Machinery's Handbook, 31st Edition
PLASTICS PROTOTYPES 611 be coated. Sputter plating uses a plasma to produce the metallic vapor, requires line- of-sight setup, and can use brass as well as the other metals mentioned. The appear- ance of gold or brass is often simulated with vacuum-metallized aluminum by tinting the overcoating lacquer. Chromium plating requires etched surfaces to ensure good adhesion. Development of Plastics Prototypes.— Model prototypes are made for testing of prop- erties, such as stress and fatigue resistance, to find ways to improve quality and reliabil - ity, to improve tooling design, and to reduce time to market. Prototyping may answer questions about finish, sink marks that result from contraction, witness lines from mold joints, ejector pin marks, knit or weld lines, texturing, moldability, shrinkage, mechani- cal strength, insert pull-out resistance electrical properties, and problems of mating with other parts. Prototypes of moldings are made in five major steps: design, refining the design, mak - ing a model (physical or computer), making a mold, and producing parts. The model may be made from wood, plaster, plastics (by machining), or metal. The majority of prototypes utilize CAD/CAM methods that enable dimensional tolerances to be held to 2 to 3 percent of drawing specifications. (See CAD/CAM on page 1390 .) However, ongoing advancements in sophisticated additive manufacturing (AM) processes, com - monly known as 3D printing, are steadily increasing the number of plastics prototypes, as well as parts, being produced using such methods. After the initial investment, use of AM processes to produce plastics prototypes can be less expensive and more efficient than producing prototypes with CNC machines, freeing up the more costly CNC machines for production. CNC systems generally re- quire more floor space than 3D printers and, as a subtractive process, use more material and produce scrap that must be recycled or otherwise disposed of, whereas AM ma- chines use only the amount of material needed. Additive Manufacturing Plastics Plastics prototyping was the motivation for development of the first AM process, known as stereolithography (SLA). This process patent was filed in 1984, granted in 1986, and commercialized in 1987 by 3D Systems. Since then, other AM processes for making plastics prototypes and production parts have been developed and refined, including selective laser sintering, fused deposition modeling, binder jetting, material jetting, and multijet fusion. In all of these processes, a part is built up one layer at a time, adding material to produce a shape. In all AM methods, as in CNC machining and other computerized manufacturing sys- tems, the objective shape is represented in a three-dimensional (3D) CAD file. The CAD file may be made by a designer or by scanning an object into a solid modeling package. Software embedded within the printer “slices” the CAD model into layers; it then deter- mines the toolpath for fabricating each layer. The “tool” in this case is an extruder, laser, or printhead, as described in the following sections. Unlike CNC systems, AM does not require external programming of the toolpath. With the new programs and interfaces, the skill level required to program and operate a 3D printer and create unique plastic models in-house is less. Alternatively, parts de - signers can take advantage of 3D printing services to produce affordable prototypes. Thus, prototypes can be produced more frequently, providing efficiencies in product design and development time. At the same time, production parts are increasingly being manufactured with this technology. Operating principles and features of various plastic AM processes follows. Table 1 summarizes attributes of plastic 3D printing processes. For more on the plastics men- tioned here, see Plastics Materials on page 556.
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