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3D Printing Polymeric Parts Reinforced With Carbon Nanotube Yarn
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3D PRINTING POLYMERIC PARTS REINFORCED WITH CARBON NANOTUBE YARN
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Dineshwaran Vijayakumar*, Dustin Lindley† 3D Printing Engineer—Simplify3D, Cincinnati, OH, United States* University of Cincinnati Research Institute, Cincinnati, OH, United States†
CHAPTER OUTLINE 1. Introduction .................................................................................................................................. 205 2. The Filament Production System .................................................................................................... 207 3. The 3D Printing Process ................................................................................................................ 209 4. Applications ................................................................................................................................. 212 5. Summary ...................................................................................................................................... 212 6. Future Recommendations ............................................................................................................... 213 References ........................................................................................................................................ 213 Further Reading ................................................................................................................................. 214
1 INTRODUCTION Three-dimensional printing provides the ability to transform complex digital designs into real products. While this technology was primarily used in rapid prototyping, it has slowly evolved into a manufacturing process. The most popular 3D printing technologies use thermoplastic polymers such as ABS, PLA, PETG, polycarbonate, nylon, and acrylic. Parts made of these materials may not be suitable for many practical applications as plastics by themselves have poor durability when exposed to mechanical and thermal stresses. This has restricted the applications to nonfunctional parts that are at the most exposed to static loading conditions. In some cases, these materials are primarily used for prototyping as a path to optimize the final design. Advancements over the past decade have led to inexpensive electronics, smarter software programs, and highly precise machine components. The only real factor that has been holding back the progress in 3D printing is the availability of high-performance printing materials. In the late 2000s, 3D printing found its way into the desktops of innovative designers. These 3D printers used one of the most popular technologies called fused filament fabrication (FFF), formerly fused deposition modeling (FDM). In FFF 3D printers, a long continuous plastic filament is melted and Nanotube Superfiber Materials. https://doi.org/10.1016/B978-0-12-812667-7.00009-4 Copyright # 2019 Elsevier Inc. All rights reserved.
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deposited in thin layers, one on top of the other to build the final part. This technique worked well for several amorphous and semicrystalline thermoplastic polymers that can be made into filaments. Hardware changes like adding a heated build platform and part cooling fan were also required to successfully print some filaments. But for most practical applications, the printed parts lacked durability, as they cannot withstand varying environmental conditions like high temperature, moisture, and in some cases chemicals like disinfectants and cleaners. The engineering grade materials like PEEK, Teflon, and Ultem (trademark PEI resin) are difficult to print on regular desktop machines. A few companies like Roboze [1], AON3D [2], and Stratasys [3] are producing machines that can successfully print at high temperatures and maintain stable ambient conditions. PEEK, in particular, is relatively strong and durable showing tremendous improvement in mechanical and thermal properties. However, they still do not show much improvement with regard to electric conductivity and strength-to-weight ratio. The next generation of plastics needs to be multifunctional with good mechanical, electric, thermal, and other properties to qualify for high-performance applications. Nanomaterial-reinforced polymer composites have been popular for several years in automotive and aircraft industries. These composites were fabricated using traditional methods such as resin transfer molding, film stacking, and pultrusion. With the 3D printing process becoming more commonplace, a few manufacturers have started incorporating additives in the homogenous plastic in an attempt to improve the multifunctional capabilities of the filament. High-performance additives such as carbon fiber, in the form of chopped fibers, short fiber, and particulates, were primarily used as reinforcements. Ning et al. [4] investigated the mechanical properties of carbon fiber-reinforced ABS parts. Carbon fiber reinforcements increased tensile strength and Young’s modulus with increased fiber length and volume fraction up to 10% [4]. Smaller particulates seemed to improve the toughness of the parts. However, there was one major drawback noted when using reinforcing short fibers, which is porosity. Increasing the volume of the carbon fiber reinforcement led to a higher chance of fracture along the interface. One of the more recently used nanomaterials is carbon nanotubes, known for their multifunctional properties. The nanoscale structure of the CNT is the primary factor that differentiates it from conventional fibers such as carbon fiber. Students from the University of Tennessee [5] analyzed the influence of multiwall CNT and graphene as additives in standard PLA parts. Graphene when added in 0.2% by weight showed a 47% increase in tensile strength, a 17% increase in elastic modulus, and a 12% increase in energy absorbed upon fracture [5]. On the other hand, multiwall CNT of 0.1% by weight showed an increase of 41% in tensile strength, 16% in elastic modulus, and 9% in impact strength [5]. Postiglione et al. [6] investigated the increase of electric properties when adding multiwall CNTs to a PLA blend. They developed a variation of the FFF process called liquid deposition modeling to print microstructure scaffolds. A significant increase in the electric conductivity of the nanocomposite of up to 100 S/m was obtained for a high concentration of MWCNT (about 5% by weight) [6]. These results suggested that reinforcing plastics with CNTs will improve the overall performance of parts. The powdered form of CNT is far more prevalent, and companies like 3DXTech [7] are selling carbon nanotube reinforced with ABS, PETG, and Ultem filaments. Small companies like Functionalize [8] and Arevo Labs [9] are also producing CNT-reinforced filaments, while Black Magic 3D [10] produces graphene-reinforced filaments. The most recent announcement has been the FilaOne GRAY, from Avante Technology that claims to manufacture a CNT-based filament that is UV resistant, water repellant, and with mechanical properties similar to polycarbonate [11]. The material properties are superior to those of standard filler materials, including carbon fiber. We believe that continuous reinforcement, in the form of thread or yarn, will produce superior results when compared with its powdered counterparts. The composite parts made of continuous CNT
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yarn will be ideal for producing complex geometries in 3D printing, as they are extremely pliable. The nanotube yarn carries the load across the component, whereas nanotube and graphene fillers only reinforce the material locally and provide only a modest improvement in properties. Lastly, electric conductivity will not be limited to the hopping mechanism of electric conduction that occurs when using powder nanotubes. Gardner et al. [12] from NASA published a paper on 3D printing highdensity CNT yarn with high-temperature engineering grade plastics. This paper demonstrated the ability to print multifunctional carbon nanotube fibers on low-cost desktop printers. The printed fiber strands were very close to each other making turns at a radius of 2.5 mm [12]. However, the mechanical properties of the printed parts were less than predicted due to the poor wettability of the Ultem matrix on the CNT fibers. This can be partially attributed to the filament manufacturing process that should ensure good fiber-matrix bonding. Nevertheless, demonstrating the printability of CNT yarn is still a huge step forward toward 3D printing high-performance materials. The continuous reinforcement in the form of thread or yarn will produce superior results when compared with its powdered counterparts. In this chapter, we will discuss some techniques and systems developed to 3D print CNT yarn superfiber.
2 THE FILAMENT PRODUCTION SYSTEM Filaments are an integral part of the FFF 3D printing ecosystem. The feedstock filament is fed through a liquefier to melt and reform into thin plastic layers that form the final product. Hence, the properties of filament directly influence the quality and performance of the 3D printed part. Filaments are manufactured by a technique called profile extrusion, which has been used in the plastic industry for several decades. The industrial filament production process can be tedious and advanced for a laboratory environment. To test the proof of concept, we developed a simplified version of the extrusion line in the Nanoworld Laboratories [13]. The composite filament used in this project needs to have a continuous core with a plastic coating. The core is usually a long continuous fiber like a CNT yarn. The coating is made from the same plastic that forms the composite part matrix. A two-stage production process is devised to manufacture the composite filament. The first stage is fiber precoating where the yarn is coated with a very fine layer of polymer. During the extrusion process, the reinforcement yarn passes through different physical environments. It will be subjected to tension, heat, and pressure. This may potentially damage the fiber surface and even break a few strands. Hence, it is imperative to precoat the fibers prior to extrusion coating. The CNT yarn used in this experiment is first preconditioned by chemical treatment to hold all the fiber strands together. The yarn is then coated with plastic through a process called immersion coating where the yarn will pass through a solvent bath (containing nylon and formic acid). Uniform and complete coating is essential to obtain good mechanical bonding. The bonding strength influences the mode of fiber failure in the final 3D printed part. The yarn is allowed to move at a very slow speed to facilitate proper infiltration of the nylon particles on the surface of the individual fiber strands. The process can be repeated multiple times until a desired thickness is obtained (about 50 μm). The wire-coating extrusion process is the second and final stage of composite filament production. Fig. 1A illustrates the design of this extrusion system. The design of this filament extruder is based on the single-screw extrusion system. But the extrusion die or the nozzle at the far end is unique as it contains two inlets and one outlet, as shown in Fig. 1D. The complete wire-coating extruder line can be seen in Fig. 1C. On the leftmost end, the precoated fiber is stored in a spool, which enters one of the
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FIG. 1 Filament extrusion setup: (A) concept design of the wire-coating extrusion setup, (B) wire-coating nozzle prototype,(C) filament extrusion production line in nanoworld, (D) cross-sectional view of the nozzle, and (E) composite filament being pulled from the nozzle.
nozzle inlets. The molten plastic from the extruder is being continuously pushed through the nozzle from the other inlet. The fiber comes in contact with the molten plastic at the mixing zone just before it leaves the nozzle. It is important to note that the fiber was prefed through the nozzle and held in the tension by the spooling system prior to the extrusion process. The spooling system consists of a filament puller, a speed control drive, and a winding apparatus. The fiber is pulled out of the nozzle at a constant speed, set on the speed control drive. The filament puller consists of two rollers placed very close to each other in order to grip the filament effectively. The rate of pulling directly influences the thickness of the coating. It may take several trial runs to obtain an optimal speed to produce the desired thickness. The final composite filament is then collected by a winding mechanism, which allows it to be stored in spools. Two different types of filaments were produced to demonstrate the extruding process: (1) CNT fiber filament and (2) Nomex fiber filament. Nomex fibers (a trademark product of DuPont) possess excellent heat- and flame-resistant properties and increased durability. They are primarily used in protective fabrics and uniforms that can perform well in high-temperature environments. Nomex, being a commercial fiber, will be a benchmark for the tests, and it can later be compared with the CNT fiber produced in nanoworld. These filaments will be used to print parts on the Mark One 3D printer, which is capable of printing continuous fiber filaments. The maximum diameter the printer can take is 0.5 mm. The filaments produced are expected to meet the specifications of the printer. The final diameters of CNT and Nomex filaments were found to be 0.265 and 0.472 mm, respectively. Both filaments were produced with high precision of 0.01 mm variation in diameter, which can improve 3D printing
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accuracy. The tensile strength of CNT fiber is greater than 400 MPa [14]. This is sufficient for the printer to keep the filament in tension while splicing and conveying it forward during the printing process. Several factors are taken into consideration to determine the printability; the most important ones are pliability, adhesion, and bonding. CNT fibers are one of the most pliable fibers by nature; they can even be tied into a knot. Fig. 2 shows microscopic images of the Nomex and CNT filament. Microscopic analysis will help in analyzing some of the finer aspects of filament quality. Air pockets are one of the few common defects that are likely to occur during the filament production. They are formed when the plastic pellets are not dehumidified before the extrusion process. Fig. 1A and B shows almost no signs of air bubbles, which is an achievement when handling hygroscopic materials like nylon. A few black spots were seen on the Nomex filament. They are most likely residues of the charred plastic left in the nozzle. Cleaning the nozzle every few cycles will eliminate this issue. Overall, the extrusion apparatus has proved to be simple and yet scalable.
3 THE 3D PRINTING PROCESS Several 3D printing techniques have been invented over the last few decades. However, FFF has been the most successful one among them. This can be attributed to ease of implementation and the availability of low-cost hardware. Most FFF 3D printers have four main components: (1) a toolhead that can melt plastic filament, (2) a motion control stage for the toolhead movements in a three-dimensional space, (3) a flat platform to build the 3D part, and (4) an electronic unit that controls this system. Interestingly, their working principle is based on material extrusion that is very similar to the filament extruders. A gear drive system is used to drive the plastic filament into a heated chamber called the liquefier. Stepper motors are used to precisely control the amount of raw plastic filament being pushed. In the liquefier, the solid filament is heated past its melting temperature, thereby allowing it to flow and be mixed (in certain cases). The feeding filament acts as a plunger pushing the molten plastic through a smaller nozzle (usually between 0.2 and 0.6 mm). The molten plastic is deposited as thin flat extrusions on the build platform along the path in which the toolhead travels. After one layer is complete, the platform moves down one layer height, and the second layer starts depositing. The newly deposited plastic fuses with the previously laid down layer of plastic, and the part grows one layer at a time.
FIG. 2 Microscopic images of composite filaments: (A) Nomex fiber composite filament, (B) CNT fiber composite filament.
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In this project, we have chosen a 3D printer called the Markforged Mark One that has the capability of printing filaments with a continuous core like a CNT yarn. It has a patented dual extrusion system where one nozzle is used to print the continuous fiber filament (CFF) and the second one for a standard thermoplastic filament as previously discussed. The CFF head just softens the incoming composite fiber filament and lays it down. The plastic coating helps the fiber to adhere to the bed or to the previously extruded layer of plastic. The CFF toolhead is also coupled with a fiber cutter that is used to splice the filaments precisely and push them through the nozzle. The software determines the length of the fiber for every splice based on the total reinforcement required in that layer. The cutter also has a tensioning system to hold the filament with tension. This will prevent kinking or coiling of fiber inside the channel. Fig. 3B demonstrates the Mark One printing the Nomex fiber filament. Parts made from the Mark One will have a high strength-to-weight ratio if the reinforcements are selectively placed to give them a structural advantage. In addition, controlling the fiber placement also provides a variety of options for engineers to manipulate the mechanical properties of the final part. Software plays a significant role in the process of transforming digital designs into 3D printed parts. The Mark One printer uses proprietary software called Eiger, a cloud-based computer-aided manufacturing (CAM) software that is specifically designed to work with continuous fiber filaments. The 3D model is sliced into several layers of toolpath instructions in the form of machine-readable codes called print files, which are more commonly referred to as G-code files. G-code is a type of numerical control programming language used to control machine tools. It contains information about the linear movements, extrusion amounts per move, heating, and fan control among other routines. The Mark One in particular also has a command to initiate the splice and tool change whenever a fiber layer needs to be printed. The firmware on the machine interprets each line of code and translates it into a specific machine function. Fig. 3D contains a screenshot from Eiger that shows an X-ray view of the model. The concentric circles are the fiber paths. Although the slicing parameters are limited, the software gives a very good control over the placement of the fiber in the part. Users have the option to pick specific layers to insert the fiber. The software calculates the length of the fiber that can be efficiently placed for each layer. Sharp edges and small-area segments are usually ignored, as fiber layup may not be possible in those areas.
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FIG. 3 Markforged Mark One composite 3D printer: (A) dual extruder toolhead with CFF nozzle and FFF nozzle, (B) printing Nomex fiber composite part, and (C) model of pipe bushing on the Eiger software; (D) concentric circles indicate carbon fiber placement in the bushing.
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Both CNT and Nomex filaments were able to successfully print on the Mark One. They met the specifications of the filament that can be used with this printer, in terms of diameter and spool size. To analyze the fiber layup and bonding, some of these prototypes were stopped midway through the print. The parts can then be studied closely under the microscope. Fig. 4A and B are microscopic images of a 3D printed Nomex fiber-reinforced part. It has two consecutive fiber layers. The CNT fiber parts were printed using a custom model that would serve as a biomedical device. Fig. 4D shows the 3D printed part made from CNT fiber. The part was flexed to determine if any fiber damage would occur, as seen in Fig. 4E. Microscopic analysis helps examine the fiber toolpath. It is important to verify that the actual fiber layup followed the theoretically predicted toolpath. This includes cutting corners and following straight lines. Discrepancies in this area can lead to dimensional inaccuracies. In Fig. 4A and B, the Nomex fiber strands seem to be slightly displaced in the lateral in-plane direction. This can happen if the nozzle is likely too close to the base layer and did not provide enough space for the fiber to be effectively laid down. When the layer height does not match the diameter of the fiber, the top fiber can force itself on the bottom fiber causing the former to slip away. It is also important to look for any defects in the print like air pockets and underextrusions. In Fig. 4B, the surface of the fiber strands is also exposed in certain regions. The fiber-matrix interface looks distorted along the curves, which is suggestive of insufficient coating. Fig. 4C shows that the bonding at the fiber-matrix interface on the CNT part is smoother and stronger. However, there are visible air gaps along the interface. This can lead to crack initiation when the part is subjected to loads. The strength and integrity of the fiber are also equally important as the majority of the strength directly comes from the fibers. The ability of the filament to maintain its structure during printing is highly important. The Nomex fiber used is a
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FIG. 4 Three-dimensional printed Nomex fiber composite part: (A) and (B) microscopic images of Nomex fiber in the printer part demonstrating the quality of the print, (C) the complete tensile test specimen printed on the Mark One, (D) 3D printed flat plate reinforced with CNT fiber, and (E) the flexible plate.
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commercially available product developed by DuPont. Nomex fibers maintained their structural integrity throughout the print as expected from a material produced in a large-scale industry. The fiber remained strong and intact even throughout the part. This is a good indication of the fiber’s performance. On the other hand, the CNT fiber yarn unraveled at several points along the print. While printing, the nozzle applies pressure on the filament against the base layer. This may have caused the individual fibers to loosen. The lack of proper plying of the yarn is clearly the reason for this behavior. On the positive side, the yarn was printed well without any damage or breakage to the fibers. This is a measure of the outstanding properties of CNT yarn, being strong but yet pliable.
4 APPLICATIONS The ability to 3D print custom composite parts will open the doors to product innovation, particularly in the medical industry. Biomedical devices like drug delivery systems, implants, prosthetics, and other fracture fixation devices can be designed more efficiently. This is called precision medicine or personalized medicine. CNT filaments, being multifunctional and 3D printable, will further enable making customized devices. In nanoworld, we are always inventing new products and applications for CNTs, and one such application is a bioremovable drug delivery system. This device is made of CNT and polycaprolactone (PCL), which is a biodegradable and biocompatible polymer. The device will carry a sensor to trigger drug delivery into the bloodstream or into tissue when needed. The device is inserted into the body using a least invasive approach. Once the drug is delivered, the CNT yarn can be removed from the device. As the rest of this device is made from PCL, it will slowly degrade over time and is eventually excreted. CNTs can also be used with flexible thermoplastics. Three-dimensional printing flexible polymers like thermoplastic elastomers are gaining interest with the introduction of 3D printed footwear, clothing, etc. When integrated with nanotubes, they can find applications in smart wearable devices. When a material like CNT filament is made available to a wider audience, it will certainly spark innovation across multiple fields. The CNT filament finds its place as a niche product in the current 3D printing material market. As the industry grows, the demand for specialty and highperformance filaments will only increase. Moreover, filaments are the only consumable in the 3D printing process. Hence, commercialization of CNT superfiber filament production will be very much sustainable and will have a great future.
5 SUMMARY The 3D printing industry now stands at the forefront of product innovation. This has led to the development of some revolutionary materials that would not have existed otherwise. Multifunctional materials like CNT yarn filaments will open the doors to innovation in the material industry. Many research groups and industries have already started experimenting with CNT in biomedical and aerospace applications. Establishing the proof of concept for 3D printing CNTs will now increase the demand of yarns among other forms such as inks and powder. A simple and scalable filament manufacturing apparatus described in this chapter produced high-quality composite filament prototypes—CNT and Nomex filaments. Tuning the fiber feed rate to the volumetric flow rate of the molten plastic was critical to get consistent filament diameters. This directly influenced the print
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quality of the composite parts. Configuring the software parameters also played a significant role in the printing process. Layer height is needed to be consistent with filament diameter to prevent defects such as incorrect filament paths. Layer height needs to be greater than the fiber diameter to prevent the fibers in the successive layers from sliding against each other. On comparing the parts printed with Nomex fiber and CNT fiber, Nomex fiber seemed to have performed better as a filament material owing to its good structural integrity. However, we know that the CNT yarn is a new kind of material, and it was lacking a good plying process. It is evident that a significant amount of trial runs is required to completely refine the composite material 3D printing process. Eiger as a software needs to offer greater control over the process parameters like software in the market. New materials have always been well received in the 3D printing industry. Introducing customized high-performance materials like CNT yarn filaments will create a new market place and could pay dividends in the next few years. In some cases, these materials face challenges in terms of acquiring safety certifications particularly for use in the biomedical industry. Products made of new materials like CNT-PCL need to go through a rigorous testing process before getting approval. Government bodies like NIH, OSHA, FDA, and similar agencies in other countries across the globe should coordinate and take proactive measures to expedite this process.
6 FUTURE RECOMMENDATIONS The next generation of 3D printers must push the envelope further by printing materials like thermoset resins and low-melting-point metals. Epoxies have already been widely accepted as a composite resin in the aerospace and automotive industry. CNTs in powder form have been implemented on a smaller scale in practical applications, namely, turbine blades, aircraft wings, and antiballistic jackets. It is therefore easier to adopt these materials in 3D printing as they have earned credibility in real-life applications. There is also a huge demand to simplify the software tool chain involved in 3D printing. The present software ecosystem that translates digital designs into printable files is rather tedious and decentralized. But several software companies including giants like Autodesk have heavily invested in improving this process. Developing a complete ecosystem to manage 3D printing projects remotely and efficiently is the need of the hour. In some cases, the entire system can be hosted on a cloud platform, which allows the users to control their projects remotely. In the future, this will lay the foundation for implementing newer technologies like machine learning and artificial intelligence in 3D printers. With advanced hardware and software development already moving in a good pace, it is time for the materials to catch up in the race. Innovation in materials including using nanotube superfiber filaments and using carbon nanotube hybrid yarn with integrated nanoparticles to customize properties will break the current material property science barriers and drive the 3D printing industry forward to widespread commercialization.
REFERENCES [1] Roboze One+ 400—3D Printer Capable of Producing High Temperature Plastics, http://www.roboze.com/ en/3d-printers/roboze-one-400.html. [2] Aon M—Industrial 3D Printer Capable of Printing Engineering Grade Materials, https://aon3d.com/aon-mindustrial-3d-printer/.
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[3] Stratasys Fortus 450mc—Industrial 3D Printer, http://www.stratasys.com/3d-printers/fortus-380mc-450mc. [4] F. Ning, W. Cong, J. Qiu, J. Wei, S. Wang, Additive manufacturing of carbon fiber reinforced thermoplastic composites using fused deposition modeling, Compos. Part B 80 (2015) 369–378. [5] A. Plymill, R. Minneci, D.A. Greeley, J. Gritton, Graphene and Carbon Nanotube PLA Composite Feedstock Development for Fused Deposition Modeling, University of Tennessee, 2016 (Honors Thesis Projects). [6] G. Postiglione, G. Natale, G. Griffini, M. Levi, S. Turri, Conductive 3D microstructures by direct 3D printing of polymer/carbon nanotube nanocomposites via liquid deposition modeling, Compos. Part A 76 (2015) 110–114. [7] ESD Safe Filaments From 3DXTech, http://www.3dxtech.com/esd-safe-filament/. [8] F-Electric From Functionalize, http://functionalize.com/product-category/conductive/. [9] Quantevo—ESD From Arevo Labs, http://arevolabs.com/additive-manufacturing-materials/. [10] Black Magic 3D—Conductive Graphene Filament, Graphene Lab Inc., http://www.blackmagic3d.com/ Conductive-p/grphn-175.htm. [11] FilaOne, GRAY From Avante Technology, https://proforma-3dprinting-store.myshopify.com/collections/ all/products/filaone-gray-advanced-composite-filament. [12] J.M. Gardner, G. Sauti, J.-W. Kim, R.J. Cano, R.A. Wincheski, C.J. Stelter, B.W. Grimsley, D.C. Working, E. J. Siochi, Additive manufacturing of multifunctional components using high density carbon nanotube yarn filaments (NF1676L-23685, Nasa Technical Reports Server), (2016). [13] University of Cincinnati Nanoworld Laboratories, http://www.min.uc.edu/nanoworldsmart. [14] G. Hou, S. Ruitao, A. Wang, V. Ng, W. Li, S. Yi, L. Zhang, M. Sundaram, V. Shanov, D. Mast, D. Lashmore, M. Schulz, Y. Liu, The effect of a convection vortex on sock formation in the floating catalyst method for carbon nanotube synthesis, Carbon 102 (2016) 513–519.
FURTHER READING [1] J.N. Coleman, U. Khan, W.J. Blau, Y.K. Gun’ko, Small but strong: a review of the mechanical properties of carbon nanotube–polymer composites, Carbon 44 (2006) 1624–1652. [2] Mark Forged—Industrial Strength 3D Printer, https://markforged.com/mark-two/. [3] Spoolhead, Extruder Design for Printing Fibers, http://reprap.org/wiki/SpoolHead. [4] 3D Printing Patents Expiry, https://3dprintingindustry.com/news/many-3d-printing-patents-expiring-soonheres-round-overview-21708/. [5] Wohlers Report 2016: 3D Printing and Additive Manufacturing State of the Industry: Annual Report, ISBN: 978-0-9913332-2-6. [6] G.T. Mark, A.S. Gozdz, Three Dimensional Printing, Patent US2014/0291886A1, October 2, 2014. [7] G.T. Mark, D. Benhaim, A. Parangi, B. Sklaroff, Methods for Fiber Reinforced Additive Manufacturing, Patent WO 2015042422 A1, September 19, 2014. [8] K. Tyler, Method and Apparatus for Continuous Composite Three-Dimensional Printing, US20140061974 A1, August 24, 2013. [9] R. Guillemette, R. Peters, Coextruded, Multilayered and Multicomponent 3D Printing Inputs, WO2015077262 A1, May 28, 2015. [10] T. Xiaoyong, Y. Cheng, C. Yi, T. Strong, Z. Yingying, Dichen, A Continuous Long Fiber Reinforced Composites 3D Printer and Printing Method, CN104149339 B, July 9, 2014. [11] B.Z. Jang, J.H. Liu, S. Chen, Z.M. Li, H. Mahfuz, A. Adnan, Nanotube Fiber Reinforced Composite Materials and Method of Producing Fiber Reinforced Composites, US6934600 B2, August 23, 2005. [12] C.B. Sweeney, M.J. Green, M. Saed, Microwave-Induced Localized Heating of CNT Filled Polymer Composites for Enhanced Inter-Bead Diffusive Bonding of Fused Filament Fabricated Parts, WO 2015130401 A2/A1/A3/A9, September 3, 2015.
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[13] J.-B. Donnet, Roop Chand Bansal Carbon Fibers, third ed., Marcel Dekker Inc., https://books.google.com/ books?id¼YkN8xIwL0dsC&dq. [14] M.F.L. De Volder, S.H. Tawfick, R.H. Baughman, A. John Hart, Carbon nanotubes: present and future commercial applications, Science 339 (6119) (2013) 535–539, https://doi.org/10.1126/science.1222453. [15] M.B. Dow, H. Benson Dexter, Development of Stitched, Braided and Woven Composite Structures in the ACT Program and at Langley Research Center (1985 to 1997), n.d. (NASA/TP-97-206234), https://ntrs. nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980000063.pdf. [16] E. Maria del Pilar Noriega, C. Rauwendaal, Troubleshooting the Extrusion Process, Hanser Publications, n.d., ISBN: 978-1-56990-470-1, https://www.hanserpublications.com/Products/273-troubleshooting-the-extrusionprocess-2e.aspx. [17] Extrusion Process, Eastman Chemical Company, http://www.eastman.com/Markets/medical_technical_ center/Processing/Extrusion/Pages/Profile_Extrusion_Process.aspx. [18] C. Rauwendaal, Polymer Extrusion, Hanser Gardner Publications, 2001 ISBN: 1569903212, 9781569903216, https://www.hanserpublications.com/Products/314-polymer-extrusion-5e.aspx. [19] Streamline Extrusion Company Manual, http://www.streamlineextrusion.com/files/manuals/paper4.pdf. [20] Conair Upstream and Downstream Extrusion, http://www.conairgroup.com/products/upstream-extrusion. [21] N.T. Alvarez, T. Ochmann, N. Kienzle, B. Ruff, M.R. Haase, T. Hopkins, S. Pixley, D. Mast, M.J. Schulz, V. Shanov, polymer coating of carbon nanotube fibers for electric microcables, Nanomaterials 4 (2014) 879–893, https://doi.org/10.3390/nano4040879. [22] Filastruder, Filament Extruder Components, http://www.filastruder.com/. [23] Filament Puller Design, Username: Wingmaster, http://www.thingiverse.com/thing:677144. [24] Nylon Data Sheet, http://www.newmantools.com/pipestoppers/NYLON_chem_resistance_nt.pdf. [25] David Standard Extruders, http://www.davis-standard.com/extruders. [26] Nomex Fibers From DuPont, http://www.dupont.com/products-and-services/fabrics-fibers-nonwovens/ fibers/products/nomex-fibers.html. [27] 3D Printing Control Schematic—DIY India, https://www.diy-india.com/make/how-to-wire-rampselectronics-prusai3.html. [28] Anon, JTEC/WTEC Panel Report: Rapid Prototyping in Europe and Japan: Volume I. Analytical Chapters, Published and Distributed by Rapid Prototyping Association of the Society of Manufacturing Engineers, 1997.http://www.wtec.org/pdf/rp_vi.pdf. [29] T.R. Kramer, F.M. Proctor, E.R. Messina, The NIST RS274NGC Interpreter-Version 3, http://www.nist.gov/ customcf/get_pdf.cfm?pub_id¼823374, 2000. [30] EEPROM, Firmware, https://en.wikipedia.org/wiki/EEPROM. [31] Marlin Documentation, https://github.com/MarlinFirmware/Marlin/wiki. [32] Tensile Test Specimen 3D Model, http://www.thingiverse.com/thing:190386. [33] C.L. Ventola, Medical applications for 3D printing: current and projected uses, PT 39 (10) (2014) 704–711. [34] M.A. Woodruff, D.W. Hutmacher, The return of a forgotten polymer—polycaprolactone in the 21st century, Prog. Polym. Sci. 35 (2010) 1217–1256. [35] D.R. Chen, J.Z. Bei, S.G. Wang, Polycaprolactone microparticles and their biodegradation, Polym. Degrad. Stab. 67 (3) (2000) 455–459. [36] J. Steuben, D.L. Van Bossuyt, C. Turner, Design for fused filament fabrication additive manufacturing, Proceedings of the ASME 2015 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference IDETC/CIE 2015 August 2–5, 2015, Boston, USA, 2015. [37] R. Jamieson, H. Hacker, Direct slicing of CAD models for rapid prototyping, Rapid Prototyp. J. 1 (2) (1995) 4–12. [38] Platform Jack Model, User—Intentional 3D, http://www.thingiverse.com/thing:925556. [39] PWC Reports, http://www.pwc.com/us/en/technology-forecast/2014/3d-printing/features/future-3d-printing. html.
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[40] Reportsn Reports: Outlook of Mergers & Acquisitions, Investments, and Patents in the 3D Printing Market (2010–2016), 2016. [41] IDTechEx, 3D Printing Materials 2016-2026: Status, Opportunities, Market Forecasts, 2016. [42] Market and Markets, 3D Printing Materials Market by Type (Plastics, Metals, Ceramics, and Others), Form (Filament, Powder and Liquid), Application, and by Region—Global Forecasts to 2021, 2016 (Report Code: CH 2921).
3D PRINTING POLYMERIC PARTS REINFORCED WITH CARBON NANOTUBE YARN
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Dineshwaran Vijayakumar*, Dustin Lindley† 3D Printing Engineer—Simplify3D, Cincinnati, OH, United States* University of Cincinnati Research Institute, Cincinnati, OH, United States†
CHAPTER OUTLINE 1. Introduction .................................................................................................................................. 205 2. The Filament Production System .................................................................................................... 207 3. The 3D Printing Process ................................................................................................................ 209 4. Applications ................................................................................................................................. 212 5. Summary ...................................................................................................................................... 212 6. Future Recommendations ............................................................................................................... 213 References ........................................................................................................................................ 213 Further Reading ................................................................................................................................. 214
1 INTRODUCTION Three-dimensional printing provides the ability to transform complex digital designs into real products. While this technology was primarily used in rapid prototyping, it has slowly evolved into a manufacturing process. The most popular 3D printing technologies use thermoplastic polymers such as ABS, PLA, PETG, polycarbonate, nylon, and acrylic. Parts made of these materials may not be suitable for many practical applications as plastics by themselves have poor durability when exposed to mechanical and thermal stresses. This has restricted the applications to nonfunctional parts that are at the most exposed to static loading conditions. In some cases, these materials are primarily used for prototyping as a path to optimize the final design. Advancements over the past decade have led to inexpensive electronics, smarter software programs, and highly precise machine components. The only real factor that has been holding back the progress in 3D printing is the availability of high-performance printing materials. In the late 2000s, 3D printing found its way into the desktops of innovative designers. These 3D printers used one of the most popular technologies called fused filament fabrication (FFF), formerly fused deposition modeling (FDM). In FFF 3D printers, a long continuous plastic filament is melted and Nanotube Superfiber Materials. https://doi.org/10.1016/B978-0-12-812667-7.00009-4 Copyright # 2019 Elsevier Inc. All rights reserved.
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deposited in thin layers, one on top of the other to build the final part. This technique worked well for several amorphous and semicrystalline thermoplastic polymers that can be made into filaments. Hardware changes like adding a heated build platform and part cooling fan were also required to successfully print some filaments. But for most practical applications, the printed parts lacked durability, as they cannot withstand varying environmental conditions like high temperature, moisture, and in some cases chemicals like disinfectants and cleaners. The engineering grade materials like PEEK, Teflon, and Ultem (trademark PEI resin) are difficult to print on regular desktop machines. A few companies like Roboze [1], AON3D [2], and Stratasys [3] are producing machines that can successfully print at high temperatures and maintain stable ambient conditions. PEEK, in particular, is relatively strong and durable showing tremendous improvement in mechanical and thermal properties. However, they still do not show much improvement with regard to electric conductivity and strength-to-weight ratio. The next generation of plastics needs to be multifunctional with good mechanical, electric, thermal, and other properties to qualify for high-performance applications. Nanomaterial-reinforced polymer composites have been popular for several years in automotive and aircraft industries. These composites were fabricated using traditional methods such as resin transfer molding, film stacking, and pultrusion. With the 3D printing process becoming more commonplace, a few manufacturers have started incorporating additives in the homogenous plastic in an attempt to improve the multifunctional capabilities of the filament. High-performance additives such as carbon fiber, in the form of chopped fibers, short fiber, and particulates, were primarily used as reinforcements. Ning et al. [4] investigated the mechanical properties of carbon fiber-reinforced ABS parts. Carbon fiber reinforcements increased tensile strength and Young’s modulus with increased fiber length and volume fraction up to 10% [4]. Smaller particulates seemed to improve the toughness of the parts. However, there was one major drawback noted when using reinforcing short fibers, which is porosity. Increasing the volume of the carbon fiber reinforcement led to a higher chance of fracture along the interface. One of the more recently used nanomaterials is carbon nanotubes, known for their multifunctional properties. The nanoscale structure of the CNT is the primary factor that differentiates it from conventional fibers such as carbon fiber. Students from the University of Tennessee [5] analyzed the influence of multiwall CNT and graphene as additives in standard PLA parts. Graphene when added in 0.2% by weight showed a 47% increase in tensile strength, a 17% increase in elastic modulus, and a 12% increase in energy absorbed upon fracture [5]. On the other hand, multiwall CNT of 0.1% by weight showed an increase of 41% in tensile strength, 16% in elastic modulus, and 9% in impact strength [5]. Postiglione et al. [6] investigated the increase of electric properties when adding multiwall CNTs to a PLA blend. They developed a variation of the FFF process called liquid deposition modeling to print microstructure scaffolds. A significant increase in the electric conductivity of the nanocomposite of up to 100 S/m was obtained for a high concentration of MWCNT (about 5% by weight) [6]. These results suggested that reinforcing plastics with CNTs will improve the overall performance of parts. The powdered form of CNT is far more prevalent, and companies like 3DXTech [7] are selling carbon nanotube reinforced with ABS, PETG, and Ultem filaments. Small companies like Functionalize [8] and Arevo Labs [9] are also producing CNT-reinforced filaments, while Black Magic 3D [10] produces graphene-reinforced filaments. The most recent announcement has been the FilaOne GRAY, from Avante Technology that claims to manufacture a CNT-based filament that is UV resistant, water repellant, and with mechanical properties similar to polycarbonate [11]. The material properties are superior to those of standard filler materials, including carbon fiber. We believe that continuous reinforcement, in the form of thread or yarn, will produce superior results when compared with its powdered counterparts. The composite parts made of continuous CNT
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yarn will be ideal for producing complex geometries in 3D printing, as they are extremely pliable. The nanotube yarn carries the load across the component, whereas nanotube and graphene fillers only reinforce the material locally and provide only a modest improvement in properties. Lastly, electric conductivity will not be limited to the hopping mechanism of electric conduction that occurs when using powder nanotubes. Gardner et al. [12] from NASA published a paper on 3D printing highdensity CNT yarn with high-temperature engineering grade plastics. This paper demonstrated the ability to print multifunctional carbon nanotube fibers on low-cost desktop printers. The printed fiber strands were very close to each other making turns at a radius of 2.5 mm [12]. However, the mechanical properties of the printed parts were less than predicted due to the poor wettability of the Ultem matrix on the CNT fibers. This can be partially attributed to the filament manufacturing process that should ensure good fiber-matrix bonding. Nevertheless, demonstrating the printability of CNT yarn is still a huge step forward toward 3D printing high-performance materials. The continuous reinforcement in the form of thread or yarn will produce superior results when compared with its powdered counterparts. In this chapter, we will discuss some techniques and systems developed to 3D print CNT yarn superfiber.
2 THE FILAMENT PRODUCTION SYSTEM Filaments are an integral part of the FFF 3D printing ecosystem. The feedstock filament is fed through a liquefier to melt and reform into thin plastic layers that form the final product. Hence, the properties of filament directly influence the quality and performance of the 3D printed part. Filaments are manufactured by a technique called profile extrusion, which has been used in the plastic industry for several decades. The industrial filament production process can be tedious and advanced for a laboratory environment. To test the proof of concept, we developed a simplified version of the extrusion line in the Nanoworld Laboratories [13]. The composite filament used in this project needs to have a continuous core with a plastic coating. The core is usually a long continuous fiber like a CNT yarn. The coating is made from the same plastic that forms the composite part matrix. A two-stage production process is devised to manufacture the composite filament. The first stage is fiber precoating where the yarn is coated with a very fine layer of polymer. During the extrusion process, the reinforcement yarn passes through different physical environments. It will be subjected to tension, heat, and pressure. This may potentially damage the fiber surface and even break a few strands. Hence, it is imperative to precoat the fibers prior to extrusion coating. The CNT yarn used in this experiment is first preconditioned by chemical treatment to hold all the fiber strands together. The yarn is then coated with plastic through a process called immersion coating where the yarn will pass through a solvent bath (containing nylon and formic acid). Uniform and complete coating is essential to obtain good mechanical bonding. The bonding strength influences the mode of fiber failure in the final 3D printed part. The yarn is allowed to move at a very slow speed to facilitate proper infiltration of the nylon particles on the surface of the individual fiber strands. The process can be repeated multiple times until a desired thickness is obtained (about 50 μm). The wire-coating extrusion process is the second and final stage of composite filament production. Fig. 1A illustrates the design of this extrusion system. The design of this filament extruder is based on the single-screw extrusion system. But the extrusion die or the nozzle at the far end is unique as it contains two inlets and one outlet, as shown in Fig. 1D. The complete wire-coating extruder line can be seen in Fig. 1C. On the leftmost end, the precoated fiber is stored in a spool, which enters one of the
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FIG. 1 Filament extrusion setup: (A) concept design of the wire-coating extrusion setup, (B) wire-coating nozzle prototype,(C) filament extrusion production line in nanoworld, (D) cross-sectional view of the nozzle, and (E) composite filament being pulled from the nozzle.
nozzle inlets. The molten plastic from the extruder is being continuously pushed through the nozzle from the other inlet. The fiber comes in contact with the molten plastic at the mixing zone just before it leaves the nozzle. It is important to note that the fiber was prefed through the nozzle and held in the tension by the spooling system prior to the extrusion process. The spooling system consists of a filament puller, a speed control drive, and a winding apparatus. The fiber is pulled out of the nozzle at a constant speed, set on the speed control drive. The filament puller consists of two rollers placed very close to each other in order to grip the filament effectively. The rate of pulling directly influences the thickness of the coating. It may take several trial runs to obtain an optimal speed to produce the desired thickness. The final composite filament is then collected by a winding mechanism, which allows it to be stored in spools. Two different types of filaments were produced to demonstrate the extruding process: (1) CNT fiber filament and (2) Nomex fiber filament. Nomex fibers (a trademark product of DuPont) possess excellent heat- and flame-resistant properties and increased durability. They are primarily used in protective fabrics and uniforms that can perform well in high-temperature environments. Nomex, being a commercial fiber, will be a benchmark for the tests, and it can later be compared with the CNT fiber produced in nanoworld. These filaments will be used to print parts on the Mark One 3D printer, which is capable of printing continuous fiber filaments. The maximum diameter the printer can take is 0.5 mm. The filaments produced are expected to meet the specifications of the printer. The final diameters of CNT and Nomex filaments were found to be 0.265 and 0.472 mm, respectively. Both filaments were produced with high precision of 0.01 mm variation in diameter, which can improve 3D printing
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accuracy. The tensile strength of CNT fiber is greater than 400 MPa [14]. This is sufficient for the printer to keep the filament in tension while splicing and conveying it forward during the printing process. Several factors are taken into consideration to determine the printability; the most important ones are pliability, adhesion, and bonding. CNT fibers are one of the most pliable fibers by nature; they can even be tied into a knot. Fig. 2 shows microscopic images of the Nomex and CNT filament. Microscopic analysis will help in analyzing some of the finer aspects of filament quality. Air pockets are one of the few common defects that are likely to occur during the filament production. They are formed when the plastic pellets are not dehumidified before the extrusion process. Fig. 1A and B shows almost no signs of air bubbles, which is an achievement when handling hygroscopic materials like nylon. A few black spots were seen on the Nomex filament. They are most likely residues of the charred plastic left in the nozzle. Cleaning the nozzle every few cycles will eliminate this issue. Overall, the extrusion apparatus has proved to be simple and yet scalable.
3 THE 3D PRINTING PROCESS Several 3D printing techniques have been invented over the last few decades. However, FFF has been the most successful one among them. This can be attributed to ease of implementation and the availability of low-cost hardware. Most FFF 3D printers have four main components: (1) a toolhead that can melt plastic filament, (2) a motion control stage for the toolhead movements in a three-dimensional space, (3) a flat platform to build the 3D part, and (4) an electronic unit that controls this system. Interestingly, their working principle is based on material extrusion that is very similar to the filament extruders. A gear drive system is used to drive the plastic filament into a heated chamber called the liquefier. Stepper motors are used to precisely control the amount of raw plastic filament being pushed. In the liquefier, the solid filament is heated past its melting temperature, thereby allowing it to flow and be mixed (in certain cases). The feeding filament acts as a plunger pushing the molten plastic through a smaller nozzle (usually between 0.2 and 0.6 mm). The molten plastic is deposited as thin flat extrusions on the build platform along the path in which the toolhead travels. After one layer is complete, the platform moves down one layer height, and the second layer starts depositing. The newly deposited plastic fuses with the previously laid down layer of plastic, and the part grows one layer at a time.
FIG. 2 Microscopic images of composite filaments: (A) Nomex fiber composite filament, (B) CNT fiber composite filament.
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In this project, we have chosen a 3D printer called the Markforged Mark One that has the capability of printing filaments with a continuous core like a CNT yarn. It has a patented dual extrusion system where one nozzle is used to print the continuous fiber filament (CFF) and the second one for a standard thermoplastic filament as previously discussed. The CFF head just softens the incoming composite fiber filament and lays it down. The plastic coating helps the fiber to adhere to the bed or to the previously extruded layer of plastic. The CFF toolhead is also coupled with a fiber cutter that is used to splice the filaments precisely and push them through the nozzle. The software determines the length of the fiber for every splice based on the total reinforcement required in that layer. The cutter also has a tensioning system to hold the filament with tension. This will prevent kinking or coiling of fiber inside the channel. Fig. 3B demonstrates the Mark One printing the Nomex fiber filament. Parts made from the Mark One will have a high strength-to-weight ratio if the reinforcements are selectively placed to give them a structural advantage. In addition, controlling the fiber placement also provides a variety of options for engineers to manipulate the mechanical properties of the final part. Software plays a significant role in the process of transforming digital designs into 3D printed parts. The Mark One printer uses proprietary software called Eiger, a cloud-based computer-aided manufacturing (CAM) software that is specifically designed to work with continuous fiber filaments. The 3D model is sliced into several layers of toolpath instructions in the form of machine-readable codes called print files, which are more commonly referred to as G-code files. G-code is a type of numerical control programming language used to control machine tools. It contains information about the linear movements, extrusion amounts per move, heating, and fan control among other routines. The Mark One in particular also has a command to initiate the splice and tool change whenever a fiber layer needs to be printed. The firmware on the machine interprets each line of code and translates it into a specific machine function. Fig. 3D contains a screenshot from Eiger that shows an X-ray view of the model. The concentric circles are the fiber paths. Although the slicing parameters are limited, the software gives a very good control over the placement of the fiber in the part. Users have the option to pick specific layers to insert the fiber. The software calculates the length of the fiber that can be efficiently placed for each layer. Sharp edges and small-area segments are usually ignored, as fiber layup may not be possible in those areas.
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FIG. 3 Markforged Mark One composite 3D printer: (A) dual extruder toolhead with CFF nozzle and FFF nozzle, (B) printing Nomex fiber composite part, and (C) model of pipe bushing on the Eiger software; (D) concentric circles indicate carbon fiber placement in the bushing.
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Both CNT and Nomex filaments were able to successfully print on the Mark One. They met the specifications of the filament that can be used with this printer, in terms of diameter and spool size. To analyze the fiber layup and bonding, some of these prototypes were stopped midway through the print. The parts can then be studied closely under the microscope. Fig. 4A and B are microscopic images of a 3D printed Nomex fiber-reinforced part. It has two consecutive fiber layers. The CNT fiber parts were printed using a custom model that would serve as a biomedical device. Fig. 4D shows the 3D printed part made from CNT fiber. The part was flexed to determine if any fiber damage would occur, as seen in Fig. 4E. Microscopic analysis helps examine the fiber toolpath. It is important to verify that the actual fiber layup followed the theoretically predicted toolpath. This includes cutting corners and following straight lines. Discrepancies in this area can lead to dimensional inaccuracies. In Fig. 4A and B, the Nomex fiber strands seem to be slightly displaced in the lateral in-plane direction. This can happen if the nozzle is likely too close to the base layer and did not provide enough space for the fiber to be effectively laid down. When the layer height does not match the diameter of the fiber, the top fiber can force itself on the bottom fiber causing the former to slip away. It is also important to look for any defects in the print like air pockets and underextrusions. In Fig. 4B, the surface of the fiber strands is also exposed in certain regions. The fiber-matrix interface looks distorted along the curves, which is suggestive of insufficient coating. Fig. 4C shows that the bonding at the fiber-matrix interface on the CNT part is smoother and stronger. However, there are visible air gaps along the interface. This can lead to crack initiation when the part is subjected to loads. The strength and integrity of the fiber are also equally important as the majority of the strength directly comes from the fibers. The ability of the filament to maintain its structure during printing is highly important. The Nomex fiber used is a
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FIG. 4 Three-dimensional printed Nomex fiber composite part: (A) and (B) microscopic images of Nomex fiber in the printer part demonstrating the quality of the print, (C) the complete tensile test specimen printed on the Mark One, (D) 3D printed flat plate reinforced with CNT fiber, and (E) the flexible plate.
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commercially available product developed by DuPont. Nomex fibers maintained their structural integrity throughout the print as expected from a material produced in a large-scale industry. The fiber remained strong and intact even throughout the part. This is a good indication of the fiber’s performance. On the other hand, the CNT fiber yarn unraveled at several points along the print. While printing, the nozzle applies pressure on the filament against the base layer. This may have caused the individual fibers to loosen. The lack of proper plying of the yarn is clearly the reason for this behavior. On the positive side, the yarn was printed well without any damage or breakage to the fibers. This is a measure of the outstanding properties of CNT yarn, being strong but yet pliable.
4 APPLICATIONS The ability to 3D print custom composite parts will open the doors to product innovation, particularly in the medical industry. Biomedical devices like drug delivery systems, implants, prosthetics, and other fracture fixation devices can be designed more efficiently. This is called precision medicine or personalized medicine. CNT filaments, being multifunctional and 3D printable, will further enable making customized devices. In nanoworld, we are always inventing new products and applications for CNTs, and one such application is a bioremovable drug delivery system. This device is made of CNT and polycaprolactone (PCL), which is a biodegradable and biocompatible polymer. The device will carry a sensor to trigger drug delivery into the bloodstream or into tissue when needed. The device is inserted into the body using a least invasive approach. Once the drug is delivered, the CNT yarn can be removed from the device. As the rest of this device is made from PCL, it will slowly degrade over time and is eventually excreted. CNTs can also be used with flexible thermoplastics. Three-dimensional printing flexible polymers like thermoplastic elastomers are gaining interest with the introduction of 3D printed footwear, clothing, etc. When integrated with nanotubes, they can find applications in smart wearable devices. When a material like CNT filament is made available to a wider audience, it will certainly spark innovation across multiple fields. The CNT filament finds its place as a niche product in the current 3D printing material market. As the industry grows, the demand for specialty and highperformance filaments will only increase. Moreover, filaments are the only consumable in the 3D printing process. Hence, commercialization of CNT superfiber filament production will be very much sustainable and will have a great future.
5 SUMMARY The 3D printing industry now stands at the forefront of product innovation. This has led to the development of some revolutionary materials that would not have existed otherwise. Multifunctional materials like CNT yarn filaments will open the doors to innovation in the material industry. Many research groups and industries have already started experimenting with CNT in biomedical and aerospace applications. Establishing the proof of concept for 3D printing CNTs will now increase the demand of yarns among other forms such as inks and powder. A simple and scalable filament manufacturing apparatus described in this chapter produced high-quality composite filament prototypes—CNT and Nomex filaments. Tuning the fiber feed rate to the volumetric flow rate of the molten plastic was critical to get consistent filament diameters. This directly influenced the print
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quality of the composite parts. Configuring the software parameters also played a significant role in the printing process. Layer height is needed to be consistent with filament diameter to prevent defects such as incorrect filament paths. Layer height needs to be greater than the fiber diameter to prevent the fibers in the successive layers from sliding against each other. On comparing the parts printed with Nomex fiber and CNT fiber, Nomex fiber seemed to have performed better as a filament material owing to its good structural integrity. However, we know that the CNT yarn is a new kind of material, and it was lacking a good plying process. It is evident that a significant amount of trial runs is required to completely refine the composite material 3D printing process. Eiger as a software needs to offer greater control over the process parameters like software in the market. New materials have always been well received in the 3D printing industry. Introducing customized high-performance materials like CNT yarn filaments will create a new market place and could pay dividends in the next few years. In some cases, these materials face challenges in terms of acquiring safety certifications particularly for use in the biomedical industry. Products made of new materials like CNT-PCL need to go through a rigorous testing process before getting approval. Government bodies like NIH, OSHA, FDA, and similar agencies in other countries across the globe should coordinate and take proactive measures to expedite this process.
6 FUTURE RECOMMENDATIONS The next generation of 3D printers must push the envelope further by printing materials like thermoset resins and low-melting-point metals. Epoxies have already been widely accepted as a composite resin in the aerospace and automotive industry. CNTs in powder form have been implemented on a smaller scale in practical applications, namely, turbine blades, aircraft wings, and antiballistic jackets. It is therefore easier to adopt these materials in 3D printing as they have earned credibility in real-life applications. There is also a huge demand to simplify the software tool chain involved in 3D printing. The present software ecosystem that translates digital designs into printable files is rather tedious and decentralized. But several software companies including giants like Autodesk have heavily invested in improving this process. Developing a complete ecosystem to manage 3D printing projects remotely and efficiently is the need of the hour. In some cases, the entire system can be hosted on a cloud platform, which allows the users to control their projects remotely. In the future, this will lay the foundation for implementing newer technologies like machine learning and artificial intelligence in 3D printers. With advanced hardware and software development already moving in a good pace, it is time for the materials to catch up in the race. Innovation in materials including using nanotube superfiber filaments and using carbon nanotube hybrid yarn with integrated nanoparticles to customize properties will break the current material property science barriers and drive the 3D printing industry forward to widespread commercialization.
REFERENCES [1] Roboze One+ 400—3D Printer Capable of Producing High Temperature Plastics, http://www.roboze.com/ en/3d-printers/roboze-one-400.html. [2] Aon M—Industrial 3D Printer Capable of Printing Engineering Grade Materials, https://aon3d.com/aon-mindustrial-3d-printer/.
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[3] Stratasys Fortus 450mc—Industrial 3D Printer, http://www.stratasys.com/3d-printers/fortus-380mc-450mc. [4] F. Ning, W. Cong, J. Qiu, J. Wei, S. Wang, Additive manufacturing of carbon fiber reinforced thermoplastic composites using fused deposition modeling, Compos. Part B 80 (2015) 369–378. [5] A. Plymill, R. Minneci, D.A. Greeley, J. Gritton, Graphene and Carbon Nanotube PLA Composite Feedstock Development for Fused Deposition Modeling, University of Tennessee, 2016 (Honors Thesis Projects). [6] G. Postiglione, G. Natale, G. Griffini, M. Levi, S. Turri, Conductive 3D microstructures by direct 3D printing of polymer/carbon nanotube nanocomposites via liquid deposition modeling, Compos. Part A 76 (2015) 110–114. [7] ESD Safe Filaments From 3DXTech, http://www.3dxtech.com/esd-safe-filament/. [8] F-Electric From Functionalize, http://functionalize.com/product-category/conductive/. [9] Quantevo—ESD From Arevo Labs, http://arevolabs.com/additive-manufacturing-materials/. [10] Black Magic 3D—Conductive Graphene Filament, Graphene Lab Inc., http://www.blackmagic3d.com/ Conductive-p/grphn-175.htm. [11] FilaOne, GRAY From Avante Technology, https://proforma-3dprinting-store.myshopify.com/collections/ all/products/filaone-gray-advanced-composite-filament. [12] J.M. Gardner, G. Sauti, J.-W. Kim, R.J. Cano, R.A. Wincheski, C.J. Stelter, B.W. Grimsley, D.C. Working, E. J. Siochi, Additive manufacturing of multifunctional components using high density carbon nanotube yarn filaments (NF1676L-23685, Nasa Technical Reports Server), (2016). [13] University of Cincinnati Nanoworld Laboratories, http://www.min.uc.edu/nanoworldsmart. [14] G. Hou, S. Ruitao, A. Wang, V. Ng, W. Li, S. Yi, L. Zhang, M. Sundaram, V. Shanov, D. Mast, D. Lashmore, M. Schulz, Y. Liu, The effect of a convection vortex on sock formation in the floating catalyst method for carbon nanotube synthesis, Carbon 102 (2016) 513–519.
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