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In theory, the Stereo Lithography Process (SLA) is deceptively simple: photosensitive liquid plastic is hardened when a bright light strikes it. By controlling the location of the light beam only portions of the liquid plastic are hardened. By hardening one layer at a time on the surface of a vat of liquid plastic, a part is built vertically layer by layer.

In practice, the technology underlying this process is quite sophisticated. The photosensitive liquid plastic that is used in the SLA process is a resin that has been developed specifically for the rapid prototyping industry. The light source is a specialized ultraviolet laser with a beam diameter of less than .020" [.5 mm]. The mechanisms that control the position of the laser beam and the hardening resin are accurate and repeatable to very tight tolerances. When all the tolerances of the laser, the resin and the machine are put together, parts are routinely fabricated with a tolerance of better than +/- .005" [.13 mm]. At Nuhill Technologies, we use the SLA3500 rapid prototyping machine built by 3D systems to create the models required by our customers

Computer software is required to create the original virtual models of the parts to be prototyped. This is usually done with solid modeling computer aided design software. While there are other model data formats that can create STL files, solid model CAD files are the most common. Examples of other file types include VRML, OBJ and MRI. The model data is translated into a form of data used for stereolithography called the STL format. The STL format is a collection of flat, triangular facets that closely approximates the original model. STL formatted files are the input data for the SLA process.

Preparation for the SLA part fabrication process begins with the analysis and verification of the STL files. If the files are good, they are oriented in SLA space: commonly called the "build platform." Part orientation in the SLA space tries to take advantage of the layering process to maintain good definition of important features and surfaces. The orientation of the part also tries to minimize the negative effect of the support structures that are required by the SLA process.

To create the machine control codes for each layer, the SLA preparation process includes a part slicing step. The supports and the STL model are sliced along the build axis and the perimeter and and fill data for each slice is recorded. This becomes the instructions for positioning the laser beam on the photosensitive liquid plastic resin.

To begin the SLA process, a platform is lowered into the liquid plastic and a scaffold like support structure is fabricated on the platform. The supports are built up for a short distance from the platform and then the first layer of the actual part is created on top of the supports. The part is then grown layer by layer in the vat of liquid resin. The platform is lowered into the liquid resin after each new layer is created until the last layer is formed. The platform then rises from the vat of resin with the newly created parts mounted on top of their supports. At this phase in the process, the parts coming directly from the SLA machine are referred to as being "green." Once the parts are removed from the SLA machine, excess uncured resin is cleaned off the parts. Then the green parts are placed in a special apparatus and cured in a bath of ultraviolet light to complete the solidification process. The support structures are removed and the ridges that may have been created by the layering nature of the SLA process can be smoothed if needed. The final finishing work may include sanding or bead blasting, painting or any other procedure needed to create a useful part.

The finished part is generally quite strong and durable. It can deform under pressure, especially with elevated temperatures. It will not be as strong as injection molded ABS, for example, so care must be taken with the part. There are many techniques in use today for rapid prototyping. However, for most purposes, SLA is the best possible prototype technique for creating a durable part both quickly and accurately.