1 Casting

Fundamental of Metal Casting

Factors in casting operation „ „ „ „

(1)Solidification (2)Fluidity of molten metal (3)Heat transfer (4)Mold material

Solidification „

„

(1)Nucleation -homogeneous -heterogeneous (2)Growth -planar -dendrite

Solidification of metals „

„

(1)Pure metals (fig.10.1-2) -skin , shell , or chill zone of fine equiaxed grains -columnar grains (2)Alloys(fig .10.3-4) -chill zone -columnar dendrites -equiaxed zone (grains )

Cast Structures of Metals Figure 10.1 Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure metals; (b) solid-solution alloys; and (c) structure obtained by using nucleating agents. Source: G. W. Form, J. F. Wallace, J. L. Walker, and A. Cibula.

Preferred Texture Development

Figure 10.2 Development of a preferred texture at a cool mold wall. Note that only favorably oriented grains grow away from the surface of the mold.

Alloy Solidification

Figure 10.3 Schematic illustration of alloy solidification and temperature distribution in the solidifying metal. Note the formation of dendrites in the mushy zone.

Solidification Patterns Figure 10.4 (a) Solidification patterns for gray cast iron in a 180-mm (7-in.) square casting. Note that after 11 min. of cooling, dendrites reach each other, but the casting is still mushy throughout. It takes about two hours for this casting to solidify completely. (b) Solidification of carbon steels in sand and chill (metal) molds. Note the difference in solidification patterns as the carbon content increases. Source: H. F. Bishop and W. S. Pellini.

Cast Structures

Figure 10.6 Schematic illustration of cast structures in (a) plane front, single phase, and (b) plane front, two phase. Source: D. Apelian.

Figure 10.5 Schematic illustration of three basic types of cast structures: (a) columnar dendritic; (b) equiaxed dendritic; and (c) equiaxed nondendritic. Source: D. Apelian.

Effects of cooling rates „

„

„

(1)Slow cooling rates (order of 102K/s ) -coarse dendritic structure (2)Faster cooling rates (order of 104K/s ) -finer with smaller dendrite arm spacing (3)Faster cooling rates (order of 106 to 108K/s ) -amorphous

Grain size of the cast alloy decreases „ „ „

(1)Strength and ductility increase (2)Microporosity decreases (3)Tendency to crack (hot tearing) decreases

Fluid flow „ „ „

(1)Bernoulli's theorem (2)Continuity law (3)Flow characteristics -turbulence or laminar flow

Riser-Gated Casting Figure 10.7 Schematic illustration of a typical riser-gated casting. Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification. See also Fig. 11.4 Source: American Foundrymen’s Society.

Fluidity of molten metal A .Molten metal „ (1)Viscosity „ (2)Surface tension „ (3)Inclusions „ (4)Solidification pattern (freezing range )

Fluidity Test Figure 10.8 A test method for fluidity using a spiral mold. The fluidity index is the length of the solidified metal in the spiral passage. The greater the length of the solidified metal, the greater is its fluidity.

Fluidity of molten metal B .Casting parameters „ (1)Mold design „ (2)Mold material and its surface characteristics „ (3)Degree of superheat „ (4)Rate of pouring „ (5)Heat transfer (fig.10.9)

Temperature Distribution Figure 10.9 Temperature distribution at the interface of the mold wall and the liquid metal during solidification of metals in casting.

Solidification time

  volume  = C   surface area 

2

Solidification Time Figure 10.10 Solidified skin on a steel casting. The remaining molten metal is poured out at the times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which is based on this principle. Source: H. F. Taylor, J. Wulff, and M. C. Flemings.

Shrinkage „ „

(1)Dimensional changes (2)Cracking

Solidification Contraction for Various Cast Metals TABLE 10.1 Metal or alloy Aluminum Al–4.5%Cu Al–12%Si Carbon steel 1% carbon steel Copper Source: After R. A. Flinn.

Volumetric solidification contraction (%) 6.6 6.3 3.8 2.5–3 4 4.9

Metal or alloy 70%Cu–30%Zn 90%Cu–10%Al Gray iron Magnesium White iron Zinc

Volumetric solidification contraction (%) 4.5 4 Expansion to 2.5 4.2 4–5.5 6.5

Defects „ „ „ „ „ „ „ „

(1)Metallic projections (2)Cavities (3)Discontinuities (4)Defective surface (5)Incomplete casting (6)Incorrect dimensions or shape (7)Inclusions (8)Porosity (fig .10.13-15)

Hot Tears

Figure 10.11 Examples of hot tears in castings. These defects occur because the casting cannot shrink freely during cooling, owing to constraints in various portions of the molds and cores. Exothermic (heatproducing) compounds may be used (as exothermic padding) to control cooling at critical sections to avoid hot tearing.

Casting Defects Figure 10.12 Examples of common defects in castings. These defects can be minimized or eliminated by proper design and preparation of molds and control of pouring procedures. Source: J. Datsko.

Internal and External Chills Figure 10.13 Various types of (a) internal and (b) external chills (dark areas at corners), used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metals, as shown in (c).

Solubility of Hydrogen in Aluminum Figure 10.14 Solubility of hydrogen in aluminum. Note the sharp decrease in solubility as the molten metal begins to solidify.

Metal-Casting Processes

Introduction „ „ „ „

(1)Metal casting processes (2)Principles (3)Advantages (4)Limitations

Impact on the casting industry „ „

(1)Mechanization and automation (2)High quality with close tolerance

Casting Examples Figure 11.2 Typical grayiron 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.

Major classification of casting processes „

(1)Expendable molds - sand , plaster , ceramics , and similar materials ,which are generally mixed with various binder, bonding agent .

Major classification of casting processes „

„

(2)Permanent molds -metals with adequate strength at high temperature (3)Composite molds

Sand casting „ „

(1)Process (2)Sands -silixca(SiO2)：naturally bonded synthetic (preferred ) -grain shape and size affect mold surface , strength , permeability -clay (a cohesive agent)

Steps in Sand Casting

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

34

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).

Sand casting „

(3)Three basic types of sand molds -green sand：a mixture of sand, clay, and water skin-dried (baked)-stronger, better dimensional accuracy and surface finish the least expensive

Sand casting -cold-box：organic and inorganic binder +sand greater strength more dimensional accuracy more expensive -no-bake mold (cold-setting): a synthetic liquid resin + sand at room temperature

Sand casting „

(4)Major component of sand casting(fig.11.4) -flask , cope , drag , cheeks -pouring basin or cup , sprue -runner system , gates -risers (blind and open) -cores -vents

Sand Mold Features

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

Sand casting „

(5)Patterns -wood , plastic , metal (table 11.2) -depends on size and shape of the casting dimensional accuracy quality molding process -parting agent

General Characteristics of Casting Processes TABLE 11.2

Process Sand Shell Expendable mold pattern

Typical materials cast

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)

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.

Sand casting „

-one-piece patterns simple shape and low quality production wood -split patterns (two-piece) cavity -match-plate pattern (fig .11.6) a popular type each half of one or more split patterns large production

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.

Sand casting „

(6)Cores -shell , no-bake , or cold-box processes -core prints (fig.11.8) -chaplets：metal supports for shifting -core blowers

Examples of Sand Cores and Chaplets

Figure 11.8 Examples of sand cores showing core prints and chaplets to support cores.

Sand casting „

(7)Sand-molding machines -jolting , squeezing , compacting(fig.11.7-8) -vertical flask less molding (fig .11.9) -impact molding -vacuum molding (V process )

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.

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.

Sand casting „

(8)Sand-casting operation(fig.11.10) -molds -gating system , risers. -solidification -surface of casting -heat treatment -finishing operations

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)

Shell Molding „

(1)Close tolerance and good surface finishing Suited for nearly any metal More economical Casting with sharper corners , thinner section , smaller projection High quality and complex shapes Typical parts : gear housings , cylinder head , connecting rods ,high-precision molding cores

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.

Dump-Box Technique Figure 11.13 A common method of making shell molds. Called dump-box 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.

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 aluminum-alloy torque converter. This part was previously cast in an all-plaster mold. Source: Metals Handbook, vol. 5, 8th ed.

Shell Molding „

(2)Process -a mounted pat tern of a ferrous or aluminum at 175-370℃ -coated with a parting agent (silicone ) -sand + 2.5-4.0 % thermosetting resin binder -sand mixture is blown over the heated pattern -shell hardens around the pattern and is removed from the pat tern -assemble of the shell

Shell Molding „

(3)Characteristics of shell -thickness depend on time , 5-l0mm -a much lower permeability produce a high volume of gas

Shell Molding „

(4)Composite molds -two or more different materials and used in shell molding and other casting processes -complex shapes ex . impellers -increase the strength of mold improve the dimensional accuracy and finish reduce overall costs and processing time

Sodium Silicate Process (CO2 Process) „

„

-sand + 1.5-6 % sodium silicate (waterglass) + CO2 gas -used as cores

Evaporative Pat tern Casting (Lost Foam) „

(1)Process -polystryrene + 5-8 % pentane or PMMA, polyalkylene carbonate placed at a preheated AL die -pattern coated with a water-base refractory slurry, dried , and placed in a flask -sand + pattern

Expendable Pattern Casting Figure 11.15 Schematic illustration of the expendable pattern casting process, also known as lost foam or evaporative casting.

Evaporative Pat tern Casting (Lost Foam) „ „

(2)Velocity of molten metal：0.1-1 m/s (3)Advantages -relatively simple process -inexpensive flasks

Evaporative Pat tern Casting (Lost Foam) -complex shapes , various sizes , and fine surface detail of inexpensive polystyrene pattern -minimum finishing and cleaning operation -economical for long production runs -automation

Evaporative Pat tern Casting (Lost Foam) „

(4)Typical parts -cylinder heads , crankshafts , brake components ,manifolds for automobiles -machine bases

Plastor-Mold Casting „

(1)Process -plaster + talc /silica flour + water placed at patter within 15 min -plaster mold dried at 120-260℃ -mold assembled and preheated to about 120℃ -pouring -shakeout

Plastor-Mold Casting „

„

(2)Pattern for plaster molding : Al alloys, thermosetting plastic , brass , or zinc alloys (3)Increasing permeability -Antioch process : -dehydrated in a pressurized oven for 6-12 hrs, then dehydrated in air for 14 hrs -foamed plaster

Plastor-Mold Casting „

(4)Characteristics -Max. temperature：about 1200℃ -used only for Al , Mg , Zn , and some Cu-base alloys casting

Plastor-Mold Casting -precision casting (Ceramic-mold , investment casting) -high dimensional accuracy -good surface finish -weight less than 10 kg , typical range of 125-250 g

Plastor-Mold Casting „

(5)Typical parts -lock component -gears -valves -fitting -tooling -ornaments

Ceramic-Mold Casting (cope-and-drag casting) „

(1)Process -slurry of fine-grained zircon , aluminum oxide , fused silica , and bonding agent poured into pattern -green mold dried , burned off -ceramic facings assembled into a complete mold -pouring -shakeout

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.

Ceramic-Mold Casting (cope-and-drag casting) „

(2)Casting materials -ferrous and other high-temperature alloys -stainless steels -tool steels

Ceramic-Mold Casting (cope-and-drag casting) „

(3)Characteristics -good dimensional accuracy -good surface finish -intricate shapes -expensive -weight up to 700 kg

Ceramic-Mold Casting (cope-and-drag casting) „

(4)Typical parts -impellers -cutters for machining -dies for metalworking -molds

Investment Casting (lost-wax process) „

(1)Process -mold to make pattern -wax pattern -pattern assembly (tree) -slurry and stucco coating -completed mold -pattern melt out -pouring molten metal -shakeout -casting

Investment Casting

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 (lost-wax process) „

(2)Characteristics -casting high melting point a1loys ex .ferrous and nonferrous metals -good surface finish -close tolerance -little or no finishing operations -intricate shapes

Investment Casting (lost-wax process) „

(3)Typical parts -gears -cams -valves -ratchets

Ceramic-Shell Investment Casting „

(1)Process -wax or plaster pattern -pattern dipped in ethyl silicate gel -pattern dipped in a fluidized bed of fine grained fused silica or zircon flour -pattern dipped in coarser grain silica to withdraw the thermal shock of pouring -the rest of the procedure is similar to lost-wax process

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: Howmet Corporation.

Investment and Conventionally Cast Rotors Figure 11.20 Crosssection and microstructure of two rotors: (top) investment-cast; (bottom) conventionally cast. Source: Advanced Materials and Processes, October 1990, p. 25 ASM International

Ceramic-Shell Investment Casting „ „

(2)Economical process (3)Precision casting of steels and hightemperature alloy

Vacuum Casting „

(1)Process - sand + urethane molded - vacuum - drawing the molten metal into the mold cavity

Vacuum-Casting Process

Figure 11.21 Schematic illustration of the vacuum-casting 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.

Vacuum Casting „

(2)Characteristics -temperature of molten metal：liquids temp.+ 55℃ -suitab1e for thin wall(1.75mm)complex shapes

Vacuum Casting -casting of carbon and low- and highalloys steel part -weight up to 70kg -inexpensive production costs (similar to green sand casting)

Permanent-Mold Casting „

„

„

(1)Mold material：cast iron, stee1, bronze, graphite, refractory metal alloys (2)Core material ：oil -bonded or resinbonded sand ,plaster , graphite , gray iron, low-carbon steel (3)Mo Id preheated to 150-200 ℃

Permanent-Mold Casting „

„

(4)Casting material ：A1 , Mg , Cu alloys , gray iron , steel (5)Good surface finish , close tolerance , Uniform and good mechanical properties High production rates

Permanent-Mold Casting „

(6)Typi calparts：automobile piston , cylinder head connecting rods , gear blanks

Slush Casting „

(1)Process -pour molten metal into mold -mold inverted -remaining liquid metal is poured out -hollow casting with thin walls

Slush Casting „ „

„

(2)Small production runs (3)Typical parts：ornamental and decorative objects and toys (4)Casting material :Zn ,Sn ,Pb alloys

Pressure Casting „

(1)Process -molten metal forced upward by gas pres sure or vacuum into a graphite or metal mold

Pressure Casting

Figure 11.22 (a) The bottom-pressure 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.

Die Casting-two basic types „

A. Hot-chamber process (1)Pressure: max.35MPa average 15MPa (2)Dies cooled by circulating water or oil (3)Casting low-melting-point alloys: Zn , Sn , Pb

Die-Casting Examples (a)

(b)

Figure 11 (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.

Die Casting-two basic types „

B .Cold-chamber process (1)Pressure: 20-70 MPa, max.150MPa (2)Casting high-melting-point alloys:A1,Mg ,Cu alloys

Hot- and Cold-Chamber DieCasting (a)

(b)

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.

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.

Hot-Chamber Die-Casting Machine (b)

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.

Die Casting-two basic types „

C. Characteristics (1)Typical parts : carburetors, motors, business machine ,hand tools ,toys (2)Die casting dies

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

Figure 11.26 Examples of cast-in- place inserts in die casting. (a) Knurled bushings. (b) Grooved threaded rod.

Die Casting-two basic types (3)High production rates Good strength, dimensional accuracy and surface finish Complex shapes (wall thickness as small as 0.5mIm) Little or no finishing operations

Centrifugal Casting-three types „

A. True centrifugal casting (1)Mold material: steel, iron, graphite coated with a refractory lining (2)Pressure: max.150g

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.

Properties and Typical Applications of Common Die-Casting Alloys TABLE 11.4 Ultimate tensile strength (MPa)

Yield strength (MPa)

Elongation in 50 mm (%)

320

160

2.5

300

150

2.5

Brass 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 Aluminum 380 (3.5 Cu-8.5 Si) 13 (12 Si)

5 (4 Al-1 Cu) Source : Data from American Die Casting Institute

Applications Appliances, automotive components, electrical motor frames and housings Complex shapes with thin walls, parts requiring strength at elevated temperatures Plumbing fiztures, lock hardware, bushings, ornamental castings Power tools, automotive parts, sporting goods Automotive parts, office equipment, household utensils, building hardware, toys Appliances, automotive parts, building hardware, business equipment

Centrifugal Casting-three types (3)Cylindrical parts: diameter: 13mm-3m length: 16m wall thickness :6-125mm (4)Typical parts: pipes, bushing, enginecylinder liners bearing rings, gun barrels

Centrifugal Casting-three types „

„

B .Semi centrifugal casting (1)Casting parts with rotational symmetry C .Centrifuging

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.

Squeeze Casting „ „

„

(1)Combination of casting and forging (2)Fine microstructure Good mechanical proper ties (3)Typical parts: automotive whee1, mortar bodies

Squeeze-Casting

Figure 11.29 Sequence of operations in the squeeze-casting process. This process combines the advantages of casting and forging.

Casting Techniques for Single-Crystal Components „

(1)Convent ional casting of turbine blades -ceramic casting: polycrystalline structure (fig.10.1c) poor resistant to creep and cracking

Casting Techniques for Single-Crystal Components „

„

(2)Directionally solidified blades -columnar grains -better resistant to creep (3)Single-crystal blades -best resistant to creep and thermal shock

Casting Techniques for Single-Crystal Components „

(4)Single-crystal growing -crystal pulling (Czochralski process ) Si, Ge ingots: dia.50-150 mm length 1m -floating zone method

Single Crystal Casting of Turbine Blades Figure 11.30 Methods of casting turbine blades: (a) directional solidification; (b) method to produce a single-crystal blade; and (c) a single-crystal 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)

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.

Melt Spinning Figure 11.32 Schematic illustration of melt-spinning to produce thin strips of amorphous metal.

Types of Melting Furnaces Figure 11.33 Two types of melting furnaces used in foundries: (a) crucible, and (b) cupola.

Inspection of Castings „ „

(1)Destructive tests (2)Nondestructive tests

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

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.

118