Metal Casting A

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 accuracy 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.

Documents

Metal Casting A