CHAPTER 11 Metal-Casting Processes
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TABLE 11.1 Process
Summary of Casting Processes
Advantages
Limitations
Sand
Almost any metal cast; no limit to size, shape or weight; low tooling cost.
Some finishing required; somewhat coarse finish; wide tolerances.
Shell mold
Gooddimensional accuracy and surface finish; high production rate.
Part size limited; expensive patterns andequipment required.
Expendable pattern
Most metals cast with no limit to size; complex shapes
Patterns have lowstrength and can be costly for lowquantities
Plaster mold
Intricate shapes; good dimensional accu- racy and finish; lowporosity.
Limitedto nonferrous metals; limitedsize andvolume of production; moldmaking time relatively long.
Ceramic mold
Intricate shapes; close tolerance parts; goodsurface finish.
Limitedsize.
Investment
Intricate shapes; excellent surface finish andaccuracy; almost any metal cast.
Part size limited; expensive patterns, molds, andlabor.
Permanent mold
Goodsurface finish and dimensional accuracy; low porosity; high production rate.
High moldcost; limitedshape andintricacy; not suitable for high-melting-point metals.
Die
Excellent dimensional accuracy andsurface finish; high production rate.
Die cost is high; part size limited; usually limitedto nonferrous metals; long lead time.
Centrifugal
Large cylindrical parts with goodquality; high production rate.
Equipment is expensive; part shape limited.
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Die-Casting Examples
(a)
(b)
Figure 11.1 (a) The Polaroid PDC-2000 digital camera with a AZ91D die-cast, high purity magnesium case. (b) Two-piece Polaroid camera case made by the hot-chamber die casting process. Source: Courtesy of Polaroid Corporation and Chicago White Metal Casting, Inc.
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General Characteristics of Casting Processes TABLE 11.2
Process Sand Shell Expendable mold pattern
Minimum
Maximum
All All
0.05 0.05
No limit 100+
5-25 1-3
4 4
1-2 2-3
0.05
No limit
5-20
4
0.05
50+
1-2
0.005
100+
0.5
300
All Nonferrous Plaster (Al, Mg, Zn, mold Cu) All (High melting Investment pt.) Permanent mold All Nonferrous (Al, Mg, Zn, Die Cu) Centrifugal All
Weig ht (kg)
Typical surface finish (µm, Ra)
Typical materials cast
Section thic kness (mm) Shape Dimensional Porosity* complexity* accuracy*
Minimum
Maximum
3 2
3 2
No limit --
1
2
2
No limit
3
1-2
2
1
--
1-3
3
1
1
1
75
2-3
2-3
3-4
1
2
50
1 3
0.5 2
12 100
<0.05 50 1-2 1-2 3-4 -5000+ 2-10 1-2 3-4 *Relative rating: 1 best, 5 worst. Note : These ratings are only general; significant variations can occur, depending on the methods used.
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Casting Examples Figure 11.2 Typical gray iron castings used in automobiles, including transmission valve body (left) and hub rotor with diskbrake cylinder (front). Source: Courtesy of Central Foundry Division of General Motors Corporation.
Figure 11.3 A cast transmission housing.
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Sand Mold Features
Figure 11.4 Schematic illustration of a sand mold, showing various features.
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Steps in Sand Casting
Figure 11.5 Outline of production steps in a typical sandcasting operation.
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Pattern Material Characteristics TABLE 11.3 Characteristic
Wood
Aluminum
Ratinga Steel
Plastic
Cast iron
Machinability E G F G G Wear resistance P G E F E Strength F G E G G Weightb E G P G P Repairability E P G F G Resistance to: Corrosionc E E P E P Swellingc P E E E E aE, Excellent; G, good; F, fair; P, poor. bAs a factor in operator fatigue. cBy water. Source : D.C. Ekey and W.R. Winter, Introduction to Foundry Technology. New York. McGraw-Hill, 1958. Kalpakjian • Schmid Manufacturing Engineering and Technology
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Patterns for Sand Casting Figure 11.6 A typical metal matchplate pattern used in sand casting.
Figure 11.7 Taper on patterns for ease of removal from the sand mold.
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Examples of Sand Cores and Chaplets
Figure 11.8 Examples of sand cores showing core prints and chaplets to support cores.
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Squeeze Heads Figure 11.9 Various designs of squeeze heads for mold making: (a) conventional flat head; (b) profile head; (c) equalizing squeeze pistons; and (d) flexible diaphragm. Source: © Institute of British Foundrymen. Used with permission.
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Vertical Flaskless Molding
Figure 11.10 Vertical flaskless molding. (a) Sand is squeezed between two halves of the pattern. (b) Assembled molds pass along an assembly line for pouring.
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Sequence of Operations for Sand Casting
Figure 11.11 Schematic illustration of the sequence of operations for sand casting. Source: Steel Founders' Society of America. (a) A mechanical drawing of the part is used to generate a design for the pattern. Considerations such as part shrinkage and draft must be built into the drawing. (bc) Patterns have been mounted on plates equipped with pins for alignment. Note the presence of core prints designed to hold the core in place. (de) Core boxes produce core halves, which are pasted together. The cores will be used to produce the hollow area of the part shown in (a). (f) The cope half of the mold is assembled by securing the cope pattern plate to the flask with aligning pins, and attaching inserts to form the sprue and risers. (continued) Kalpakjian • Schmid Manufacturing Engineering and Technology
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Sequence of Operations for Sand Casting (cont.)
Figure 11.11 (g) The flask is rammed with sand and the plate and inserts are removed. (g) The drag half is produced in a similar manner, with the pattern inserted. A bottom board is placed below the drag and aligned with pins. (i) The pattern, flask, and bottom board are inverted, and the pattern is withdrawn, leaving the appropriate imprint. (j) The core is set in place within the drag cavity. (k) The mold is closed by placing the cope on top of the drag and buoyant forces in the liquid, which might lift the cope. (l) After the metal solidifies, the casting is removed from the mold. (m) The sprue and risers are cut off and recycled and the casting is cleaned, inspected, and heat treated (when necessary). Kalpakjian • Schmid Manufacturing Engineering and Technology
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Surface Roughness for Various Metalworking Processes
Figure 11.12 Surface roughness in casting and other metalworking processes. See also Figs. 22.14 and 26.4 for comparison with other manufacturing processes.
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Dump-Box Technique Figure 11.13 A common method of making shell molds. Called dumpbox technique, the limitations are the formation of voids in the shell and peelback (when sections of the shell fall off as the pattern is raised). Source: ASM International.
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Composite Molds
Figure 11.14 (a) Schematic illustration of a semipermanent composite mold. Source: Steel Castings Handbook, 5th ed. Steel Founders' Society of America, 1980. (b) A composite mold used in casting an aluminumalloy torque converter. This part was previously cast in an allplaster mold. Source: Metals Handbook, vol. 5, 8th ed.
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Expendable Pattern Casting Figure 11.15 Schematic illustration of the expendable pattern casting process, also known as lost foam or evaporative casting.
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Ceramic Molds Figure 11.16 Sequence of operations in making a ceramic mold. Source: Metals Handbook, vol. 5, 8th ed.
Figure 11.17 A typical ceramic mold (Shaw process) for casting steel dies used in hot forging. Source: Metals Handbook, vol. 5, 8th ed. Kalpakjian • Schmid Manufacturing Engineering and Technology
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Figure 11.18 Schematic illustration of investment casting, (lostwax process). Castings by this method can be made with very fine detail and from a variety of metals. Source: Steel Founders' Society of America.
Investment Casting
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Investment Casting of a Rotor
Figure 11.19 Investment casting of an integrally cast rotor for a gas turbine. (a) Wax pattern assembly. (b) Ceramic shell around wax pattern. (c) Wax is melted out and the mold is filled, under a vacuum, with molten superalloy. (d) The cast rotor, produced to net or nearnet shape. Source: Howmet Corporation.
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Investment and Conventionally Cast Rotors Figure 11.20 Cross section and microstructure of two rotors: (top) investmentcast; (bottom) conventionally cast. Source: Advanced Materials and Processes, October 1990, p. 25 ASM International
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Vacuum-Casting Process
Figure 11.21 Schematic illustration of the vacuumcasting process. Note that the mold has a bottom gate. (a) Before and (b) after immersion of the mold into the molten metal. Source: From R. Blackburn, "Vacuum Casting Goes Commercial," Advanced Materials and Processes, February 1990, p. 18. ASM International. Kalpakjian • Schmid Manufacturing Engineering and Technology
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Pressure Casting
Figure 11.22 (a) The bottompressure casting process utilizes graphite molds for the production of steel railroad wheels. Source: The Griffin Wheel Division of Amsted Industries Incorporated. (b) Gravity pouring method of casting a railroad wheel. Note that the pouring basin also serves as a riser. Railroad wheels can also be manufactured by forging.
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Hot- and Cold-Chamber Die-Casting (a)
(b)
Figure 11.23 (a) Schematic illustration of the hotchamber diecasting process. (b) Schematic illustration of the coldchamber diecasting process. Source: Courtesy of Foundry Management and Technology.
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Cold-Chamber Die-Casting Machine
(a)
Figure 11.24 (a) Schematic illustration of a coldchamber diecasting machine. These machines are large compared to the size of the casting because large forces are required to keep the two halves of the dies closed.
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Hot-Chamber Die-Casting Machine (b)
Figure 11.24 (b) 800ton hotchamber diecasting machine, DAM 8005 (made in Germany in 1998). This is the largest hotchamber machine in the world and costs about $1.25 million.
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Die-Casting Die Cavities Figure 11.25 Various types of cavities in a diecasting die. Source: Courtesy of American Die Casting Institute.
Figure 11.26 Examples of castin place inserts in die casting. (a) Knurled bushings. (b) Grooved threaded rod.
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Properties and Typical Applications of Common Die-Casting Alloys TABLE 11.4 Ultimate tensile strength (M Pa)
Yield strength (M Pa)
Elongation in 50 mm (%)
Aluminum 380 (3.5 Cu-8.5 Si)
320
160
2.5
13 (12 Si)
300
150
2.5
Bras s 858 (60 Cu)
380
200
15
Magnesium AZ91 B (9 Al-0.7 Zn)
230
160
3
Zinc No. 3 (4 Al)
280
--
10
320
--
7
Alloy
5 (4 Al-1 Cu)
Applications Appliances, automotive components, electrical motor frames and housings Complex shapes with thin walls , parts requiring s trength at elevated temperatures Plumbing fiztures, lock hardware, bushings , ornamental cas tings Power tools, automotive parts, sporting goods Automotive parts , office equipment, hous ehold utensils , building hardware, toys Appliances, automotive parts, building hardware, busines s equipment
Source : Data from American Die Cas ting Ins titute
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Centrifugal Casting Process
Figure 11.27 Schematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped parts can be cast with this process. Kalpakjian • Schmid Manufacturing Engineering and Technology
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Semicentrifugal Casting
Figure 11.28 (a) Schematic illustration of the semicentrifugal casting process. Wheels with spokes can be cast by this process. (b) Schematic illustration of casting by centrifuging. The molds are placed at the periphery of the machine, and the molten metal is forced into the molds by centrifugal force.
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Squeeze-Casting
Figure 11.29 Sequence of operations in the squeezecasting process. This process combines the advantages of casting and forging.
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Single Crystal Casting of Turbine Blades Figure 11.30 Methods of casting turbine blades: (a) directional solidification; (b) method to produce a singlecrystal blade; and (c) a singlecrystal blade with the constriction portion still attached. Source: (a) and (b) B. H. Kear, Scientific American, October 1986; (c) Advanced Materials and Processes, October 1990, p. 29, ASM International. (c)
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Single Crystal Casting Figure 11.31 Two methods of crystal growing: (a) crystal pulling (Czochralski process) and (b) the floatingzone method. Crystal growing is especially important in the semiconductor industry. Source: L. H. Van Vlack, Materials for Engineering. AddisonWesley Publishing Co., Inc., 1982.
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Melt Spinning Figure 11.32 Schematic illustration of meltspinning to produce thin strips of amorphous metal.
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Types of Melting Furnaces Figure 11.33 Two types of melting furnaces used in foundries: (a) crucible, and (b) cupola.
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