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Transcript
Introduction to Manufacturing
Chapters 10 & 11: Metal Casting
Processes and Equipment
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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Mold Patterns
• 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
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Mold Components
• Cores- inner part of mold
• Chaplets- anchors, supports for cores
• Chill- insert for preferential cooling
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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, Evaporativepattern, or Lost Pattern Casting
• Polystyrene beads, bonded by hot die
• Flask formed vaporizes during pouring
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Expendable Foam
• 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.
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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 Casting
Process
Sand Casting
Shell-mold Casting
Plaster Casting
Investment Casting
Permanent-mold Casting
Die Casting
Centrifugal Casting
Mold/Die
L
L-M
L-M
M-H
M
H
M
Cost
Equipment
L
M-H
M
L-M
M
H
H
Labor
L-M
L-M
M-H
H
L-M
L-M
L-M
Production
rate (parts/hr)
< 20
< 50
< 10
< 1000
< 60
< 200
< 50
L - low, M - medium, H - high
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