casting processes 1(modif)fkm.utm.my/~zulkepli/notescast/casting_lec2.pdf ·  · 2005-08-15lecture...

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CASTING PROCESSES I

Lecture Notesby

Zulkepli Muhamad

Solidification Processes

Starting work material is either a liquid or is in a highly plastic condition, and a part is created through solidification of the materialSolidification processes can be classified according to engineering material processed:

MetalsCeramics, specifically glassesPolymers 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

CompositemoldSand

ShellExpandable patternPlasterCeramicinvestment

SlushPressureDieCentrifugalSqueezesemisolid

Single-crystal for microelectronicsSingle-crystal turbine bladeDirectional solidification

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Casting Processes

1. Expendable mold processes – uses an expendable mold which must be destroyed to remove casting

Mold materials: sand, plaster, and similar materials, plus binders

2. Permanent mold processes – uses a permanent mold which can be used many times to produce many castings

Made of metal (or, less commonly, a ceramic refractory material

Overview of Casting Technology

Casting is usually performed in a foundry Foundry = factory equipped for making molds, melting and handling molten metal, performing the casting process, and cleaning the finished castingWorkers who perform casting are called foundrymen

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Casting

Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavityThe term casting also applies to the part made in the processSteps in casting seem simple: 1. Prepare mould/die2. Melt the metal3. Pouring melt into a mould/die4. Let it solidify5. 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 MakingGating system

Furnaces Solidification ShakeoutRemoval of risersand gates

Cleaning &Inspection-additioanl heattreatment-defects-dimensions

Why Castings ?

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Capabilities and Advantages of Casting

Can create complex part geometriesCan create both external and internal shapesSome casting processes are net shape;others are near net shapeCan produce very large parts Some casting methods are suited to mass 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 pump housings 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 castingPattern materials:

Wood - common material because it is easy to work, but it warps Metal - more expensive to make, but lasts much longer 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 CoreFull-scale model of interior surfaces of part

It is inserted into the mold cavity prior to pouringThe molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces May require supports to hold it in position in the mold cavity during pouring, called chaplets

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

Green sand casting: the most common and the cheapest. Associated problems are that the mould has low strength and high moisture content.Dry sand: core boxes are used instead of

patterns. Expensive and time consuming.Skin-dried sand: the mould is dried to a

certain depth. Used in the casting of steels.

Sand Casting - Economic considerations

Production rates of 1-60 pieces/hour, but dependent on size.Lead time ranges from days to several weeks

depending on complexity and size of casting.Material utilization is low to moderate - 20-50% of material lost in runners and risers.Both mould material and runners and risers may

he recycled.Patterns are easy to make and set, and are

reusable.

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Sand Casting-Economic considerations

Pattern material dependent on the number of castings required. Easy to change design during production. Economical for low production runs. Can be used for one-offs. Tooling costs are low. Equipment costs are low. Direct labour costs are moderate to high. Can be labour intensive. Finishing costs can be high. Cleaning and fettling are important before secondary processing.

Process Parameters

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Pouring

Desirable Mold Properties and Characteristics

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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SandsThe sands used may be one of the following

categories;naturally bonded (bank) - less expensive synthetic (lake) - this sand can have a variety of controlled compositions.

Types of sand include,Zircon (ZrSiO4) - low thermal expansion Olivine (Mg2SiO4) - low thermal expansion Iron Silicate (Fe2SiO4) - low thermal expansion Chromite (FeCr2O4) - high heat transfer

Foundry SandsSilica (SiO2) or silica mixed with other minerals

Good refractory properties - capacity to endure high temperatures Small grain size yields better surface finish on the cast part Large grain size is more permeable, to allow escape of gases during pouring Irregular grain shapes tend to strengthen molds due to interlocking, compared to round grains

Disadvantage: interlocking tends to reduce permeability

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Types of Sand Mold

Green-sand molds - mixture of sand, clay, and water;

“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 MoldingVacuum MoldingExpanded Polystyrene ProcessInvestment CastingPlaster Mold and Ceramic Mold Casting