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Figure 9-2. Food-safe ceramic, via powder bed/
inkjet process, followed by heat treatment
Figure 9-3. Fused nylon powder, via selective laser
sintering
Plastics
SLS is one of the most economical 3D print-
ing methods and is forgiving in terms of
design guidelines. Most vendors charge by
volume of powder consumed, so you can
often save money by “hollowing out” solid
models and printing them as shells. Most
vendors also charge for “trapped” powder,
however, so models printed as shells will
usually need to include at least one small
hole so that the powder can be recovered
when the print is done.
Stereolithography (SLA)
Stereolithography is the original 3D printing
process, in which a liquid plastic resin is se-
lectively hardened by exposure to high-
intensity light, often from a laser. After the
laser has drawn a 2D path along the surface,
the freshly polymerized model layer is low-
ered into the surrounding resin bath. The la-
ser traces over the fresh surface, curing and
joining the resin to the previous layer. SLA
produces prints of exceptional smoothness.
Photopolymer Jetting
Photopolymer jetting uses movable heads,
like an inkjet printer, to deposit droplets of
resin onto a build platform through a num-
ber of very small jets. Once the droplets are
in position, a UV lamp moves across the plat-
form to harden the resin. A support material
may be printed surrounding the droplets
and can be removed, manually or by wash-
ing, once the print is complete. Photopoly-
mer jetting can create very finely detailed
models with smooth surfaces and multiple
materials—tinted, clear, rigid, flexible, etc.—
in a single print ( Figure 9-4 ). It is not widely
available from 3D printing services, yet.
Selective Laser Sintering (SLS)
Selective laser sintering uses a high-power
laser to melt and fuse particles of very fine
plastic powder, often nylon ( Figure 9-3 ). The
laser scans across a leveled and compacted
powder surface, and when each layer is com-
pleted, the entire bed is lowered and fresh
powder is spread on top. As the laser works
its way across the new layer, molten powder
particles in the top surface fuse to each other
and to the layer below. The unfused powder
acts as support material, so SLS fabrication
works well for models that have thin sec-
tions, overhangs, or complex geometries.
 
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