Introduction: Additive/3D Printing Materials—Filaments, Functionalized Inks, and Powders
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Additive manufacturing (AM) is usually defined as the process of joining materials to make objects from 3D model data, typically layer upon layer, as opposed to subtractive manufacturing methodologies, which remove material (American Society for Testing and Materials, ASTM). We refer the reader to the recent overview article by (Huang et al. in Journal of manufacturing science and engineering 137:014001–1, 2015)  on the wide array of activities in the manufacturing community in this area.
To enhance overall filament-based material properties, by embedding particles within a binder, which is then passed through a heating element and deposited onto a surface,
To “functionalize” inks by adding particles to freely flowing solvents forming a mixture, which is then deposited onto a surface, and
To directly deposit particles, as dry powders, onto surfaces and then to heat them with a laser, e-beam, or other external source, in order to fuse them into place.
Heated filament-based materials (historically for prototyping) are comprised of thermoplastics. To extend the materials to applications beyond prototyping, second-phase particles are added to the heated mixture which solidify (cure) to form the overall material properties comprised of particles in a binding matrix when deposited onto a substrate. The particles are used to “tune” the binding matrix properties to the desired overall state. Specifically, much of the commercial additive manufacturing processes are polymer-based, with second-phase particles added to enhance the properties of the binder, which is typically either (1) polylactic acid or polylactide (PLA), which is a biodegradable thermoplastic aliphatic polyester or (2) acrylonitrile butadiene styrene (ABS) which is a common thermoplastic polymer. In 2015, PLA had the second highest consumption volume of any bioplastic of the world. PLA is derived from renewable resources, such as plants (corn starch, sugarcane, etc.). ABS is a terpolymer that is significantly stronger than PLA. It is made by polymerizing styrene and acrylonitrile in the presence of polybutadiene. The styrene gives the plastic a reflective surface, while the rubbery polybutadiene endows toughness. The overall properties are created by rubber toughening, where fine particles of elastomer are distributed throughout the rigid matrix. Typically, metal and ceramic particles are also added to endow specific mechanical, thermal, electrical, and magnetic effective overall properties.
Functionalized ink materials (primarily for printed electronics) are comprised of particles in a solvent/lubricant which cure when deposited. Oftentimes, these inks are used to lay down electric circuit lines or to have some other specific electromagnetic function on a surface. One application where such functionalized inks are important is printed electronics on flexible foundational substrates, such as flexible solar cells and smart electronics. One important technological obstacle is to develop inexpensive, durable electronic material units that reside on flexible platforms or substrates which can be easily deployed onto large surface areas. Ink-based printing methods involving particles are, in theory, ideal for large-scale electronic applications and provide a framework for assembling electronic circuits by mounting printed electronic devices on flexible plastic substrates, such as polyimide and “PEEK” (polyether ether ketone, a flexible thermoplastic polymer) film. There are many variants of this type of technology, which is sometimes referred to as flexible electronics or flex circuits. Flex circuits can be, for example, screen-printed silver circuits on polyester. For an early history of the printed electronics field, see Gamota . In order to develop flexible micro-/nanoelectronics for large area deployment, traditional methods of fabrication using silicon-based approaches have become limited for applications that involve large area coverage, due to high cost of materials and equipment (which frequently need a vacuum environment). For flexibility and lower cost, the ability to develop these electronics on plastics is necessary. To accomplish this task, print-based technologies are starting to become popular for these applications. In many cases, this requires the development of nanoparticle-functionalized “inks.” These nanoparticles include germanium (which has higher mobility and better tailorable absorption spectrum for ambient light than silicon) and silver (which is being studied due to the possibility to sinter the particles without the need of directly applied intense heating). Other semiconductor nanoparticles, including zinc- and cadmium-based compounds and metals, such as gold and copper, can be considered. Precise patterning of (nanoparticle-functionalized) prints is critical for a number of different applications. For example, some recent applications include optical coatings and photonics (Nakanishi et al. ), MEMS applications (Fuller et al. , Samarasinghe et al. , and Gamota et al. ), and biomedical devices (Ahmad et al. ). In terms of processing techniques, we refer the reader to Sirringhaus et al. , Wang et al. , Huang et al. , Choi et al. [11, 12, 13, 14], and Demko et al. [36, 37] for details.1 We further mention that electromagnetically sensitive fluids are typically constructed (“functionalized”) by embedding charged or electromagnetically sensitive particles in a neutral fluid. Such fluids date back, at least, to Winslow [19, 20] in 1947. While the most widely used class of such fluids are electrorheological fluids, which are comprised of extremely fine suspensions of charged particles (on the order of 50 microns) in an electrically neutral fluid, there has been a renewed interest in this class of materials because of so-called e-inks (electrically functionalized inks) driven by printed electronics. Inkjet printing is attractive due to its simplicity, high throughput, and low material loss. However, patterning with inkjet printing is limited to a resolution of around 20–50 \(\upmu \)m with current printers (Ridley et al. ) with higher resolution possible by adding complexity to the substrate prior to printing (Wang et al. ). Electrohydrodynamic printing has also been proposed to increase the resolution beyond the limits of inkjet printing, achieving a line resolution as small as 700 nm (Park et al. ).
Dry powder-based materials (primarily for sintered load-bearing structures) are deposited onto a surface and then heated by a laser, e-beam, or other external source, in order to fuse them into place. These types of applications and associated technology are closely related to those in the area of spray coatings, and we refer the reader to the extensive works of Sevostianov and Kachanov [23, 24, 25], Nakamura and coworkers: Dwivedi et al. , Liu et al. [27, 28], Nakamura and Liu , Nakamura et al.  and Qian et al.  and to Martin [32, 33] for the state of the art in deposition technologies. In powder-based processes, after deposition, laser processing is applied to heat particles in a powder to desired temperatures to either subsequently soften, sinter, melt or ablate them. Selective laser sintering was pioneered by Householder  in 1979 and Deckard and Beaman  in the mid-1980s.2 Laser-based heating is quite attractive because of the degree of targeted precision that it affords.3 Because of the monochromatic and collimated nature of lasers, they are a highly controllable way to process powdered materials, in particular with pulsing, via continuous beam chopping or modulation of the voltage. Carbon dioxide (\(CO_2\)) and yttrium aluminum garnet (YAG) lasers are commonly used. The range of power of a typical industrial laser is relatively wide, ranging from approximately 100–10000 W. Typically, the initial beam produced is in the form of collimated (parallel) rays, which are then focused with a lens onto a small focal point as fine as 0.00001 m in diameter. However, a chief concern of manufacturers are residual stresses and the microstructural defects generated in additively manufactured products, created by imprecisely controlled heat-affected zones, brought on by miscalibration of the laser power needed for a specific goal. In particular, because many substrates can become thermally damaged, for example, from thermal stresses, ascertaining the appropriate amount of laser input is critical.
Basic continuum mechanics,
Continuum characterization of particle-functionalized materials,
Continuum properties of mixtures and optimization,
CM approaches for ascertaining time-transient thermo-mechanical responses, residual stresses, and laser processing,
DEM approaches for modeling the deposition of dry powders,
DEM approaches for modeling laser–particle interaction, and
DEM approaches for modeling of advanced processing and the associated multiphysical effects.
Monograph Appendix 1: A review of essential mathematics,
Monograph Appendix 2: Continuum electrical properties of mixtures,
Monograph Appendix 3: Continuum properties of multiphase mixtures,
Monograph Appendix 4: Continuum fluid properties of mixtures, and
Monograph Appendix 5: Combining DEM and continuum approaches.
A closely related method, electron beam melting, fully melts the material and produces dense solids that are void-free.
There are a variety of other techniques that may be involved in an overall additive manufacturing processes, such as: (a) electron beam melting, which is a process by where powder is bonded together layer per layer with an electron beam in a high vacuum, (b) aerosol jetting, which consists of utilizing streams of atomized particles at high velocities toward a substrate, and (c) inkjet printing, which works by projecting small droplets of ink toward a substrate through a small orifice by pressure, heat, and vibration. The deposited material is then heated by UV light or other means to rapidly dry.
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