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CASTING PROCESSES I

Lecture Notes

by

Zulkepli Muhamad

Solidification Processes

Starting work material is either a liquid or is in ahighly plastic condition, and a part is createdthrough solidification of the material

Solidification processes can be classified

according to engineering material processed:z Metals

z Ceramics, specifically glasses

z Polymers and polymer matrix composites(PMCs)

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Classification of solidification processes

Classification of casting processes

Metal casting processes Foundries

Expandable mold Permanent mold Single-crystal growing

Composite

moldSandShell

Expandable pattern

Plaster

Ceramic

investment

Slush

Pressure

Die

Centrifugal

Squeeze

semisolid

Single-crystal for microelectronics

Single-crystal turbine blade

Directional solidification

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Casting Processes

1. Expendable mold processes uses an expendable

moldwhich must be destroyed to remove casting

Mold materials: sand, plaster, and similar

materials, plus binders

2. Permanent mold processes uses apermanent

moldwhich can be used many times to produce

many castings

Made of metal (or, less commonly, a ceramic

refractory material

Overview of Casting Technology

Castingis usually performed in a foundry

Foundry = factory equipped for making

molds, melting and handling molten metal,

performing the casting process, andcleaning the finished casting

Workers who perform casting are called

foundrymen

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Casting

Process in which molten metal flows by gravity or otherforce into a mold where it solidifies in the shape ofthe mold cavity

The term castingalso applies to the part made in theprocess

Steps in casting seem simple:1. Prepare mould/die

2. Melt the metal

3. Pouring melt into a mould/die4. Let it solidify

5. Finishing processes

Metal casting components

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Metal casting components

Metal casting components

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Production steps

Mold

Molding

Sand

Melting Pouring Casting Heat treatment Finishing

Pattern makingCore Making

Gating system

Furnaces Solidification ShakeoutRemoval of risers

and gates

Cleaning &

Inspection-additioanl heat

treatment

-defects

-dimensions

Why Castings ?

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Capabilities and Advantages

of Casting

Can create complex part geometries

Can create both external and internalshapes

Some casting processes are net shape;others are near net shape

Can produce very large parts

Some casting methods are suited tomass production

Parts Made by Casting

Big parts: engine blocks and heads for

automotive vehicles, wood burning stoves,

machine frames, railway wheels, pipes,

church bells, big statues, and pumphousings

Small parts: dental crowns, jewelry, small

statues, and frying pans

All varieties of metals can be cast, ferrous

and nonferrous

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Sand Casting process

680 Kg Compressor Frame

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Sand mould

Sand mould

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Sand Moulding Processes

Sand Moulding Processes (cont.)

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Sand Moulding Processes (cont.)

Two forms of mold: (a) open mold, simply a

container in the shape of the desired part;

and (b) closed mold, in which the mold

geometry is more complex and requires a

gating system (passageway) leading into the

cavity

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The Pattern

A full-sized model of the part, slightly

enlarged to account for shrinkage and

machining allowances in the casting

Pattern materials:

z Wood - common material because it is easy to

work, but it warps

z Metal - more expensive to make, but lasts much

longerz Plastic - compromise between wood and metal

Types of patterns used in sand casting:

(a) solid pattern

(b) split pattern

(c) match-plate pattern

(d) cope and drag pattern

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The Core

Full-scale model of interior surfaces of part

It is inserted into the mold cavity prior topouring

The molten metal flows and solidifiesbetween the mold cavity and the core to formthe casting's external and internal surfaces

May require supports to hold it in position in

the mold cavity during pouring, calledchaplets

Core held in place in the mold cavity by

chaplets

(b) possible chaplet design

(c) casting with internal cavity

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Sand Casting - Process variations

z Green sand casting: the most common and

the cheapest. Associated problems arethat the mould has low strength and high

moisture content.

z Dry sand: core boxes are used instead of

patterns. Expensive and time consuming.

z Skin-dried sand: the mould is dried to a

certain depth. Used in the casting of steels.

Sand Casting - Economicconsiderations

z Production rates of 1-60 pieces/hour, but

dependent on size.

z Lead time ranges from days to several weeks

depending on complexity and size of casting.

z Material utilization is low to moderate - 20-50% of

material lost in runners and risers.

z Both mould material and runners and risers may

he recycled.

z Patterns are easy to make and set, and are

reusable.

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Sand Casting-Economic

considerations

z Pattern material dependent on the number of

castings required.z Easy to change design during production.

Economical for low production runs. Can be used

for one-offs.

z Tooling costs are low.

z Equipment costs are low. Direct labour costs are

moderate to high. Can be labour intensive.

z Finishing costs can be high. Cleaning and fettling

are important before secondary processing.

Process Parameters

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Pouring

Desirable Mold Properties andCharacteristics

Strength - to maintain shape and resist

erosion

Permeability- to allow hot air and gases to

pass through voids in sand

Thermal stability- to resist cracking on

contact with molten metal

Collapsibility- ability to give way and allow

casting to shrink without cracking the casting

Reusability- can sand from broken mold be

reused to make other molds?

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Sands

The sands used may be one of the following

categories;naturally bonded (bank) - less expensive

synthetic (lake) - this sand can have avariety of controlled compositions.

Types of sand include,

Zircon (ZrSiO4) - low thermal expansion

Olivine (Mg2SiO4) - low thermal expansion

Iron Silicate (Fe2SiO4) - low thermal

expansionChromite (FeCr2O4) - high heat transfer

Foundry Sands

Silica (SiO2) or silica mixed with other minerals

Good refractory properties - capacity toendure high temperatures

Small grain size yields better surface finish onthe cast part

Large grain size is more permeable, to allowescape of gases during pouring

Irregular grain shapes tend to strengthenmolds due to interlocking, compared to roundgrainsz Disadvantage: interlocking tends to reduce

permeability

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Types of Sand Mold

Green-sand molds - mixture of sand, clay,

and water;

z Green" means mold contains moisture at time of

pouring

Dry-sand mold- organic binders rather than

clay and mold is baked to improve strength

Skin-dried mold- drying mold cavity surface

of a green-sand mold to a depth of 10 to 25

mm, using torches or heating lamps

Other Expendable Mold

Casting Processes

Shell Molding

Vacuum Molding

Expanded Polystyrene Process

Investment Casting

Plaster Mold and Ceramic Mold Casting