Classification of Different 3D Printing Methods

There are numerous ways to distinguish and classify different 3D printing technologies. One popular method considers the dimensional order of the manufacturing process, such as point movement or line movement of the material unit to form an item [28, 35]. The other popular approach is to identify the source material morphology, like powder, wire, liquid, and sheet [35, 36]. The most widely accepted method is to classify 3D printing according to the material reshaping process such as laser heating, cumulative 2D printing, material extrusion, and liquid curing method. [1, 37]. However, some processes get lumped together in what seems to be odd combinations (like selective laser sintering being grouped

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with cumulative 2D printing). And some processes that may appear to produce similar results end up being separated (like Stereolithography and Objet technology) [38, 39]. Therefore, a more understandable categorisation method according to the type of layer formation is introduced in this section. They are material extrusion, photopolymer curing, powder binding, powder melting, and sheet binding [35, 40, 41].

Material extrusion method

Material extrusion method refers to those AM processes that extruding source material from a nozzle, and reshaping the material keeping its spatial position by different methods [1]. The material reshaping can be realized by powder melting [38, 42], wire melting [43], and chemical reaction [44]. Figure 2-3 demonstrates an example using Direct Ink Writing (DIW) method [44]. It fabricates a 3D model using the ink loaded into a syringe, and then it mounted on the robotic arm to control the movement and material extrusion of the machine [45].

This project selected the FDM method for thermoplastic manufacturing. The FDM is the cheapest, most widely adopted and most accessible 3D printing method worldwide [18]. However, poor surface roughness of FDM manufactured objects greatly limits its applications to toys, figures, or prototypes which only require shape realisation regardless of surface quality nor colour [27].

Photopolymer curing method

This method uses liquid photosensitive resin as source material to build an item. A liquid bed is treated under curing light and the exposed material is solidified, then a fresh layer of liquid photopolymer will be deposited on the top for the next layer. As shown in Figure 2-4, Digital Light Processing (DLP) and

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Figure 2-3 Multi-material DIW for complex 3D metallic structures with removable supports [44].

Figure 2-4 Photopolymer curing methods, (a) digital light apparatus (DLP), (b) stereolithographic apparatus (SLA) [46].

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Stereolithography Apparatus (SLA) are the two main methods used with projection or laser light as a curing source [46]. This method usually achieves high spatial resolution (generally 0.015mm at spatial direction) [47]. A disadvantage is that the wax-like material is usually not strong enough for load- bearing purposes, and the material and small print volume largely constraint the applications of this method. In real cases, lost-wax-casting technology is usually applied to replace the resin by functional materials but maintain the as-printed geometry. [29]

Powder binding method

This method uses material powder binding for its reshaping process. As demonstrated in Figure 2-5, the fine powder is paved evenly on a workbench [48]. The machine binds a thin layer of material powder at the designed place, and then another fresh layer is paved onto the surface for sequential binding, repeatedly accomplishing a 3D part printing [49, 50]. The method can either extrude binder to stick the powder together, or extrude powder into the binding bed [49, 51] [52]. The machine can also use sticky coloured ink as binder to fabricate multi-colour object, and use functional powder such as ceramic for wear resistance ability [1, 28]. The object usually has well recognizable colour, as demonstrated in Figure 2-6 [53]. The excellent performance and accuracy ensure the powder binding technology has extensive applications in industry [54].

Powder melting method

This method melts the material powder by heating sources. The melt will cool down and solidify very quickly to maintain its spatial position and bonded together to build up an item [55]. For example, Figure 2-7 demonstrate the diagram of selective laser melting (SLM) method. A thin layer of material on powder bed is

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Figure 2-5 Schematic of 3D printing method, (a) powder distribution on build tray, (b) binder deposition on powder, (c) 3D object near completion [48].

Figure 2-6 Polyjet fabricated multi-material fetal replica with internal structures, including major organs and the developing skeletal system clearly visible [53].

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melted by powerful laser scanning and then deposit on the substrate, layer by layer accumulatively to form a 3D part [1].

Metal 3D printing is usually realised by the powder melting method. In the industry an electron beam, laser, or plasma is used as a powerful heating source to melt metal powder. The powder bed method and the powder extrusion method are both applied in metal 3D printing production [56]. However, high cost and poor surface quality are the two main obstructions of its development [1, 39]. SLM is selected in this study to fabricate medical product, which is able to fabricate high- performance metal components and has been widely accepted in industries such as automotive and aerospace. On the other hand, the SLM machine is too expensive for small-scale enterprises, educational institutions nor individuals, which narrows its applications to large companies and government-funded institutions.

Sheet binding method

The idea of a sheet binding method is to cumulate 2D printing pages and cut out the external contour of each layer. As shown in Figure 2-8, the upper layer is sticked onto the layer beneath it, and the 2D printing and contour cutting process is repeated to finalize the final parts. The printed component will be in the stack of paper, and wastes will be removed when the printing process is finished by breaking the outer shell cut by laser [57, 58].