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Solutions Ch11

Solutions for Fundamentals of Modern Manufacturing, 3/e (published by Wiley) © MPGroover 2007 11 METAL CASTING PROCESSES Review Questions 11.1 11.2 Name the two basic categories of casting processes? Answer. The two categories are: (1) expendable mold processes, and (2) permanent mold processes. There are various types of patterns used in sand casting. What is the difference between a split pattern and a match-plate pattern? Answer. A split pattern is a pattern that consists of two pieces; a ma

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  Solutions for  Fundamentals of Modern Manufacturing , 3/e (published by Wiley) © MPGroover 2007   11 METAL CASTING PROCESSES Review Questions 11.1   Name the two basic categories of casting processes? Answer . The two categories are: (1) expendable mold processes, and (2) permanent mold processes.11.2   There are various types of patterns used in sand casting. What is the difference between a splitpattern and a match-plate pattern? Answer . A split pattern is a pattern that consists of two pieces; a match-plate pattern consists of thetwo split patterns attached to opposite sides of a plate.11.3   What is a chaplet? Answer . Chaplets are metal supports of various designs used to hold the core in place in the sandmold.11.4   What properties determine the quality of a sand mold for sand casting? Answer . The usual properties are (1) strength - ability to maintain shape in the face of the flowingmetal, (2) permeability - ability of the mold to allow hot air and gases to escape from the cavity, (3)thermal stability - ability to resist cracking and buckling when in contact with the molten metal, (4)collapsibility - ability of the mold to give way during shrinkage of the solidified casting, and (5)reusability - can the sand be reused to make other molds?11.5   What is the Antioch process? Answer . The Antioch process refers to the making of the mold. The mold is 50% sand and 50%plaster heated in an autoclave and then dried. This mold has greater permeability than a plastermold.11.6   What is the difference between vacuum permanent-mold casting and vacuum molding? Answer . Vacuum permanent-mold casting is a form of low-pressure casting in which a vacuum isused to draw molten metal into the cavity. Vacuum molding is sand casting in which the sand moldis held together by vacuum pressure rather than by a chemical binder.11.7   What are the most common metals used in die casting? Answer . Common die-casting metals include zinc, tin, lead, aluminum, brass, and magnesium.11.8   Which die casting machines usually have a higher production rate, cold-chamber or hot-chamber,and why? Answer . Hot-chamber machines are faster because cold- chamber die casting machines requiremolten metal to be ladled into the chamber from an external source.11.9   What is flash in die casting? Answer . Flash is a thin portion of metal at the exterior of a casting that results from molten metalbeing squeezed into the spaces between the die halves of the mold at the parting line, or into theclearances around the cores and ejector pins.11.10   What is the difference between true centrifugal casting and semicentrifugal casting? Answer . In true centrifugal casting, a tubular mold is used and a tubular part is produced. Insemicentrifugal casting, the shape is solid; an example is a railway wheel. The mold is rotated so Excerpts from this work may be reproduced by instructors for distribution on a not-for-profit basis for testing or instructional purposes only tostudents enrolled in courses for which the textbook has been adopted.  Any other reproduction or translation of this work beyond that permitted by Sections 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful.   51  Solutions for  Fundamentals of Modern Manufacturing , 3/e (published by Wiley) © MPGroover 2007   that centrifugal force is used to distribute the molten metal to the exterior of the mold so that thedensity of the final metal is greater at the outer sections.11.11   What is a cupola? Answer . A cupola is a vertical cylindrical furnace equipped with a tapping spout near its base.Cupolas are used for melting cast irons.11.12   What are some of the operations required in sand casting after the casting is removed from the mold? Answer . The operations include (1) trimming, in which the sprues, runners, risers, and flash areremoved, (2) core removal, (3) surface cleaning, (4) inspection, (5) repair if needed, (6) heattreatment, and (7) machining.11.13   What are some of the general defects encountered in casting processes? Name and briefly describethree. Answer . General defects include: (1) misruns, in which the casting solidifies before filling the moldcavity; (2) cold shuts, in which two portions of metal flow together but there is lack of fusion at the joint; (3) cold shots, where solid globules of cast metal become entrapped in the casting; (4)shrinkage cavity, which is a depression on the casting surface or an internal void in the castingcaused by solidification shrinkage; (5) microporosity, which is a network of small voids throughoutthe casting caused by localized solidification shrinkage; and (6) hot tearing, which is a crack in thecasting caused by a mold that does not yield to the metal during the early stages of solidificationshrinkage.11.14   ( Video ) What is the composition of green sand in the green-sand molding process? Answer: The sand is composed of silica sand, clay, and water.11.15   ( Video ) What are the advantages and disadvantages of sand casting over investment casting? Answer: Sand casting provides low production cost for a wide variety of metals, shapes andsizes. The size of the casting is unlimited. The disadvantage is the surface finish an dimensionalcontrol are not very good.11.16   ( Video ) Explain the difference between horizontal and vertical die casting machines. Which ismore popular? Answer: The direction in the machine indicates the direction which the metal is injected.Horizontal is injected from the side and vertical from the top. Horizontal is the most commontype used in industry.11.17   ( Video ) Why are aluminum and copper alloys unsuitable for use in hot chamber die casting? Answer: Molten aluminum and copper alloys attack the metal pot used to hold the shot for thenext casting. Over time, they would chemically attack and erode the die casting feedingmechanism.11.18   ( Video ) According to the die casting video, what materials are most common for die casting dies? Answer: Common materials for die casting dies are hot-work tool steels, mold steels, maragingsteels, and refractory metals such as tungsten alloys or molybdenum alloys. Multiple Choice Quiz There is a total of 27 correct answers in the following multiple choice questions (some questions havemultiple answers that are correct). To attain a perfect score on the quiz, all correct answers must be given.Each correct answer is worth 1 point. Each omitted answer or wrong answer reduces the score by 1 point,and each additional answer beyond the correct number of answers reduces the score by 1 point. Percentagescore on the quiz is based on the total number of correct answers. Excerpts from this work may be reproduced by instructors for distribution on a not-for-profit basis for testing or instructional purposes only tostudents enrolled in courses for which the textbook has been adopted.  Any other reproduction or translation of this work beyond that permitted by Sections 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful.   52  Solutions for  Fundamentals of Modern Manufacturing , 3/e (published by Wiley) © MPGroover 2007   11.1   Which one of the following casting processes is the most widely used: (a) centrifugal casting, (b) diecasting, (c) investment casting, (d) sand casting, or (e) shell casting? Answer . (d).11.2   In sand casting, the volumetric size of the pattern is (a) bigger than, (b) the same size as, or (c)smaller than the cast part? Answer . (a).11.3   Silica sand has which one of the following compositions: (a) Al 2 O 3 , (b) SiO, (c) SiO 2 , or (d) SiSO 4 ? Answer . (c).11.4   For which one of the following reasons is a green mold named: (a) green is the color of the mold, (b)moisture is contained in the mold, (c) mold is cured, or (d) mold is dry? Answer . (b).11.5   Given that W  m = weight of the molten metal displaced by a core and W  c = weight of the core, thebuoyancy force is which one of the following: (a) downward force = W  m + W  c , (b) downward force = W  m - W  c , (c) upward force = W  m + W  c , or (d) upward force = W  m - W  c ? Answer . (d).11.6   Which of the following casting processes are expendable mold operations (four correct answers): (a)centrifugal casting, (b) die casting, (c) investment casting, (d) low pressure casting, (e) sand casting,(f) shell molding, (g) slush casting, and (h) vacuum molding? Answer . (c), (e), (f), and (h).11.7   Shell molding is best described by which one of the following: (a) casting operation in which themolten metal has been poured out after a thin shell has been solidified in the mold, (b) castingprocess in which the mold is a thin shell of sand binded by a thermosetting resin, (c) sand castingoperation in which the pattern is a shell rather than a solid form, or (d) casting operation used tomake artificial sea shells? Answer . (b).11.8   Investment casting is also known by which one of the following names: (a) fast-payback molding,(b) full-mold process, (c) lost-foam process, (d) lost pattern process, or (e) lost-wax process? Answer . (e).11.9   In plaster mold casting, the mold is made of which one of the following materials: (a) Al 2 O 3 , (b)CaSO 4 -H 2 O, (c) SiC, or (d) SiO 2 ? Answer . (b).11.10   Which of the following qualifies as a precision casting process (two correct answers): (a) ingotcasting, (b) investment casting, (c) plaster mold casting, (d) sand casting, and (c) shell molding? Answer . (b) and (c).11.11   Which of the following casting processes are permanent mold operations (three correct answers): (a)centrifugal casting, (b) die casting, (c) expanded polystyrene process, (d) sand casting, (e) shellmolding, (f) slush casting, and (g) vacuum molding. Answer . (a), (b), and (f).11.12   Which of the following metals would typically be used in die casting (three best answers): (a)aluminum, (b) cast iron, (c) steel, (d) tin, (e) tungsten, and (f) zinc? Excerpts from this work may be reproduced by instructors for distribution on a not-for-profit basis for testing or instructional purposes only tostudents enrolled in courses for which the textbook has been adopted.  Any other reproduction or translation of this work beyond that permitted by Sections 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful.   53  Solutions for  Fundamentals of Modern Manufacturing , 3/e (published by Wiley) © MPGroover 2007   Answer . (a), (d), and (f).11.13   Which of the following are advantages of die casting over sand casting (four best answers): (a) bettersurface finish, (b) closer tolerances, (c) higher melting temperature metals, (d) higher productionrates, (e) larger parts can be cast, and (f) mold can be reused? Answer . (a), (b), (d), and (f).11.14   Cupolas are furnaces used to melt which of the following metals (one best answer): (a) aluminum,(b) cast iron, (c) steel, or (d) zinc? Answer . (b).11.15   A misrun is which one of the following defects in casting: (a) globules of metal becoming entrappedin the casting, (b) metal is not properly poured into the downsprue, (c) metal solidifies before fillingthe cavity, (d) microporosity, and (e) pipe formation? Answer . (c).11.16   Which one of the following casting metals is most important commercially: (a) aluminum and itsalloys, (b) bronze, (c) cast iron, (d) cast steel, or (e) zinc alloys? Answer . (c). Problems Buoyancy Force 11.1   An aluminum-copper alloy casting is made in a sand mold using a sand core that weighs 20 kg.Determine the buoyancy force in Newtons tending to lift the core during pouring. Solution : Core volume V  = 20/1605.4 = 0.01246 m 3 Weight of displaced Al-Cu W  = 35.17 kgDifference = (35.17 - 20) x 9.815 = 149 N 11.2   A sand core located inside a mold cavity has a volume of 157.0 in 3 . It is used in the casting of a castiron pump housing. Determine the buoyancy force that will tend to lift the core during pouring. Solution : From Table 13.1, density of cast iron  ρ  = 0.26 lb/in 3 F  b   = W  m - W  c   W  c = 157(0.058) = 9.106 lb W  m = 157(0.26) = 40.82 lb F  b = 40.82 - 9.11 = 31.71 lb 11.3   Caplets are used to support a sand core inside a sand mold cavity. The design of the caplets and themanner in which they are placed in the mold cavity surface allows each caplet to sustain a force of 10 lbs. Several caplets are located beneath the core to support it before pouring; and several othercaplets are placed above the core to resist the buoyancy force during pouring. If the volume of thecore = 325 in. 3 , and the metal poured is brass, determine the minimum number of caplets that shouldbe placed (a) beneath the core, and (b) above the core. Solution : From Table 13.1, density of brass  ρ  = 0.313 lb/in 3 .(a) W  c = 325(0.058) = 18.85 lb  At least 2 caplets are required beneath to resist the weight of the core. Probably 3 or 4 capletswould be better to achieve stability.(b) W  m = 325(.313) = 101.73 lb F  b = 101.73 - 18.85 = 82.88 lb A total of  9 caplets are required above the core to resist the buoyancy force. Excerpts from this work may be reproduced by instructors for distribution on a not-for-profit basis for testing or instructional purposes only tostudents enrolled in courses for which the textbook has been adopted.  Any other reproduction or translation of this work beyond that permitted by Sections 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful.   54