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					                                         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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