4
Phase-Change Processes

Phase-change processes produce a solid part from liquid material. They include the commercial processes of metal casting, infiltration of composites, and injection molding of polymers. These processes may reside early in the process stream (e.g., ingot casting of wrought metallic products) or produce a finished component (e.g., a molded polymeric beverage container). Control of the part shape and workpiece microstructure to specific levels is a high priority in these processes and often establishes the economics of the manufacturing process.

This chapter discusses the phase-change processes used to process metals, polymers, and metal-matrix composites.

Metals

Manufacturing processes that change the phase of metals by melting and subsequently resolidifying materials into finished or semifinished products are categorized as molding and casting processes. Primary metal industries use melting, casting, and solidification processes to produce semifinished products. Molten metals and alloys are cast by continuous methods or into individual ingot molds. Subsequently, these primary castings are rolled into semifinished products such as sheet, plate, rod, and bar, or they are forged into semifinished shapes (Weidmann, 1990).

In contrast to the primary metals industry, the foundry industry uses a wide variety of molding and casting processes to produce discrete, shaped products. These products range in size from steel castings of several hundred tons that are used in power generation plants to small, precision castings, such as delicate jewelry produced in investment molds. These processes are typically characterized by their molding process or by the casting process itself. For example, in sand casting, large volumes of castings are poured into sand molds.



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--> 4 Phase-Change Processes Phase-change processes produce a solid part from liquid material. They include the commercial processes of metal casting, infiltration of composites, and injection molding of polymers. These processes may reside early in the process stream (e.g., ingot casting of wrought metallic products) or produce a finished component (e.g., a molded polymeric beverage container). Control of the part shape and workpiece microstructure to specific levels is a high priority in these processes and often establishes the economics of the manufacturing process. This chapter discusses the phase-change processes used to process metals, polymers, and metal-matrix composites. Metals Manufacturing processes that change the phase of metals by melting and subsequently resolidifying materials into finished or semifinished products are categorized as molding and casting processes. Primary metal industries use melting, casting, and solidification processes to produce semifinished products. Molten metals and alloys are cast by continuous methods or into individual ingot molds. Subsequently, these primary castings are rolled into semifinished products such as sheet, plate, rod, and bar, or they are forged into semifinished shapes (Weidmann, 1990). In contrast to the primary metals industry, the foundry industry uses a wide variety of molding and casting processes to produce discrete, shaped products. These products range in size from steel castings of several hundred tons that are used in power generation plants to small, precision castings, such as delicate jewelry produced in investment molds. These processes are typically characterized by their molding process or by the casting process itself. For example, in sand casting, large volumes of castings are poured into sand molds.

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--> Alternative molding methods, such as shell molding, investment casting (also known as the "lost wax" process), and expendable pattern (or "lost foam") casting, provide improved dimensional control and often more intricate shapes. Major production systems are based on metal mold processes, such as die, or permanent mold casting, which use either pressure or gravity feed in the casting process. Another metal mold process is centrifugal casting, which takes advantage of radial forces on the solidifying product to reduce casting porosity; it is, for example, used to produce pipes. More recently, technologies involving squeeze casting have taken advantage of mechanical forces on the solidifying metal. Rheocasting (or thixocasting) is based on thixotropy, a physical state wherein a liquid/solid mixture or semisolid materials flow more easily when shear stresses are applied. Casting of single crystal materials is a critical new technology. The molding and casting methods used include crystal growing by techniques such as crystal pulling, zone melting, and vapor deposition. Applications range from gas turbine blades to semiconductor materials. Manufacturing entities involved in basic primary metals operations are large energy consumers and require significant environmental controls. Consequently, primary metals producers benefit from advances in energy conservation and pollution control. Continuous or direct casting processes produce a cast sheet or thin slab close to the final hot-rolled dimensions, thereby eliminating hot rolling as a processing step with a corresponding reduction in investment and operating costs. Direct casting has been used in commercial production in the nonferrous industries for more than 70 years. However, the difficulty of controlling liquid steel prevented serious consideration of continuous casting until recently. Nucor Steel has led the implementation of thin-slab steel technology, breaking ground for the first U.S. plant in Crawfordsville, Indiana, in September 1987. Strip casting, a variation of thin-slab casting, eliminates the need for the hot strip mill operation, using small, inexpensive, and cost-efficient equipment (Schwaha et al., 1987). This further integration of the casting and rolling processes offers additional substantial energy savings in the manufacture of primary metals. Recent reports have described the evolution of technologies that have made this possible and the quality assurance requirements necessary to gain wide adoption (Tsubakihara, 1987), as well as the corresponding operating considerations (Harabuchi and Pehlke, 1988). Future improvements, including direct charging of the hot casting to the reheat furnaces for rolling or for directing-line rolling of cast slabs, can be expected in the areas of equipment design and process development, with anticipated improvements in productivity and energy savings.

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--> Recently, integrated and networked computer control systems have been installed in a number of major slab-casting installations for steel. These systems involve a computer architecture composed of mainframe computers with backup; several minicomputers; and a number of programmed logic controllers, digital controllers, and microprocessor-based controlled operator stations. This computer hardware provides for a distributed control system hierarchy and a database that supports the entire production system. Management of the melting function and its correlation with caster scheduling is implemented at a low control level, along with data collection and reporting and tracking of the slabs in the system. Workstations for operator guidance and the implementation of supervisory set-point control have been installed at the low control level, along with data acquisition/reduction and reporting functions. This control level also supports variable monitoring with alarm systems and local control, such as for mold level, and certain control and sequencing functions, such as tundish movement and ladle turret rotation. This control structure has provided a dramatic improvement in plant productivity and quality. The productivity gain has been achieved through improved scheduling and extended sequence casting. Product quality has been improved through process optimization; process control; and a substantial upgrade of overall product flow, monitoring, and integrated supervision of downstream functions including slab cutting, marking, and direct charging to the reheat furnace at the hot strip mill. Many sensors now monitor the quality of the continuous casting process. Further upgrading of continuous casting productivity and quality depends to a large extent on the development of improved sensors for monitoring and control functions. Direct linking of continuous steel casting and hot rolling, either by direct charging or in-line rolling, will be widely adopted in the twenty-first century. Wider acceptance of this technology will depend on assured product quality and maximum productivity and will require the development and implementation of a number of technology areas, including improved overall control of steel-making operations, manufacturing a defect-free strand, ensuring rollable temperatures when delivered for hot rolling, and flexibility of on-line width changing of slabs. A variation of direct casting is spray forming. It uses atomization onto a substrate to produce near-net shape castings and eliminates traditional hot working processes. Spray forming has been used in the manufacture of rings, tubes, small billets, and pipes for both ferrous and nonferrous metals (Rickinson et al., 1981; Evans et al., 1985). Of these direct casting processes, strip casting appears to have the greatest potential for commercialization in the longer term. The concept has been successful in the production of steel sheet (Preston, 1991a,b). This process could replace costly ingot casting and hot-rolling facilities, resulting in significant

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--> overall cost savings. The limitations in direct strip casting for manufacturing sheet metal products lie in the ability to identify, monitor, and control relevant process parameters. A suitable process control strategy must be developed that will allow production of high-quality strip on a reproducible basis and within narrow product specifications. Such a control strategy requires improved understanding of process dynamics, so that available computing capabilities can be used to interpret sensory inputs and to apply the resulting information to control the casting process in real time. Shaped castings are the product of the metal casting industry, which is composed of 3,100 foundries in the United States. Since 1980, the number of foundries in the nation has decreased by over 26 percent. The U.S. industry has identified several key factors that have affected its competitiveness (AFS, 1994). A primary factor is market loss due to the development of new materials and the downsizing of end products (e.g., plastic pipe is replacing iron pipe, the average weight of automobiles is 30 percent less than fifteen years ago, etc.). Another key factor is the fact that the industry is primarily composed of small companies, with 80 percent of U.S. foundries having 100 or fewer employees. This makes research support, research program identification, and education and training difficult. The American Foundrymen's Society is the major technical organization representing the entire industry. The society is composed of over 850 individuals from the North American metal casting and supplier industry. The society has developed a research and technology plan for the U.S. metal casting industry with four specific objectives, which are summarized in Table 4-1 (AFS, 1994). A specific research agenda for the cast-metals industry has been set recently by an Advisory Board to the Department of Energy in their effort to respond to the Metal Casting Competitiveness Research Act of 1990. Their recommended research agenda is summarized in Table 4-2. Polymers The use of polymeric materials as viable structural materials has grown steadily because of their lower cost and weight. Consequently, research efforts have focused on the development of new materials and product applications, rather than on the processes required for their manufacture. The following discussion primarily addresses thermomechanical or thermochemical processes used for batch manufacturing of discrete parts. Technical areas related to processing (such as: melt rheology, microstructure development, sensors, and process control issues) are also considered.

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--> Table 4-1 Objectives of the American Foundrymen's Society Research and Technology Plan MARKETS Research that expands cast component markets and applications that will serve to further diversify and stabilize the metal-casting industry PRODUCTIVITY Research that maximizes the production efficiency of a foundry. Programs that improve total metal yield and reduce casting rework are of major importance. ENVIRONMENTAL Research that reduces or eliminates the environmental risks affecting foundries and their employees. TECHNOLOGY TRANSFER (EDUCATION) Research that includes plans to transfer technology and effectively educate the segment(s) of industry that the results address. There are two basic families of polymeric materials: (1) thermoplastics consisting of a linear chain structure that exhibits reversibility during phase-change processes and (2) thermosetting polymers for which the phase-change process is irreversible because chemical reactions and chain cross-linking take place. Some unit processes can produce components of both materials, while others can only process either thermoplastics or thermosets (Table 4-3). Two factors are critical in the process science of polymeric materials: the material behavior during processing and process dynamics (Isayev, 1987; Tucker, 1989; Hieber and Shen, 1980; Wang et al., 1986; and Chiang et al., 1991). A thorough understanding of each is essential in the prediction of product characteristics and in the precise control of the process. The current R&D emphasis has centered around these factors and has included the following areas: melt rheology during processing; modeling and simulation of process dynamics; polymerization and kinetics in reactive processing; structural changes under processing conditions; material properties characterization; unit processes for composites; and process control and sensor technologies.

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--> Table 4-2 Recommended Metal-Casting Research Priorities PRIMARY RESEARCH PRIORITIES: • Solidification and Casting Technologies    Dimensional Control of Castings    Clean Cast-Metal Technology • Computational Modeling and Design    Computer Integrated Processing Methods for Productivity and Quality Improvements • Processing Technologies and Design for Energy Efficiency, Material Conservation, Environmental Protection, or Industrial Productivity    Aluminum Furnace Optimization    Process Improvements for Lightweight Components of Aluminum Magnesium and Thin-Wall Metal Castings    Sand Reclamation    Characterization of Waste Streams SECONDARY RESEARCH PRIORITIES: • Solidification and Casting Technologies    Expendable Pattern Casting Technology • Emerging Areas Needing Research    On-Line Process Controls and Sensors for Molding Melting and Coremaking    Plasma Melting • Processing Technologies and Design for Energy Efficiency, Material Conservation, Environmental Protection, or Industrial Productivity    Cupola Furnace Optimization    Gating System Removal and Finishing Operation Technologies The payoff to the polymer processing industry that results from the R&D effort in the polymer unit processes is becoming more noticeable. For example, according to a recent survey, computer-aided engineering technology for injection molding has been gaining acceptance in the industry (Naitove and DeGaspari, 1992). Many users of mold analysis software tools are reporting better product quality and shorter lead time and, in some cases, are eliminating the need for prototyping. Recent developments in processes for resin transfer molding and

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--> Table 4-3 Polymer Phase-Change Processes Materials Processes Thermoplastics Thermosets Injection Molding X X Compression Molding X X Polymer Casting X X Rotational Molding X X Co-Injection Molding X   Gas-Assisted Injection Molding X   Foam Molding X   Calendering X   Cold Forming X   Extrusion X   Blow Molding X   Themoforming X   Transfer Molding   X Reaction Injection Molding   X Liquid Injection Molding   X Rubber Injection Molding   X Reinforced Injection Molding   X structural reaction injection molding have shown great potential for mass producing low-cost structural parts made from composite materials. Effective use of gas-assisted injection molding could significantly increase the flexibility of plastic part design.

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--> Metal-Matrix Composites Metal-matrix composite (MMC) materials are reinforced with continuous fibers, discontinuous whiskers, or particulates that exhibit properties that are widely different from the matrix. These reinforcements are made from nonmetallic materials, such as carbon, alumina, silicon carbide, or organic materials such as polyaramid or polyethylene. Reinforcement orientation can range from highly aligned to random. The resultant properties of the composite material are the result of the reinforcement composition, amount, distribution, orientation, and interaction with the matrix. Continuous-fiber MMCs are expensive and thus have found use principally in defense and space applications that demand high-performance materials. For example, carbon fiber-reinforced aluminum costs more than ten times the cost of its constituents (carbon fiber itself is much more expensive than aluminum) due to the highly labor intensive nature of its manufacturing process: First liquid-metal, infiltrated-fiber tows must be made; these are laid-up on a platen press and interlaid with aluminum sheets. Finally the laminate is consolidated by diffusion bonding. Discrete parts of discontinuously reinforced MMCs are being produced much more cost-effectively using phase-change processes, such as aluminum with discontinuous particles of Al2O3 or SiC. Casting technology for discontinuously reinforced MMCs experienced a development spurt roughly fifteen years ago. Cast composite technology then suffered from the stigma of castings having highly segregated microconstituents that resulted in nonuniform and variable product properties. The demonstration of rheocasting and thixocasting (i.e., the processing of a semisolid slurry) has led to processes and process design methodology that are geared to optimizing of cast structure and properties (Sawtell, 1990). The technology itself is apparently well developed, as is exemplified by the thixocasting, investment-casting, and pressure-casting technologies. These types of composites are relatively isotropic compared with the continuous fiber types. With the proper selection of matrix and reinforcements materials, they do not exhibit many of the problems associated with fiber interaction during service and fiber degradation during processing. Also, these composites can typically be easily joined; for example, some can be welded. Their primary drawback is their inferior mechanical properties compared with the properties of the continuous fiber composites in the principal axis direction. Cast MMC parts with discontinuous reinforcement are potentially economically acceptable for a wide range of applications. For example, thixocastbased processes will likely find wide use in automotive, defense/aerospace, and heavy applications in which weight savings translate to increased fuel efficiently

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--> and, therefore, cost savings. MMCs processed by thixocasting are lighter than cast iron, exhibit better wear resistance and higher thermal conductivity and are thus excellent candidates for disc brake rotors. Similarly, investment-cast MMCs are being employed in microelectronic packaging and in stiffness-driven structures, while pressure-cast MMCs, with their superior surface finish and good dimensional tolerances, are increasingly used for machine tools and products requiring high thermal conductivity. Several new matrix alloys and materials are currently under consideration. Probable applications include large structural components such as aircraft frames, surgical implants, and structures for transatmospheric hypersonic vehicles, as well as smaller applications such as constraining cores for printed circuit boards. Matrix materials include beryllium and titanium. For aerospace propulsion systems with lighter engines and reduced cooling requirements, reinforced intermetallics are being investigated. Those such as Ti3Al, NiAl, FeAl, and NbAl3 offer improvements in weight, stiffness, and corrosion resistance but suffer from low toughness at low temperatures and strength degradation at high temperatures. These limitations can be overcome by reinforcing the matrix with high-strength discontinuous fibers and particulates. When these considerations are coupled with the technical challenge of incorporating reinforcing fibers into the aluminide matrix without adverse interfacial reactions, another major barrier to MMC application becomes evident. The need for high-temperature performance led to the use of MMCs in automotive pistons, as demonstrated by Toyota's ceramic fiber-reinforced aluminum alloy pistons in diesel engines in the early 1980s. The development program for the National Aerospace Plane in the 1990s has furthered the challenge of integrating and optimizing the manufacturing steps for composite material processing. To date, highly controlled processing techniques such as powder metallurgy, thermal spraying, and diffusion bonding have not been found to be cost-effective. Although extensive effort has been devoted to manufacturing processes that emphasize techniques to optimize structure and properties of the MMC, the lag in the development of such techniques that are also cost-effective represents one major barrier to the widespread use of MMCs. Another major barrier prohibiting more-extensive use of cast MMCs is the development of matrix/reinforcement systems that can be readily produced in the foundry. These systems would have castable matrices with reinforcing agents that are nonreactive during the high temperatures encountered during processing. Thus, control or elimination of interfacial reactions is another serious consideration in the selection of composite constituents. In summary, much additional fundamental research will be necessary before the full potential of MMCs is realized in commercial products. Many design

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--> engineers have the incorrect perception that casting processes yield parts and products with properties that are inconsistent and inferior to those of wrought processes. Part of this stigma reflects the lack of a unified focus on optimal processing techniques. A well-characterized composite system produced under strictly controlled processes would help to remove this perception. Research Opportunities Metal-Casting Processes Specific research opportunities in metal-casting processes can be summarized as the following: Advanced or emerging technologies. A number of new processes have shown considerable promise and are in various stages of commercialization. These processes include near-net shape casting, pressure die casting, rheocasting, rapid solidification, metal-matrix composites, powder process technologies, and vapor/solid processes for films and coatings (Flemings and Brown, 1988). Potential benefits for these processes include lower costs and safe, environmentally clean processing. Modeling and design. Modeling and design is the key to advancement of process understanding and optimization for molding, melting, and casting processes. Computer modeling of fluid flow, heat transfer, and solidification mechanisms is a consistent need for every casting process. For instance, improved models of materials behavior would result in prediction of residual stresses and the resulting geometric distortions of castings. Developing such models requires additional mechanical property data (e.g., elastic and plastic) for the casting material from room temperature to the melting point and specific volume data over this temperature range, as well as accurate multicomponent phase diagrams. Modeling also requires the development of pertinent thermal property data for the mold and casting materials, as well as models of the heat transfer situation at the mold-casting interfaces. For instance, there is a need for modeling the thixotropic process, including the flow of the material into the mold coupled with cooling gradients and solidification rates. Particular opportunities may exist in processes such as magnesium alloy die casting. Automation control and sensors. Process automation, control, and the development of required supporting sensor technologies are key aspects in the future advancement of the casting industries.

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--> Energy conservation and environmental protection. Significant R&D needs relate to energy conservation, waste reduction, and environmental control for the casting industries. The molding, melting, and casting industries are energy intensive and environmentally sensitive. As these industries evolve and undergo modifications, improvements, and advances in technology, productivity, and quality, R&D efforts must be directed toward energy conservation and environmental control. The competitiveness and viability of these industries depends on success in addressing these basic issues. Advanced Polymeric Unit Processes There are many research opportunities for advanced polymeric unit processes despite the considerable progress that has been made to date. The following generic research areas support the further development of polymers: Material characterization and testing techniques. Methodologies and instruments for accurate measurement of dynamic properties of polymer materials under processing conditions are required. In addition, accurate mathematical representations of the measured properties (i.e., viscosity, specific heat, thermal conductivity, thermodynamic behavior, and constitutive relationships) are needed to support process simulation development. For instance, in order to predict and control the shrinkage and warpage of injection-molded parts, improvements in the characterization of the viscoelastic and thermodynamic properties of the material during processing are required. In addition, improved processing of crystalline thermoplastics and reactive polymers depends on the determination of the crystallization and chemical reaction kinetics, respectively. In processing polymer composite materials, the basic knowledge of the influence of fiber orientation on injection or compression molding of short-fiber-reinforced polymers at high fiber concentration requires further development. Similarly, the flow behavior of polymers through preformed fiber mats during the process of resin transfer molding requires substantial characterization. Expansion of the characterization to include commercial production materials is critical to the realization of enhanced manufacturing productivity in the polymer processing industries. Process modeling and simulation. Improvements in numerical techniques for three-dimensional simulations of anisotropic material behavior are required for process design of polymer composite parts. Expanded simulation of solidification events in polymer processing, detailed predictions of component thermal shrinkage, and thermomechanical modeling of part distortion are also key future needs for improved part quality and process productivity. In addition,

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--> three-dimensional simulation capability will extend the level of complexity of accurate part design. Innovative unit processes and equipment. For economical production of quality components, processes and equipment designed from a basic understanding of the behavior of materials is necessary. New and emerging advanced polymeric materials (e.g., supermicrocellular foams, long-fiber-filled polymers, or liquid-crystal polymers) are particularly sensitive to this issue. In addition, innovative processes of increased capability in component precision and geometric complexity offer added opportunities in the flexibility of part design. For example, a novel injection molding process, gas-injection molding, allows the manufacture of parts with increased structural stiffness and improved surface finish and tolerances; it is one of the most promising types of polymer injection molding. Development of advanced process-control methodologies. Increased process productivity can result from better process control that is enabled by improved sensors and process control software. Improved part quality is projected to be an added benefit. Application of traditional polymeric processing to advanced composites . Dramatic reductions in production costs are feasible with the adaptation of commercially available equipment to the processing of high-performance advanced composites. Process design methodologies offer the potential for both high-volume and low-volume products at lower cost. Recycling techniques and considerations. Recycling is a major issue in using plastics for the decades to come. Considerations of recycling during materials development and in product-process design are high priorities. Future efforts should also include the development of recycling technologies for emerging materials. MMCs Research opportunities for unit processes in support of producing discrete parts made from MMCs include the following: More emphasis on characterizing MMC behavior during realistic processing conditions. Such characterization should occur during the early stage of development. Improved product consistency using existing unit processes. This process development must include consideration of the influence of process conditions on the property levels and durability of the MMC components. Realistic modeling and simulation approaches to identify optimal processing conditions are

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--> required. The development of criteria functions for the production of sound-cast MMC products using unit manufacturing processes would improve the efficiency and the economics of MMC commercialization. Development of adaptive-control techniques for the processing of cast composites. The success of this endeavor depends a great deal on the accuracy of the control's software knowledge of the behavior of the unit process and the MMC workpiece. The material must be accurately characterized on microscopic and macroscopic bases, and the property changes as functions of the process and the accompanying mechanical responses must be predictable. Accordingly, enhanced measurement techniques and process modeling capabilities are also required. Development of fiber coatings and fiber preforms for in-mold liquid metal infiltration techniques of continuous-fiber MMC. Research on improving the manufacture of continuous fiber MMCs requires an integration of process technologies, sensors, controls, and materials characterization. Studies of suitable coatings that inhibit interfacial reactions and, at the same time, improve wetting are necessary to advance this technology, together with the development of processes to efficiently apply these coatings. In the long term, development of new fiber materials that are nonreactive in the matrix would be highly desirable. Research and analytical modeling of liquid-metal infiltration processes are also needed. Processes utilizing liquid-metal infiltration of fiber preforms with suitable fiber coatings offer a promising route to reducing the cost of continuous-fiber MMC materials.

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--> References AFS (American Foundrymen's Society). 1994. Foundry Industry Research Plan-1994. Des Plaines, Illinois: AFS. Chiang, H.H., C.A. Hieber, and K.K. Wang. 1991. A unified simulation of the filling and postfilling stages in injection molding, part I: Formulation. Journal of Polymer Science and Engineering 31(2):116-124. Evans, R.W., A.G. Leatham, and R.G. Brooks. 1985. The Osprey preform process. Powder Metallurgy 28(1): 13-20. Flemings, M.C., and S.B. Brown. 1988. Pp. 9-15 in Casting of Near Net Shape Products. Warrendale, Pennsylvania: The Metallurgical Society. Harabuchi, T.B., and R.D. Pehlke. 1988. Continuous Casting—Design and Operations, Volume 4, Pp. 79-129. Pittsburgh, Pennsylvania: Iron and Steel Society—American Institute of Mining, Metallurgical, and Petroleum Engineers. Hieber, C.A., and S.F. Shen. 1980. A finite-element/finite-difference simulation of injection-molding filling process. Journal of Non-Newtonian Fluid Mechanics 7(1):1-32. Isayev, A.I., ed. 1987. Injection and Compression Molding Fundamentals. New York: Marcel Dekker. Naitove, M.H., and J. De Gaspari. 1992. Mold analysis makes the grade (user survey). Plastics Technology 38(Apr):62-73. Preston, R. 1991a. Annals of enterprise—Hot metal—Part I. The New Yorker 67(2):43. Preston, R. 1991b. Annals of enterprise—Hot metal—Part II. The New Yorker 67(3):41. Rickinson, B.A., F.A. Kirk, and D.R.G. Davies. 1981. CSD—a novel process for particle metallurgy products. Powder Metallurgy 24(1):1-7

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--> Sawtell, R.R. 1990. Industrial perspectives of pressure casting. Advanced Materials and Processes 137(2):41. Schwaha, K., W. Egger, H. Rametsteiner, R. Landerl, A. Niedermayer, and F. Hirschmanner. 1987. Strip Casting of Low-Carbon Steel at VOESTALPINE. Proceedings of International Symposium on Near-Net-Shape Casting of Strip. Toronto: Canadian Institute of Mining and Metallurgy. Tsubakihara, O. 1987. Technologies that have made direct concatenation of continuous casting and hot rolling possible. Transactions of the Iron and Steel Institute of Japan 27(2):81-102. Tucker, C.L., III, ed. 1989. Fundamentals of Computer Modeling for Polymer Processing. New York: Hanson. Wang, V.W., C.A. Hieber, and K.K. Wang. 1986. Dynamic simulation and graphic for the injection molding of three-dimensional thin parts. Journal of Polymer Engineering 7(1):21-45. Weidmann, G., P. Lewis, and N. Reid, ed. 1990. Structural Materials. London: Butterworth Scientific Ltd.

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