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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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Steps in Shell Molding
(3) Box isrepositioned so
loose uncuredparticles dropaway
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Steps in Shell Molding
(4) Sand shell isheated in oven for
several minutes tocomplete curing
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Steps in Shell Molding
(5) shell moldis stripped
from pattern
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Steps in Shell Molding
(6) Two halves of the shellmold are assembled,
supported by sand or metal shot in a box, andpouring is accomplished
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Steps in Shell Molding
(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
Expanded Polystyrene Process
(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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Expanded Polystyrene Process
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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Steps in Investment Casting
(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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Steps in Investment Casting
(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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Steps in Investment Casting
(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 Die Casting
Hot-chamber diecasting cycle: (1)
with die closedand plunger withdrawn,molten metalflows into thechamber
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Hot-Chamber Die Casting
(2) plunger forcesmetal in chamber to
flow into die,maintaining pressureduring cooling andsolidification.
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Hot-Chamber Die Casting
(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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Cold-Chamber Die Casting Cycle
(2) Ram forces metal to flow into die, maintainingpressure during cooling and solidification
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Cold-Chamber Die Casting Cycle
(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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Product Design Considerations
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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Product Design Considerations
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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Product Design Considerations
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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Product Design Considerations
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)
Recommended