Machinery's Handbook, 31st Edition
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ADDITIVE MANUFACTURING PLASTICS Table 1. Plastics Processes and Materials for 3D Printing
20–50
Stereolithography (SLA).—Under the action of light energy supplied by a laser beam, individual molecules in a photosensitive liquid resin cross-link into a long-chain mole - cule to form a solid polymer. Motion of the laser spot is controlled by galvanometer-driven mirrors to trace a two-dimensional (2D) slice of the three-dimensional (3D) CAD model. Commonly, the part is built from bottom up, as shown in Table 2A. First, the bottom slice of the part is produced on a platform that moves in the vertical direction. The plat - form is then lowered into a vat of photosensitive liquid resin, by one layer thickness (see Table 1). An arm sweeps across the surface to smooth it, and the next slice is traced by the laser. This action sequence is repeated until the part specified by the CAD file is produced. In a variation of this process, the platform is moved upward, while the laser beam proj - ects from below onto the bottom of a transparent tray containing photosensitive liquid resin, as shown in Table 2B . In this case, the top slice of the part is produced first, the build platform moves upward, in one layer thickness, and each new slice is added in a top-down approach. This method initially requires much less resin than the bottom-up approach. A version of the top-down SLA method, shown in Table 2C, uses a digital light proces- sor (DLP) to convert each layer of liquid resin into a solid 2D slice with a flash of light; this sequence is repeated until each slice of the 3D part is produced. The DLP consists of millions of tiny, movable micro-mirrors, whose tilt angles are individually computer con - trolled to reflect a single light source toward or away from the layer of resin. Those mirrors tilted toward the resin solidify the entire shape at once, while those tilted away reflect light onto a nonreflecting surface. In all SLA approaches, support structures are printed with the part to support overhang - ing sections of the part that otherwise would sag under gravity. Supports are removed from the finished part and the affected surfaces smoothed, though SLA produces the smoothest finish of all plastic 3D printing processes. It also produces the most accurate parts (approximately 0.002 in. or 0.05 mm) of the 3D printing processes. With SLA’s abil- ity to produce parts with high resolution and good surface finish, this process is suitable for functional prototypes of mechanisms, patterns for fine investment casting (such as jewelry), and form-and-fit models.
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