UNIVERSITI MALAYSIA PAHANG
BMM3643 (SEM II_2012-13)
a) What metals are frequently cast into products?
b) What materials are used to produce the expendable patterns for investment casting?
c) Explain why a casting may have to be subjected to various heat treatments.
a) Cast parts can range in size from a fraction of an inch and a fraction of an ounce to over 30 feet and many tons. Moreover, casting can incorporate complex shapes, hollow sections or internal cavities, and irregular curved surfaces.
b) In investment casting a pattern is formed from a low melting temperature, low vaporization temperature material, often wax. The mold is produced by surrounding the pattern with the mold material. The mold cavity is produced when the pattern is removed by melting/vaporizing the pattern. In early process development with porous mold materials the melted wax from the pattern would migrate into the mold material and be lost.
c) Heat treatments (described in Chapter 4) such as quenching and tempering, among others, are carried out to optimize the grain structure of metal castings, thereby controlling and enhancing mechanical properties. Heat treating can control microporosity, which is a main reason that castings are weak in tension.
a) What are some of the attractive features of die casting compared to alternative casting methods? b) For the cast metal wheel illustrated in Figure below, show how (a) riser placement, (b) core placement, and (c) chills may be used to help feed molten metal and eliminate porosity in the isolated hub boss.
c) What are some of the general defects encountered in casting processes? Name and briefly describe three. (8 marks)
a) Die casting is characterized by extremely smooth surface finishes, excellent dimensional accuracy, and high production rates. A single set of dies can produce many thousand castings without significant changes in dimension. b) Solutions;
c) General defects include;
iv) misruns, in which the casting solidifies before filling the mold cavity v) cold shuts, in which two portions of metal flow together but there is lack of fusion at the joint; vi) cold shots, where solid globules of cast metal become entrapped in the casting; vii) shrinkage cavity, which is a depression on the casting surface or an internal void in the casting caused by solidification shrinkage; viii) microporosity, which is a network of small voids throughout the casting caused by localized solidification shrinkage; and ix) hot tearing, which is a crack in the casting caused by a mold that does not yield to the metal during the early stages of solidification shrinkage.
a) How does the fabrication of a thermoplastic polymer differ from the processing of a thermosetting polymer? b) What are the significant differences in the equipment and operating procedures between injection mold- ing of thermoplastics and injection molding of thermosets? c) Can thermosetting plastics be used in injection molding? Explain. (8 marks)
a) Thermoplastic polymers can be heated to a temperature at or near the melting temperature so that the material becomes either a formable solid or a liquid. The polymer can than be cast, injected into a mold, or forced through a die to produce the desired shape. With thermosetting polymers, once the polymerization has occurred, no further deformation can occur. Thus, the polymerization reaction and the shape-forming process must be accomplished simultaneously. b) The differences in injection molding of thermosets are (1) shorter barrel length, (2) lower temperatures in the barrel, these first two reasons to prevent premature curing; and (3) use of a heated mold to cause cross-linking of the...