Machinery's Handbook, 31st Edition
ADDITIVE MANUFACTURING PLASTICS 615 Binder droplets can be thought of as dots that combine to produce each slice of the CAD file. The binder not only bonds particles within each droplet, it also bonds each dot to its neighbors and penetrates to the slice below to ensure strength for subsequent handling. Because the droplet mass is negligible, bonded powder particles have the same density as unbonded particles, and supports are not required. The finished part is removed from the build box, and the unbonded powder is removed and reused. Starch or plaster powder is commonly used in binder jetting. To add strength to the part, it may be infiltrated or painted with epoxy and cured. Material Jetting (MJ).—Something of a hybrid process, material jetting involves print - ing fine droplets of light-curable resin onto a build plate. Immediately after the printhead deposits the resin droplets, a light bar passes over the build plate, solidifying the resin in a polymerization process similar to SLA, producing thermoset plastics. Overhangs require supports to prevent sagging of solidified material. Some advanced versions of MJ involve multiple printheads; each one can be set up to jet a different resin. Various resins harden into solid polymers of different color and hard- ness. As a result, material jetting can be used to produce product prototypes and parts with varying material properties, such as a toothbrush with a rigid handle, soft grip, and stiff bristles. Multijet Fusion (MJF).—A powder bed fusion process like binder jetting, multijet fu- sion uses two types of binders for selective fusion. A layer of plastic powder is spread, a printhead deposits the two binders, and intense infrared radiation is applied to the layer. One binder is a dark color that absorbs infrared radiation and fuses the polymer particles together; it is deposited by multiple jets where the part is to be made. The second, lighter binder, which prevents fusion, is deposited in other areas to provide sharp definition of part boundaries and features. MJF produces highly accurate parts and is useful for proto- type development and form-and-fit models. AM Considerations for Tooling and Parts All plastic AM processes build parts layer by layer. This characteristic enables produc- tion of much more complex shapes than can be made by conventional machining, casting and molding, or other forming operations. Because AM processes do not involve forming tools, overhangs and re-entrant corners can be produced readily. In addition, internal cav - ities and channels that are neither round nor straight are feasible—overcoming limitations of conventional drilling. On the other hand, the per-part cost of AM parts is greater than that of conventional mass production processes. The cost of powder or filament feedstocks is higher than the same materials in conventional form, and the processes are much slower than conventional pro - cesses. These advantages and limitations are illustrated as cost break-even points in Fig. 30a and Fig. 30b in Additive Manufacturing Workflow on page 1555. A major application that fits within these limits is production of tooling for conventional processes. Applications include jigs and fixtures for machining, grinding, and welding; dimension-checking fixtures and gages for quality control; masking and non-marring of critical surfaces; organizing tools; and robotic end effectors. In addition, high-strength AM plastic dies are used for short-run thermoforming and injection molding. Compos- ite layup tools and sand casting patterns also are produced economically by plastic AM processes. Probably the most widely used AM tooling application is in fixtures for dimensioning first production articles and/or for periodic checks. Such fixtures normally are CNC ma - chined from aluminum or assembled from machined aluminum segments, but in most cases, the strength of aluminum is not required. Furthermore, fixtures can be produced by the FDM process in a day or two, rather than the far longer lead time required for CNC programming and production or assembly.
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