MENG 3324: Manufacturing Processes, UTPB, ch11

7/27/2019 MENG 3324: Manufacturing Processes, UTPB, ch11

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2012 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 5/e

METAL CASTING PROCESSES

1. Sand Casting2. Other Expendable Mold Casting Processes3. Permanent Mold Casting Processes4. Foundry Practice5. Casting Quality6. Metals for Casting7. Product Design Considerations


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Two Categories of

Casting Processes1. Expendable mold processes - mold is sacrificed to

remove part

Advantage: more complex shapes possibleDisadvantage: production rates often limited bythe time to make mold rather than casting itself

2. Permanent mold processes - mold is made of metaland can be used to make many castings

Advantage: higher production ratesDisadvantage: geometries are limited by the needto open the mold


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Overview of Sand Casting

Most widely used casting process, accounting for asignificant majority of total tonnage cast

Nearly all alloys can be sand casted, including metalswith high melting temperatures, such as steel, nickel,and titaniumCastings range in size from small to very large

Production quantities from one to millions


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Sand casting for anindustrial pump. Holes

and surfaces have beenmachined (courtesy of George E. KaneManufacturingTechnology Laboratory,

Lehigh University)


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Steps in Sand Casting

1. Pour the molten metal into sand mold2. Allow time for metal to solidify3. Break up the mold to remove casting4. Clean and inspect casting

Separate gating and riser system5. Heat treatment of casting is sometimes required to

improve metallurgical properties


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Making the Sand Mold

The cavity in the sand mold is formed by packingsand around a pattern, then separating the mold into

two halves and removing the patternThe mold must also contain gating and riser systemIf casting is to have internal surfaces, a core must beincluded in mold

A new sand mold must be made for each partproduced


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Sand Casting Production

Sequence

Production sequence in sand casting, includingpattern-making and mold-making


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The Pattern

Full-sized model of the part, slightly enlarged toaccount for shrinkage and machining allowances in

the castingPattern materials:

Wood - common material because it is easy towork, but it warps

Metal - more expensive to fabricate, but lastslonger Plastic - compromise between wood and metal


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Types of Patterns

Types of patterns used in sand casting: (a) solidpattern, (b) split pattern, (c) match-plate pattern, (d)

cope and drag pattern


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Core

Full-scale model of interior surfaces of partInserted into mold cavity prior to pouringThe molten metal flows and solidifies between themold cavity and the core to form the casting'sexternal and internal surfacesMay require supports to hold it in position in themold cavity during pouring, called chaplets


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Core in Mold

(a) Core held in place in the mold cavity by chaplets,(b) possible chaplet design, (c) casting


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Desirable Mold Properties

Strength - to maintain shape and resist erosionPermeability - to allow hot air and gases to passthrough voids in sandThermal stability - to resist cracking on contact withmolten metalCollapsibility - ability to give way and allow casting toshrink without cracking the castingReusability - can sand from broken mold be reused tomake other molds?


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Foundry Sand

Silica (SiO 2) or silica mixed with other mineralsGood refractory properties - for high temperaturesSmall grain size for better surface finish on partLarge grain size is more permeable, allowinggases to escape during pouringIrregular grain shapes strengthen molds due tointerlocking, compared to round grains

Disadvantage: interlocking reducespermeability


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Binders Used with

Foundry Sand

Sand is held together by a mixture of water andbonding clay

Typical mix: 90% sand, 3% water, and 7% clayOther bonding agents also used in sand molds:

Organic resins (e g , phenolic resins)Inorganic binders (e g , sodium silicate andphosphate)

Additives are sometimes combined with the mixtureto increase strength and/or 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

pouringDry-sand mold - organic binders rather than clay

Mold is baked to improve strengthSkin-dried mold - drying mold cavity surface of agreen-sand mold to a depth of 10 to 25 mm, usingtorches or heating lamps


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Buoyancy in a

Sand Casting Operation

During pouring, buoyancy of the molten metal tendsto displace the core, which can cause casting to be

defectiveForce tending to lift core = weight of displaced liquidless the weight of core itself

F b = W m - W c

where F b = buoyancy force; W m = weight of moltenmetal displaced; and W c = weight of core


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Other Expendable Mold

Processes

Shell MoldingVacuum Molding

Expanded Polystyrene ProcessInvestment CastingPlaster Mold and Ceramic Mold Casting


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Shell Molding

Casting process inwhich the mold is a

thin shell of sand heldtogether bythermosetting resinSteps: (1) A metalpattern is heated andplaced over a boxcontaining sand mixedwith TS resin


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Steps in Shell Molding

(2) Box is inverted sothat sand and resin

fall onto the hotpattern, causing alayer of the mixture topartially cure on thesurface to form ahard shell


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(3) Box isrepositioned so

loose uncuredparticles dropaway


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(4) Sand shell isheated in oven for

several minutes tocomplete curing


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(5) shell moldis stripped

from pattern


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(6) Two halves of the shellmold are assembled,

supported by sand or metal shot in a box, andpouring is accomplished


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(7) Finished castingwith sprue removed


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Shell Molding: Advantages and

Disadvantages Advantages:

Smoother cavity surface permits easier flow of

molten metal and better surface finishGood dimensional accuracyMold collapsibility minimizes cracks in castingCan be mechanized for mass production

Disadvantages:More expensive metal patternDifficult to justify for small quantities


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Vacuum Molding

Uses sand mold held together by vacuum pressurerather than by a chemical binder

The term "vacuum" refers to mold making rather than casting operation itself Developed in Japan around 1970


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Vacuum Molding: Advantages

and Disadvantages Advantages:

Easy recovery of the sand, since no binders

Sand does not require mechanical reconditioningdone when binders are usedSince no water is mixed with sand,moisture-related defects are avoided

Disadvantages:Slow processNot readily adaptable to mechanization


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Expanded Polystyrene Process

Uses a mold of sand packed around a polystyrenefoam pattern which vaporizes when molten metal ispoured into mold

Other names: lost-foam process, lost patternprocess, evaporative-foam process, and full-moldprocessPolystyrene foam pattern includes sprue, risers,gating system, and internal cores (if needed)Mold does not have to be opened into cope anddrag sections


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Steps in


(1) Polystyrenefoam pattern iscoated withrefractorycompound


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Steps in

Expanded Polystyrene Process(2) Foam pattern isplaced in mold box,and sand iscompacted aroundthe pattern


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Steps in

Expanded Polystyrene Process(3) Molten metal is pouredinto the portion of the

pattern that forms thepouring cup and sprue As the metal enters themold, the polystyrenefoam is vaporized aheadof the advancing liquid,thus filling the mold cavity


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Expanded Polystyrene Process:

Advantages and Disadvantages Advantages of expanded polystyrene process:

Pattern need not be removed from the mold

Simplifies and speeds mold-making, because twomold halves are not required as in a conventionalgreen-sand mold

Disadvantages:

A new pattern is needed for every castingEconomic justification of the process is highlydependent on cost of producing patterns


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Applications:Mass production of castings for automobile

engines Automated and integrated manufacturingsystems are used to1. Mold the polystyrene foam patterns and then

2. Feed them to the downstream castingoperation


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Aluminum engine head produced by

expanded polystyrene casting process



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Investment Casting

(a.k.a. Lost Wax Process) A pattern made of wax is coated with a refractorymaterial to make the mold, after which wax is melted

away prior to pouring molten metal"Investment" comes from a less familiar definitionof "invest" - "to cover completely," which refers tocoating of refractory material around wax pattern

It is a precision casting processCapable of producing castings of high accuracyand intricate detail


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Steps in Investment Casting

(1) Wax patterns areproduced

(2) Several patternsare attached to asprue to form apattern tree


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(3) Pattern tree iscoated with a thin

layer of refractorymaterial(4) Full mold isformed by coveringthe coated tree withsufficient refractorymaterial to make itrigid


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(5) Mold is held in aninverted position and

heated to melt thewax and permit it todrip out of the cavity(6) Mold is preheatedto a high temperature,the molten metal ispoured, and itsolidifies


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(7) Mold is brokenaway from the finished

casting and the partsare separated fromthe sprue


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One-piece compressor stator with 108separate airfoils made by investment

casting (courtesy of Alcoa Howmet)


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Investment Casting:

Advantages and Disadvantages Advantages:

Parts of great complexity and intricacy can be cast

Close dimensional control and good surface finishWax can usually be recovered for reuseThis is a net shape process - additional machiningis not normally required

Disadvantages:Many processing steps are requiredRelatively expensive process


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Plaster Mold Casting

Similar to sand casting except mold is made of plaster of Paris (gypsum - CaSO 4-2H 2O)

In mold-making, plaster and water mixture ispoured over plastic or metal pattern and allowedto set

Wood patterns not generally used due to

extended contact with water Plaster mixture readily flows around pattern,capturing its fine details and good surface finish


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Plaster Mold Casting:

Advantages and Disadvantages Advantages:

Good accuracy and surface finish

Capability to make thin cross sectionsDisadvantages:

Mold must be baked to remove moistureMoisture can cause problems in casting

Mold strength is lost if over-bakedPlaster molds cannot stand high temperatures

Limited to lower melting point alloys


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Ceramic Mold Casting

Similar to plaster mold casting except that mold ismade of refractory ceramic material that can

withstand higher temperatures than plaster Can be used to cast steels, cast irons, and other high-temperature alloys

Applications similar to those of plaster mold

casting except for the metals cast Advantages (good accuracy and finish) alsosimilar


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Permanent Mold

Casting ProcessesEconomic disadvantage of expendable mold casting:

A new mold is required for every casting

In permanent mold casting, the mold is reused manytimesProcesses include:

Basic permanent mold castingDie castingCentrifugal casting


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The Basic Permanent Mold

ProcessUses a metal mold constructed of two sectionsdesigned for easy, precise opening and closing

Molds used for casting lower melting point alloysare commonly made of steel or cast ironMolds used for casting steel must be made of refractory material, due to the very high pouring

temperatures


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Steps in

Permanent Mold Casting(1) Mold is preheated and coated for lubrication andheat dissipation


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Steps in

Permanent Mold Casting(2) Cores (if anyare used) are

inserted andmold is closed


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Steps in

Permanent Mold Casting

(3) Molten metal ispoured into the mold,

where it solidifies


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Steps in

Permanent Mold Casting(4) Mold is opened



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Permanent Mold Casting:

Advantages and Limitations

Advantages of permanent mold casting:Good dimensional control and surface finish

Rapid solidification caused by metal mold results in afiner grain structure, so castings are stronger

Limitations:Generally limited to metals of lower melting pointSimpler part geometries compared to sand castingbecause of need to open the moldHigh cost of mold


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Die Casting

A permanent mold casting process in which moltenmetal is injected into mold cavity under high pressure

Pressure is maintained during solidification, thenmold is opened and part is removedMolds in this casting operation are called dies ;hence the name die casting

Use of high pressure to force metal into die cavityis what distinguishes this from other permanentmold processes


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Die Casting Machines

Designed to hold and accurately close two moldhalves and keep them closed while liquid metal is

forced into cavityTwo main types:1. Hot-chamber machine2. Cold-chamber machine


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Hot-Chamber Die Casting

Metal is melted in a container, and a piston injectsliquid metal under high pressure into the die

High production rates500 parts per hour not uncommon

Applications limited to low melting-point metalsthat do not chemically attack plunger and other

mechanical componentsCasting metals: zinc, tin, lead, and magnesium


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Hot-chamber diecasting cycle: (1)

with die closedand plunger withdrawn,molten metalflows into thechamber


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(2) plunger forcesmetal in chamber to

flow into die,maintaining pressureduring cooling andsolidification.


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(3) Plunger iswithdrawn, die is

opened, andcasting is ejected


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Cold-Chamber Die Casting

MachineMolten metal is poured into unheated chamber fromexternal melting container, and a piston injects metal

under high pressure into die cavityHigh production but not usually as fast ashot-chamber machines because of pouring stepCasting metals: aluminum, brass, and magnesium

alloys Advantages of hot-chamber process favor its use onlow melting-point alloys (zinc, tin, lead)


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Cold-Chamber Die Casting Cycle

(1) With die closed and ram withdrawn, molten metal ispoured into the chamber


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(2) Ram forces metal to flow into die, maintainingpressure during cooling and solidification


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(3) Ram is withdrawn, die is opened, and part isejected


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Molds for Die Casting

Usually made of tool steel, mold steel, or maragingsteel

Tungsten and molybdenum (good refractory qualities)used to die cast steel and cast ironEjector pins required to remove part from die when itopens

Lubricants must be sprayed onto cavity surfaces toprevent sticking


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A die casting thatmeasures about 400

mm diagonally for atruck cab floor (courtesy of George E. KaneManufacturingTechnologyLaboratory)


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Die Casting:

Advantages and Limitations Advantages:

Economical for large production quantities

Good accuracy and surface finishThin sections possibleRapid cooling means small grain size and goodstrength in casting

Disadvantages:Generally limited to metals with low metal pointsPart geometry must allow removal from die


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Squeeze Casting

Combination of casting and forging in which a moltenmetal is poured into a preheated lower die, and the

upper die is closed to create the mold cavity after solidification beginsDiffers from usual closed-mold casting processesin which die halves are closed before introductionof the molten metalCompared to conventional forging, pressures areless and finer surface details can be achieved


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Semi-Solid Metal Casting

Family of net-shape and near net-shape processesperformed on metal alloys at temperatures between

liquidus and solidusThus, the alloy is a mixture of solid and moltenmetals during casting (mushy state)To flow properly, the mixture must consist of solid

metal globules in a liquid Achieved by stirring the mixture to preventdendrite formation


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Semi-Solid Metal Casting:

AdvantagesComplex part geometriesThin part walls possible

Close tolerancesZero or low porosity, resulting in high strength of thecasting


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Centrifugal Casting

A family of casting processes in which the mold isrotated at high speed so centrifugal force distributes

molten metal to outer regions of die cavityThe group includes:True centrifugal castingSemicentrifugal casting

Centrifuge casting


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True Centrifugal Casting

Molten metal is poured into rotating mold to producea tubular part

In some operations, mold rotation commencesafter pouring rather than beforeParts: pipes, tubes, bushings, and ringsOutside shape of casting can be round, octagonal,

hexagonal, etc , but inside shape is (theoretically)perfectly round, due to radially symmetric forces


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True Centrifugal Casting

Setup for true centrifugal casting


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Semicentrifugal Casting

Centrifugal force is used to produce solid castingsrather than tubular parts

Molds use risers at center to supply feed metalDensity of metal in final casting is greater in outer sections than at center of rotationOften used on parts in which center of casting is

machined away, thus eliminating the portionwhere quality is lowestExamples: wheels and pulleys


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Semicentrifugal Casting


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Centrifuge Casting

Mold is designed with part cavities located away fromaxis of rotation, so molten metal poured into mold isdistributed to these cavities by centrifugal force

Used for smaller partsRadial symmetry of part is not required as in other centrifugal casting methods


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Centrifuge Casting

(a) The process and (b) one of the parts cast


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Furnaces for Casting Processes

Furnaces most commonly used in foundries:Cupolas

Direct fuel-fired furnacesCrucible furnacesElectric-arc furnacesInduction furnaces


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Cupolas

Vertical cylindrical furnace equipped with tappingspout near base

Used only for cast irons Although other furnaces are also used, thelargest tonnage of cast iron is melted in cupolas

The "charge," consisting of iron, coke, flux, and

any alloying elements, is loaded through acharging door located less than halfway up heightof cupola


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Cupola for melting castiron


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Direct Fuel-Fired Furnaces

Small open-hearth in which charge is heated bynatural gas fuel burners located on side of furnace

Furnace roof assists heating action by reflectingflame down against charge At bottom of hearth is a tap hole to release moltenmetal

Generally used for nonferrous metals such ascopper-base alloys and aluminum


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Crucible Furnaces

Metal is melted without direct contact with burningfuel mixture

Sometimes called indirect fuel-fired furnaces Container (crucible) is made of refractory materialor high-temperature steel alloyUsed for nonferrous metals such as bronze,

brass, and alloys of zinc and aluminumThree types used in foundries: (a) lift-out type, (b)stationary, (c) tilting


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Three Types of

Crucible Furnaces(a) Lift-out crucible, (b) stationary pot - molten metalmust be ladled, and (c) tilting-pot furnace


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Electric-Arc Furnaces

Charge is melted by heat generated from an electricarc

High power consumptionBut electric-arc furnaces can be designed for high melting capacity

Used primarily for melting steel


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Induction Furnaces

Uses alternating current passing through a coil todevelop magnetic field in metal

Induced current causes rapid heating and meltingElectromagnetic force field also causes mixingaction

Since metal does not contact heating elements,

environment can be closely controlled to producemolten metals of high quality and purityCommon alloys: steel, cast iron, and aluminum


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Induction Furnace


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Ladles

Two common types of ladles to transfer molten metalsto molds: (a) crane ladle, and (b) two-man ladle


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Additional Steps After

SolidificationTrimmingRemoving the core

Surface cleaningInspectionRepair, if requiredHeat treatment


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Trimming

Removal of sprues, runners, risers, parting-line flash,fins, chaplets, and any other excess metal from thecast part

For brittle casting alloys and when cross sectionsare relatively small, appendages can be broken off Otherwise, hammering, shearing, hack-sawing,band-sawing, abrasive wheel cutting, or varioustorch cutting methods are used


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Removing the Core

If cores have been used, they must be removedMost cores are bonded, and they often fall out of

casting as the binder deterioratesIn some cases, they are removed by shaking thecasting, either manually or mechanicallyIn rare cases, cores are removed by chemically

dissolving bonding agentSolid cores must be hammered or pressed out


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Surface Cleaning and Inspection

Removal of sand from casting surface and otherwiseenhancing appearance of surface

Cleaning methods: tumbling, air-blasting withcoarse sand grit or metal shot, wire brushing,buffing, and chemical picklingSurface cleaning is most important for sandcasting

Defects are possible in casting, and inspection isneeded to detect their presence


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Heat Treatment

Castings are often heat treated to enhance propertiesReasons for heat treating a casting:

For subsequent processing operations such asmachiningTo bring out the desired properties for theapplication of the part in service


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Casting Quality

There are numerous opportunities for things to gowrong in a casting operation, resulting in qualitydefects in the productThe defects can be classified as follows:

General defects common to all casting processesDefects related to sand casting process


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A casting that hassolidified before

completely fillingmold cavity

General Defects:Misrun


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Two portions of metalflow together but there

is a lack of fusion dueto premature freezing

General Defects:Cold Shut


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Metal splatters duringpouring and solidglobules form andbecome entrapped incasting

General Defects:Cold Shot


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Depression in surfaceor internal void caused

by solidificationshrinkage that restrictsamount of moltenmetal available in lastregion to freeze

General Defects:

Shrinkage Cavity


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Balloon-shaped gascavity caused by

release of moldgases during pouring

Sand Casting Defects:Sand Blow


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Formation of many smallgas cavities at or slightly

below surface of casting

Sand Casting Defects:

Pin Holes


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When fluidity of liquidmetal is high, it may

penetrate into sand moldor core, causing castingsurface to consist of amixture of sand grainsand metal

Sand Casting Defects:

Penetration


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A step in the castproduct at parting

line caused bysidewise relativedisplacement of cope and drag

Sand Casting Defects:Mold Shift


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Foundry Inspection Methods

Visual inspection to detect obvious defects such asmisruns, cold shuts, and severe surface flaws

Dimensional measurements to insure that toleranceshave been metMetallurgical, chemical, physical, and other testsconcerned with quality of cast metal


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Metals for Casting

Most commercial castings are made of alloys rather than pure metals

Alloys are generally easier to cast, and propertiesof product are better Casting alloys can be classified as:

Ferrous

Nonferrous


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Ferrous Casting Alloys:

Cast IronMost important of all casting alloysTonnage of cast iron castings is several times that of

all other metals combinedSeveral types: (1) gray cast iron, (2) nodular iron, (3)white cast iron, (4) malleable iron, and (5) alloy castirons

Typical pouring temperatures 1400 C (2500 F),depending on composition


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Ferrous Casting Alloys:

SteelThe mechanical properties of steel make it anattractive engineering material

The capability to create complex geometries makescasting an attractive shaping processDifficulties when casting steel:

Pouring temperature is high 1650 C (3000 F)

At such temperatures, steel readily oxidizes, somolten metal must be isolated from air Molten steel has relatively poor fluidity


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Nonferrous Casting Alloys:

AluminumGenerally considered to be very castableLow pouring temperatures due to low melting

temperaturePure Aluminum T m = 660 C (1220 F)Properties:

Light weight

Range of strength properties by heat treatmentEasy to machine


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Nonferrous Casting Alloys:

Copper AlloysIncludes bronze, brass, and aluminum bronzeProperties:

Corrosion resistance Attractive appearanceGood bearing qualities

Limitation: high cost of copper

Applications: pipe fittings, marine propeller blades,pump components, ornamental jewelry


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Nonferrous Casting Alloys:Zinc Alloys

Very castable, commonly used in die castingLow pouring temperatures due to low melting

temperaturePure zinc T m = 419 C (786 F)

Good fluidity for ease of castingProperties:

Low creep strength, so castings cannot besubjected to prolonged high stresses


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Product Design Considerations

Geometric simplicity Although casting can be used to produce complexpart geometries, simplifying the part designusually improves castability

Avoiding unnecessary complexities:Simplifies mold-making

Reduces the need for coresImproves the strength of the casting


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Corners on the castingSharp corners and angles should be avoided,since they are sources of stress concentrationsand may cause hot tearing and cracksGenerous fillets should be designed on insidecorners and sharp edges should be blended


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Draft GuidelinesIn expendable mold casting, draft facilitatesremoval of pattern from mold

Draft = 1 for sand castingIn permanent mold casting, purpose is to aid inremoval of the part from the mold

Draft = 2 to 3 for permanent mold processesSimilar tapers should be allowed for solid cores


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Draft

Design change to eliminate need for using a core: (a)original design, and (b) redesign


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Dimensional Tolerances and Surface FinishDimensional accuracy and finish vary significantly,depending on process

Poor dimensional accuracies and finish for sandcastingGood dimensional accuracies and finish for diecasting and investment casting


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Machining Allowances Almost all sand castings must be machined toachieve the required dimensions and part features

Additional material, called the machining allowance , is left on the casting in those surfaceswhere machining is necessaryTypical machining allowances for sand castingsare around 1.5 and 3 mm (1/16 and 1/4 in)

Documents

MENG 3324: Manufacturing Processes, UTPB, ch11