Plastics - Injection Molding: Converting Metal Parts to Plastic
Historically, John Wesley Hyatt proposed the use of celluloid, the world's first commercial synthetic organic resin, as a substitute for ivory to manufacture billiard balls. Hyatt began making machines to process the material, and in 1872, his first machine was patented. A German dentist first used cellulose acetate in an injection molding machine in 1916 to form plastic dentures. Others in Germany continued using plastics, and in 1922, W.D. Grote of Kentucky imported a German machine into the United States. In 1923, he built his own machine based on the German design. This manual machine was replaced in 1929 by the first fully automatic injection molding press. Buttons and bobbins were the first items molded, but when the machine was improved and powered by 500 psi of hydraulic pressure, molding larger parts became more feasible. Development of new materials in the 1930s prompted improvements in the machines and their capabilities.
Machine functions: Injection molding converts resin pellets into molded parts. Plastic pellets are fed through a heated screw and barrel. Under high pressure, the liquefied material moves through a runner system and into the mold. The cavity of the mold determines the external shape of the product while the core shapes the interior. When the material enters the chilled cavities, it starts to re-plasticize and return to a solid state and the configuration of the finished part. The machine then ejects the finished parts or products.
The variety of products being produced by injection molding continues to increase. Consequently, manufacturers design and build (with computer-aided design systems) and run specialized molds per customer specifications. Specialized molds speed up cycle times and reduce waste and subsequent processes, and multi-cavity molds speed up production. Mold sizes, resins types, cycle times, wall thickness and tolerances vary the molding processes.
The size of injection molding machines varies from microinjection to large-tonnage machines well over 3,000 tons. Converting metal parts to plastic has precipitated the need for these large machines capable of molding products such as bumpers for the automotive industry, housings for the appliance industry and containers for refuse and shipping. Although applications include molded screws for medical applications that weigh as little as 6.5 mg, microinjection machines are currently being used most often for manufacturing compact discs (CDs).
Raw materials: Injection-molding raw materials include two types: commodity resins and engineering resins. Polyethylene is one of the common commodity resins. The low cost, durability and recycling abilities of these materials make them suitable for use in the packaging applications, the medical-supply industry and toy manufacturing. As recycling expands and technology improves, the industry continues to increase its use of recycled materials.
Engineered plastic materials (glass-fiber filled, carbon-fiber filled, mineral filled and the like) offer more than 17,000 choices, which permit a wide range of applications. However, the number of different engineered-plastic materials continues to increase. Engineered plastics are used for specialized industrial requirements such as those of the automotive and appliance industries and, on a smaller scale, in the electronics and business-machine industries.
As the quality and strengths of materials improve, opportunities increase daily to use plastic parts to replace all sorts of steel, aluminum and brass parts. Although the automotive industry leads all others in the amount of plastic used in their products, most other manufacturers are also evaluating the benefits of plastics. Benefits include no rust, lightweight, elimination of secondary operations and combining multiple parts into one. Currently, the heating/air-conditioning, fitness-equipment and door-hardware industries are among others converting parts from metal to plastic.
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