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

1. 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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2. 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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3. Considerations
Solidification – mold design and material effect cooling rate (Heat transfer).
Metal flow into mold cavity – Flow.
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4. 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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5. Cooling Rates Slow- course dendrites, large spacing.
Moderate- fine with small dendrite spacing.
Fast- amorphous structure.
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6. 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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7. Alloy Segregation Microsegregation. Normal Segregation.
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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8. 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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9. Fluid Flow Basic Casting System Pouring basin (cup).
Sprue, runners- channels
Gate- entry point for mold
Riser- reservoir
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10. Fluid Flow (Cont.) Bernoulli's Theorem. Continuity Law
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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11. Fluidity Capability of molten metal to fill the mold.
Metal Characteristics.
Casting Parameters.
(see p. 249)
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12. 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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13. Defects Metallic projections. Cavities. Discontinuities.
flash, fins, swells.
Cavities.
blow holes, pinholes, shrinkage.
Discontinuities.
cracks, cold or hot tearing, cold shuts.
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14. Defects (Cont.) Defective surface. Incomplete casting.
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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15. Porosity Caused by shrinkage or gasses.
Chills are used to increase the rate of solidification
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16. Molds Expendable molds. Permanent 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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17. 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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18. 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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19. 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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20. 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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21. Types of Sand Molds Green Sand: Cold-box: sand, clay, and water.
least expensive.
Cold-box:
organic and inorganic binders.
greater dimensional accuracy.
greater cost.
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22. 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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23. 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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24. 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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25. Mold Components Cores- inner part of mold
Chaplets- anchors, supports for cores
Chill- insert for preferential cooling
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26. 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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27. 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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28. 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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29. 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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30. 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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31. 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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32. 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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33. 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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34. Other Processes Permanent Mold Casting. Slush Casting.
Centrifugal Casting.
Squeeze Casting
Semi-solid Metal Forming.
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35. Economics of Casting
L - low, M - medium, H - high
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