Chapter 1 - Sand Casting.ppt

Chapter 1Foundry Processes

CastingOne of the oldest manufacturing processes 4000 B.C. with stone and metal molds for casting copperPour molten metal into a mold cavity to produce solidified parts that take on the shape of the cavityMany different casting processes, each with its own characteristics, applications and materials, advantages, limitations, and costs

CastingCasting can produce complex shapes with internal cavities or hollow sectionsCasting can produce very large partsCompetitive with other processesGood net-shape manufacturing for metals

Casting ConsiderationsFlow of molten metal into the mold cavityMold designFluid flow characteristics

Solidification and cooling of the metal in the moldMetallurgical and thermal properties of the molten metal and mold materialGeometric relationship between volume and surface area of the castingMold shape

Influence and type of mold materialRate of cooling of the molten metalFactors that affect defect formationSurface finish

Cast Structures of MetalsFigure 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.

Types of CastingExpandable mold casting

Permanent mold casting

Cont.Sand castingShell mould castingInvestment castingCentrifugal castingDie casting

CASTING CHARACTERISTICSA mould cavity must have the designed shape and size, and include allowances for shrinkage. The mould material must be capable of producing the shape and not react with the molten metal.

Moulds can be produced for one-off castings, but economics make it desirable to reuse moulds in some way, if possible. Permanent moulds can be made of graphite or metals but are expensive.

CASTING CHARACTERISTICSA suitable method of melting the alloy must be available; this includes adequate temperature, satisfactory quality and quantity at low cost.

When the molten metal is poured, all gases must be able to escape allowing the cavity to be filled and keeping the casting dense and defect free; this is the reason for risers.

Casting DefectsFigure 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.

*Summary of Casting Processes

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TABLE 11.1

ProcessAdvantagesLimitations

SandAlmost any metal cast; no limit to size, shape or weight; low tooling cost.Some finishing required; somewhat coarse finish; wide tolerances.

Shell moldGood dimensional accuracy and surface finish; high production rate.Part size limited; expensive patterns and equipment required.

Expendable patternMost metals cast with no limit to size; complex shapesPatterns have low strength and can be costly for low quantities

Plaster moldIntricate shapes; good dimensional accuracy and finish; low porosity.Limited to nonferrous metals; limited size and volume of production; mold making time relatively long.

Ceramic moldIntricate shapes; close tolerance parts; good surface finish.Limited size.

InvestmentIntricate shapes; excellent surface finish and accuracy; almost any metal cast.Part size limited; expensive patterns, molds, and labor.

Permanent moldGood surface finish and dimensional accuracy; low porosity; high production rate.High mold cost; limited shape and intricacy; not suitable for high-melting-point metals.

DieExcellent dimensional accuracy and surface finish; high production rate.Die cost is high; part size limited; usually limited to nonferrous metals; long lead time.

CentrifugalLarge cylindrical parts with good quality; high production rate.Equipment is expensive; part shape limited.

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TABLE 11.1

Typicalht (kg)Typical surfaceSection thickness (mm)

Processmaterials castMinimumMaximumfinish (m, Ra)Porosity*Shape complexity*Dimensional accuracy*MinimumMaximum

SandAll0.05No limit5-2541-233No limit

ShellAll0.05100+1-342-322--

Expendable mold patternAll0.05No limit5-204122No limit

Plaster moldNonferrous (Al, Mg, Zn, Cu)0.0550+1-231-221--

InvestmentAll (High melting pt.)0.005100+1-3311175

Permanent moldAll0.53002-32-33-41250

DieNonferrous (Al, Mg, Zn, Cu)

General Characteristics of Casting Processes

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TABLE 11.1 Summary of Casting Processes, Their Advantages and Limitations











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Sheet2

TABLE 11.2

TypicalWeight (kg)Typical surfaceSection thickness (mm)

Processmaterials castMinimumMaximumfinish (mm, Ra)Porosity*Shape complexity*Dimensional accuracy*MinimumMaximum


ShellAll0.05100+1-342-322--






Sand Casting

Sand Casting - Products

Characteristics of ProductsLarge size High StrengthLow surface finish

Sand Mold FeaturesFigure 11.4 Schematic illustration of a sand mold, showing various features.

*Figure 11.5 Outline of production steps in a typical sand-casting operation.Steps in Sand Casting

Pattern Material Characteristics

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TABLE 11.1 Summary of Casting Processes, Their Advantages and Limitations











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TABLE 11.2 General Characteristics of Casting Processes

TypicalWeight (kg)Typical surfaceSection thickness (mm)

Processmaterials castMinimumMaximumfinish (m, Ra)Porosity*Shape complexity*Dimensional accuracy*MinimumMaximum


ShellAll0.05100+1-342-322--






Patterns for Sand CastingFigure 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.

Sequence of Operations for Sand CastingFigure 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)

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.)

Surface Roughness for Various Metalworking ProcessesFigure 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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Chapter 1 - Sand Casting.ppt

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