30079_09b

30079_09b - 9.4.1 Polymer Matrix Composites There are a...

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9.4.1 Polymer Matrix Composites There are a large and increasing number of processes for making PMC parts. Many are not very labor-intensive and can make near-net shape components. For thermoplastic matrices reinforced with discontinuous fibers, one of the most widely used processes is injection molding. However, as dis- cussed in Section 9.3, the stiffness and strength of resulting parts are relatively low. This section focuses on processes for making composites with continuous fibers. Many PMC processes combine fibers and matrices directly. However, a number use an interme- diate material called a prepreg, which stands for preimpregnated material, consisting of fibers em- bedded in a thermoplastic or partially cured thermoset matrix. The most common forms of prepreg are unidirectional tapes and impregnated tows and fabrics. Material consolidation is commonly achieved by application of heat and pressure. For thermo- setting resins, consolidation involves a complex physical-chemical process, which is accelerated by subjecting the material to elevated temperature. However, some resins undergo cure at room temper- ature. Another way to cure resins without temperature is by use of electron bombardment. As part of the consolidation process, uncured laminates are often placed in an evacuated bag, called a vacuum bag, which applies atmospheric pressure when evacuated. The vacuum-bagged assembly is typically cured in an oven or autoclave. The latter also applies pressure significantly above the atmospheric level. PMC parts are usually shaped by use of molds made from a variety of materials: steel, aluminum, bulk graphite, and also PMCs reinforced with E-glass and carbon fibers. Sometimes molds with embedded heaters are used. The key processes for making PMC parts are filament winding, fiber placement, compression molding, pultrusion, prepreg lay-up, resin film infusion and resin transfer molding. The latter process uses a fiber preform which is placed in a mold. 9.4.2 Metal Matrix Composites An important consideration in selection of manufacturing processes for MMCs is that reinforcements and matrices can react at elevated temperatures, degrading material properties. To overcome this problem, reinforcements are often coated with barrier materials. Many of the processes for making MMCs with continuous fiber reinforcements are very expensive. However, considerable effort has been devoted to development of relatively inexpensive processes that can make net shape or near-net shape parts that require little or no machining to achieve their final configuration. Manufacturing processes for MMCs are based on a variety of approaches for combining constit- uents and consolidating the resulting material: powder metallurgy, ingot metallurgy, plasma spraying, chemical vapor deposition, physical vapor deposition, electrochemical plating, diffusion bonding, hot pressing, remelt casting, pressureless casting, and pressure casting. The last two processes use preforms. Some MMCs are made by
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This note was uploaded on 05/02/2010 for the course ME 100 taught by Professor Any during the Spring '10 term at Purdue.

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30079_09b - 9.4.1 Polymer Matrix Composites There are a...

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