Additive manufacturing (AM) alias 3D printing translates computer-aided design (CAD) virtual 3D models into physical objects. By digital slicing of CAD, 3D scan, or tomography data, AM builds objects layer by layer without the need for molds or machining. AM enables decentralized fabrication of customized objects on demand by exploiting digital information storage and retrieval via the Internet. The ongoing transition from rapid prototyping to rapid manufacturing prompts new challenges for mechanical engineers and materials scientists alike. Because polymers are by far the most utilized class of materials for AM, this Review focuses on polymer processing and the development of polymers and advanced polymer systems specifically for AM. AM techniques covered include vat photopolymerization (stereolithography), powder bed fusion (SLS), material and binder jetting (inkjet and aerosol 3D printing), sheet lamination (LOM), extrusion (FDM, 3D dispensing, 3D fiber deposition, and 3D plotting), and 3D bioprinting....

Polymers for 3D Printing and Customized Additive Manufacturing

An introduction to heat transfer

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

New microproducts need the utilization of a diversity of materials and have complicated three-dimensional (3D) microstructures with high aspect ratios. To date, many micromanufacturing processes have been developed but specific class of such processes are applicable for fabrication of functional and true 3D microcomponents/assemblies. The aptitude to process a broad range of materials and the ability to fabricate functional and geometrically complicated 3D microstructures provides the additive manufacturing (AM) processes some profits over traditional methods, such as lithography-based or micromachining approaches investigated widely in the past. In this paper, 3D micro-AM processes have been classified into three main groups, including scalable micro-AM systems, 3D direct writing, and hybrid processes, and the key processes have been reviewed comprehensively. Principle and recent progress of each 3D micro-AM process has been described, and the advantages and disadvantages of each process have been presented.

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A review on 3D micro-additive manufacturing technologies

3D printing has the potential to significantly change the field of microfluidics. The ability to fabricate a complete microfluidic device in a single step from a computer model has obvious attractions, but it is the ability to create truly three dimensional structures that will provide new microfluidic capability that is challenging, if not impossible to make with existing approaches. This critical review covers the current state of 3D printing for microfluidics, focusing on the four most frequently used printing approaches: inkjet (i3DP), stereolithography (SLA), two photon polymerisation (2PP) and extrusion printing (focusing on fused deposition modeling). It discusses current achievements and limitations, and opportunities for advancement to reach 3D printing's full potential.

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3D printed microfluidic devices: enablers and barriers

Multi-lattice inner structures for high-strength and light-weight in metal selective laser melting process

Experimental studies on the fabrication of sub-30-nm nanofibers using two-photon initiated photopolymerization (TPP) have been carried out. To generate nanofibers at the interior region of microstructures, a photopolymerization method involving a long laser-exposure technique (LET) is proposed. A multitude of nanofibers with a notably high resolution (about 22nm) in TPP were produced using the LET. Furthermore, it is also demonstrated that thin interconnecting networks were created regularly in a weakly polymerized region existing around the boundary of a densely polymerized voxel, allowing for the creation of various embossing patterns. By controlling the distance between adjacent voxels or lines, a selective generation of nanofibers in a local area is possible, which leads to the fabrication of high-functional filters and mixers. Embossing patterns and microchannels including nanofibers inside were fabricated by the LET so as to demonstrate the practical feasibility of this approach. These sub-30-nm nan...

Fabrication of a bunch of sub-30-nm nanofibers inside microchannels using photopolymerization via a long exposure technique

A subregional slicing method (SSM) is proposed to increase the nanofabrication efficiency of a nanostereolithography (NSL) process based on two-photon polymerization (TPP). The NSL process can be used to fabricate three-dimensional (3D) microstructures via the accumulation of layers of uniform thickness; hence, the precision of the final 3D microstructure depends on the layer thickness. The use of a uniform layer thickness means that, to fabricate a precise microstructure, a large number of thin slices is inevitably required, leading to long processing times. In the SSM proposed here, however, the 3D microstructure is divided into several subregions on the basis of the geometric slope, and then each of these subregions is uniformly sliced with a layer thickness determined by the geometric slope characteristics of each subregion. Subregions with gentle slopes are sliced with thin layer thicknesses, whereas subregions with steep slopes are sliced with thick layer thicknesses. Here, we describe the procedure of the SSM based on TPP, and discuss the fabrication efficiency of the method through the fabrication of a 3D microstructure.

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Subregional slicing method to increase three-dimensional nanofabrication efficiency in two-photon polymerization

Generally, it is known that a welding technology is not suitable for repairing casting parts due to metallurgical issues and high carbon contents. So, the repairing of cast iron by welding has not been satisfactorily realized yet, resulting in microstructure changes in the welding area, which greatly impacts on crack initiation and growth as well as Martensite formation in the repaired area and heat affected zone (HAZ). In this study, we evaluated how the HAZ affected the mechanical properties of gray cast iron after it was repaired by the traditional welding process (shielded metal arc welding) and by the additive metal-layer deposition (AMD) process using a laser. The results indicated that the ultimate strength of each repaired part reduced by up to 16% when compared to that of the original specimen composed of only cast iron, FC300. However, the elongations of the specimens varied according to the repair methods. In the case of repair welding, the elongation decreased by approximately 20%. However, in the case of repairing by AMD, the elongation increased dramatically to approximately 60%. Hence, the proposed repairing method based on AMD appears to be a promising method for repairing castings.

Repairing casting part using laser assisted additive metal-layer deposition and its mechanical properties

A technique in ultraviolet nanoimprint lithography (UV-NIL) for the creation of three-dimensional (3D) nanopatterns in a single step is proposed. The single-step fabrication of 3D or multilevel structures has a multitude of benefits. Inherent in this is the elimination of a need for alignment for multilevel fabrications as well as being a cost effective and simple process. For 3D UV-NIL, a trial in the fabrication of multilayered stamps has been conducted employing two-photon polymerization and diamondlike carbon (DLC) coating technique. The DLC coating layer enables the polymer patterns to be used effectively as a stamp without the need for an antiadhesion material. Additionally, O2-plasma ashing has the potential for an epoch-making improvement of the precision of polymer patterns with a linewidth of 60nm. Overall, several fine patterns are imprinted using the multilayered stamp onto a UV-curable resist via a single-step process without any identifiable damage.

Sang-Hu Park

Papers

Multi-lattice inner structures for high-strength and light-weight in metal selective laser melting process

Fabrication of a bunch of sub-30-nm nanofibers inside microchannels using photopolymerization via a long exposure technique

Subregional slicing method to increase three-dimensional nanofabrication efficiency in two-photon polymerization

Repairing casting part using laser assisted additive metal-layer deposition and its mechanical properties

Effective fabrication of three-dimensional nano/microstructures in a single step using multilayered stamp