PerspectiveENGINEERING

Rethinking digital manufacturing with polymers

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Science  15 Dec 2017:
Vol. 358, Issue 6369, pp. 1384-1385
DOI: 10.1126/science.aaq1351

Additive manufacturing (AM) is poised to radically change the way objects are manufactured, ranging from critical applications such as aircraft components and medical devices to more commonplace, yet highly engineered, products such as running shoes. The ability to produce three-dimensional (3D) objects from a digital template can have advantages over traditional manufacturing techniques (such as machining, injection molding, and thermoforming), including mass customization, formation of complex part geometries that are not readily molded or cast, on-demand inventory, elimination of tooling costs, and reduced lead time. To realize these advantages, digital manufacturing requires materials that not only achieve the requisite mechanical properties and economic targets but are also designed to work in software-controlled, data-centric, fabrication technologies. We focus here on this challenge in the realm of polymeric materials.

Despite the initial excitement about AM dating back to the 1980s, a lack of suitable materials that can be printed economically and with sufficient quality for many production applications has prevented 3D printing from reaching its potential (1). One major limitation for polymers is that 3D-printed parts often behave differently than their injection-molded counterparts, which greatly limits their use in manufacturing applications. This limitation is especially problematic for AM techniques, such as fused deposition modeling (FDM) and powder bed fusion (PBF), that use heat to process industrially relevant thermoplastics such as acrylonitrile butadiene styrene (ABS), polylactic acid, and polyamides. Although the inherent material properties are suitable for a wide range of applications, the layer-by-layer process by which the starting materials are deposited or sintered results in anisotropic mechanical properties resulting from poor adhesion between deposited layers of powder or filament (24).

However, not all manufacturing applications require fully isotropic properties or defect-free parts, and the ability of FDM and PBF to process high-performance thermoplastic materials, such as Ultem polyimide and polyether ether ketone (PEEK), is attractive for low-volume applications in aerospace and medical devices. For example, Airbus, in conjunction with Stratasys, announced in 2015 that the A350 aircraft contained more than 1000 polymeric 3D-printed parts, developed using FDM, that meet U.S. Federal Aviation Administration regulations for flame and smoke toxicity. Compared to traditional methods, up to 90% less energy and raw materials were used to produce the parts, and the reduced weight of the parts led to operational savings (5). With inventory-on-demand enabled by AM, Airbus can manage an inventory exceeding 3.5 million replacement parts for their airliners (6).

Light-based AM technologies, such as stereolithography, use digital projection or lasers to cure a photopolymer resin and produce parts with resolutions of 10 to 100 µm. Light has advantages over heat in that it offers excellent spatial and temporal resolution and allows for direct synthesis of polymers from monomers contained in the photopolymer resin. Despite being the largest category of AM materials by sales ($350 million in 2016) (7), photopolymer resins have poor mechanical properties and machinability (for example, they cannot easily have a hole drilled into them) compared to injection-molded thermoplastics and other AM techniques. Because of their brittle, highly cross-linked nature, production applications for these materials have been limited to parts such as dental models, where fit and accuracy are prioritized over mechanical properties.

However, liquid photopolymer resins present a rich opportunity to tune final materials properties by introducing various monomers, oligomers, additives, and additional reactive functionalities. To be considered for AM applications, these materials should not only be designed to replicate the mechanical properties of tough, impact-resistant materials—such as polypropylene, polycarbonate-ABS blends, and glass-filled nylon—but also be customized for specific applications.

Continuous liquid-interface production allows for faster, layerless printing of polymers that can avoid internal stresses that decrease mechanical performance (8). “Dual-cure” photopolymer resins recently developed by Carbon, Inc., can be used with their printing technology, digital light synthesis (DLS) (9). This process uses light to set the shape of an object during printing, but the final properties are obtained after a thermal cure, which activates latent chemistries within the part (see the figure). The resulting materials are interpenetrating polymer networks that have superior properties compared to traditional photopolymer resins and are more comparable to thermoplastics used in injection molding. Isotropic properties can be obtained because of the layerless nature of DLS in combination with dual-cure resins (10). This approach enables AM to access different classes of chemistry and, in turn, additional categories of mechanical and thermal properties. Dual-cure resins containing polyurethane, epoxy, cyanate ester, and silicone functionalities have properties that resemble traditional materials, such as ABS, thermoplastic elastomers, silicones, and glass-filled nylon and polyester (9).

In addition to the photopolymerization process, DLS governs all processes by which the data contained in a 3D file are transformed into a final production part. Not only can specific pixels be turned on and off, but complex algorithms can optimize a build for accuracy, mechanical properties, speed, and surface finish. For example, finite-element analysis can be used to predict time-dependent forces during printing that determine the exact timing of irradiation and movement. With DLS, designers and engineers can take full advantage of state-of-the-art software tools such as digital surface texturing, whereby textures are added by simply modifying the digital file of a part.

Such materials are useful only if downstream manufacturers can validate the performance of materials in given applications. One key example is Adidas's decision to use DLS to mass-produce midsoles for running shoes, with plans for over 100,000 pairs in 2018 (11). The ability to rapidly print complex lattices in high-performance polyurethane elastomers enables the production of midsole geometries not previously available. The ultimate goal is “bespoke sneakers”—footwear tailored to an individual's size, shape, and gait (see the figure).

Digitally processing polymers

Additive manufacturing of complex shapes with tough polymers can be enabled by creating a digital model and then using 3D printing with robust resins. Digital light synthesis encompasses all aspects of production.

CREDITS: (TOP TWO) CARBON, INC.; (BOTTOM TWO) ADIDAS

Innovative fabrication techniques and applications demand innovative approaches to materials (12, 13). For example, advances in photoresist materials drove the miniaturization of electronic circuits. The resulting capabilities in 3D digital manufacturing could create business models built around digital part design, data management, and emerging chemistries designed for the digital age. The impact of polymer-based AM processes on how goods are manufactured is expected to increase in the coming years as recent innovations address challenges and emerging technologies shift the perception that AM is not suitable for manufacturing.

Perhaps even more exciting, AM holds the promise to connect the digital thread from design to production, enabling so-called “smart” factories. Using network-connected manufacturing tools to collect and manage data opens exciting possibilities in part serialization, tracking, and troubleshooting. Furthermore, integrating software-controlled fabrication tools with advances in materials science presents new capabilities in “digital chemistry,” where final part properties can be controlled and adjusted simply by using software.

References and Notes

  1. www.3dprint.com/63169/airbus-a350-xwb-3d-print/.
  2. See www.forbes.com/sites/airbus/2014/07/15/how-adding-a-new-dimension-to-airplanes-is-delivering-parts-on-demand/#46801e7b3d55.
  3. See www.carbon3d.com/materials/.
  4. See www.technologyreview.com/s/607964/carbon-prints-amazing-materials/.
  5. Acknowledgments: Carbon, Inc., holds several patents on DLS technology.

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