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Introduction: Additive/3D Printing Materials—Filaments, Functionalized Inks, and Powders

  • Tarek I. ZohdiEmail author
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Part of the Lecture Notes in Applied and Computational Mechanics book series (LNACM, volume 60)

Abstract

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) [1] on the wide array of activities in the manufacturing community in this area.

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. [1] on the wide array of activities in the manufacturing community in this area. One subclass of AM, so-called 3D printing (3DP), has received a great deal of attention over the last few years. Typically, such a process takes CAD drawings and slices them into layers, printing layer by layer. 3DP was pioneered by Hull [2] of the 3D Systems Corporation in 1984. 3DP was a 2.2 billion dollar industry in 2014, with applications ranging from motor vehicles, consumer products, medical devices, military hardware, and the arts.
Fig. 1.1

Typical printing ingredients: top left: finely ground metallic powder (iron). Top right: extruded PLA. Bottom left: ABS pellets and bottom right: coarsely ground steel flakes

A key ingredient of these processes is the specialized materials and the precise design of their properties, enabled by the use of fine-scale “functionalizing” particles. The rapid rise in the use of particle-based materials has been made possible by the large-scale production of consistent, high-quality particles, which are produced in a variety of ways, such as: (a) sublimation from a raw solid to a gas, which condenses into particles that are recaptured (harvested), (b) atomization of liquid streams into droplets by breaking jets of metal, (c) reduction of metal oxides, and (d) comminution/pulverizing of bulk material. As mentioned in the preface, particle-functionalized materials play a central role in this field, in three main ways:
  1. (1)

    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,

     
  2. (2)

    To “functionalize” inks by adding particles to freely flowing solvents forming a mixture, which is then deposited onto a surface, and

     
  3. (3)

    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.

     
In more detail, we have (see Fig. 1.1):
  • 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 [3]. 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. [4]), MEMS applications (Fuller et al. [5], Samarasinghe et al. [6], and Gamota et al. [3]), and biomedical devices (Ahmad et al. [7]). In terms of processing techniques, we refer the reader to Sirringhaus et al. [8], Wang et al. [9], Huang et al. [10], 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. [21]) with higher resolution possible by adding complexity to the substrate prior to printing (Wang et al. [9]). 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. [22]).

  • 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. [26], Liu et al. [27, 28], Nakamura and Liu [29], Nakamura et al. [30] and Qian et al. [31] 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 [34] in 1979 and Deckard and Beaman [35] 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.

Fig. 1.2

Left: a linkage schematic of a 3D printer. Right: a multiphase droplet representation using the Discrete Element Method

1.1 Objectives

In order for emerging additive manufacturing approaches to succeed, such as the ones mentioned, one must draw upon rigorous theory and computation to guide and simultaneously develop design rules for the proper selection of particle, binder, and solvent combinations for upscaling to industrial manufacturing levels (Fig. 1.2 ). This motivates the content of this monograph. This monograph is broken into two main methodologies: “Continuum Method” (CM) approaches and “Discrete Element Method” (DEM) approaches. The materials associated with heated filament and functionalized-ink methods are closely related types of continua (particles embedded in a continuous binder) and are analyzed using continuum approaches. The dry powder materials, which are of a discrete particulate character, are analyzed using discrete element methods for the deposition phase of the analysis, and continuum approaches are used for the curing (cooling) stress analysis. This monograph seeks to introduce the reader to some of the main approaches for modeling and simulation of particle-based materials used in additive manufacturing, namely:
  • 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.

In addition to appendices within the chapters themselves (labeled “Chapter Appendices”), background material is also included in the “Monograph Appendices” on the following related topics:
  • 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.

Footnotes

  1. 1.

    For reviews of optical coatings and photonics, see Nakanishi et al. [4] and Maier and Atwater [15], for biosensors, see Alivisatos [16], for catalysts, see Haruta [17], and for MEMS applications, see Fuller et al. [5] and Ho et al. [18].

  2. 2.

    A closely related method, electron beam melting, fully melts the material and produces dense solids that are void-free.

  3. 3.

    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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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.University of CaliforniaBerkeleyUSA

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