60105277 3 1 Metal Casting for Students

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M t l C tiMetal Casting

By S K MondalCompiled By: S K Mondal Made Easy

Sand castingSand casting uses ordinary sand as the primarymould material.The sand grains are mixed with small amounts ofother materials, such as clay and water, to improvemouldability and cohesive strength and are thenmouldability and cohesive strength, and are thenpacked around a pattern that has the shape of thedesired casting.The pattern must be removed before pouring, themold is usually made in two or more pieces.An opening called a sprue hole is cut from the top ofthe mold through the sand and connected to asystem of channels called runners. Contd….Compiled By: S K Mondal Made Easy

The molten metal is poured into the sprue hole, flowsthrough the runners, and enters the mold cavitythrough an opening called a gate.Gravity flow is the most common means ofintroducing the metal into the mold.After solidification, the mold is broken and thefinished casting is removed.The casting is then “fettled” by cutting off the ingateand the feeder head.Because the mold is destroyed, a new mold must bemade for each casting.

Contd…Compiled By: S K Mondal Made Easy Compiled By: S K Mondal Made Easy

Sequential steps in making a sand castingA pattern board is placed between the bottom (drag) and top (cope) halves of a flask, with the bottom side up.

Sand is then packed into the drag half of the mold.

A bottom board is positioned on top of the packed sand, and the mold is turned over, showing the top (cope) half of pattern with sprue and riser pins in place.

The cope half of the mold is then packed with sand. Contd…Compiled By: S K Mondal Made Easy

The mold is opened, the pattern board is drawn(removed), and the runner and gate are cut into thesurface of the sand.

The mold is reassembled with the pattern boardd d l l d h h hremoved, and molten metal is poured through the

sprue.

The contents are shaken from the flask and the metalsegment is separated from the sand, ready for furtherprocessing.

Compiled By: S K Mondal Made Easy

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Compiled By: S K Mondal Made Easy Compiled By: S K Mondal Made Easy


Casting TermsFlask: A moulding flask is one which holds the sand

mould intact. It is made up of wood for temporary

applications or metal for long‐term use.pp g

Drag: Lower moulding flask.

Cope:Upper moulding flask.

Cheek: Intermediate moulding flask used in three‐

piece moulding.Contd…Compiled By: S K Mondal Made Easy

Pattern: Pattern is a replica of the final object to be

madewith some modifications.

Parting line: This is the dividing line between the two

ld fl k h k h d ldmoulding flasks that makes up the sand mould.

Bottom board: This is a board normally made of wood,

which is used at the start of the mould making.

Contd…Compiled By: S K Mondal Made Easy

Moulding sand: The freshly prepared refractory

material used for making the mould cavity. It is a

mixture of silica, clay and moisture in appropriate

proportions.

Backing sand: This is made up of used and burnt

sand.

Core:Used for making hollow cavities in castings.


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Pouring basin: A small funnel‐shaped cavity at the top

of the mould into which the molten metal is poured.

Sprue: The passage through which the molten metal

from the pouring basin reaches the mould cavityfrom the pouring basin reaches the mould cavity.

Runner: The passage ways in the parting plane through

which molten metal flow is regulated before they reach

the mould cavity.

Gate: The actual entry point through which molten

metal enters the mould cavity in a controlled rate. Contd…Compiled By: S K Mondal Made Easy

Chaplet: Chaplets are used to support cores inside the

mould cavity.

Chill: Chills are metallic objects, which are placed in

h ld i h li f ithe mould to increase the cooling rate of castings.

Riser: It is a reservoir of molten metal provided in the

casting so that hot metal can flow back into the mould

cavity when there is a reduction in volume of metal due

to solidificationContd…Compiled By: S K Mondal Made Easy

PaddingTapering of thinner section towards thicker sectionis known as 'padding'.This will require extra material.If padding is not provided, centre line shrinkage orp g p gporosity will result in the thinner section.


IES‐2001

Q. The main purpose of chaplets is

(a) To ensure directional solidification

(b) To provide efficient venting

(c) For aligning the mold boxes

(d) To support the cores


IES‐1996Q. Which of the following methods are used for

obtaining directional solidification for riser design

1. Suitable placement of chills1. Suitable placement of chills

2. Suitable placement of chaplets

3. Employing padding

Select the correct answer.

(a) 1 and 2 (b) 1 and 3 (c) 2 and 3 (d) 1, 2 and 3Compiled By: S K Mondal Made Easy

IES 2007

Q. Which one of the following is the correctstatement?Gate is provided in moulds to(a) Feed the casting at a constant rate(b) Give passage to gases(c) Compensate for shrinkage(d) Avoid cavities


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GATE‐2009

Q. Match the items in Column I and Column II.Column I Column IIP Metallic Chills 1 Support for the coreP. Metallic Chills 1. Support for the coreQ. Metallic Chaplets 2. Reservoir of the molten metalR. Riser 3. Control cooling of critical

sectionsS. Exothermic Padding 4. Progressive solidification(a) P‐1,Q‐3, R‐2, S‐4 (b) P‐1,Q‐4, R‐2, S‐3(c) P‐3, Q‐4, R‐2, S‐1 (d) P‐4, Q‐1, R‐2, S‐3


GATE‐1992In a green‐sand moulding process, uniform ramming leads to(a) Less chance of gas porosity(b) Uniform flow of molten metal into the mould (b) Uniform flow of molten metal into the mould cavity(c) Greater dimensional stability of the casting(d) Less sand expansion type of casting defect


GATE 2011Green sand mould indicates that(a) polymeric mould has been cured(b) mould has been totally dried(c) mould is green in colour( ) g(d) mould contains moisture


PatternA pattern is a replica of the object to be made by thecasting process, with some modifications.

Themainmodifications areThe addition of pattern allowancesThe addition of pattern allowances,The provision of core prints, andElimination of fine details, which cannot be obtainedby casting and hence are to be obtained by furtherprocessing


Pattern Allowances1. Shrinkage or contraction allowance

2. Draft or taper allowance

M hi i fi i h ll3. Machining or finish allowance

4. Distortion or camber allowance

5. Rapping allowance


Shrinkage allowanceAll metals shrink when cooling except perhaps

bismuth.

This is because of the inter‐atomic vibrations which

are amplified by an increase in temperature.

The shrinkage allowance is always to be added to the

linear dimensions. Even in case of internal dimensions.


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Liquid shrinkage and solid shrinkageLiquid shrinkage refers to the reduction involume when the metal changes from liquid tosolid state at the solidus temperature. To accountfor this, risers are provided in the moulds.

Solid shrinkage is the reduction in volumecaused, when a metal loses temperature in thesolid state. The shrinkage allowance is provided totake care of this reduction.


Pattern AllowancesCast Iron 10 mm/mBrass, Copper, Aluminium 15 mm/mSteel 20 mm/mZinc, Lead 25 mm/m

In grey cast iron and spheroidal graphite iron, theamount of graphitization controls the actualshrinkage. When graphitization is more, theshrinkage would be less and vice versa.


IES‐1995Q. Which one of the following materials will require

the largest size of riser for the same size of casting?

(a) Aluminium( )

(b) Cast iron

(c) Steel

(d) Copper.


GATE‐1999

Q. Which of the following materials requires thelargest shrinkage allowance, while making apattern for casting?( ) l(a) Aluminium(b) Brass(c) Cast Iron(d) Plain Carbon Steel


IES‐1999In solidification of metal during casting,

compensation for solid contraction is

(a) Provided by the oversize pattern

(b) Achieved by properly placed risers

(c) Obtained by promoting directional

solidification

(d) Made by providing chills


GATE‐2001Q. Shrinkage allowance on pattern is provided tocompensate for shrinkage when(a) The temperature of liquid metal drops from

pouring to freezing temperaturepouring to freezing temperature(b) The metal changes from liquid to solid state at

freezing temperature(c) The temperature of solid phase drops from

freezing to room temperature(d) The temperature of metal drops from pouring

to room temperatureCompiled By: S K Mondal Made Easy

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GATE‐2004

Q. Gray cast iron blocks 200 x 100 x 10 mm are to becast in sand moulds. Shrinkage allowance forpattern making is 1%. The ratio of the volume of

tt t th t f th ti ill bpattern to that of the casting will be

(a) 0.97 (b) 0.99 (c) 1.01 (d) 1.03


GATE‐2008

Q. While cooling, a cubical casting of side 40 mmundergoes 3%, 4% and 5% volume shrinkageduring the liquid state, phase transition and solidt t ti l Th l f t lstate, respectively. The volume of metalcompensated from the riser is

(a) 2% (b) 7% (c) 8% (d) 9%


GATE 2011A cubic casting of 50 mm side undergoes volumetricsolidification shrinkage and volumetric solidcontraction of 4% and 6% respectively. No riser isused. Assume uniform cooling in all directions. Theside of the cube after solidification and contraction isside of the cube after solidification and contraction is(a) 48.32 mm(b) 49.90 mm(c) 49.94 mm(d) 49.96 mm


IAS‐1995Assertion (A): A pattern is made exactly similar tothe part to be cast.Reason (R): Pattern is used to make the mouldcavity for pouring inmolten for casting.( ) B h A d R i di id ll d R i h(a) Both A and R are individually true and R is thecorrect explanation of A(b) Both A and R are individually true but R is not thecorrect explanation of A(c) A is true but R is false(d) A is false but R is true


IAS‐2003Match List I (Material to be cast) with List II(Shrinkage Allowance in mm/m) and select thecorrect answer using the codes given below the lists:List‐I List‐II(Material to Cast) (Shrinkage Allowance in mm/m)(Material to Cast) (Shrinkage Allowance in mm/m)(A) Grey cast iron 1. 7 ‐ 10(B) Brass 2. 15(C) Steel 3. 20(D) Zinc 4. 24

Codes:A B C D A B C D(a) 1 2 3 4 (b) 3 4 1 2(c) 1 4 3 2 (d) 3 2 1 4


DraftTo reduce the chances of the damage of the mould

cavity at the time of pattern removal, the vertical faces

of the pattern are always tapered from the parting line.

This provision is called draft allowance.

Inner surfaces of the pattern require higher draft than

outer surfaces.

Draft is always provided as an extra metal.


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DRAFT ALLOWANCECompiled By: S K Mondal Made Easy

Shake Allowance

At the time of pattern removal, the pattern is rapped

all around the vertical faces to enlarge the mould

cavity slightly to facilitates its removalcavity slightly to facilitates its removal.

It is a negative allowance and is to be applied only to

those dimensions, which are parallel to the parting

plane.


Distortion Allowance A metal when it has just solidified is very weak andtherefore is likely to be distortion prone.

This is particularly so for weaker sections such as longThis is particularly so for weaker sections such as longflat portions, V, U sections or in a complicated castingwhich may have thin and long sections which areconnected to thick sections.

The foundry practice should be to make extramaterial provision for reducing the distortion.


Pattern MaterialsWood patterns are relatively easy to make. Wood is notvery dimensionally stable. Commonly used teak, whitepine and mahogany wood.Metal patterns are more expensive but are moredimensionally stable and more durable Commonly useddimensionally stable and more durable. Commonly usedCI, Brass, aluminium and white metal.Hard plastics, such as urethanes, and are often preferredwith processes that use strong, organically bonded sandsthat tend to stick to other pattern materials.In the full‐mold process, expanded polystyrene (EPS) isused.Investment casting useswax patterns.


The pattern material should beEasily worked, shaped and joined

Light in weight

Strong, hard and durable

Resistant to wear and abrasion

Resistant to corrosion, and to chemical reactions

Dimensionally stable and unaffected by variations in

temperature and humidity.

Available at low cost.Compiled By: S K Mondal Made Easy

IES‐1994Q. Which of the following materials can be used for

making patterns?

1. Aluminium 2. Wax 3. Mercury 4. Lead3 y 4

Select the correct answer using the codes given below:

Codes:

(a) 1,3 and 4 (b) 2,3 and 4 (c) 1, 2 and 4 (d) 1, 2 and 3


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GATE‐2000

Disposable patterns are made of

(a) Wood

(b) Rubber

(c) Metal

(d) Polystyrene


Types of PatternSingle Piece Pattern

These are inexpensive and the simplest type ofpatterns. As the name indicates, they are made of asingle piece.

Gated PatternGating and runner system are integral with thepattern. This would eliminate the hand cutting ofthe runners and gates and help in improving theproductivity of a moulding.


Types of PatternSplit Pattern or Two Piece PatternThis is the most widely used type of pattern for intricatecastings. When the contour of the casting makes itswithdrawal from the mould difficult, or when the depthof the casting is too high then the pattern is split into twoof the casting is too high, then the pattern is split into twoparts so that one part is in the drag and the other in thecope.


Types of PatternCope and Drag Pattern

These are similar to split patterns. In addition tosplitting the pattern, the cope and drag halves ofthe pattern along with the gating and riser systemsp g g g yare attached separately to the metal or woodenplates along with the alignment pins. They arecalled the cope and drag patterns.


Types of PatternMatch Plate Pattern

The cope and drag patterns along with thegating and the risering are mounted on a singlematching metal or wooden plate on either side.


Types of PatternLoose Piece Pattern

This type of pattern is also used when thecontour of the part is such that withdrawing thepattern from the mould is not possiblepattern from the mould is not possible.


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Types of PatternFollow Board Pattern

This type of pattern is adopted for thosecastings where there are some portions, whichare structurally weak and if not supportedproperly are likely to break under the force ofproperly are likely to break under the force oframming.


IES‐2008

Q. The pattern adopted for those castings wherethere are some portions which are structurallyweak and are likely to break by the force of

i ll dramming are called:(a) Loose piece pattern(b) Follow board pattern(c) Skelton pattern(d) Single piece pattern


Types of PatternSweep Pattern

It is used to sweep the complete casting by meansof a plane sweep. These are used for generatinglarge shapes, which are axi‐symmetrical orprismatic in nature such as bell shaped orprismatic in nature such as bell‐shaped orcylindrical.


Types of PatternSkeleton Pattern

A skeleton of the pattern made of strips of woodis used for building the final pattern by packingsand around the skeleton. After packing thesand the desired form is obtained with the helpsand, the desired form is obtained with the helpof a strickle. This type of pattern is usefulgenerally for very large castings, required insmall quantities where large expense oncomplete wooden pattern is not justified.


Cooling Curve


FluidityThe ability of a metal to flow and fill a mold is knownas fluidity.

Pouring TemperatureThe most important controlling factor of fluidity is thepouring temperature or the amount of superheat.p g p pHigher the pouring temperature, the higher the fluidity.Excessive temperatures should be avoided, however. Athigh pouring temperatures, metal‐mold reactions areaccelerated and the fluidity may be so great as to permitpenetration.Penetration is a defect where the metal not only fills themold cavity but also fills the small voids between the sandparticles in a sand mold.


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CoreUsed for making cavities and hollow projections.

All sides of core are surrounded by the molten metaland are therefore subjected to much more severeand are therefore subjected to much more severethermal and mechanical conditions and as a result thecore sand should be of higher strength than themoulding sand.


Desired characteristics of a core

Green Strength: A core made of green sand shouldbe strong enough to retain the shape till it goes forbaking.

h h ld h d d hDry Strength: It should have adequate dry strengthso that when the core is placed in the mould, itshould be able to resist the metal pressure acting onit.Refractoriness: Since in most cases, the core issurrounded all around it is desirable that the corematerial should have higher refractoriness.


Permeability: Gases evolving from the molten metaland generated from the mould may have to gothrough the core to escape out of the mould. Hencecores are required to have higher permeability.

Collapsibility: At the time of cooling, castingshrinks, and unless the core has good collapsibility(ability to decrease in size) it is likely to provideresistance against shrinkage and thus can cause hottears.

Contd..Compiled By: S K Mondal Made Easy

Friability: The ability to crumble should be a very

important consideration at the time of removal.

Smoothness: Surface of the core should be smooth

f d fi i h t th tifor good finish to the casting.

LowGas Emission


Core SandsUsed clay free silica sand.

Binders used are linseed oil, core oil, resins, dextrin,

molasses, etc.

Core oils are mixtures of linseed, soy, fish and

petroleum oils and coal tar.

The general composition of a core sand mixture could

be core oil (1%) and water (2.5 to 6%).


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Carbon Dioxide MouldingSodium silicate (water glass, SiO2:Na2O) is used as a binder.This is essentially a quick process of core or mouldpreparation.The mould is prepared with a mixture of sodium silicate andsand and then treated with carbon dioxide for two to threeminutes such that a dry compressive strength of over 1.4MPa is arrived.The carbon dioxide is expected to form a weak acid, whichhydrolyses the sodium silicate resulting in amorphous silica,which forms the bond.The introduction of CO2 gas starts the reaction by forminghydrated sodium carbonate (Na2CO3 + H2O).


The compressive strength of the bond increases with

standing time due to dehydration.

Because of the high strength of the bond, the core need not

be provided with any other reinforcements.p y

It does not involve any distortions due to baking and also

better dimensional accuracies are achieved.

The sand mixture does not have good shelf life and

therefore should be used immediately after preparation.


IES‐2002Assertion (A): In CO2 casting process, the mould orcore attains maximum strength.Reason (R): The optimum gassing time of CO2through the mould or core forms Silica Gel whichimparts sufficient strength to the mould or coreimparts sufficient strength to the mould or core.(a) Both A and R are individually true and R is thecorrect explanation of A(b) Both A and R are individually true but R is not thecorrect explanation of A(c) A is true but R is false(d) A is false but R is true


Moulding Sand CompositionSand: Ordinary silica Sand (SiO2), zircon, or olivine

sands.

Clay: Acts ss binding agents mixed to the moulding

sands

Kaolinite or fire clay (Al2O3 2SiO2 2H2O), and

Bentonite (Al2O3 4SiO2 H2O nH2O).

Water: Clay is activated by water.


Other AdditivesCereal binder up to 2% increases the strength.

Pitch if used up to 3% would improve the hot

strength.g

Saw dust up to 2% may improve the collapsibility by

slowly burning, and increase the permeability.

Other materials: sea coal, asphalt, fuel oil, graphite,

molasses, iron oxide, etc.Compiled By: S K Mondal Made Easy

Moulding Sand PropertiesPorosity or Permeability: Permeability or porosity ofthe moulding sand is the measure of its ability topermit air to flow through it.Strength: It is defined as the property of holdingt th f d i A ldi d h ld htogether of sand grains. A moulding sand should haveample strength so that the mould does not collapse orget partially destroyed during conveying, turning overor closing.Refractoriness: It is the ability of the moulding sandmixture to withstand the heat of melt without showingany signs of softening or fusion.


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Plasticity: It is the measure of the moulding sand to flow around and over a pattern during ramming and to uniformly fill the flask. Collapsibility: This is the ability of the moulding sand to decrease in volume to some extent under the compressive forces developed by the shrinkage of metal during freezing p y g g gand subsequent cooling. Adhesiveness: This is the property of sand mixture to adhere to another body (here, the moulding flasks). The moulding sand should cling to the sides of the moulding boxes so that it does not fall out when the flasks are lifted and turned over. This property depends on the type and amount of binder used in the sand mix.


Other SandsFacing sand: The small amount of carbonaceousmaterial sprinkled on the inner surface of the moldcavity to give a better surface finish to the castings.Backing sand: It is what constitutes most of therefractory material found in the mould This is maderefractory material found in the mould. This is madeup of used and burnt sand.Green Sand: The molding sand that containsmoisture is termed as green sand. The green sandshould have enough strength so that the constructedmould retains its shape.Dry sand: When the moisture in the moulding sand iscompletely expelled, it is called dry sand.Compiled By: S K Mondal Made Easy

IES‐2008

Q. Small amount of carbonaceous materialsprinkled on the inner surface of mould cavity iscalled( ) k d(a) Backing sand(b) Facing sand(c) Green sand(d) Dry sand


Grain size numberASTM (American Society for Testing and Materials)grain size number, defined as

NWhere N is the number of grains per square inch

-n12Where N is the number of grains per square inchvisible in a prepared specimen at 100X and n is theASTM grain‐size number. Low ASTM numbers mean afew massive grains; high numbers refer to many smallgrains.


IES‐2002

Q. In the grain ‐size determination using standard charts, the relation between the given size number n and the average number of grains 'N' per square inch at a magnification of 100 X isper square inch at a magnification of 100 X is(a) N = 2n

(b) N = 2n‐l

(c) N = 2n + 1

(d) N = 2n + 1


Casting YieldThe casting yield is the proportion of the actual casting mass, w, to the mass of metal poured into the mould, W, expressed as a percentage.

= ×Casting yield 100wW


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Gating System


Gating SystemPouring basin: A small funnel shaped cavity at thetop of the mould into which the molten metal ispoured.

S Th th h hi h th lt t lSprue: The passage through which the molten metal,from the pouring basin, reaches the mould cavity. Inmany cases it controls the flow of metal into themould.

Runner: The channel through which the moltenmetal is carried from the sprue to the gate.


Ingate: A channel through which the molten metal enters the mould cavity. Vent: Small opening in the mould to facilitate escape of air and gases.


IES 2011In light metal casting, runner should be so designed that:

1. It avoids aspiration2. It avoids turbulence3. The path of runner is reduced in area so that

l l f fl h h hunequal volume of flow through each gatetakes place

(a) 1 and 2 only(b) 1 and 3 only(c) 2 and 3 only(d) 1, 2 and 3


IES 2011Match List –I with List –II and select the correct answer usingthe code given below the lists :

List –I List –II

A. Top gate 1. Heavy and large castings

B. Bottom gate 2. Most widely used and economical

CodesA B C D A B C D

(a) 3 4 2 1 (b) 1 4 2 3(c) 3 2 4 1 (d) 1 2 4 3

g y

C. Parting gate 3. Turbulence

D. Step gate 4. Unfavourable temperature gradient


IES‐1998Q. A sand castingmould assembly isshown in the abovefigure. The elementsmarked A and B aremarked A and B arerespectively(a) Sprue and riser(b) Ingate and riser(c) Drag and runner(d) Riser and runner


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GATE‐2002Q. The primary purpose of a sprue in a castingmould is to(a)Feed the casting at a rate consistent with the rateof solidificationof solidification

(b)Act as a reservoir for molten metal(c)Feed molten metal from the pouring basin to thegate

(d)Help feed the casting until all solidification takesplace


The goals for the gating system To minimize turbulence to avoid trapping gasses intothe moldTo get enough metal into the mold cavity before themetal starts to solidifyTo avoid shrinkageEstablish the best possible temperature gradient in thesolidifying casting so that the shrinkage if occurs mustbe in the gating system not in the required cast part.Incorporates a system for trapping the non‐metallicinclusions.


IES‐1998Q. Which of the following are the requirements of an ideal gating system?

1. The molten metal should enter the mould cavity with as high a velocity as possible.

2 It should facilitate complete filling of the mould cavity2. It should facilitate complete filling of the mould cavity.3. It should be able to prevent the absorption of air or gases

from the surroundings on the molten metal while flowing through it.Select the correct answer using the codes given below:

(a) 1, 2 and 3 (b) 1 and 2 (c) 2 and 3 (d) 1 and 3


IES‐2009Q. Consider the following statements:1.The actual entry point through which the molten metal enters the mould cavity is called ingate.2.Bottom gate in case of a mould creates unfavourablegtemperature gradient.3.Sprue in case of a mould is made tapered to avoid air inclusion.Which of the above statements is/are correct?(a) 1 only (b) 1 and 2 (c) 2 and 3 (d) 1 and 3


Types of Gating Systems

The gating systems are of two types:

Pressurized gating systemPressurized gating system

Un‐pressurized gating system


Pressurized Gating SystemThe total cross sectional area decreases towards themold cavityBack pressure is maintained by the restrictions in themetal flowFlow of liquid (volume) is almost equal from all gatesBack pressure helps in reducing the aspiration as thesprue always runs fullBecause of the restrictions the metal flows at highvelocity leading to more turbulence and chances ofmold erosion.


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Un‐Pressurized Gating SystemThe total cross sectional area increases towards the

mold cavity

Restriction only at the bottom of spruey p

Flow of liquid (volume) is different from all gates

Aspiration in the gating system as the system never

runs full

Less turbulence.Compiled By: S K Mondal Made Easy

Sprue DesignSprue: Sprue is the channel through which the moltenmetal is brought into the parting plane where it enters therunners and gates to ultimately reach the mould cavity.The molten metal when moving from the top of the cope tothe parting plane gains in velocity and some low‐pressurep g p g y parea would be created around the metal in the sprue.Since the sand mould is permeable, atmospheric air wouldbe breathed into this low‐pressure area which would thenbe carried to the mould cavity.To eliminate this problem of air aspiration, the sprue istapered to gradually reduce the cross section as it movesaway from the top of the cope as shown in Figure below (b).


The exact tapering can be obtained by the equation of continuity. Denoting the top and choke sections of The sprue by the subscripts’t’ and 'c' respectively, we get

t t c cA V A V= = ct c

t

VA AV


Since the velocities are proportional to the square of the potential heads, as can be derived from Bernoulli's equation,

= ct c

t

hA Ah

Where H = actual sprue heightand ht = h + H


GATE‐2001Q. The height of the down‐sprue is 175 mm and itscross‐sectional area at the base is 200 mm2. Thecross‐sectional area of the horizontal runner isl 2 A i l i di t thalso 200 mm2. Assuming no losses, indicate the

correct choice for the time (in seconds) required tofill a mould cavity of volume 106 mm3. (Use g = 10m/s2).

(a)2.67 (b)8.45 (c)26.72 (d)84.50


GATE‐2007Q. A 200 mm long down sprue has an area of crosssection of 650 mm2 where the pouring basin meets thedown sprue (i.e. at the beginning of the down sprue).A constant head of molten metal is maintained by thepouring basin. The Molten metal flow rate is 6.5 × 105p g 5mm3/s. Considering the end of down sprue to be opento atmosphere and an acceleration due to gravity of104mm/s2, the area of the down sprue in mm2 at its end(avoiding aspiration effect) should be

(a)650.0 (b)350.0 (c)290.7 (d)190.0Contd…Compiled By: S K Mondal Made Easy

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Gating ratioGating ratio is defined as: Sprue area: Runner area:

Ingate area.

For high quality steel castings, a gating ratio of 1: 2: 2 or

1: 2: 1.5 will produce castings nearly free from erosion,

will minimize oxidation, and will produce uniform

flow.

A gating ratio of 1: 4: 4 might favour the formation of

oxidation defects.Compiled By: S K Mondal Made Easy

IES‐2003Q. A gating ratio of 1: 2: 4 is used to design the gatingsystem for magnesium alloy casting. This gating ratiorefers to the cross∙ section areas of the various gatingelements as given below:1 Down sprue 2 Runner bar 3 Ingates1. Down sprue 2. Runner bar 3. IngatesThe correct sequence of the above elements in theratio 1: 2: 4 is(a) 1, 2 and 3(b) 1,3 and 2(c) 2, 3 and 1(d) 3, 1 an 2


IES‐2005

Q. The gating ratio 2: 8: 1 for copper in gating systemdesign refers to the ratio of areas of:(a) Sprue: Runner: Ingate(b) Runner: Ingate: Sprue(c) Runner: Sprue: Ingate(d) Ingate: Runner: Sprue


GATE‐2010

Q. In a gating system, the ratio 1:2:4 represents

(a) Sprue base area: runner area: ingate area

(b) Pouring basin area : ingate area : runner area

(c) Sprue base area : ingate area : casting area

(d) Runner area : ingate area : casting area


IAS‐1999Assertion (A): The rate of flow of metal through sprueis NOT a function of the cross‐sectional areas of sprue, runner and gate.Reason (R): If respective cross‐sectional areas of sprue, runner and gate are in the ratio of 1: 2: 2, the system is known as unpressurised gating systemsystem is known as unpressurised gating system.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true


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Risers and Riser DesignRisers are added reservoirs designed to feed liquidmetal to the solidifying casting as a means ofcompensating for solidification shrinkage.To perform this function, the risers must solidify afterthe castingthe casting.According to Chvorinov's rule, a good shape for a riserwould be one that has a long freezing time (i.e., a smallsurface area per unit volume).Live risers (also known as hot risers) receive the lasthot metal that enters the mold and generally do so at atime when the metal in the mold cavity has alreadybegun to cool and solidify.Compiled By: S K Mondal Made Easy

IES‐1994Q. Assertion (A): In a mould, a riser is designed and placed so that the riser will solidify after the casting has solidified.Reason (R): A riser is a reservoir of molten metal which will supply molten metal where a shrinkage cavity would have occurred.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true


Chvorinov’s ruleTotal solidification time (ts) = B (V/A) n

where n = 1.5 to 2.0[Where, B = mould constant and is a function of (mould material, casting material, and condition of casting]

n = 2 and triser = 1.25 tcasting

⎛ ⎞ ⎛ ⎞=⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

2 2

riser casting

V V1.25A A

( )= π

π= π +

2

2

V D H / 4DA DH 2 4

For cylinder of diameter D and height H

or


IES 2011The relationship between total freezing time t,volume of the casting V and its surface area A,according to Chvorinov’s rule is :

( ) Va t kA

⎛ ⎞= ⎜ ⎟⎝ ⎠

Where K is a constant

2

2

( )

( )

( )

AAb t kV

Ac t kV

Vd t kA

⎜ ⎟⎝ ⎠⎛ ⎞= ⎜ ⎟⎝ ⎠

⎛ ⎞= ⎜ ⎟⎝ ⎠

⎛ ⎞= ⎜ ⎟⎝ ⎠


IES‐1998Q. A spherical drop of molten metal of radius 2 mm

was found to solidify in 10 seconds. A similar drop

of radius 4 mmwould solidify inof radius 4 mmwould solidify in

(a) 14.14 seconds

(b) 20 seconds

(c) 28.30 seconds

(d) 40 secondsCompiled By: S K Mondal Made Easy

GATE‐2003

Q. With a solidification factor of 0.97 x 106 s/m2, thesolidification time (in seconds) for a sphericalcasting of 200 mm diameter is

(a) 539 (b) 1078 (c) 4311 (d) 3233


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IES‐2006Q. According to Chvorinov's equation, the

solidification time of a casting is proportional to:

(a) v2(a) v

(b) v

(c) 1/v

(d) 1/v2

Where, v = volume of castingCompiled By: S K Mondal Made Easy

GATE‐2007

Q. Volume of a cube of side 'l' and volume of a sphere ofradius ‘r’ are equal. Both the cube and the sphere aresolid and of same material. They are being cast. Theti f th lidifi ti ti f th b t thratio of the solidification time of the cube to the same

of the sphere is:

( ) ( ) ( ) ( )3 6 2 2 3 2 44 r 4 r 4 r 4 ra b c d

6 l 6 l 6 l 6 lπ π π π⎛ ⎞ ⎛ ⎞ ⎛ ⎞⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞

⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠



Conventional Question ESE 2003:

Compare the solidification time of two optimum side – risers of the same volume i h h li d i l h d h i with one has cylindrical shape and other is

parallopiped. [30 Marks]


Notes


Modulus MethodIt has been empirically established that if the modulus

of the riser exceeds the modulus of the casting by a

factor of 1.2, the feeding during solidification would be

satisfactory.

MR = 1.2 Mc

Modulus = volume/Surface area

In steel castings, it is generally preferable to choose a

riser with a height‐to‐diameter ratio of 1.Contd…Compiled By: S K Mondal Made Easy

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22

4D Dπ π+4


Conventional Question IES‐2008Calculate the size of a cylindrical riser (height and diameter

equal) necessary to feed a steel slab casting of dimensions

30 x 30 x 6 cm with a side riser casting poured horizontally30 x 30 x 6 cm with a side riser, casting poured horizontally

into the mould.

[Use Modulus Method]

[10 ‐Marks]


Caine’s MethodFreezing ratio = ratio of cooling characteristics of casting to the riser.

Th i h ld lidif l

( )( )

casting

riser

AVx

AV

=

The riser should solidify last so x > 1

According to Caine X =

Y = and a, b, c are constant.

a cY b

+−

riser

casting

VV


Table: Constants in Caine’s Method


Conventional Question IES‐2007

Calculate the size of a cylindrical riser (height and

diameter equal) necessary to feed a steel slabdiameter equal) necessary to feed a steel slab

casting of dimensions 25 x 25 x 5 cm with a side

riser, casting poured horizontally into the mould.

[Use Caine’s Method]


Solution:Volume of the casting = 25 × 25 × 5 = 3125 cm3

Surface area of the casting = 2 × 25 × 25 + 4 × 25 × 5 = 1750cm2

Volume of the riser =Wh D i h i di

2.D4

π

Where D is the riser diameterSurface area of the riser =

Freezing ratio, X=

Y =

22 2.D.D 1.25 .D

4π

π + = π

2 31750 / 3125 0.112 D

1.25 D / 0.25 D=

π π3

3Volume of riser 0.25 D 0.000251DVolume of casting 3125

= =


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Substituting this in the Caines’s equation for steels, we get

On simplification, we get

30.100.112 D 1.0

0.000251 D 0.03= +

−

p , g

By trial and error, we get

4 3D 8.9286 D 119.52 D 2490− − =

D 11.44 cm 12 cm= ≈


ChillsExternal chills are masses of high‐heat‐capacity, high‐thermal‐conductivity material that are placed in the mould (adjacent tothe casting) to accelerate the cooling of various regions.Chills can effectively promote directional solidification orincrease the effective feeding distance of a riser. They can oftenb d d h b f i i d f ibe used to reduce the number of risers required for a casting.Internal chills are pieces of metal that are placed within themould cavity to absorb heat and promote more rapidsolidification. Since some of this metal will melt during theoperation, it will absorb not only the heat‐capacity energy, butalso some heat of fusion. Since they ultimately become part ofthe final casting, internal chills must be made from the samealloy as that being cast.


IES‐1995

Q. Directional solidification in castings can be

improved by using

(a) Chills and chaplets

(b) Chills and padding

(c) Chaplets and padding

(d) Chills, chaplets and padding.


GATE‐1998,2007

Q. Chills are used in moulds to

(a) Achieve directional solidification

(b) Reduce the possibility of blowholes

(c) Reduce freezing time

(d) Smoothen metal flow for reducing splatter.


IAS 1994Chills are used in casting moulds to(a) Achieve directional solidification(b) Reduce possibility of blow holes(c) Reduce the freezing time(c) Reduce the freezing time(d) Increase the smoothness of cast surface


CupolaSteel can be melted in hot blast cupola.

In hot blast cupola, the flue gases are used to preheat

the air blast to the cupola so that the temperature in p p

the furnace is considerably higher than that in a

conventional cupola.

Cupola has been the most widely used furnace for

melting cast iron.


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IES‐1997Q. Assertion (A): Steel can be melted in hot blast cupola.Reason (R): In hot blast cupola, the flue gases are used topreheat the air blast to the cupola so that the temperature inthe furnace is considerably higher than that in a

i l lconventional cupola.(a) Both A and R are individually true and R is the correctexplanation of A(b) Both A and R are individually true but R is not thecorrect explanation of A(c) A is true but R is false(d) A is false but R is true


Electric Arc Furnace For heavy steel castings, the open‐hearth type of

furnaces with electric arc or oil fired would be generally

suitable in view of the large heat required for melting.

Electric arc furnaces are more suitable for ferrous

materials and are larger in capacity.


Crucible FurnaceSmaller foundries generally prefer the crucible furnace.The crucible is generally heated by electric resistanceor gas flame.

Induction FurnaceThe induction furnaces are used for all types ofmaterials, the chief advantage being that the heatsource is isolated from the charge and the slag and fluxget the necessary heat directly from the charge insteadof the heat source.


LadlesTwo types of ladles used in the pouring of castings.


Casting Cleaning (fettling)Impurities in the molten metal are prevented from reaching the mould cavity by providing a (i) Strainer (ii) Button well(iii) Skim bob


GATE‐1996

Q. Light impurities in the molten metal areprevented from reaching the mould cavity byproviding a( )(a) Strainer(b) Button well(c) Skim bob(d) All of the above


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Pouring timeTime taken to fill the mould with top gate

Where A = Area of mould H = Height of mouldAg = Area of Gate

Ag m

A.HtA 2gh

=

Hm = Gate height

Time taken to fill the mould with bottom gate

( )B m mg

2At h h HA 2g

= − −


GATE‐2005

Q. A mould has a downsprue whose length is 20 cmand the cross sectional area at the base of thedownsprue is 1cm2. The downsprue feeds ah i t l l di i t th ld it fhorizontal runner leading into the mould cavity ofvolume 1000 cm3. The time required to fill themould cavity will be

(a)4.05 s (b)5.05 s (c)6.05 s (d)7.25 s


GATE‐2006Q. In a sand casting operation, the total liquid head ismaintained constant such that it is equal to the mouldheight. The time taken to fill the mould with a top gateis tA. If the same mould is filled with a bottom gate,then the time taken is tB. Ignore the time required tofill h d f i i l ff Afill the runner and frictional effects. Assumeatmospheric pressure at the top molten metal surfaces.The relation between tA and tB is(A) 2(B) 2

(C)2

(D) 2 2

B A

B A

AB

B A

t tt t

tt

t t

=

=

=

= Compiled By: S K Mondal Made Easy

Expression for choke area

Where m = mass of the casting, kg

= 2mCA mmcρt 2gH

= Density of metal, kg / m3

t = pouring timec = Efficiency factor and is the function of gate

system used H = Effective head of liquid metal

= h for top gate

ρ


H=h‐ for bottom gate

=h‐ for parting line gate

mh2

2c

m

h2h

top gate parting line gate bottom gate

mh

P i li

mhCh

mh


Casting Defects The following are the major defects, which are likely to

occur in sand castings:

Gas defects

Shrinkage cavities

Molding material defects

Pouring metal defects

Mold shift.Compiled By: S K Mondal Made Easy

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Gas DefectsA condition existing in a casting caused by thetrapping of gas in the molten metal or by mold gasesevolved during the pouring of the casting.The defects in this category can be classified intog yblowholes and pinhole porosity.Blowholes are spherical or elongated cavities presentin the casting on the surface or inside the casting.Pinhole porosity occurs due to the dissolution ofhydrogen gas, which gets entrapped during heating ofmolten metal.


Shrinkage CavitiesThese are caused by liquid shrinkage occurring during the solidification of the casting.To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mold.Sprues may be too thin too long or not attached in the proper Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities.It is recommended to use thick sprues to avoid shrinkage cavities.


Molding Material Defects

Cuts and washes,

Scab

Metal penetration,

Fusion, and

Swell


Cut and washesThese appear as rough spots and areas of excess metal, andare caused by erosion of molding sand by the flowingmetal.This is caused by the molding sand not having enoughstrength and the molten metal flowing at high velocitystrength and the molten metal flowing at high velocity.The former can be taken care of by the proper choice ofmolding sand and the latter can be overcome by theproper design of the gating system.


ScabThis defect occurs when a portion of the face of a mould lifts or breaks down and the recess thus made is filled by metal.When the metal is poured into the cavity, gas may be disengaged with such violence as to break up the sand, hi h i h h d d h l i i fill d which is then washed away and the resulting cavity filled

with metal.The reasons can be: ‐ too fine sand, low permeability of sand, high moisture content of sand and uneven mould ramming.


Metal penetrationWhen molten metal enters into the gaps between sand grains, the result is a rough casting surface.This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration.g p


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FusionThis is caused by the fusion of the sand grains with

the molten metal, giving a brittle, glassy appearance

on the casting surface.

The main reason for this is that the clay or the sand

particles are of lower refractoriness or that the

pouring temperature is too high.


SwellUnder the influence of metallostatic forces, the moldwall may move back causing a swell in the dimensionof the casting. A proper ramming of the mold willcorrect this defect.

InclusionsParticles of slag, refractory materials sand ordeoxidation products are trapped in the casting duringpouring solidification. The provision of choke in thegating system and the pouring basin at the top of themold can prevent this defect.


Pouring Metal DefectsThe likely defects in this category are

Mis‐runs andCold shuts

A mis‐run is caused when the metal is unable to fillthe mold cavity completely and thus leaves unfilledcavities.A cold shut is caused when two streams while meetingin the mold cavity, do not fuse together properly thusforming a discontinuity in the casting.


The mis‐run and cold shut defects are caused either bya lower fluidity of the mold or when the sectionthickness of the casting is very small. Fluidity can beimproved by changing the composition of the metaland by increasing the pouring temperature of themetalmetal.


GATE‐2004

Q.Misrun is a casting defect which occurs due to

(a) Very high pouring temperature of the metal

(b) Insufficient fluidity of the molten metal

(c) Absorption of gases by the liquid metal

(d) Improper alignment of the mould flasks


GATE‐2009Q. Two streams of liquid metal which are not hot

enough to fuse properly result into a casting defect

known as

(a) Cold shut

(b) Swell

(c) Sand wash

(d) ScabCompiled By: S K Mondal Made Easy

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Mold ShiftThe mold shift defect occurs when cope and drag

or molding boxes have not been properly aligned.


IES‐2001

Q. Scab is a

(a) Sand casting defect

(b) Machining defect

(c) Welding defect

(d) Forging defect


IAS‐2004Match List‐I (Casting Defects) with List‐II (Explanation) and select the correct answer using the codes given below the lists:List‐I List‐II(Casting Defects) (Explanation)A. Metallic projections 1. Consist of rounded or rough internal or exposed cavities

including blow holes and pin holesB. Cavities 2. Formed during melting, solidification and moulding.C. Inclusions 3. Includes single folds, laps, scars adhering sand layers and

oxide scaleD. Discontinuities 4. Include cracks, cold or hot tearing and cold shuts

5.Consist of fins, flash or massive projections and rough surfaces

Codes: A B C D A B C D(a) 1 5 3 2 (b) 1 5 2 4(c) 5 1 2 4 (d) 5 1 3 2


GATE‐2003Hardness of green sand mould increases with(a) Increase in moisture content beyond 6 percent(b) Increase in permeability(c) Decrease in permeability(c) Decrease in permeability(d) Increase in both moisture content and permeability


IES‐1998Assertion (A): Stiffening members, such as webs and ribs, used on a casting should be liberally provided. Reason (R): They will provide additional strength to a cast memberto a cast member.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true


IES‐2005In gating system design, which one of the following is the correct sequence in which choke area, pouring time, pouring basin and sprue sizes are calculated?(a) Choke area ‐ Pouring time ‐ Pouring basin – Sprue(b) Pouring basin ‐ Sprue ‐ Choke area ‐ Pouring time(c) Choke area ‐ Sprue ‐ Pouring basin ‐ Pouring time(d) Pouring basin ‐ Pouring time ‐ Choke area ‐ Sprue


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IES‐1997If the melting ratio of a cupola is 10: 1, then the coke requirement for one ton melt will be(a) 0.1 ton(b) 10 tons(b) 10 tons(c) 1 ton(d) 11 tons


IES‐2009In which one of the following furnaces most of the non‐ferrous alloys are melted?(a) Reverberatory furnace(b) Induction furnace(b) Induction furnace(c) Crucible furnace(d) Pot furnace


IAS‐2001Which of the following pattern‐materials are used in Precision Casting?1. Plaster of Paris2. Plastics2. Plastics3. Anodized Aluminium Alloy 4. Frozen MercurySelect the correct answer using the codes given below:(a) 1 and 2 (b) 2 and 4 (c)3 and 4 (d) 1 and 3


IAS‐2004Which one of the following gating systems is best suited to obtain directional solidification?(a) Top grating(b) Part‐line grating(b) Part line grating(c) Bottom grating(d) Stepped grating


IES 2011In the designation of Aluminium casting A514.0indicates :(a) Aluminium purity(b) Aluminium content(c) Percentage of alloy element(d) Magnesium Content

Compiled By: S K Mondal Made Easy Compiled By: S K Mondal Made Easy

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60105277 3 1 Metal Casting for Students