page_title: 3D printing processes
section_title: Stereolithography
hierarchical_section_title: 3D printing processes / Processes / Stereolithography
caption_attribution_description: English: Schematic representation of stereolithography; a light-emitting device a) (laser or dlp) selectively illuminate the bottom of a tank b) filled with a liquid photo-polymerizing resin; the solidified resin d) is progressively dragged up by a lifting platform e).
context_page_description: A variety of processes, equipment, and materials are used in the production of a three-dimensional object via additive manufacturing. 3D printing is also known as additive manufacturing, therefore the numerous available 3D printing process tend to be additive in nature with a few key differences in the technologies and the materials used in this process.
  Some of the different types of physical transformations which are used in 3D printing include melt extrusion, light polymerization, continuous liquid interface production and sintering.
context_section_description: The Stereolithography (SLA) process is based on light curing (photopolymerization) of liquid materials into a solid shape; it  was patented in 1986 by Chuck Hull.
  In this process a vat of liquid polymer is exposed to controlled lighting (like a laser or a digital light projector) under safelight conditions. Most commonly the exposed liquid polymer hardens through cross-linking driven by the addition reaction of carbon carbon double bonds in acrylates.  Polymerization occurs when photopolymers are exposed to light when photopolymers contain chromophores, otherwise, the addition of molecules that are photosensitive are utilized to react with the solution to begin polymerization. Polymerization of monomers lead to cross-linking, which creates a polymer. Through these covalent bonds, the property of the solution is changed. The build plate then moves down in small increments and the liquid polymer is again exposed to light. The process repeats until the model has been built. The liquid polymer is then drained from the vat, leaving the solid model. The EnvisionTEC Perfactory is an example of a DLP rapid prototyping system.
  Inkjet printer systems like the Objet PolyJet system spray photopolymer materials onto a build tray in ultra-thin layers (between 16 and 30 µm) 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. The gel-like support material, which is designed to support complicated geometries, is removed by hand and water jetting. It is also suitable for elastomers. There is another type of inkjet printing system available in the market that can print a photopolymer in a layer-by-layer manner, with intermediate UV curing, to produce ophthalmic corrective lenses. No support structures are required in this case, as ophthalmic lenses do not need overhangs. Luxexcel, a Dutch company, has commercialized this technology and printing platform. 
  Ultra-small features can be made with the 3D micro-fabrication technique used in multiphoton photopolymerisation. This approach uses a focused laser to trace the desired 3D object into a block of gel. 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. The technique has been used to create objects composed of multiple materials that cure at different rates. In research systems, the light is projected from below, allowing the resin to be quickly spread into uniform thin layers, reducing production time from hours to minutes. Commercially available devices such as Objet Connex apply the resin via small nozzles.
  Continuous liquid interface production (CLIP) is another form of additive manufacturing that uses the DLP based photo polymerization process to create smooth-sided solid objects of a wide variety of shapes.  The continuous process of CLIP begins with a pool of liquid photopolymer resin. Part of the pool bottom is transparent to ultraviolet light (the "window"). Like DLP systems before it, ultraviolet light beam shines through the window, illuminating the precise cross-section of the object. The light 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. CLIP is different from traditional DLP processes, due to an oxygen-permeable membrane which lies below the resin, creating a "dead zone" (persistent liquid interface) preventing the resin from attach
caption: Schematic representation of Photopolymerization; 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)
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