the highest amount of CFRP parts, constituting 50 percent of the aircraft.
Performance automobiles heavily feature carbon fiber reinforcement, but due to the price of the material, CFRP parts have been slow to make their way into most mass-produced vehicles. For that reason, you'll more likely see carbon fiber reinforcement used in racecars than in a four-door sedan. That being said, manufacturers are attempting to introduce CFRP components into mainstream cars, such as the BMW i3, which features a largely CFRP chassis.
If you are a serious cyclist, your bike may well have a carbon fiber bike frame. If you are driving across a bridge, it might be reinforced with carbon fiber, sometimes applied when retrofitting old structures. If you’re carrying a tennis racket or catching a wave on a surfboard, there's a good chance you are doing it with the help of carbon fiber. If you have microelectrodes in your pocket for measuring dopamine concentrations, those are made with carbon fiber, too.
How Carbon Fiber Composites Are Made
Invented by various engineers in the late 19th century, carbon fibers are made up of individual strands of carbon atoms about 5 to 10 microns thick. About 90 percent of carbon fibers are made by heating up a polymer called polyacrylonitrile (PAN) in several stages until it sheds all atoms, including hydrogen and nitrogen, but the carbon atoms.
Outside of this PAN process, roughly 10 percent of carbon fiber is produced through a pitch process, which involves the heating of plants, crude oil or coal into a gelatinous material and then depositing it over a cooling wheel before applying subsequent procedures. While PAN-made carbon fiber, known as turbostratic carbon fibers, has a high tensile strength, carbon fibers made via the latter process have a high stiffness and thermal conductivity.
While the carbon fiber can be wound up into a reel, known as a tow, and used itself, it is more frequently woven into sheets and combined with a polymer to form carbon fiber–reinforced composites. In this case, the resinous polymer, often called a “matrix,” acts as a binder that binds the carbon fiber to or within the end part. These matrix materials, often thermoset plastics, can vary from nylon and polyether ether keytone (PEEK) to Kevlar and polyester.
The process of creating carbon fiber–reinforced plastic/polymer parts depends on the type of object that is being made. For instance, carbon fiber cloth may be laid into a mold in the shape of the end product before the mold is filled with the matrix material and heated or air cured.
Otherwise, a mold may be lined with reinforcement material before being placed into a vacuum bag, which is then filled with the matrix material. Both of these processes may also be performed with a fiber composite that has already been impregnated with the matrix material (a “pre-preg”) to add efficiency to the process. Another method employed by companies like BMW sees the reinforcement and matrix materials compressed between male and female portions of a metal mold.
Most methods of making CFRP parts have been labor intensive, but new methods for automation are starting to be developed. Computer numerically controlled (CNC) machines with price tags ranging in the tens of millions of dollars can apply strips of reinforcement material to a polymer part, cut the strips to the appropriate length and apply heat to fuse them together before the part is put into an autoclave for final curing.
These processes are often labor intensive and expensive with laying carbon fiber either performed tediously by hand or using automated machines that are too costly for any but larger manufacturers to afford. By 3D printing CFRP parts, it’s possible to reduce the manual elements involved, while also introducing the ability to custom manufacture one-off parts or short production runs with increased geometric complexity.
Carbon Fiber 3D Printing Filament
At the moment, the easiest method for introducing carbon fiber into the 3D printing process may be the use of CFRP filament. This material typical combines chopped carbon fiber with a thermoplastic to create a composite filament that can be extruded from fused filament fabrication (FFF) technologies. FFF 3D printers are often low-cost, entry-level systems but can also be professional and industrial-grade machines.
There are several manufacturers of CFRP filaments using varying degrees of carbon fiber reinforcement and different matrix materials. colorFabb, based in the Netherlands, produces XT-CF20, a material that combines Eastman Chemical’s Amphora polyethylene terephthalate glycol-modified (PETG) copolyester with 20 percent chopped carbon fiber. Proto-pasta’s Carbon Fiber PLA is a mix of polylactic acid (PLA), a corn starch–derived plastic, and chopped carbon fiber.
Markforged, which makes its own carbon fiber 3D printer that will be discussed in the following section, manufactures a nylon-carbon fiber composite as well. 3DXTECH makes a variety of different carbon fiber filaments, ranging from PLA and acrylonitrile butadiene styrene (ABS) to PETG, nylon and PEEK.
Each variety of filament offers different characteristics. While a PLA composite may be the easiest to print with, ABS or PETG may be a bit stronger, without breaking the bank. Nylon will be even tougher and more wear resistant than these other options, but paying the higher price for PEEK will enable you to construct parts that may truly be industrial grade. Though all may be stronger than their non–carbon fiber counterparts, PEEK will be the strongest and most heat, chemical and moisture resistant of the bunch.
These filaments may be more than twice as strong as materials without carbon fiber; however, chopped carbon fiber is limited in terms of the strength it can offer because the material is, well, chopped. Continuous carbon fiber reinforcement can be even more durable 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.
For this reason, Arevo Labs has developed not just forms of filament with chopped carbon fiber, but materials with continuous carbon fiber filament as well as materials with carbon nanotubes. As Hemant Bheda, CEO of Arevo Labs, explained, “We have a process to combine continuous fibers with thermoplastic material. This process is separate from the 3D printer. Carbon nanofibers are treated similar to any other continuous fibers. Carbon nanofibers have much higher strength compared to carbon fibers.”
Whether they use chopped carbon fiber, carbon nanotubes or continuous carbon fiber, the majority of 3D printing technologies still suffer from a characteristic known as delamination, in which layers on the Z-axis are not fully fused and come apart. For this reason, Arevo Labs has developed a five-axis 3D printing technique, which prints not just in the X, Y and Z axes, but from virtually every angle.
Bheda described the benefits of this process: “We extend [traditional] deposition to [our] True 3D deposition. Classical ‘3D printing’ deposits material in the XY plane only. We call this ‘2.5D printing.’ 2.5D printing gives rise to weakness in the Z direction. This is also called delamination. Because our five-axis robotic arm driven by our software algorithms can deposit material on a 3D surface (not limited to the XY plane), it results in higher strength in the Z direction and improved aesthetics.”
3D Printing with Continuous Carbon Fiber
At the moment, Markforged's continuous filament fabrication (CFF) is one of the few methods on the market for 3D printing with carbon fiber and one of only two capable of using continuous carbon fiber that I’m aware of. The technology is similar to FFF, in that it extrudes a thermoplastic from a print head onto a substrate layer by layer until the object is complete. However, CFF adds a second extruder that feeds strands of fiber filament into the print during the process.
Markforged launched with the Mark One 3D printer at SOLIDWORKS World 2014 but has since upgraded the system to be a bit larger, higher quality and more reliable with the Mark Two. The Mark Two is capable of 3D printing with either nylon or carbon fiber–reinforced nylon as the matrix material along with a variety of reinforcement materials, including carbon, fiberglass, high-strength and high-temperature fiberglass and Kevlar.
Cynthia Gumbert, vice president of marketing for Markforged, explained that the company’s technology makes it possible to produce such objects as tooling, production fixtures, end-use parts and even orthotics, all high strength and lightweight due to the CFF process. “Many of our customers now print parts that used to be machined out of metal,” Gumbert said. “The savings in cost, time and materials with CFF is creating a new economy for 3D printing, with significant savings over machining and traditional composites production processes.”
She pointed out that Markforged printers also produce no wasted material, unlike, say, selective laser sintering, and completed parts require no post-processing. The use of nylon or carbon fiber–reinforced nylon as the shell material also results in low-friction parts, which is beneficial for producing tooling and fixtures. Gumbert added, “[CFF is the] only advanced fiber routing [process that follows] specifically designed contours. This is a major advance over the traditional sheet-based approach—we can follow a contour and reinforce it. For example, we can print rings around a hole to reinforce the hole. In the old layer method, all the fibers leading up to the hold are loose, cut ends, which need to be reinforced with inserts. Our continuous fiber is laid up to the hole, around the hole and then back out. We further alternate the seam in each layer as to avoid having any weak spots.”
The Mark One and Mark Two have proven themselves to be uniquely capable of providing custom fiber-reinforced parts at an affordable price. The Mark Two starts at $5,499, giving small labs and large companies the ability to prototype parts or produce end products reinforced with carbon fiber and other materials.
However, Markforged is not the only company that has developed a method for laying continuous carbon fiber within a print. A California company called Orbital Composites Inc. has created an extruder that is also capable of filling prints with carbon fiber filament. So far, at least one firm is relying on this technology for its own carbon fiber printer and even plans to send a version outfitted for operation in microgravity to the International Space Station.
In space or on terra, one disadvantage to this technology is the fact that some complex geometries and fine details may not be printable with carbon fiber reinforcement. Though CFF is easily capable of 3D printing objects that might typically be CNC milled, an intricate lattice may not be printable with reinforcement. Additionally, like all FFF platforms, CFF may not be all that fast, and at 320 mm x 132 mm x 154 mm (12.6 in x 5.2 in x 6.1 in), the build platform is relatively small.
Impossible Speed and Flexibility with Carbon Fiber 3D Printing
While Markforged machines are desktop printers, emerging carbon fiber 3D printing technologies may require a dedicated production facility or machine shop. This may be true of Impossible Objects' composite-based additive manufacturing method (CBAM) technology. However, the firm's CBAM process may solve some of the limitations faced by desktop carbon fiber printers by combining fiber reinforcement with any number of matrix materials at potentially high speeds and at scalable sizes.Once a CAD file has been sliced into individual bitmap layers, the printer deposits an aqueous-based printing solution into the shape of that bitmap onto a substrate sheet made from a given reinforcement material. The substrate sheet is subsequently flooded with the thermoplastic matrix material, which sticks only to the aqueous solution. The powder is then blown or vacuumed off, leaving only the plastic that has adhered to the liquid.
This process is repeated with each layer of the CAD file, with all of the substrate sheets finally stacked, compressed and placed into an oven to fuse the matrix material together. The object is then taken out of the oven, and the excess reinforcement material is removed via chemical bath or sand blasting. The result is a thermoplastic print reinforced with anything from carbon fiber, fiberglass, polyester, polyvinyl alcohol and PLA to silk and cotton.
These prints can be up to 10 times stronger than components made with FFF or other 3D printing processes. Additionally, because CBAM does not melt the thermoplastic material, as occurs with FFF, a wider variety of materials are available for 3D printing, such as PEEK.
Moreover, because inkjet heads can deposit millions of drops per second, it’s possible to print much more quickly. The company's founder and CEO, Robert Swartz, believes that Impossible Objects can create a CBAM machine capable of 3D printing at rates of 100 meters per minute. Currently, the prototype machine can print with sheets 12 in x 16 in (305 mm x 406 mm) in size, but he thinks that this size can be scaled up to fabricate entire car hoods.
The geometries that can be printed with CBAM are dependent on the reinforcement material used. 3D printing with carbon fiber, in particular, requires the use of sandblasting to take away support structures and excess material, so that interior parts may not be easily removed during post-processing. When using chemical processes, however, the geometric complexity is much greater, as excess material is dissolved away.
While not quite as strong as traditionally manufactured parts reinforced with carbon fiber, CBAM may be able to fabricate components much stronger than many other 3D printing technologies. It may, therefore, be ideal for creating complex, durable parts more quickly and affordably than those produced with traditional technologies. As with CFF, this process is also much more automated than conventional carbon fiber reinforcement methods.
Swartz spoke to how the firm’s technology advances the state of the art in 3D printing. “The biggest challenge for AM is the ability to build lightweight functional parts and to do it quickly,” he said. “Carbon fiber–based CBAM produces parts with great mechanical strength, which can be used in applications to replace metals, and much better mechanical properties than existing AM processes like selective laser sintering, fused deposition or stereolithography. In addition, CBAM is faster than these existing processes.”
He added, “This opens up a number of markets, including medical, aerospace and performance sports as well as other areas, like lightweighting in the auto industry, where carbon fiber’s superior strength-to-weight ratios are important.”
There is, however, not yet a CBAM 3D printer on the market. Impossible Objects plans to ship beta preproduction machines in early 2017. When CBAM systems are available for purchase though, they will likely be less expensive than high-end Stratasys, EOS and HP 3D printers.
Massive Carbon Fiber 3D Printing
CBAM is not the only new carbon fiber 3D printing technology on the horizon. At RAPID 2016, digital light processing pioneer EnvisionTEC unveiled its massive SLCOM 1 3D printer, which relies on the company's patent-pending selective lamination composite object manufacturing (SLCOM) technology.
Remember “pre-pregs” from earlier in this article? SLCOM uses rolls of woven reinforcement material—including carbon fiber, fiberglass and Kevlar—pre-impregnated with a thermoplastic, such as PEEK, polyetherketoneketone, polycarbonate, Nylon 6, Nylon 11 or Nylon 12. The roll is fed into the print chamber layer by layer with a heated roller passing over to melt the thermoplastic within. At the same time, an inkjet head deposits wax and a binding agent to the metal. A carbon blade with an attached ultrasonic emitter follows up by cutting away any area with wax.
The SLCOM 1 is capable of 3D printing parts as large as 24 in x 30 in x 24 in (610 mm x 762 mm x 610 mm) and up to 500 lbs (226.8 kg). Such a massive print comes with a massive price tag of roughly $1 million when the machine is released this winter.
John Hartner, chief operating officer at EnvisionTEC, was able to shed light on the possibilities offered by the SLCOM process. He pointed out that the printer is targeted toward the aerospace, automotive and defense industries and, therefore, designed to solve some of the key challenges associated with creating composite parts, such as the limitations of hand layup composites and the challenges of processing pre-pregs.
“Our new 3D printer is capable of making large woven fiber parts and custom parts that are strong and lightweight and have certain functional characteristics made possible with different composite matrices,” Hartner explained. “Specifically, our technology simplifies the process of creating composite parts and allows for 24/7 production of complete and nearly complete, high-quality composite parts that are capable of being machined if desired. We believe this marks the beginning of an exciting, new period of advancement for composite production.”
Hartner elaborated that aerospace and defense firms have shown a particular interest in the technology, with an eye towards flexible and rapid production. However, he believes that other sectors will soon follow given the SLCOM 1's ability to produce high-strength, lightweight parts or parts with special functions.
“For example, the SLCOM technology can be used to produce parts in a wide variety of custom-tailored composite materials, and composite combinations can be selected for a variety of functions, such as low flammability, high wear, X-ray transparency and more,” Hartner said. “Aside from the obvious benefit this offers the aerospace, automotive and defense industries, it’s easy to see how medical parts, sporting goods, lighting and a variety of consumer products could be transformed by this new technology.”
Given the immense size of the machine and the tough materials being printed, I wondered about the post-processing required for removing such large parts from excess materials. Hartner explained that this is not as arduous as one might think: “All 3D processes require some post-processing, but one of the distinguishing factors of our technology is the use of a lamination inhibition fluid during the build process, which essentially applies an anti-stick material at the desired edge of the part or product. This reduces the post-processing to basically an easy release, or breaking away, of the part from the waste area. It’s a time-saving improvement.”
Research
3D printing with carbon fiber is still in the early stages of development. If Impossible Objects and EnvisionTEC are any indication, however, we may be on the cusp of an exciting new area of 3D printing technology. Currently, there are university and government labs engaged in research that may turn out new methods for reinforcing 3D-printed objects with carbon fiber.
One lab that is keenly interested in this topic is the U.S. Department of Energy's Oak Ridge National Laboratory (ORNL), which has already helped to develop one new carbon fiber 3D printing method and may have more to come. The lab actually has a 42,000-square-foot facility devoted to the production of carbon fiber, including a 390-foot-long processing line that can make up to 25 tons of carbon fiber per year.
In 2014, ORNL partnered with machine manufacturer Cincinnati Inc. and crowdsourcing auto manufacturer Local Motors to retrofit existing Cincinnati equipment for 3D printing. The result is the Big Area Additive Manufacturing (BAAM) machine. With a build volume of 7 ft x 13 ft x 3 ft (2.1 m x 4.0 m x 0.9 m), the BAAM uses a hopper to feed raw material pellets into an extruder in order to 3D print at an alarming rate of 40 lbs/hr.
The technology has proven ideal for 3D printing the chassis of Local Motors vehicles, including the upcoming LM3D Swim, which will be the first road-ready, 3D-printed car series, and Olli, a 3D-printed, autonomous public transit vehicle. Made from only 5 percent carbon fiber and 95 percent ABS, the firm’s first 3D-printed auto, the Strati, had very little carbon fiber content. ORNL later went on to 3D print a Shelby Cobra made up of 20 percent carbon fiber with finish so fine that it is impossible to tell that the car was 3D printed.
However, given the research currently being performed by ORNL, the lab may be upping the amount of carbon fiber reinforcement that’s fit to 3D print. ORNL is working with Texas startup Cosine Additive to develop a Medium Area Additive Manufacturing system, which will be similar to a smaller version of the BAAM. The lab will also be working with Impossible Objects to 3D print tooling for making carbon fiber composite parts.
3D Printing with Graphene
3D printing with carbon fiber may be more immediately implemented, but some are already looking to carbon fiber’s miraculous cousin, graphene. With a thickness of a single carbon atom, graphene is about 100 times stronger than steel, incredibly lightweight and electrically and thermally conductive.
Right now, anyone can 3D print with graphene-PLA filament from Graphene 3D Lab; however, several research endeavors have demonstrated the ability to 3D print purer versions of the stuff, including scientists at the Imperial College London and Lawrence Livermore National Laboratory (LLNL).
The difficulty of 3D printing with graphene is the inability to deposit this hydrophobic wonder material from a print head. While the Imperial College London team actually 3D prints with graphene-oxide combined with a responsive polymer in order to extrude the material as a paste, LLNL 3D printed graphene-oxide into a silica gel.
As this technology is commercialized, it may be used to create highly conductive electronic components, as well as protective coatings for aircraft, among other applications. Researchers at Imperial College London, for instance, aim to create smart skin for robots with the process.
In the near term, however, carbon fiber will more quickly make its way into the portfolio of 3D printing materials. As it does, this author’s favorite manufacturing technology may be able to further demonstrate the ability to pull off feats that conventional production techniques could only dream of.