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SLA (Stereolithography Apparatus) – Process using photosensitive
resins cured by a laser that traces the parts cross sectional geometry layer
by layer. SLA produces accurate models with a variety of material choices.
The SLA rapid prototyping process was the first entry into the rapid prototyping field
during the 1980’s and continues to be the most widely used technology. The SLA method
uses liquid photopolymer resins that are solidified by a laser to generate parts. An
SLA machine consists of the following parts: a build platform, resin vat, recoating
blade, ultraviolet laser and a scanning device. The build platform, which translates
up and down, is suspended in the vat of resin. The build platform is placed slightly
under the surface of the resin. A laser beam hardens the resin when it makes surface
contact. A scanning device, which controls the laser beam, traces the first cross section
of the prototype. The laser will trace the part as well as support structures necessary
to support any overhanging features. Once the first cross section is complete, the build
platform lowers one layer thickness into the vat. A recoating blade is then used to hasten
the process of covering the cross section with liquid resin. Once the first cross section
is suitably covered, the next cross section is scanned. This process repeats until the
part is complete. Once the part is completed, the build platform is raised and the excess
resin is allowed to drain. Depending on the material, a post cure operation is sometimes
needed to attain the desired material properties. After any final curing, the support
structures that were built to prevent any sagging are removed.
The SLA method of rapid prototyping provides a good combination of speed, accuracy and
surface finish. The main drawback is limited selection of materials for use as functional models.
The material selection for SLA is limited by the need to use a photo-curable resin. However,
material advancements are beginning to allow parts to be used more rigorously than they were
previously. In addition, newer materials can also be used to create color highlighted models.