Stereolithography Apparatus (SLA)

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Stereolithography Apparatus (SLA)
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What is SLA?

• Stereolithography Apparatus (SLA) is a
  liquid-based process which builds parts directly
  from CAD software.
• SLA uses a low-power laser to harden
  photo-sensitive resin and achieve polymerization.
• A hopper full of the resin lies under a laser
  which uses its beam to harden the part in layers
  a Min. of 0.001 and up to 150lbs. Part weight.
• Parts can reach a Max. size of 20x20x23.

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Click here to view a diagram of a RP
Stereolithography Apparatus (SLA).
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SLA
Click here to view an SLA machine along with an
introduction.
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Companies

• 3D Systems Corp. in Valencia, CA.
• Other vendors are in the market but do not have
  significant market share.

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The Process

• The process begins with a 3D CAD file. The file
  is digitally sliced  into a series of parallel
  horizontal cross-sections which are then provided
  to a StereoLithography Apparatus (SLA) one at a
  time. A laser traces the cross-section onto a
  bath of photopolymer resin which solidifies the
  cross-section. The part is lowered a layer
  thickness into the bath and additional resin is
  swept onto the surface. The laser then solidifies
  the next cross-section. This process is repeated
  until the part is complete. Most parts are
  completed in a matter of hours, thereby defined
  as a "Rapid Prototype".

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The Process cont

• The process is still in use however, the software
  has changed dramatically.
• The technology first appeared on UNIX-Systems and
  MSDOS.
• Now, Windows NT, 3D Lightyear, and Buildstation
  are compatible.

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The Process cont

• A platform is lowered into the resin (via an
  elevator system), such that the surface of the
  platform is a layer-thickness below the surface
  of the resin. The laser beam then traces the
  boundaries and fills in a two-dimensional cross
  section of the model, solidifying the resin
  wherever it touches. Once a layer is complete,
  the platform descends a layer thickness, resin
  flows over the first layer, and the next layer is
  built. This process continues until the model is
  complete.

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The Process cont

• Once the model is complete, the platform rises
  out of the vat and the excess resin is drained.
  The model is then removed from the platform,
  washed of excess resin, and then placed in a UV
  oven for a final curing. The model is then
  finished by smoothing the "stair-steps."

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Advantages

• Probably the most accurate functional prototyping
  on the market.
• Smooth surface finish, high dimensional
  tolerance, and finely detailed features
  (thin-walls, sharp corners, etc)
• Used in Investment Casting, Wind Tunnels, and
  Injection Molding as tooling
• Resins can be custom engineered to meet different
  needs higher-temps, speed, finish

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Cost

• Cost of materials the resin costs as much as
  600-800 per gallon. (For a 20x20x23 cube
  thats 39.8 gallons! 27,860)
• Post processing Requirements
• Careful practices are required to work with the
  resins.
• Frameworks must be removed from the finished
  part.
• Alcohol baths then Ultraviolet ovens are used to
  clean and cure the parts.

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Development

• The Rapid Prototyping Stereolithography (SLA)
  process was developed by 3D Systems of Valencia,
  California, USA, founded in 1986. A vat of
  photosensitive resin contains a vertically moving
  platform. The part under construction is
  supported by the platform that moves downward by
  a layer thickness (typically about 0.1 mm / 0.004
  inches) for each layer. A laser beam traces out
  the shape of each layer and hardens the
  photosensitive resin.

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Development cont.

• The SLA rapid prototyping process was the first
  entry into the rapid prototyping field during the
  1980s and continues to be the most widely used
  technology.

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Highlights

• First Rapid Prototyping technique.
• Inexpensive compared to other techniques.
• Uses a light-sensitive liquid polymer.
• Requires post-curing.
• Long-term curing can lead to warping.
• Parts are quite brittle and have a tacky surface.
• No milling steps so z accuracy suffers.
• Support structures are typically required.
• Simple process No milling or masking steps
• Uncured material is toxic.

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Materials

• The laser can be either HeCd or Solid State and
  can range in power from 12 800mW
• The original resin was acrylate based, then
  epoxy-based ACES (Acrylic Clear Epoxy System).
• The resin can be modified to improve different
  characteristics depending on the users needs.

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Materials cont

• SLA Somos 7120 - A high speed general use resin
  that is heat and humidty resistant. For more
  information on this material please read the
  material technical data sheet. Somos 9120 - A
  robust accurate resin for functional parts. For
  more information on this material please read the
  material technical data sheet. Somos 9920 - A
  durable resin whose properties mimic
  polypropylene. Offers superior chemical
  resistance, fatigue properties, and strong memory
  retention. For more information on this material
  please read the material technical data sheet.
  Somos 10120 WaterClear - A general purpose resin
  with mid range mechanical properties. Transparent
  parts are possible if finished properly. Note
  that finishing for transparent parts must be
  special ordered. For more information on this
  material please read the material technical data
  sheet.

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Materials cont

• Somos 11120 WaterShed - Produces strong, tough,
  water-resistant parts. Many of its mechanical
  properties mimic that of ABS plastic. For more
  information on this material please read the
  material technical data sheet.
• Somos 14120 White - A low viscosity liquid
  photopolymer that produces strong, tough,
  water-resistant parts. For more information on
  this material please read the material technical
  data sheet.
• Somos ProtoTool - ProtoTool is a high density
  material that transcends currently available
  stereolithography resins by offering superior
  modulus and temperature resistance. For more
  information on this material please read the
  material technical data sheet.

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Models
Click here to view product examples from
AcuCast.com.
Click on Photo 009 to view models from their
StereoLithography Apparatus (SLA).
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REFERENCES

• http//www.acucast.com/rapid_prototyping.htm
• http//www.milparts.net/sla.html
• http//rpdrc.ic.polyu.edu.hk/old_files/sla_introdu
  ction.htm