10 FUNDAMENTALS OF METAL CASTING Review Questions 10.1 Identify some of the important advantages of shape casting processes. Answer. Advantages include (1) complex part geometries are possible; (2) some casting operations are net shape processes, meaning that no further manufacturing operations are needed to accomplish the final part shape; (3) very large parts are possible; (4) they are applicable to any metal that can be melted; and (5) some casting processes are suited to mass production. 10.2 What are some of the limitations and disadvantages of casting? Answer. Disadvantages include (1) limitations on mechanical strength properties; (2) porosity; (3) poor dimensional accuracy; (4) safety hazards due to handling of hot metals; and (5) environmental problems. 10.5 Name the two basic mold types that distinguish casting processes. Answer. The two mold types are (1) expendable molds and (2) permanent molds. 10.6 Which casting process is the most important commercially? Answer. Sand casting is the most important casting process. 10.7 What is the difference between a pattern and a core in sand molding? Answer. The pattern determines the external shape of the cast part, while a core determines its internal geometry if the casting includes a cavity. 10.8 What is meant by the term superheat? Answer. Superheat is the temperature difference above the melting point at which the molten metal is poured. The term also refers to the amount of heat that is removed from the molten metal between pouring and solidification. 10.9 Why should turbulent flow of molten metal into the mold be avoided? Answer. Turbulence causes the following problems: (1) it accelerates formation of oxides in the solidified metal, and (2) it causes mold erosion or gradual wearing away of the mold due to impact of molten metal. 10.13 How does solidification of alloys differ from solidification of pure metals? Answer. Pure metals solidify at a single temperature equal to the melting point. Most alloys (exceptions are eutectic alloys) start to solidify at the liquidus and complete solidification occurs at the solidus, where the liquidus is a higher temperature than the solidus. 10.16 Identify the three sources of contraction in a metal casting after pouring. Answer. The three contractions occur due to (1) contraction of the molten metal after pouring, (2) solidification shrinkage during transformation of state from liquid to solid, and (3) thermal contraction in the solid state. 10.17 What is a chill in casting? Answer. A chill is a heat sink placed to encourage rapid freezing in certain regions of the casting. Multiple Choice Quiz 10.1 Sand casting is which of the following types: (a) expendable mold or (b) permanent mold? Answer. (a). 10.4 In foundry work, a runner is which one of the following: (a) channel in the mold leading from the downsprue to the main mold cavity, (b) foundryman who moves the molten metal to the mold, or (c) vertical channel into which molten metal is poured into the mold? Answer. (a). 10.6 Total solidification time is defined as which one of the following: (a) time between pouring and complete solidification, (b) time between pouring and cooling to room temperature, (c) time between solidification and cooling to room temperature, or (d) time to give up the heat of fusion? Answer. (a). 10.8 Chvorinov's Rule states that total solidification time is proportional to which one of the following quantities: (a) (A/V)n, (b) Hf, (c) Tm, (d) V, (e) V/A, or (f) (V/A)2; where A = surface area of casting, Hf = heat of fusion, Tm = melting temperature, and V = volume of casting? Answer. (f). 10.10 In a sand casting mold, the V/A ratio of the riser should be (a) equal to, (b) greater than, or (c) smaller than the V/A ratio of the casting itself? Answer. (b). Problems 10.9 A mold cavity has the shape of a cube, 100 mm on a side. Determine the dimensions and volume of the final cube after cooling to room temperature if the cast metal is copper. Assume that the mold is full at the start of solidification and that shrinkage occurs uniformly in all directions. Use the shrinkage values given in Table 10.1. Solution: For copper, solidification shrinkage is 4.9%, solid contraction during cooling is 7.5%. Volume of cavity V = (100)3 = 1,000,000 mm3 Volume of casting V = 1,000,000(1-0.049)(1-0.075) = 1,000,000(.951)(.025) = 879,675 mm3 Dimension on each side of cube = (879,675)0.333 = 95.82 mm
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