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Polymer Science and Engineering: The Shifting Research Frontiers
functional uses such as coatings for surface protection, films for a wide variety of uses, fibers for fabrics and carpeting, and an enormous variety of molded shapes.
Melt processing is the most widely used and generally the preferred processing method. It is used for polymers that become liquid at elevated temperatures so that they can be extruded into fibers, films, tubes, or other linear shapes or molded into parts of complex shape. Such processes involve much more than simply changing the physical shape of the polymer; they also influence phase morphology, molecular conformations, and so on and ultimately have an important role in the performance of the product.
A mold is a hollow form that imparts to the material its final shape in the finished article. The term "molding" is employed for processes involving thermosets and thermoplastics and includes injection, transfer, compression, and blow molding. The injection molding process is the most common method of making plastic parts. In that process, thermoplastic pellets are melted and pumped toward a melt reservoir by a rotating screw. When enough molten plastic has accumulated, the screw plunges forward to push the melt into a steel mold. The plastic solidifies on cooling, and the mold is opened for removal of the part. Injection molding cycle times vary from a few seconds to minutes, depending on the plastic and the part size. Molding machines have become very sophisticated, and they are capable of turning out large numbers of molded articles with little or no operator attention. The heated plastic conforms intimately to the polished mold surface, which may be of complex shape, and the part produced usually requires little or no further machining or polishing. The mold and the machine that delivers plastic to the mold can be quite costly; therefore, the technology is suited only to parts needed in large numbers. Even so, injection molding is a process capable of exceptionally low cost in comparison with production processes for metal or ceramic parts.
Some of the current challenges in polymer processing include developing new materials, achieving greater precision, pursuing process modeling and development, and recycling. Some examples of new materials are special blends of existing polymers, polymer composites with fiber reinforcement, and liquid crystalline polymers. Some of these new materials are expensive and may be difficult to form into desired shapes; however, they are of value to the defense and aerospace industries in applications in which weight and performance are more important than cost and processibility. In contrast, automotive and appliance industries use materials that are less expensive, readily molded, and dimensionally