L5 Metal Casting Ppt

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MEL120: Manufacturing Practices 1

Metal Casting

Lecture 5


Steps in Casting

Pattern and MouldMelting and PouringSolidification and CoolingRemoval, Cleaning, Finishing and Inspection

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Casting Process Flow


Pattern and MouldA pattern is

a replica of the final product and is used for preparing mould cavitymade of wood or metal

Mould cavitywhich contains molten metal is essentially a negative of the final product

Mould materialshould posses refractory characteristics and with stand the pouring temperature

When the mold is used for single casting, it made of sand and known as expendable moldWhen the mold is used repeatedly for number of castings and is made of metal or graphite are called permanent mouldFor making holes or hollow cavities inside a casting, cores made of sand are used.

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Melting and PouringSeveral types of furnaces are available for melting metals Furnace selection depends on

the type of metal, the maximum temperature required and the rate and the mode of molten metal delivery.

Before pouring, provisions are made for the escape of dissolved gases. The gating system should be designed to minimize the turbulent flow and erosion of mould cavity.The other important factors are the pouring temperature and the pouring rate.


Solidification & CoolingThe properties of the casting significantly depends on the solidification time cooing ratesolidification time cooing rate.Shrinkage of casting, during cooling of solidified metal should not be restrained by the mould material, otherwise internal stresses may develop and form cracks in casting.Proper care should be taken at the design stage of casting so that shrinkage can occur without casting defects.

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Removal, Cleaning, Finishing and Inspection

After the casting is removed from the mouldit is thoroughly cleaned and the excess material usually along the parting line and the place where the molten metal was poured, is removed using a potable grinder.

White light inspection, pressure test, magnetic particle inspection, radiographic test, ultrasonic inspection etc. are used


Pattern & Mould

Open and Closed Mould

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Sand Casting

Expendable Mould

Permanent Pattern


Chaplets: To avoid Core Shifting

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Pattern Geometry

a) Solid Pattern b) Split Pattern c) Match-Plate Pattern b) Cope and Drag Pattern


PatternsChoice of pattern depends on:

Configuration of casting Number of casting required

Pattern TypesSingle-piece patternSplit patternFollow board patternCope and drag patternMatch plate patternLoose-piece patternSweep patternSkeleton pattern

a) Split pattern, b) Follow-board, c) Match Plate, d) Loose-piece, e) Sweep, f) Skeleton pattern

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Pattern AllowancesPattern always made larger than final jobExcess dimensions – Pattern Allowance

Shrinkage allowanceContraction of casting

Liquid – Pouring Temp to Freezing TempChange of phase – Liquid to SolidSolid casting – Freezing Temp to Room temp

Draft allowanceTo withdraw pattern from mould

Machining allowanceFor final shape


Mould MaterialMajor part of Moulding material in sand casting are1. 70-85% silica sand (SiO2)2. 10-12% bonding material e.g., clay etc.3. 3-6% waterRequirements of molding sand are:(a) Refractoriness – ability to remain solid at high temp(b) Cohesiveness – bonding(c) Permeability – gas flow through mould(d) Collapsibility – ability to permit metal to shrink after solidificationThe performance of mould depends on following factors:(a) Permeability(b) Green strength(c) Dry strength

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Desirable properties of a Sand based Molding material

Inexpensive in bulk qtyRetain properties through transportation and storageUniformly fills a flask or containerCompacted or set by simple methodsSufficient elasticity to remain undamaged during pattern withdrawalWithstand high temperatures and maintain its dimensions until metal solidifiesSufficient permeable to allow the escape of gases


Desirable properties of a Sand based Molding material

Sufficiently dense to prevent metal penetrationSufficiently cohesive to prevent washout of mold material into the pour streamChemically inert to metal being castGood collapsibility to permit easy removal and separation of castingCan be recycled

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Effect of moisture, grain size and shape on mould quality


Melting and PouringThe quality of casting depends on the method of melting.Molten metal is prevented from oxidation by covering the molten metal with fluxes Ladles which pour the molten metal from beneath the surface are usedThe two main consideration during pouring are the temperatureand pouring rateFluidity of molten metal is more at higher temperature but it results into more amount of dissolved gases and high temperature also damage the mould walls and results into poor surface quality of the castingFluidity:Fluidity: Capability of molten metal to fill mold cavities

Characteristics of molten metalCasting parameters

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FluidityCharacteristics of molten metal

ViscositySurface tensionInclusionsSolidification pattern of the alloy

Casting ParametersMold designMold material and surface characteristicsDegree of superheatRate of pouringHeat transfer


Gating system1. Minimize turbulent flow to reduce

• absorption of gases, • oxidation of metal and • erosion of mould surfaces

2. Regulate the entry of molten metal into the mould cavity

3. Ensure complete filling of mould cavity, and4. Promote a temperature gradient within the casting

so that all sections irrespective of size and shape could solidify properly

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Gating system

A: pouring basinB: WeirC: SprueD: Sprue wellE: RunnerF: IngatesG: Runner break upH: BlindJ: Riser


Cooling and Solidification

Pure metal

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Mechanism of SolidificationPure metals solidifies at a constant temp. equal to its freezing point, which same as its melting point.The change form liquid to solid does not occur all at once. The process of solidification starts with nucleation, the formation of stable solid particles within the liquid metal. Nuclei of solid phase, generally a few hundred atom in size, start appearing at a temperature below the freezing temperature. The temp. around this goes down and is called supercooling or undercooling. In pure metals supercooling is around 20% of the freezing temp. A nuclei, more than a certain critical size grows, and causes solidification.


Mechanism of SolidificationBy adding, certain foreign materials (nucleating agents) the undercooling temp. is reduced which causes enhanced nucleation. In case of pure metalspure metals fine equi-axed grains are formed near the wall of the mold and columnar grain growth takes place upto the centre of the ingot.In typical alloyalloy, the columnar grains do not extend uptothe center of casting but are interrupted by an inner zone of equiaxed graines.By adding typical nucleating agents like sodium, sodium, magnesium or bismuthmagnesium or bismuth the inner zone of equiaxedgrained can be extended in whole casting.

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Crystal structure in Castings


Dendrite formation

• In alloys, such as Fe-C, freezing and solidification occurs over a wide range of temp. There is no fine line of demarcation exists between the solid and liquid metal.

• Here, ‘start of freezing’ implies that grain formation while progressing towards the center does not solidify the metal completely but leaves behind the islands of liquid metals in between grains which freeze later and there is multidirectional multidirectional tree like growthtree like growth.

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RiserRisers are added reservoirsadded reservoirs designed to feed liquid metal to the solidifying casting as a means for compensating for solidification shrinkagecompensating for solidification shrinkage.Riser must solidify after castingRiser must solidify after casting.Riser should be located so that directional solidification occurs from the extremities of mold cavity back toward the riser.Thickest part of casting Thickest part of casting –– last to freezelast to freeze, Riser should feed directly to these regions.


Why Risers?The shrinkage occurs in three stages,1. When temperature of liquid metal drops from Pouring

to Freezing temperature2. When the metal changes from liquid to solid state, and3. When the temperature of solid phase drops from

freezing to room temperatureThe shrinkage for stage 3 is compensated by providing shrinkage allowance on pattern, while the shrinkage during stages 1 and 2 are compensated by providing risers.

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Riser – Location & Types

• Top riserTop riser has the advantage of additional pressure head and smaller feeding distance over the side riser.

• Blind risersBlind risers are generally bigger in size because of additional area of heat conduction.


Cleaning and Finishing1. Casting is taken out of the mould by ShakingShaking and the

Moulding sand is recycled often with suitable additions.2. The remaining sand, some of which may be embedded

in the casting, is removed by means of Shot blastingShot blasting.3. The excess material in the form of sprue, runners,

gates etc., along with the flashes formed due to flow of molten metal into the gaps is broken manually in case of brittle casting or removed by sawing and grinding in case of ductile grinding.

4. The entire casting is then cleaned by either shot shot blastingblasting or chemical picklingchemical pickling.

5. Sometimes castings are heat treatedheat treated to achieve better mechanical properties.

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Casting Defects

Defects may occur due to one or more of the following reasons:

Fault in design of casting patternFault in design on mold and coreFault in design of gating system and riserImproper choice of molding sandImproper metal compositionInadequate melting temperature and rate of pouring


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Classification of Casting Defects

Surface DefectsBlow, Scar, Blister, Drop, Scab, Penetration, Buckle

Internal DefectsBlow holes, Porosity, Pin holes, Inclusions, Dross

Visible DefectsWash, Rat tail, Swell, Mis run, Cold shut, Hot tear, Shrinkage/Shift


Surface Defects

BlowBlow is relatively large cavity produced by gases which displace molten metal from convex surface.

ScarScar is shallow blow generally occurring on a flat surface.

A scar covered with a thin layer of metal is called blisterblister. These are due to improper permeability or venting. Sometimes excessive gas forming constituents in moulding sand.

DropDrop is an irregularly-shaped projection on the cope surface caused by dropping of sand.

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Surface DefectsA scabscab when an up heaved sand gets separated from the mould surface and the molten metal flows between the displaced sand and the mold.

PenetrationPenetration occurs when the molten metal flows between the sand particles in the mould. These defects are due to inadequate strength of the mold and high temperature of the molten metal adds on it.

BuckleBuckle is a v-shaped depression on the surface of a flat casting caused by expansion of a thin layer of sand at the mould face.


Internal DefectsThe internal defects found in the castings are mainly due to trapped gases and dirty metal.

Gases get trapped due to hard ramming or improper venting.

These defects also occur when excessive moisture or excessive gas forming materials are used for mould making.

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Internal DefectsBlow holesBlow holes are large spherical shaped gas bubbles

PorosityPorosity indicates a large number of uniformly distributed tiny holes.

Pin holesPin holes are tiny blow holes appearing just below the casting surface.

InclusionsInclusions are the non-metallic particles in the metal matrix, Lighter impurities appearing the casting surface are drossdross..


Visible Defects

Insufficient mould strength, insufficient metal, lowpouring temperature, and bad design of casting are some of the common causes.

WashWash is a low projection near the gate caused by erosion of sand by the flowing metal.

Rat tailRat tail is a long, shallow, angular depression caused by expansion of the sand.

SwellSwell is the deformation of vertical mould surface due to hydrostatic pressure caused by moisture in the sand.

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Visible Defects

MisrunMisrun and cold shutcold shut are caused by insufficient superheat provided to the liquid metal.

Hot tearHot tear is the crack in the casting caused by high residual stresses.

Shrinkage is essentially solidification contraction and occurs due to improper use of Riser.

ShiftShift is due to misalignment of two parts of the mould or incorrect core location.


Shell Molding

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Shell Molding1. A match plate or cope-drag metal pattern is heated and

placed over a box containing sand mixed with thermosetting resin.

2. Box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell.

3. Box is repositioned so that loose, uncured particles drop away.

4. Sand shell is heated in oven for several minutes for complete curing.

5. Shell mold is stripped from the pattern6. Two halves of the shell mold are assembled, supported by

sand or metal shot in a box, and pouring is accomplished. The finished casting with sprue is removed.


Shell MoldingAdvantages & Limitations

Shell thickness typically 9 mm is usedSurface of shell mold cavity is smoother than sand mold.Easy flow of molten metal, good surface qualityFinish is of the order of 2.5 micrometer. Good dimensional accuracyCan be mechanized for mass production and is very economicalGears, valve bodies, bushings, and cam shafts are typical productsExpensive metal pattern as compared to sand castingDifficult to justify for small quantities manufacturingPossible on small to medium size partsSuitable for steel castings less than 10 kg.

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Investment Casting


Investment Casting Advantages & Limitations

Parts of greater complexity and intricacy can be castClose dimensional control ±0.075mmGood surface finishThe lost wax can be reusedAdditional machining is not required in normal courseAl, Cu, Ni, Carbon and alloy steels, tool steels etc. are the common materialsPreferred for casting weight less than 5 kg, maximum dimension less than 300 mm, Thickness is usually restricted to 15mm

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Die Casting

Graphite+oil


Die Casting Machine

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Die CastingIn Die casting the molten metal is forced to flowinto a permanent metallic mold under moderate to high pressures, and held under pressure during solidificationThis high pressure forces the metal into intricate details, produces smooth surface and excellent dimensional accuracyHigh pressure causes turbulence and air entrapment. In order to minimize this larger in-gates are used and in the beginning, pressure is kept low and is increased gradually


Hot Chamber Casting

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Cold Chamber Casting


Centrifugal Casting

A permanent mold made of metal or ceramic is rotated at high speed (300 to 3000 rpm). The molten metal is then poured into the mold cavity and due to centrifugal action the molten metal conform to the cavity provided in the mould.Castings are known for their higher densities in the outer most regions.The process gives good surface finishApplications: pipes, bushings, gears, flywheels etc.

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Comparison of Casting Processes


Comparison of Casting Processes

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L5 Metal Casting Ppt