Processes and printers
A large number of additive processes are available. The main differences
between processes are in the way layers are deposited to create parts
and in the materials that are used. Each method has its own advantages
and drawbacks, which is why some companies offer a choice of powder and
polymer for the material used to build the object. Others sometimes use
standard, off-the-shelf business paper as the build material to produce
a durable prototype. The main considerations in choosing a machine are
generally speed, costs of the 3D printer, of the printed prototype,
choice and cost of the materials, and color capabilities. Printers that
work directly with metals are generally expensive. However less
expensive printers can be used to make a mold, which is then used to
make metal parts.
ISO/ASTM52900-15 defines seven categories of Additive Manufacturing (AM)
processes within its meaning: binder jetting, directed energy
deposition, material extrusion, material jetting, powder bed fusion,
sheet lamination, and vat photopolymerization.
Some methods melt or soften the material to produce the layers. In Fused
filament fabrication, also known as Fused deposition modeling (FDM), the
model or part is produced by extruding small beads or streams of
material which harden immediately to form layers. A filament of
thermoplastic, metal wire, or other material is fed into an extrusion
nozzle head (3D printer extruder), which heats the material and turns
the flow on and off. FDM is somewhat restricted in the variation of
shapes that may be fabricated. Another technique fuses parts of the
layer and then moves upward in the working area, adding another layer of
granules and repeating the process until the piece has built up. This
process uses the unfused media to support overhangs and thin walls in
the part being produced, which reduces the need for temporary auxiliary
supports for the piece.
Laser sintering techniques include selective laser sintering, with both
metals and polymers, and direct metal laser sintering. Selective laser
melting does not use sintering for the fusion of powder granules but
will completely melt the powder using a high-energy laser to create
fully dense materials in a layer-wise method that has mechanical
properties similar to those of conventional manufactured metals.
Electron beam melting is a similar type of additive manufacturing
technology for metal parts (e.g. titanium alloys). EBM manufactures
parts by melting metal powder layer by layer with an electron beam in a
high vacuum. Another method consists of an inkjet 3D printing system,
which creates the model one layer at a time by spreading a layer of
powder (plaster, or resins) and printing a binder in the cross-section
of the part using an inkjet-like process. With laminated object
manufacturing, thin layers are cut to shape and joined together.???
Schematic representation of Stereolithography; a light-emitting device
a) (laser or DLP) selectively illuminate the transparent bottom c) of a
tank b) filled with a liquid photo-polymerizing resin; the solidified
resin d) is progressively dragged up by a lifting platform e)
Other methods cure liquid materials using different sophisticated
technologies, such as stereolithography. Photopolymerization is
primarily used in stereolithography to produce a solid part from a
liquid. Inkjet printer systems like the Objet PolyJet system spray
photopolymer materials onto a build tray in ultra-thin layers (between
16 and 30 Ķm) until the part is completed. Each photopolymer layer is
cured with UV light after it is jetted, producing fully cured models
that can be handled and used immediately, without post-curing.
Ultra-small features can be made with the 3D micro-fabrication technique
used in multiphoton photopolymerisation. Due to the nonlinear nature of
photo excitation, the gel is cured to a solid only in the places where
the laser was focused while the remaining gel is then washed away.
Feature sizes of under 100 nm are easily produced, as well as complex
structures with moving and interlocked parts. Yet another approach uses
a synthetic resin that is solidified using LEDs.
In Mask-image-projection-based stereolithography, a 3D digital model is
sliced by a set of horizontal planes. Each slice is converted into a
two-dimensional mask image. The mask image is then projected onto a
photocurable liquid resin surface and light is projected onto the resin
to cure it in the shape of the layer. Continuous liquid interface
production begins with a pool of liquid photopolymer resin. Part of the
pool bottom is transparent to ultraviolet light (the “window”), which
causes the resin to solidify. The object rises slowly enough to allow
resin to flow under and maintain contact with the bottom of the object.
In powder-fed directed-energy deposition, a high-power laser is used to
melt metal powder supplied to the focus of the laser beam. The powder
fed directed energy process is similar to Selective Laser Sintering, but
the metal powder is applied only where material is being added to the
part at that moment.
As of December 2017, additive manufacturing systems were on the market
that ranged from $99 to $500,000 in price and were employed in
industries including aerospace, architecture, automotive, defense, and
medical replacements, among many others. For example, General Electric
uses the high-end model to build parts for turbines. Many of these
systems are used for rapid prototyping, before mass production methods
are employed. Higher education has proven to be a major buyer of desktop
and professional 3D printers which industry experts generally view as a
positive indicator. Libraries around the world have also become
locations to house smaller 3D printers for educational and community
access. Several projects and companies are making efforts to develop
affordable 3D printers for home desktop use. Much of this work has been
driven by and targeted at DIY/Maker/enthusiast/early adopter
communities, with additional ties to the academic and hacker
communities.