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Polymer Science and Engineering: The Shifting Research Frontiers
physical properties. Compact disks can be made because of high flow grades of polycarbonate and the injection-compression molding process.
The importance of injection molding, and of precision injection molding in particular, can hardly be overstated. The economies that can be realized in the production of mechanically complex parts can contribute to the feasibility of large-scale manufactured products. For example, communications systems of the future, such as fiber optics to the home providing broad-band information, depend on many manufactured details that must be reduced in cost if the concepts are to succeed. Polymer-based solutions are essential to the realization of the promise of progress in diverse areas.
Extensive progress has also been made recently in blow molding, especially to form bottles for various packaging applications. Special grades of polymers with uniquely tailored rheological properties, via broad molecular weight distribution and chain branching, have been developed for this market. Stretch blow molding processes allow control of the development of chain orientation and crystalline structure for materials such as poly(ethylene terephthalate) to gain better barrier properties.
Melt extrusion processes are usually the most convenient, economical, and environmentally favorable for film and sheet manufacturing. Screw extruders, in which a rotating screw transports material through a heated barrel and a shape-forming die, are the heart of such processes. Extruder screws, which can be very sophisticated, are designed with the help of extensive computer-assisted modeling. Frequently, mixing, compounding, and devolatilization are also involved to process formulations that include special additives, such as antioxidants, plasticizers, flame retardants, lubricants, pigments, fillers, and other polymers. Films are formed through film blowing of thin-walled tubes or drawing and tentering of cast films. Optimization of the process requires fundamental understanding of material properties and processing characteristics.
The properties of the fabricated product are strongly dictated by the details of the fabrication process. Influential variables include uniaxial or biaxial orientation, degree of crystallinity, morphology of amorphous and/or crystalline regions, internal stress, and dimensional control. The complete system—from material design, synthesis, and formulation to product design and processing—must be addressed to achieve optimal material selection for a specific application. All elements of the system are important. Materials scientists and engineers, who understand structure-property relationships and can manipulate molecular design, work closely with process engineers and product designers in a systems approach to meet the growing demand for extrudable polymers that have specific characteristics.