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Plastics - Morgan State University

By Sheila Evans,2014-03-17 22:09
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Plastics - Morgan State University

    Plastics

    The word polymer is derived from “plasticos.” It means capable of being shaped or

    molded. Plastics are also known as polymers. Polymers are the most widely used of the new synthetic materials even though they are much simpler in comparison than the natural polymers of muscles, ligaments and bones. Polymers have large molecules called monomers which when linked together, form polymers. A process called polymerization forms polymers.

     The uses of polymers vary. Polymers are used in beverage and food containers, toys, packaging and insulating materials. The list of usages of polymers goes on. Plastics are formed in a wide variety of applications. Polymers are formed in such a wide variety of applications because of their varying properties. Typical plastics have corrosion resistance, good electrical and thermal conductivity, low density, high strength-to-weight ratio, noise reduction, wide choice of colors and transparencies and ease of manufacturing.

     Two main types of plastics are thermosets and thermoplastics. Thermoplastics melt or soften when heated and harden when cooled. They maintain their original hardness or strength and can be reprocesses. Examples of thermoplastics are acrylics, nylons, polyethylenes, and polyvinyl chloride. Thermosets on the other hand have a shape that is permanently set because of strong covalent bonds. If a thermoset plastic is heated, the form taken on when heated is permanent and not reversible. Examples of thermosets are epoxy, polyester, and methane.

    How Both Types of Plastics are Formed

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     There are six types of plastic manufacturing processes: they are extrusion, expansion, foaming, molding, lamination, and thermoforming. Extrusion requires the squeezing of the softened plastic through a die of the desired shape. This is a means of molding a part of a constant cross section and continuous length. This process results in pipes, straws, and plastic bags.

    How Thermoplastics are Formed

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Blow Film Extrusion

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    Expansion is a process, which results, in packing materials and in wall and ceiling insulators. This plastic is processed by combing a mixture of gases into the plastic under a high pressure. Foaming is a process that combines liquid chemicals with the plastic. This results in light materials used in cushions and insulation. Molding is a process used only in thermoplastics. Two types of molding are compression and injection. The plastic is heated, forged into a mold and then solidified. This processes is rapid and results in the production of many parts. Lamination involves the stacking and compression of a fibrous sheet. These sheets are coated with resin and binded by heat and pressure. Laminated plastics are used in refrigerator doors and on cotton. Thermoforming involves the forming of material through the application of heat ad pressure. A sheet of material is heated, and then applied to the mold by a vacuum. Thermoforming parts are used in advertising signs, packaging, etc.

     Plastics are a major class of materials that posses a wide range of mechanical, physical, chemical, and optical properties. These properties depend on the molecular weight degree of polymerization and crystallinity and additives. The two major classes of polymers are thermosets and thermoplastics. Plastics have a wide variety of application.

    Bibliography

    http://encarta.msn.com/find/MediaMax.asp?pg=3&ti=761553604&idx=461548298

    http://encarta.msn.com/find/MediaMax.asp?pg=3&ti=761553604&idx=461547469

    http://encarta.msn.com/find/MediaMax.asp?pg=3&ti=761553604&idx=461547688

    Cinquanti, Michael and Robert Roy. Plastics. General Electric-Unit 2. Genius

    Publishing Corporation, 1990.

    Kalpakjian, Serope, and Steven Schmid. Manufacturing Engineering and Technology.

    Upper Saddle River, NJ: Prentice Hall Publishers, 2001. Shane, Robert and James Young. Materials and Processes. 3rded. New York,

    NY: Marcel Dekker Inc, 1985.

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