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8/3/2019 Metal Casting A
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Metal Casting
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Lesson Outcomes
By the end of this lessons, students should be able to
explain:
1. The generic metal casting process
2. The sand casting process and example products3. The investment casting process and example
products
4. The die casting process and example products
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The metal casting processThe casting process generally involves:
1. Pouring molten metal into a mold
patterned after the part to be manufactured
2. Allowing the metal to cool and solidify
3. Removing the solid part from the mold
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Types of moldsCasting processes can be classified by the type of mold
used:
Expendable molds After the casting has solidified, mold is broken to remove the
casting
Typically made of sand, plaster, ceramics
Permanent molds Used repeatedly (not broken after solidification)
Made of metals
Composite molds Consists of permanent and expendable portions
Made of two or more different materials (eg: sand, graphite,
metal)
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Overview
Shell mold Good dimensional accuracyand surface finish; high
production rate.
Part size limited; expensivepatterns 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; gooddimensional accu- racy and
finish; low porosity.
Limited to nonferrous metals;limited s ize 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 cos t; limited shape
and intricacy; not suitable for
high-melting-point metals.
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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 s ize limited;
usually limited to nonferrous
metals; long lead time.
Centrifugal Large cylindrical parts with
good quality; high productionrate.
Equipment is expensive; part
shape limited.
Overview
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Examples
(a) (b)
(a) The Polaroid PDC-2000 digital camera with a AZ91D die-cast,high purity magnesium case. (b) Two-piece Polaroid camera casemade by the hot-chamber die casting process. Source: Courtesy ofPolaroid Corporation and Chicago White Metal Casting, Inc.
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Sand Casting
Most common type of casting
The mold is made of sand
There are two types of sand used:
Naturally bonding
Synthetic
Example products: machine bases,
propellers, impellers, etc.
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Sand casting: A typical sand mold
Schematic illustration of a sand mold, showing various features.
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Sand Casting: Sand mold features
1. Flask Supports the mold
2. Pouring basin A hole into which the molten metal is poured
3. Sprue
A vertical channel that allow the metal to flow downward4. Runner System
Channels that allow metal to flow from the sprue to mold cavity
5. Risers Store and supply additional molten metal to the casting as it
shrinks during solidification6. Cores
Inserts made from sand. Placed in mold to form hollow regions.
7. Vents Allow gasses and exhaust air to exit
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Sand casting: Mold Patterns
A typical metal match-platepattern used in sand casting. Taper on patterns for ease of removal
from the sand mold.
Used to shape the sand mixture into the shape of the castingUsed repeatedly to make sand molds
Can be made of wood, plastic or metal
Coated with parting agent to ease removal ofmold from pattern
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Sand casting: Steps
Outline of production steps in a typical sand-casting operation.
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Investment casting
Example products: components for office
equipment, gears, cams, valves, etc.
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Investment Casting: StepsSchematicillustration ofinvestment casting,
(lost-wax process).Castings by thismethod can bemade with veryfine detail and froma variety of metals.Source: SteelFounders' Societyof America.
1. Mold is normally made ofmetal
2. Pattern is then made byinjecting wax or plastic intomold
3. A series of patterns can beassembled into a pattern tree
4. The pattern tree is then dipped
into a slurry (eg: fine silicaand binders)
5. Stucco coating is appliedrepeatedly to increasethickness, forming a one-piece mold
6. The one-piece mold is driedand heated to melt out thewax (original pattern).
7. The one-piece mold is heatedfurther to remove water andburn off any remaining wax.
8. Molten metal is poured intothe one-piece mold. Aftersolidification, the one-piecemold is broken up and thecasting is removed.
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1. Mold is normally made of metal
2. Pattern is then made by injecting wax or plasticinto mold
3. A series of patterns can be assembled into apattern tree
4. The pattern tree is then dipped into a slurry (eg:fine silica and binders)
5. Stucco coating is applied repeatedly to increasethickness, forming a one-piece mold
6. The one-piece mold is dried and heated to melt
out the wax (original pattern).7. The one-piece mold is heated further to remove
water and burn off any remaining wax.
8. Molten metal is poured into the one-piece mold.After solidification, the one-piece mold isbroken up and the casting is removed.
Investment Casting: Steps
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Die Casting In permanent-mold casting, two halves of a mold are made
from materials that resist erosion and thermal fatigue suchas cast iron, steel and graphite.
The die casting process: A piston traps a volume of molten metal
Piston then forces molten metal into the die cavity through anozzle
Metal is held under pressure (up to 35 MPa) until it solidifies.
The part is then ejected by an ejector mechanism
The die is normally cooled using circulating water or oil
Example products: Engine blocks, hand tools, motor housings
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Hot-chamber die-casting
Heated chamber
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Cold-chamber die casting
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Die Casting Cavities
Various types of cavities in a die-casting die. Source: Courtesy of American DieCasting Institute.
Examples of cast-in- placeinserts in die casting. (a)Knurled bushings. (b)Grooved threaded rod.
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Hot Chamber Die
Casting Machine(b)
(b) 800-ton hot-chamber die-casting machine, DAM 8005 (made in Germanyin 1998). This is the largest hot-chamber machine in the world and costsabout $1.25 million.
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Summary
Metal casting involves pouring molten metal into amold, allowing the metal to solidify and removingthe part from the mold
Casting processes can be classified as permanentmold and expendable mold processes
Each process is suitable for manufacturing aparticular class of products.