ADDITIVE MANUFACTURING PLASTICS Machinery's Handbook, 31st Edition
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Table 2. Three Configurations of the Stereolithography (SLA) Process
(a) Bottom-up build by SLA with ultraviolet (UV) laser projecting from above: The build platform descends into a vat of photosensitive polymer, one layer at a time. A UV laser scans the cross section of the part on the top surface. As the laser moves across this surface, it solidifies the polymer, incrementally transforming each slice of the model into a layer of the 3D part.
A
B
C
(b) Top-down build by SLA with UV laser projecting from below: The build platform moves upward, one layer at a time, over a build tray containing photosensitive polymer. A UV laser scans the cross section of the part through the transparent base of the build tray. As the laser moves across and projects through the build tray, it solidifies the polymer, incrementally transforming each slice of the model into a layer of the 3D part.
A
B
C
(c) Top-down build by SLA with digital light processor (DLP) projecting from below: The build platform moves upward, one layer at a time, over a build tray containing photosensitive polymer. A DLP projects through the transparent base of the build tray. As the DLP projects the complete shape of each slice on the build tray, it solidifies the polymer, transforming the slice of the model into a layer of the 3D part all at once.
B
C
Selective Laser Sintering (SLS).—In this powder-bed layered process, parts are built within a build box on a build plate that moves vertically. First, a thin layer of polymer powder is spread on the build plate by a roller, blade, or hopper. Then an infrared laser travels across the surface, guided by galvanometer-driven mirrors (as in the bottom-up SLA process described earlier) and traces the 2D layer of the 3D part to be produced. Polymer powders within the laser spot are heated and fused together by sintering. Thus, thermoplastic materials are used in SLS processes. The build plate is moved down by one layer thickness (see Table 1) and a new layer of powder is spread, followed by the laser trace. This sequence is repeated until the 3D part is completed. Unused powder is removed from the build box, the part is brushed off to remove residual powder, and the unused powder is saved for reuse (supplemented by new powder) in future builds. Unprinted powder provides support for powder bonded by the laser, so supports are not added to parts produced using SLS printers. As a result, material is not wasted in supports, and multiple layers of parts can be produced in one build, improving productivity of SLS
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