Stereolithography is a 3-D layering manufacturing process that applies ultraviolet lasers to cure liquid photopolymer resins to produce solid, plastic, 3-D objects that simulate properties of polyethylene, polypropylene and polycarbonate materials.

The SLA machine’s software sections the 3-D CAD model into multiple thin layers (typically five to 10 layers per millimeter). In the SLA machine, the ultraviolet lasers basically paint one of the model layers on the liquid plastic, curing it. The machine drops down a minuscule amount in the liquid, and the laser then paints another layer again. This process is repeated until the model (or stack up of layers) is complete.

 Typically the time for this process is six to 12 hours for small to medium sized parts. Accuracy of the machine is approximately ±0.005-inches—depending on part size. Once complete, the parts are finally fully cured in an ultraviolet oven. Typical costs for these services range from $55 to $75/hr depending on part complexity and the material used. More advanced prototyping materials are being developed continuously.

 Selective laser sintering (SLS) is another type of prototyping process. With SLS, the model is made from heated powder with a laser that creates the 3-D layers by fusing the particles together.

   These powdered materials can be nylon, glass-filled nylon, polystyrene or alumide resins. Like SLA, the computer software of the machine takes 3-D data of the part, slices it into layers and laser sinters (melts) the powder, layer by layer. Once one layer is completed, new powder is deposited on top of the solid layer and sintered into another layer. Typical costs for these services range from $55 to $75/hr—depending on part complexity.

  SLA and SLS both produce very good prototypes, but their individual application depends on the desired requirements for the final model. For example, if the prototype requires durability and good structural properties for functionality testing and assembly trials, then SLS is the recommended process. If the desire is for a model with excellent finish properties—like an exhibition piece or the part is to be used as a master to produce RTV or epoxy molds—then SLA should be used.

  The Process Breakdown
The prototyping process can be broken down into three stages after the initial 3-D CAD design data is submitted:
Rapid prototype a functional model of the part by either SLA or SLS—depending on requirements.

  Validate the design, fit and finish.
Either machine the finished mold at that point or produce an RTV silicone or epoxy mold from the master and pour cast urethane to simulate the part. Although there may be some minimal elongation issues, parts produced from cast urethane have good surface quality and will exhibit 85 to 90 percent of the characteristics of the thermoplastic that will ultimately be used for the production parts. Also, with silicone or epoxy molds, B-surface attachments and clips can be molded right in place.

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