Machinery's Handbook, 31st Edition
614 ADDITIVE MANUFACTURING PLASTICS machines. SLS parts have a rougher surface finish than SLA parts, and feature resolu - tion is less. Applications of SLS include functional prototypes, design evaluation models, short-run bridge production, and casting patterns. Fused Deposition Modeling (FDM).—Also called material extrusion , this process in- volves feeding a thermoplastic filament from a coil into a heated nozzle, which melts the plastic and extrudes it into a fine filament, typically 0.004 in. (0.102 mm) in diameter (Fig. 1 ). The nozzle is attached to a precision XYZ motion system that traces the first 2D layer, as defined by the sliced CAD file, onto a platform. The platform then moves down one layer thickness, and the next layer is traced; this process is repeated until the defined model is completed. Since the extruded plastic is soft, it requires supports, extending from the build platform upward to any overhangs of the part; after the part is completed and separated from the build platform, supports are easily broken off. Because the part is made of extruded fila - ments, the surface will be bumpy. Surface finishing can be accomplished by sanding or exposing the part to boiling acetone, which dissolves the surface slightly and allows it to reflow to a smooth finish.
Filament
Rollers
Insulating Plate
Filament Heater
Nozzle
Fig. 1. Fused Deposition Modeling (FDM) Process FDM machines are the least expensive plastics prototyping machines because they do not involve a laser and the heated extrusion head is widely available. Filaments for FDM are readily made by extrusion of thermoplastic feedstocks. Some feedstocks include re- inforcing particles of glass or chopped graphite fiber, adding strength to printed parts. One FDM method uses two nozzles and can produce multicolored parts. Or one of the nozzles can extrude a water-soluble support material that can be removed easily by soak - ing parts in water, reducing post-processing labor. Another FDM process includes a separate nozzle for inserting continuous fibers of graphite, fiberglass, or Kevlar. This type of machine produces parts with selective rein - forcement in high-stress areas. Applications for FDM are based on its low cost but limited resolution. These include proof-of-concept models, low-cost prototyping, sand casting patterns, and jigs and fix - tures. Because of its low cost, FDM has brought AM to a large number of nontechnical users, such as hobbyists, experimenters, and model-makers. In addition, FDM machines have provided a convenient and low-cost vehicle for introduction of students at all levels to the basic concepts of design and manufacturing. Binder Jetting (BJ).—In this process, a layer of powder is spread on a build plate in a build box, similar to SLS. However, instead of bonding powder by sintering with the heat of an infrared laser, droplets of binder are jetted from a printhead, with thousands of jets, as it passes over the powder layer, creating a 2D layer. After each pass, the build platform is lowered by one layer thickness, a new layer of powder is spread, and the print head passes over the new layer. This process is repeated until the complete 3D part is produced.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online