| SLA (STEREO LITHOGRAPHY APPARATUS) |
      SLA(Stereo Lithography Apparatus) prototyping parts are usually used to check appearance and fine particular, but fragile performance is weakness of SLA prototyping. SLA(Stereo Lithography Apparatus/Stereolithography) was developed by Chuck W. Hull in 1986. SLA is an additive manufacturing technology for producing models, prototypes, patterns, and in some cases, production parts. Technology description Stereolithography is an additive manufacturing process using a vat of liquid UV-curable photopolymer "resin" and a UV laser to build parts a layer at a time. On each layer, the laser beam traces a part cross-section pattern on the surface of the liquid resin. Exposure to the UV laser light cures, or, solidifies the pattern traced on the resin and adheres it to the layer below. After a pattern has been traced, the SLA's elevator platform descends by a single layer thickness, typically 0.05 mm to 0.15 mm. Then, a resin-filled blade sweeps across the part cross section, re-coating it with fresh material. On this new liquid surface, the subsequent layer pattern is traced, adhering to the previous layer. A complete 3-D part is formed by this process. After building, parts are cleaned of excess resin by immersion in a chemical bath and then cured in a UV oven. SLA requires the use of support structures to attach the part to the elevator platform and to prevent certain geometry from not only deflecting due to gravity, but to also accurately hold the 2D cross sections in place such that they resist lateral pressure from the re-coater blade.Supports are generated automatically during the preparation of 3D CAD models for use on the SlA machine, although they may be manipulated manually. Supports must be removed from the finished product manually; this is not true for all rapid prototyping technologies. Highlights of Stereo Lithography Apparatus * The first Rapid Prototyping technique and still the most widely used. * Inexpensive compared to other techniques. * Uses a light-sensitive liquid polymer. * Requires post-curing since laser is not of high enough power to completely cure. * Long-term curing can lead to warping. * Parts are quite brittle and have a tacky surface. * No milling step so accuracy in z can suffer. * Support structures are typically required. * Process is simple: There are no milling or masking steps required. * Uncured material can be toxic. Ventilation is a must. Introduction of Stereo Lithography Apparatus Stereo Lithography Apparatus (SLA), the first Rapid Prototyping 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. The Stereo Lithography Apparatus (SLA) System overall arrangement:  Stereo Lithography Apparatus Process The sequence of steps for producing an Stereo Lithography Apparatus (SLA) layer is shown in the following figures:      Uncured resin is removed and the model is post-cured to fully cure the resin. Because of the layered process, the model has a surface composed of stair steps. Sanding can remove the stair steps for a cosmetic finish. Model build orientation is important for stair stepping and build time. In general, orienting the long axis of the model vertically takes longer but has minimal stair steps. Orienting the long axis horizontally shortens build time but magnifies the stair steps. For aesthetic purposes, the model can be primed and painted. During fabrication, if extremities of the part become too weak, it may be necessary to use supports to prop up the model. The supports can be generated by the program that creates the slices, and the supports are only used for fabrication. The following three figures show why supports are necessary:    |
