CHAPTER 11

Metal-Casting Processes

2

Summary of Casting

Processes

TABLE 11.1Process Advantages LimitationsSand Almost any metal cast; no limit

to size, shape or weight; low tooling cost.

Some finishing required; somewhat coarse finish; wide tolerances.

Shell mold Good dimensional accuracy and surface finish; high production rate.

Part size limited; expensive patterns and equipment required.

Expendable pattern Most metals cast with no limit to size; complex shapes

Patterns have low strength and can be costly for low quantities

Plaster mold Intricate shapes; good dimensional accu- racy and finish; low porosity.

Limited to nonferrous metals; limited size and volume of production; mold making time relatively long.

Ceramic mold Intricate shapes; close tolerance parts; good surface finish.

Limited size.

Investment Intricate shapes; excellent surface finish and accuracy; almost any metal cast.

Part size limited; expensive patterns, molds, and labor.

Permanent mold Good surface finish and dimensional accuracy; low porosity; high production rate.

High mold cost; limited shape and intricacy; not suitable for high-melting-point metals.

Die Excellent dimensional accuracy and surface finish; high production rate.

Die cost is high; part size limited; usually limited to nonferrous metals; long lead time.

Centrifugal Large cylindrical parts with good quality; high production rate.

Equipment is expensive; part shape limited.

3

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.

4

General Characteristics of Casting Processes

TABLE 11.2

Typical Weig ht (kg)Typical surface Section thic kness (mm)

Processmaterials

cast Minimum Maximum finish (µm, Ra) Porosity*

Shape complexity*

Dimensional accuracy* Minimum Maximum

Sand All 0.05 No limit 5-25 4 1-2 3 3 No limitShell All 0.05 100+ 1-3 4 2-3 2 2 --Expendable mold pattern All 0.05 No limit 5-20 4 1 2 2 No limit

Plaster mold

Nonferrous (Al, Mg, Zn,

Cu) 0.05 50+ 1-2 3 1-2 2 1 --

Investment

All (High melting

pt.) 0.005 100+ 1-3 3 1 1 1 75Permanent mold All 0.5 300 2-3 2-3 3-4 1 2 50

Die

Nonferrous (Al, Mg, Zn,

Cu) <0.05 50 1-2 1-2 3-4 1 0.5 12Centrifugal All -- 5000+ 2-10 1-2 3-4 3 2 100*Relative rating: 1 best, 5 worst.Note : These ratings are only general; significant variations can occur, depending on the methods used.

5

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.

6

Casting Examples

Figure 11.2 Typical gray-iron castings used in automobiles, including transmission valve body (left) and hub rotor with disk-brake cylinder (front). Source: Courtesy of Central Foundry Division of General Motors Corporation.

Figure 11.3 A cast transmission housing.

7

Chapter 11

…

mold

mold

1 個 workpiece

n 個 workpiece

Expendable moldExpendable mold

Permanent moldPermanent mold

8

Sand Mold Features

Figure 11.4 Schematic illustration of a sand mold, showing various features.

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Figure 11.5 Outline of production steps in a typical sand-casting operation.

Steps in Sand Casting

10

Ch11.2vCh 11.2.2

green molding sand 生砂 (砂、黏土、水)vCh 11.2.3q1 pattern→n mold ( pattern使用鋁或木頭等較易加工的材質)qTaper 錐度 (利於拔模，通常是1°~3 °)qCore print 砂心撐

vFigure 11.9vFigure 11.11vFigure 11.15

Core print

11

Pattern Material Characteristics

TABLE 11.3Ratinga

Characteristic Wood Aluminum Steel Plastic Cast ironMachinability E G F G GWear resistance P G E F EStrength F G E G GWeightb E G P G PRepairability E P G F GResistance to:Corrosionc E E P E PSwellingc P E E E EaE, 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.

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Patterns for Sand Casting

Figure 11.6 A typical metal match-plate 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.

14

Squeeze HeadsFigure 11.9 Various designs of squeeze heads for moldmaking: (a) conventional flat head; (b) profile head; (c) equalizing squeeze pistons; and (d) flexible diaphragm. Source: ©Institute of British Foundrymen. Used with permission.

15

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. (b-c) Patterns have been mounted on plates equipped with pins for alignment. Note the presence of core prints designed to hold the core in place. (d-e) 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)

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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).

Sequence of Operations for Sand Casting (cont.)

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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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Ch11.7v Investment casting 脫蠟鑄造

q優點：• 可做複雜造型 (∵wax易加工修整，可拼裝複雜造型)• 表面光滑 (∵ slurry磨粒細)

q缺點：• 無法做大型鑄件

Pattern (wax)Pattern (wax)

Mold (ceramics)Mold (ceramics)

Metal castingMetal casting

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Figure 11.18 Schematic illustration of investment casting, (lost-wax 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 near-net shape. Source: HowmetCorporation.

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Ch 11.12vDie casting 壓鑄

(die casting的材質通常是Al或Cu，熔點比steel低，故使用steel die。當材質是steel，就必須使用熔點更高的模具材料)

…die

n 個 casting

22

Hot- and Cold-Chamber Die-Casting

Figure 11.23 (a) Schematic illustration of the hot-chamber die-casting process. (b) Schematic illustration of the cold-chamber die-casting process. Source: Courtesy of Foundry Management and Technology.

(a) (b)

23

Cold-Chamber Die-Casting Machine

(a)

Figure 11.24 (a) Schematic illustration of a cold-chamber die-casting 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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Figure 11.24 (b) 800-ton hot-chamber die-casting machine, DAM 8005 (made in Germany in 1998). This is the largest hot-chamber machine in the world and costs about $1.25 million.

(b)

Hot-Chamber Die-Casting Machine

25

Ch11.12vDie castingqHot- chamber Figure 11.23 (a)• For ↓Tm metals (Zn. Sn. Pb)• 103/hr• P 15MPa

qCold-chamber Figure 11.23 (b)• For ↑Tm metals (Al. Cu. Mg)• Avoid comtamination in chamber at high temperature

(因為化學反應在高溫易發生，所以高溫時容易產生許多雜質)• P=20~150MPa → fine detail and thin wall

≈

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Die-Casting Die Cavities

Figure 11.25 Various types of cavities in a die-casting die. Source: Courtesy of American Die Casting Institute.

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Ch11.12vFigure 11.24qDie：Workpiece=1000：1q25~3000 ton 鎖模力

vEstimate of Clamping force 鎖模力

ton55N1050forceClampingmm10sectioncrossofcavitydiea

50N/mm50MPapressurecastingdie

4

24

2

=×=→

≈

=≈

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Properties and Typical Applications of Common Die-Casting Alloys

TABLE 11.4

Alloy

Ultimate tensile

strength (MPa)

Yield strength (MPa)

Elongation in 50 mm

(%) ApplicationsAluminum 380 (3.5 Cu-8.5 Si) 320 160 2.5 Appliances, automotive components,

electrical motor frames and housings 13 (12 Si) 300 150 2.5 Complex shapes with thin walls, parts

requiring strength at elevated temperatures

Brass 858 (60 Cu) 380 200 15 Plumbing fiztures, lock hardware, bushings, ornamental castings

Magnesium AZ91 B (9 Al-0.7 Zn) 230 160 3 Power tools, automotive parts, sporting goods

Zinc No. 3 (4 Al) 280 -- 10 Automotive parts, office equipment, household utensils, building hardware, toys

5 (4 Al-1 Cu) 320 -- 7 Appliances, automotive parts, building hardware, business equipment

Source : Data from American Die Casting Institute

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Ch 11vAdvantage of die castingqFine detail (thin wall<0.5 mm)qFine surface finishqFine grain structure (fast cooling→grain size

small)qHighly automated process

vDisadvantage of die castingqHigh equipment cost

(machine：106~107 NTD；die：104~105 NTD)

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Single Crystal Casting

Figure 11.31 Two methods of crystal growing: (a) crystal pulling (Czochralski process) and (b) the floating-zone method. Crystal growing is especially important in the semiconductor industry. Source: L. H. Van Vlack, Materials for Engineering. Addison-Wesley Publishing Co., Inc., 1982.

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Ch 11

精讀：11.1~11.2.4, 11.2.6, 11.7, 11.12略讀：11.2.5, 11.4, 11.15(d)免讀：11.3, 11.5~11.6, 11.8~11.11,

11.15 (a)~(c), 11.17~11.19