MET 34800 – Engineering Materials Chapter 10

MET 34800 – Engineering Materials Chapter 10 -...

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Unformatted text preview: MET 34800 – ENGINEERING MATERIALS IUPUI ENGINEERING TECHNOLOGY ENT Department CHAPTER 10 SELECTION OF PLASTIC/POLYMERIC MATERIALS Engineering Materials: properties and selection, 9th ed. Kenneth G. Budinski, Michael K. Budinski Upon completion of this chapter, the student should: have an understanding of how to select an appropriate plastic for an application have an understanding of adhesives and how to select them have an understanding of paints and how to select them CHAPTER GOALS Review properties – list required Collect property data Plastic idiosyncrasies Match properties to requirements 10.1 METHODOLOGY OF SELECTION Table 10 –1 Some commonly specified ASTM property tests 10.1 METHODOLOGY OF SELECTION 10.1 METHODOLOGY OF SELECTION Idiosyncrasies: Plastics expand with temperature at a rate that is usually 10 times that of metals (coefficient of thermal expansion). Steels are 30 times stiffer than the stiffest unreinforced plastics (modulus of elasticity). Plastics can burn and sometimes give off toxic fumes in the process (flammability, toxicity). Plastics can deteriorate by aging, by exposure to sunlight, and by atmosphere-induced cracking (environmental resistance). Plastics are lighter than metals (density). Plastics are electrical insulators (electrical resistivity, dielectric constant, etc.). Plastics are poor conductors of heat (thermal conductivity). The upper use temperature for 99% of all plastics is less than 500°F (260°C) (heat deflection temperature ). 6 10.1 METHODOLOGY OF SELECTION Idiosyncrasies: con’t Plastics are soft; the hardest plastic is much softer than the softest steel (penetration hardness). Plastics cannot be shaped by the cold-forming processes that apply to most metals (resistance to plastic deformation). Plastics normally cannot be made to the close dimensional tolerances that are common in metals; they are difficult to accurately machine (machinability, stability). Plastics cannot resist impact as well as most metals (impact strength). Plastics may permanently deform in use (creep strength). Plastics may fail under sustained load (stress rupture strength). Plastics are not as strong as most metals (tensile, yield, and shear strengths). Plastics may fail under cyclic loading (fatigue strength). Plastics can absorb moisture from their use environment and change; size or properties can be affected (moisture absorption in 24 h). 7 10.1 METHODOLOGY OF SELECTION Good attributes: Plastics are low in cost; the material cost is low, and they usually can be shaped with low-cost processes (injection molding, etc.). They usually do not need to be painted; the color can be "free" and exists throughout the part thickness....
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This note was uploaded on 12/01/2010 for the course MET 34800 taught by Professor Flechinsine during the Spring '10 term at IUPUI.

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MET 34800 – Engineering Materials Chapter 10 -...

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