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Polymer Era and the birth of 3D printing
As a result of the Industrial Revolution and the technological advances in petroleum and chemical engineering, a new class of materials emerged: the polymers. The first example of synthetic polymer was reported in 1907 by Leo Baekeland with a thermo-setting phenol-formaldehyde resin called Bakelite [4]. Polymers completely changed the materials world. Indeed, for the first time humans were able to manufacture objects starting from synthetic, not pre-existing, materials. World War II boosted the need for new polymers to replace materials that were hard to supply such as silk and natural rubber. Nylon and synthetic rubber were discovered and later advanced polymers were introduced, as Teflon and Kevlar [5]. Nowadays, polymers are used in almost every field, since they combine lightweight with unique properties such as chemical resistance, thermal stability, electric and barrier properties [6]. The possibility to start from a synthetic material allows a large control of its properties. Due to the low processing temperatures, it is possible to add specific additives and/or constituents to the final material. For these reasons, polymers science reached important goals, and nowadays it is almost impossible to think about a device without polymers.
The cold war and the space race between the Soviet Union and the United States boosted the research on innovative materials and technology. If materials have always been a pivotal point in human civilization progression, it is only during the Cold War that materials science began to be more widely recognized as a specific and distinct field of science and engineering. The pivot point was the document released by the US President Science Advisory Committee (PSAC) that stated materials were the bottleneck for space discovery and military technology. Thanks to both academic and industrial interests and through an interdisciplinary approach, incorporating elements of physics, chemistry, and engineering, materials science has grown rapidly since the 1950s. At the same time, major technical universities around the world created dedicated schools for its study. The first “material science and engineering” department was created in 1955 at Northwestern University (Evanston, Illinois, USA), while in Europe it was only inaugurated in the 1970s [7]. During the relatively peaceful period after World War II, several new technologies were developed such as electronics, telecommunications, and informatics, driving to the so-called “Digital Era” that, with a materials interpretation, it easy to describe as “Silicon” or “Polymer” Age, since the large make use of these materials in digital applications. Nowadays, computer numerical control (CNC) pushes for the development of electronics. CNCs are motorized machines that can be programmed by an external computer accomplishing precisely specific instructions. CNC limited once again the role of the human being in the manufacturing process, increasing the precision and the reliability of the produced objects. Simultaneously, the development of automation techniques increased the interest for an easier approach to the CNC machines. The effort on this topic led to the release of graphical computer-aided design (CAD) software, improving the programmability and the precision of CNC. The invention of CAD software is the first step for today’s manufacturing revolution: 3D printing. Until 1980s, the manufacturing techniques were based on the concepts developed in ancient times of material removal or molding, where only the procedure evolved from a hand-made approach to a large-scale fully industrialized production. Generally, in these processes a large amount of wasted material is produced. Moreover, not all the morphologies can be obtained, as often the intrinsic deformability of the material limits the possibility of having all shapes. The two abovementioned drawbacks have been solved by the development of the 3D printing technologies. Indeed, the ultimate object shape is no longer led by material removal, but by the addition of subunits of a row material, driven by a virtual construction of the object. Furthermore, 3D printing allows better control of materials properties, and recently, it is also opening new perspectives in materials micro/nano structuration, and programming smart and functional materials, boosting progresses in several application fields such as electronics, robotics, and biomedicine [8], [9]. Technological innovation is one of the main characters in the civilization transformation and evolution. In a synergic cycle, the availability of new materials and manufacturing processes often boosts the advances in several applications, and the last ones foster again the development of new materials. In the case of 3D printing technology, its birth was determined by the development of informatics and electronics; but the last two were possible by the progress in silicon technology. In the next section, we will describe the historical evolution of the 3D printing technologies, their spread in industrial applications, and the potential socio-economic implications and technological trends.
Additive Manufacturing and 3D printing
3D printing, also known as Additive Manufacturing (AM), is a groundbreaking technology that is rapidly becoming popular in many applications. This section aims to faithfully painting the historical effort made to widen the palette of available materials and to improve the printing technologies. The success of AM over the conventional manufacturing approaches are discussed, and several examples of industrial applications are reported. Besides, by this historical and economic analysis, the importance of a design-free approach, and the development of advanced materials in the growth of the 3D printing technology will be underlined.
3D Printing: An historical overview
The advances in graphic software as CAD, coupled with the important goals achieved in the polymers science, brought in the ’80s to the introduction of the 3D printing concept. In 1986, Charles Hull from Colorado University took advantage of his expertise in photopolymerization processes, realizing and patenting the first 3D printer device, as well as the STL file format used in the stereolithography software. At the same time, he founded 3D Systems, the first company specialized in 3D printing. The first 3D printer was a stereolithography (SLA) one building the physical product by the point-by-point polymerization of a photocurable acrylate resins according to the instructions contained in the virtual shapefile (STL file). SLA technique works by selectively exposing liquid resins to a laser light source, to form layers of solid polymeric materials that stack up to create a solid object. Owing to the large success of SLA, different printing techniques were developed by several research groups in the following years. Among them, Scott Crump in 1990 invented the Fused Deposition Modeling™ (FDM), also known as Fused Filament Fabrication (FFF), the most popular 3D printing technique, where a filament of a thermoplastic polymer is first molten and then deposited by a nozzle on a building platform according to the instruction given by the STL file. A detailed description of the operating principles of the most diffused 3D printing techniques is given in section 1.4. Another aspect that should not be overlooked is the development of new printable materials and design solutions. For instance, at the beginning of the 3D printing era, only acrylated polymers were available for SLA, and a restricted palette of low-temperature melting thermoplastic materials were processable via FFF. Later, not only the typologies of available polymeric material were increased, but also in 1993 waxy materials were successfully 3D printed by adding clays to the starting formulations or filament [12]. The introduction on the market of Selective Laser Sintering (SLS) opened the path to the 3D printing of polymeric, metal, and ceramic powders [12], [13]. In addition, from the 2000s onwards nanocomposites [14], organic and edible materials were profitably 3D printed [12]. Nowadays, advances are made to increase the volume and speed production, or to downscale the precision of manufactured objects in the micro and nanoscale, as for the Two Photons 3D Printing (2PP) [15]. From the materials point of view, researchers are developing structural and functional materials suitable for 3D printing techniques. Indeed, the possibility to manufacture materials with unique properties with no shape-limitation can help to design new devices in several application fields such as electronics, robotics, and medicines [15], [16]. Looking at the industrial applications, due to the low reliability and accuracy of the first SLA and FDM 3D printers, as well as the small palette of available materials, their use was only intended for rapid prototyping. However, nowadays AM technologies gained an important place in the automotive and aerospace industries [17]. The birth of new based-AM starts-up and companies boosted the 3D printing economy by introducing innovative technologies and materials, and, in turn, decreasing the costs related to the printing processes. Nowadays additive manufacturing is present in other industrial fields such as architecture and civil engineering for the realization of structural models, in medical and pharmaceutics where the clearer examples are the 3D printed dental prosthesis, and lately, fashion and food industries are just around the corner [18].
Economic Considerations
“A once-shuttered warehouse is now a state-of-the-art lab where new workers are mastering the 3D printing that has the potential to revolutionize the way we make almost everything” – Barack Obama, 2013 Although its fast expansion, there is still a remarkable gap between the capabilities reported by the media and the effective spread of 3D printing in industries and everyday life. The manufacturing revolution linked to 3D printing is in fact in contradiction with the general economic concept of large-scale production, and the transition may result hard. In large-scale economies, the rate and volume production of a good has a strong impact on the costs required to manufacture it, thus on the price of the object on the market. Instead, by additive manufacturing, the volume-cost-price correlation is no longer valid, since the production costs are only linked to the materials and energy consumption. The choice of the manufacturing technique is strictly related to the number of pieces to produce. In 2012 it was estimated that additive manufacturing was competitive to conventional inject molding for productions lower than 5,000 units [19]. If in 2012 the average price for a 3D printer was 10,000$ nowadays is possible to buy a DLP printer for 5,000$ and an FFF printer for less than 1,000$ [19]. Thanks to advances in technology and new materials available the threshold limit of 5,000 pieces is going to increase. The costs related to the fabrication of a single object are doubtless lower in 3D printing than in conventional manufacturing techniques, but in an economy of scale, where the production of multiple copies of the same identical object is required, conventional manufacturing processes result by far the best option [20]. Analyzing the diffusion of additive manufacturing in industrial fields, it is no wonder that their expansion concerns those fields where large-scale production is not a fundamental requirement: aerospace, fashion, architecture, education, and high-performance automotive [16].
Although the drawbacks of 3D printing technologies compared to large-scale production, in 2014 about 11% of the producers had already switched to volume production of 3D printed parts or products [19]. Indeed, additive manufacturing shows other advantages that traditional manufacturing techniques cannot achieve such as the reduction of inventory costs and risks. The on-demand production methodology guaranteed by additive manufacturing, avoid the storage of a large number of components and semi-finished products to assemble; moreover, the number of unsold finished goods is strongly decreased. Other advantages come from the low energetic consumption and the reduction of waste. It is estimated that with 3D printing is possible to save 40% of the waste material compared to subtractive manufacturing techniques, furthermore, almost 98% of the wasted material produced during a printing process can be recycled or even re-use [21]. In Tab. 1.1 the differences between additive manufacturing and traditional manufacturing are resumed. As an example of the side-benefits assured by 3D printing, and their exploitation in the industrial fabrication, the General Electrics (GE) case-study, concerning the production of fuel nozzles for jet engines, is here reported. GE expects to produce more than 45,000 units of the same object by year, so by considering only the production volume, traditional large-scale production would represent the best option. However, in the conventional manufacturing process, the gear is built by assembling 20 separately cast parts, while on the counterpart, additive manufacturing allows the production of fuel nozzle in a single process. Considering the costs related to the production and shipment of the cast parts, more the storage costs, GE convinced that 3D printing technology would cut the manufacturing costs by about 75% [19].
Table of contents :
1. INTRODUCTION TO 4D PRINTING
1.1 A short historical overview of the development of materials science and technologies: from
the Stone Age to 3D printing
1.1.1 From stones hitting to industrialized manufacturing process
1.1.2 Polymer Era and the birth of 3D printing
1.2 Additive Manufacturing and 3D printing
1.2.1 A problem of definition
1.2.2 3D Printing: An historical overview
1.2.3 Economic Considerations
1.3 Smart Materials and 4D Printing
1.3.1 Introduction to Smart Materials
1.3.2 The need for 4D Printing
1.3.3 Shape Morphing Materials
1.4 3D Printing Technology
1.4.1 From virtual file to real object
1.4.2 3D Printers Classification
1.4.3 Extrusion 3D Printers
1.4.4 Photopolymerization
1.4.5 Vat photopolymerization 3D Printers: Stereolithography and Digital Light Processing
2. MAGNETO RESPONSIVE POLYMERS: A SHORT OVERVIEW
2.1 Magnetic fillers
2.1.1 Properties of a magnetic materials
2.1.2 Magnetism at the nanoscale
2.2 Polymer matrix
2.3 Synthesis and properties of magneto responsive polymers
2.3.1 The intrinsic properties of the magneto-responsive polymers
2.3.2 The role of the design
2.4 Examples of magneto-responsive polymer composites
2.4.1 Magneto-rheological elastomers
2.4.2 Composite Hard Magnets
2.5 Magneto-driven soft actuators
2.51. Common strategies for MPAs fabrication, and their actuation mechanisms
2.5.2 Programmable magnetic anisotropy and microstructures
2.5.3 Magnetically Assisted Vat Photopolymerization 3D Printers
2.6 Final Remarks
3. 3D PRINTING OF MAGNETO-RESPONSIVE POLYMERS WITH RANDOMLY DISPERSED MAGNETIC NANOPARTICLES
3.1 Motivations of the work
3.2 Optimization of the Photocurable Resin Containing Magnetic Nanofillers
3.2.1 Selection of the photocurable formulation: the urethane-acrylate resin
3.2.2 Optimization of the formulation: the effect of the reactive diluent
3.2.3 Optimization of the formulation: the effect of the magnetite nanoparticles
3.3 Optimization of the 3D printing parameters
3.4 Magnetic properties of 3D printed samples
3.5 3D printing of magneto-responsive nanocomposite polymers with dispersed Fe3O4 nanoparticles
3.6 Conclusions
4. SELF-ASSEMBLY OF FE3O4 NPS DISPERSED IN PHOTOCURABLE RESINS AND INVESTIGATIONS ON THE ROTATION OF THE ASSEMBLED CHAINS
4.1 Introduction & motivation of the work
4.2 Physical considerations of the self-assembly process
4.2.1 Physical models of the self-assembly process in the Literature
4.2.2 The effect of external parameters
4.3 Physical model for self-assembly description
4.4 Investigations of the self-assembly process of magnetic particles at the nanoscale
4.5 Optical microscopy investigations on the self-assembly process of Fe3O4 NPs
4.5.1 Effect of the magnetic field intensity
4.5.2 Effect of the nanoparticles concentration
4.5.3 Effect of the viscosity of the liquid medium
4.5.4 Final remarks on the self-assembly process of Fe3O4 nanoparticles
4.6 Rotation of the assembled magnetic chains
4.6.1 Physical considerations about magnetic chains rotation
4.6.2 Preliminary studies on the rotation of magnetic chains
4.6.3 Limit cases ΔθB=30° and ΔθB=40°
4.6.4 Study on the I rotational regime of magnetic chains
4.6.5 Final remarks about the magnetic chains rotation
4.7 Conclusions
5. SET-UP OF A MAGNETICALLY ASSISTED DLP 3D PRINTER
5.1 Introduction
5.2 Introducing a magnetic field in the a DLP printer
5.2.1 Introducing a magnetic field in X-Y plane
5.2.2 Mapping of the magnetic field distribution in X-Y plane
5.2.3 Control of magnetic chains direction in the X-Y plane during the 3D printing process: a proof of concept.
5.3 Magnetic field in the Z direction
5.3.1 Solenoid set-up
5.3.2 Control the orientation of magnetic chains in the whole space: a proof of concept
5.4 Implementation of the set up into a DLP printer
5.5 Conclusion
6. 4D PRINTING OF MAGNETO RESPONSIVE COMPOSITES WITH PROGRAMMABLE MICROSTRUCTURES
6.1 Introduction
6.2 SEM investigations on the microstructure of the 3D printed samples
6.3 Mechanical Properties
6.4 Magnetic properties of the 3D printed samples
6.4.1 Measuring the magnetic anisotropy varying the orientation of the microstructures
6.4.2 Effect of NPs concentration
6.4.3 Comparison with the 50Eb50BA_6NPs system
6.5 Exploiting the magnetic anisotropies of 3D printed polymers: the role of the magnetic torques
6.6 Programmable magnetic-driven hammer-like actuators
6.6.1 Magnetic-responsive hammers flexible actuators
6.7 3D printed magneto-responsive polymeric devices
6.7.1 Magnetic-driven spur gears
6.7.2 Magnetic-driven gear-trains
6.7.3 Spur gear coupled with a rack
6.7.4 Magneto-responsive gripper
6.8 Conclusions
GENERAL CONCLUSIONS AND PERSPECTIVES
APPENDICES
A1 – Composition of the 3D printed formulations
A2 – Analysis of the optical microscopy images
A3 – List of tables
A4 – List of figures
REFERENCES
