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].
Next:
Advantages of Additive Manufacturing
