Introduction to Manufacturing Chapters 10 & 11: Metal Casting Processes and Equipment

Introduction to Manufacturing

Chapters 10 & 11: Metal Casting Processes and Equipment

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Metal Casting• Process in which molten metal is poured into

a mold (shaped after the part to be manufactured), then allowed to cool and solidify.

• After solidification occurs the part is removed from the mold to cool further.

• The main objective is to produce parts free of defects and with the desired properties.

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Process Characteristics• Complex shapes that may have internal cavities

• Large or small parts.

• Can use materials which are otherwise hard to

shape.

• Economical.

• Near net shape manufacturing.

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Considerations• Solidification – mold design and material

effect cooling rate (Heat transfer).

• Metal flow into mold cavity – Flow.

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Solidification of Metals

• Pure metals have single cooling temperature.

• Chill zone- (skin) small equiaxed grains.

• Columnar zone- crystal growth inward.

• Homogeneous nucleation- grains grow

upon each other.

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Cooling Rates

• Slow- course dendrites, large spacing.

• Moderate- fine with small dendrite

spacing.

• Fast- amorphous structure.

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Cooling Rates (Cont.)

• When grain size decreases• strength and ductility increase.• microporosity decreases.• cracks decrease.

Lack of uniformity in grain size gives anisotropic properties

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Alloy Segregation• Microsegregation.

• higher concentration of alloying elements at surface.

• Normal Segregation.• higher concentration of alloying elements at

center. Lower melting alloys forced to center.

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Alloy Segregation (Cont.)• Inverse Segregation

• Metals with high concentrations of alloy elements -lower concentration of alloying elements at center. Dentrites shrink, alloys fill

• Gravity Segregation• high density inclusions or compounds sink, lighter

elements float.

• Inoculation• heterogeneous nucleation.

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Fluid Flow• Basic Casting System

• Pouring basin (cup).

• Sprue, runners- channels

• Gate- entry point for mold

• Riser- reservoir

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Fluid Flow (Cont.)• Bernoulli's Theorem.

• pressure, velocity, elevation at any location, and frictional losses.

• Continuity Law • rate of flow, permeability.• aspiration (non tapered sprue)

• Flow Characteristics• laminar or turbulent.

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Fluidity

• Capability of molten metal to fill the mold.

• Metal Characteristics.

• Casting Parameters.

• (see p. 249)

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Heat Transfer During Cooling• Temperature Distribution.

• flow, premature chilling- (Fig. 10-9)

• Solidification Time (shapes).• ratio of volume to surface area.

• Shrinkage- (Table 10.1).• contraction of the metal when cooled.• Grey iron expands.

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Defects• Metallic projections.

• flash, fins, swells.

• Cavities.• blow holes, pinholes, shrinkage.

• Discontinuities.• cracks, cold or hot tearing, cold shuts.

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Defects (Cont.)• Defective surface.

• folds, laps, scars, adhering sand, oxide.

• Incomplete casting.• misruns, insufficient metal, runout.

• Incorrect dimensions or shape.

• Inclusions.• non-metallic, stress risers.

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Porosity• Caused by shrinkage or gasses.

• Chills are used to increase the rate of solidification

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Molds• Expendable molds.

• made of sand, plaster, or ceramics (mixed with bonding agents/binders).

• broken up to remove casting.

• Permanent molds• used repeatedly.• made from metals which maintain strength at

high temperatures.

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Other (Molding) Methods• Composite molds.

• two or more types of materials.• used to improve mold strength, cooling rates,

cost of process.

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Sand Casting• Consists in placing a pattern in sand to make

an imprint, incorporating a gating system, filling the cavity with molten metal, letting it cool, breaking the mold to remove the casting.• Traditional casting method.

• Loose tolerances.

• “poor” surface finish.

• low cost.

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Sands• Silica based (SiO2), inexpensive, resistance

to high temperature.

• Naturally bonded (bank sands).

• Synthetic (lake sands).

• Grain size (permeability- heat transfer/gases out,

collapsibility- sand breaks down easily).

• Sand is typically conditioned

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Sands (Cont.)• Mulling- mixing sand with additives.

• Additives:

• Clay (bentonite)- for bonding/strength

• Zircon, Olivine, and Iron silicate- to lower

thermal expansion

• Chromite- for high heat transfer

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Types of Sand Molds

• Green Sand:• sand, clay, and water.• least expensive.

• Cold-box:• organic and inorganic binders.• greater dimensional accuracy.• greater cost.

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Types of Sand Molds (Cont.)

• No-bake:• synthetic liquid resin mixed with sand.• Cold-setting process- bonding of mold

takes place without heat

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Mold Components (Fig. 11-4, p.265)

• Flask.• Cope/Drag.

• Pouring basin or pouring cup.• Sprue.• Runner and gates.• Risers (blind and open).• Cores.• Vents.

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• Used to create sand molds• Made of wood, aluminum, steel, plastic, cast

iron.• One piece (loose pattern).

• simple shapes, low quantity production.

• Split pattern• Two piece patterns, complex shapes.

• Match plate• Split patterns secured to plate

Mold Patterns

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• Cores- inner part of mold• Chaplets- anchors, supports for cores

• Chill- insert for preferential cooling

Mold Components

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Shell Mold Casting

• Uses binder which hardens in CO2

• Shell is formed from injected/poured sand over a mold

• Close tolerance good surface finish, low cost.

• Thin walled-low permeability

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Expendable-Pattern Casting

• Also known as Lost Foam, Evaporative-pattern, or Lost Pattern Casting

• Polystyrene beads, bonded by hot die

• Flask formed vaporizes during pouring

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• Advantages (P. 275)• simple process, no parting lines, or riser

system.• inexpensive flasks, minimum finishing and

cleaning.• polystyrene is cheap and gives good detail.• economical for long production runs

(pattern mold cost).• can be automated.

Expendable Foam

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Plaster-Mold Casting

• Plaster of paris with talc and silica flour.• Water mix- pour as a slurry.• Low permeability (gas cannot escape).• Good surface and details.• Cools slowly.• “Lower” temperature alloys (Mg, Al, Zn)

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Ceramic-Mold Casting• Also referred to as cope and drag investment

casting.• Ceramic slurry is poured over a pattern,

removed, and baked• Slurry: refractory mold materials (fine grained

zircon, aluminum oxide, fused silica).• Good dimensional accuracy and surface

finish, but expensive.

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Investment Casting (P. 278)

• Lost-Wax Process• Consists in coating a pattern, made of

wax or plastic, with a refractory material. Once the coating agent has dried, the mold is heated to remove the wax.

• Superb finishing• Trends (RP).

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Vacuum Casting• Counter-gravity low pressure process.• Sand and urethane molded over metal

die.• Gate is on the bottom.• Immersed into molten metal, which is

drawn into mold cavity.• Thin wall, complex shapes, uniform

properties, high volume, low cost.

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Die Casting

• Molten metal is forced into a permanent mold (die) at high pressure.

• Hot chamber and Cold chamber pressure casting

• High production rates, high quality parts, good dimensions, complex shapes, good surface (net shape).

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Other Processes

• Permanent Mold Casting.

• Slush Casting.

• Centrifugal Casting.

• Squeeze Casting

• Semi-solid Metal Forming.

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Economics of CastingProduction

Mold/Die Equipment Labor rate (parts/hr)Sand Casting L L L-M < 20Shell-mold Casting L-M M-H L-M < 50Plaster Casting L-M M M-H < 10Investment Casting M-H L-M H < 1000Permanent-mold Casting M M L-M < 60Die Casting H H L-M < 200Centrifugal Casting M H L-M < 50

CostProcess

L - low, M - medium, H - high