topic 4 - metal casting

7/23/2019 Topic 4 - Metal Casting

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Metal Casting
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Lesson Outcomes

By the end of this lessons, students should be able toexplain:

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 process

The 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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Why Casting?Casting process are most often selected over other

manufacturing method because:

1. Casting can produce complex shape

2. Can incorporate internal cavities

3. Can produce large parts in one piece

4. Can utilize material difficult or uneconomic to

process by other means
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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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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, showin 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 downward

4. 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 solidification

6. 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 casting

Used repeatedly to make sand molds

Can be made of wood, plastic or metal

Coated with parting agent to ease removal of mold 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: Steps

Schematicillustration ofinvestmentcasting, (lost-wax process).Castings by thismethod can be

made with veryfine detail andfrom a variety ofmetals. Source:Steel Founders'Society ofAmerica.

1. Mold to make pattern isnormally made of metal

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 thendipped into a slurry (eg: finesilica and 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 isheated further to remove

water and burn off anyremaining wax.

8. Molten metal is poured intothe one-piece mold. Aftersolidification, the one-piecemold is broken up and thecasting is removed.

C i S
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1. Mold is normally made of metal

2. Pattern is then made by injecting wax or plastic

into mold3. A series of patterns can be assembled into a

pattern tree

4. The pattern tree is then dipped into a slurry (eg:

fine silica and binders)5. Stucco coating is applied repeatedly to increase

thickness, 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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Sand Casting & Investment Casting - video


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Inpermanent-mold casting, (also called hard-mold

casting), two halves of a mold are made from materials

with high resistance to erosion and thermal fatigue, such

as cast iron, steel, bronze, graphite, or refractory metalalloys.

The mold cavity and gating system are machined into the

mold and, thus, become an integral part of it.

To produce castings with internal cavities, cores made of

metal or sand aggregate are placed in the mold prior to

casting.

Permanent-Mold Casting


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In order to increase the life of permanent molds, the surfaces of the

mold cavity usually are coated with a refractory slurry (such as

sodium silicate and clay) or sprayed with graphite every few

castings.

The molds are clamped together by mechanical means and heatedto about 150 to 200C to facilitate metal flow and reduce thermal

damage to the dies due to high-temperature gradients.



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Although the permanent-mold casting operation can be performed

manually, it can be automated for large production runs.

This process is used mostly for aluminum, magnesium, copper

alloys, and gray iron because of their generally lower melting

points, although steels also can be cast using graphite or heat-

resistant metal molds.



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Various types: -

Die Casting

Vacuum Casting

Centrifugal Casting

Squeeze Casting

Composite Mold Casting



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Die Casting The two halves of a mold are made from materials that

resist erosion and thermal fatigue such as cast iron, steeland graphite.

The die casting process: A piston traps a volume of molten metal

Piston then forces molten metal into the die cavity through a

nozzle at pressures ranging from 0.7 to 700 MPa.

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, engine blocks,business-machine and appliance components, hand tools, and toys.

Example products:

There are two basic types of die-casting machines:

hot-chamberand cold-chamber.


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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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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.

Die-casting dies may besingle cavity, multiple cavity (with

several identical cavities), combination cavity (with severaldifferent cavities), or unit dies (simple small dies that can

be combined in two or more units in a master holding die).


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Process capabil i ties

Die casting has the capability for rapid production of

strong, high-quality parts with complex shapes, especially

with aluminum, brass, magnesium, and zinc.

It also produces good dimensional accuracy and surfacedetails, so that parts require little or no subsequent

machining or finishing operations (net-shape forming).

Die Casting


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Die Casting


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Die Casting - video


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Vacuum Casting


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Centrifugal Casting


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Squeeze casting

Summary


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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 moldprocesses

Each process is suitable for manufacturing aparticular class of products.

topic 4 - metal casting

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