Additive Manufacturing Processes

There are many techniques for layer by layer addition of materials such as Fused Filament Fabrication (FFF), Stereo Lithography (SL), and Laser Sintering (LS) etc. which will be explained below but common to all of them, the first step is to design the model using CAD tools. The CAD model is then converted to suitable machine understandable format like STL, VRML, AMF or G Code. The converted file is then input to the 3D printer. The software installed in the 3D printer reads the file and slices the model into different layers. The thickness of the layer can be adjusted. As the thickness decreases the quality increases but processing time also increases. The head of the 3D printer moves according to the cross section of the model and depending upon the process, one layer gets made. This process repeats so that the entire 3D model is manufactured. The support structures if any can be removed by using different techniques such as immersing in a special liquid after the production stage. The different steps are shown in the Fig.1.2.

Fig.1.2 3D Printing Steps (Medfab, 2018)

Stereolithography:

Stereolithography (SL) is the first commercialized 3D printing process. SL is a laser-based process that works with photopolymer resins. As illustrated in Fig.1.3, when the laser hit on the resin, it cures to form a solid in a very precise way. The photopolymer resin is held in a vat with a movable platform inside. A laser beam is directed in the X-Y axes across the surface of the resin according to the 3D data supplied to the machine (the .stl file). The resin hardens precisely where the laser hits the surface. Once the layer is completed, the platform within the vat drops down by a fraction (in the Z axis) and the subsequent layer is traced out by the laser. This continues until the entire object is completed and the platform can be raised out of the vat for removal (Think3d.in, 2018).

Fig.1.3 Stereolithography Process (Reddy and Upputuri, 2018a)

Because of the nature of the SL process, it requires support structures for some parts, specifically those with overhangs or undercuts. These structures need to be manually removed. In terms of other post processing steps, many objects 3D printed using SL need to be cleaned and cured. Curing involves subjecting the part to intense light in an oven-like machine to fully harden the resin. It is one of the most accurate 3D printing processes with excellent surface finish. However limiting factors include the post-processing steps required and the stability of the materials over time which can become more brittle (Ngo et al., 2018).

Laser Sintering:

Laser sintering is also a laser based 3D printing process but works with powdered materials. As shown in Fig.1.4, according to the 3D data fed to the machine, the laser is traced across a powder bed of tightly compacted powdered material. As the laser interacts with the surface of the powdered material, it sinters or fuses the particles to each other forming a solid. As each layer is completed, the powder bed drops incrementally and a roller smoothens the powder over the surface of the bed prior to the next pass of the laser for the subsequent layer to be formed and fused with the previous layer (Srinivas and Babu, 2017).

Fig.1.4 Laser Sintering Process (Reddy and Upputuri, 2018b)

The build chamber is completely sealed as it is necessary to maintain a precise temperature during the process specific to the melting point of the powdered material of choice. Once finished, the entire powder bed is removed from the machine and the excess powder can be removed to leave the printed parts.

One of the key advantages of this process is that the powder bed serves as an in-process support structure for overhangs and undercuts therefore complex shapes that could not be manufactured in any other way are possible with this process. However on the downside, because of the high temperatures required for laser sintering, cooling times can be considerable (Think3d.in, 2018).

Free Form Fabrication:

3D printing utilizing the extrusion of thermoplastic material is the most common and recognizable 3DP process. Fused Filament Fabrication (FFF) is one of such process. It works by melting plastic filament that is deposited via a heated extruder a layer at a time, onto a build platform according to the 3D data supplied to the printer. Each layer hardens as it is deposited and bonds to the previous layer (See Fig.1.5).

Fig.1.5 Fused Filament Fabrication Process (Atta, 2018)

The most common materials used by are ABS and PLA (Think3d.in, 2018). The FFF processes require support structures for any applications with overhanging geometries. It is one of the limitations of the FFF 3D printers. However as the systems have evolved and improved to incorporate dual extrusion heads, it has become less of an issue. In terms of models produced, the FFF process produces much less accurate models but things are constantly improving. The process can be slow for some part geometries and layer-to-layer adhesion can be a problem, resulting in parts that are not watertight. Again, post-processing using Acetone can resolve these issues (En.wikipedia.org, 2018a).

References:

Atta, E. (2018). Comparison Between Fused Deposition Modelling (FDM) and Selective Laser Sintering (SLS). [online] Green-mechanic.com. Available at: https://www.green-mechanic.com/2016/12/comparison-between-fused-deposition.html [Accessed 13 Jan. 2018].

En.wikipedia.org. (2018a). Fused filament fabrication. [online] Available at: https://en.wikipedia.org/wiki/Fused_filament_fabrication [Accessed 10 Jan. 2018].

Medfab. (2018). Post-Processing - medfab. [online] Available at: http://medfab.de/post-processing/ [Accessed 12 Jan. 2018].

Ngo, T., Kashani, A., Imbalzano, G., Nguyen, K. and Hui, D. (2018). Additive manufacturing (3D printing): A review of materials, methods, applications and challenges. Composites Part B: Engineering, 143, 172-196.

Reddy, P. and Upputuri, R. (2018a). Stereolithography (SLA) Technology Overview | think3D. [online] Think3D. Available at: https://www.think3d.in/stereolithography-sla-technology-overview/ [Accessed 13 Jan. 2018].

Reddy, P. and Upputuri, R. (2018b). Direct Metal Laser Sintering (DMLS) Overview | think3D. [online] Think3D. Available at: https://www.think3d.in/direct-metal-laser-sintering-dmls-technology-overview/ [Accessed 12 Jan. 2018].

Srinivas, M. and Babu, B. (2017). A Critical Review on Recent Research Methodologies in Additive Manufacturing. Materials Today: Proceedings, 4(8), 9049-9059.

Think3d.in. (2018). [online] Available at: https://www.think3d.in/landing-pages/beginners-guide-to-3d-printing.pdf [Accessed 7 Jan. 2018].

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