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Polymer Science and Engineering: The Shifting Research Frontiers
accurate. Yet there are limitations to what can be done, even with common materials. Molten plastics are viscous, and making thin parts may require high pressures. Further, the plastic shrinks as it cools, and this tendency must be compensated for by using oversized mold cavities. Molds are expensive, from several thousands to millions of dollars each.
Plastic materials are rheologically complex, and as a result many factors can affect the properties and dimensional accuracy of parts made from them. There are variations in operation of the molding machine, small temperature fluctuations, and differences in molecular orientation caused by flow into the mold. However, injection molding has been brought to levels that allow tolerances on small parts in the micrometer range. Among the high-performance plastics that have been introduced to meet the demands of the high-precision market are the thermotropic liquid crystalline polymers and low-viscosity versions of high-temperature materials such as polyetherimides and polyaryl sulfones.
Advances in processing are occurring at a rapid rate as on-line sensing, computing, and process feedback allow control and optimization of the molding process that were undreamed of only a few years ago. Parameters of importance include injection speed, peak pressure, hold pressure, and mold temperature, along with less obvious factors such as "cushion length" and position-or pressure-dependent cutoff. As the processing industry learns to take advantage of the capabilities of the new machines and materials, precision injection molding can be expected to make further inroads into the domain of machined metal parts.
Process dynamics and the properties of the finished article are critically dependent on the conditions of flow and solidification, down to the molecular level. As the mold is filling, the molten polymer solidifies first along the walls. The material that is farthest from the wall flows more rapidly, leading to a shearing and molecular elongation in the wall area. After the flow front has passed and the mold is full, the central regions solidify under conditions in which shear elongation is not a major factor. This solidification process leads to a morphology characterized by a skin of highly oriented polymer around a core of less oriented material. The two layers are mechanically and optically distinct. Control of these components through polymer composition and processing technology is a central issue in the production of precision, high-performance parts. Skin effects are most obvious in parts made from polymers that crystallize. Amorphous polymers are much less influenced. These morphological factors have important consequences for the failure mode and fracture mechanics of the finished part.
Additional processing techniques, such as gas-assisted molding and injection-compression molding, are gaining industrial acceptance. Materials suppliers are developing new plastics with enhanced flow characteristics and better