1 sand casting

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06-11-2014 1 Metal Casting Sand casting Sand casting uses ordinary sand as the primary mould material. The sand grains are mixed with small amounts of other materials, such as clay and water, to improve mouldability and cohesive strength, and are then packed around a pattern that has the shape of the desired casting. The pattern must be removed before pouring, the mold is usually made in two or more pieces. An opening called a sprue hole is cut from the top of the mold through the sand and connected to a system of channels called runners. Contd…. The molten metal is poured into the sprue hole, flows through the runners, and enters the mold cavity through an opening called a gate. Gravity flow is the most common means of introducing the metal into the mold. After solidification, the mold is broken and the finished casting is removed. The casting is then “fettled” by cutting off the ingate and the feeder head. Because the mold is destroyed, a new mold must be made for each casting. Contd… Sequential steps in making a sand casting A 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… The mold is opened, the pattern board is drawn (removed), and the runner and gate are cut into the surface of the sand. The mold is reassembled with the pattern board removed, and molten metal is poured through the sprue. The contents are shaken from the flask and the metal segment is separated from the sand, ready for further processing.

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1 Sand Casting

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Page 1: 1 Sand Casting

06-11-2014

1

Metal Casting Sand casting Sand casting uses ordinary sand as the primary

mould material. The sand grains are mixed with small amounts of

other materials, such as clay and water, to improvemouldability 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….

The molten metal is poured into the sprue hole, f lowsthrough the runners, and enters the mold cavitythrough an opening called a gate.

Gravity f low 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…

Sequential steps in making a sand casting A pattern board is placed between the bottom (drag)

and top (cope) halves of a f lask, 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…

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 boardremoved, and molten metal is poured through thesprue.

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

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Casting Terms Flask: 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.

Drag: Lower moulding flask.

Cope: Upper moulding flask.

Cheek: Intermediate moulding flask used in three-

piece moulding.Contd…

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

made with some modifications.

Parting line: This is the dividing line between the two

moulding 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…

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.

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 cavity.

Runner: The passage ways in the parting plane through

which molten metal f low 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…

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Chaplet: Chaplets are used to support cores inside the

mould cavity.

Chill: Chills are metallic objects, which are placed in

the 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…

Padding Tapering of thinner section towards thicker section

is known as 'padding'. This will require extra material. If padding is not provided, centre line shrinkage or

porosity will result in the thinner section.

IES-2001

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-1996Which of the following methods are used for

obtaining directional solidification for riser design

1. 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 3

IES 2007

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

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

The main modifications are The addition of pattern allowances, The provision of core prints, and Elimination of fine details, which cannot be obtained

by casting and hence are to be obtained by furtherprocessing

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Pattern Allowances1. Shrinkage or contraction allowance

2. Draft or taper allowance

3. Machining or finish allowance

4. Distortion or camber allowance

5. Rapping allowance

Shrinkage allowance All 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.

Contd…

Liquid shrinkage and solid shrinkage Liquid shrinkage refers to the reduction in

volume 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-1995Which 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.

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

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Draft To 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.

DRAFT ALLOWANCE

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 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 and

therefore is likely to be distortion prone.

This 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 Materials Wood patterns are relatively easy to make. Wood is not

very dimensionally stable. Commonly used teak, whitepine and mahogany wood.

Metal patterns are more expensive but are moredimensionally 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 uses wax patterns.

The pattern material should be Easily 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.

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IES-1994Which of the following materials can be used for

making patterns?

1. Aluminium 2. Wax 3. Mercury 4. Lead

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

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 Pattern

This 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 twoparts so that one part is in the drag and the other in thecope.

Types of Pattern Cope 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 systemsare attached separately to the metal or woodenplates along with the alignment pins. They arecalled the cope and drag patterns.

Types of Pattern Match 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 Pattern Loose Piece Pattern

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

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Types of Pattern Follow 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 oframming.

IES-2008

The pattern adopted for those castings where thereare some portions which are structurally weak andare likely to break by the force of ramming arecalled:(a) Loose piece pattern(b) Follow board pattern(c) Skelton pattern(d) Single piece pattern

Types of Pattern Sweep 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 orcylindrical.

Types of Pattern Skeleton 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 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 f low and fill a mold is knownas f luidity.

Pouring Temperature The most important controlling factor of fluidity is the

pouring temperature or the amount of superheat. Higher the pouring temperature, the higher the fluidity. Excessive temperatures should be avoided, however. At

high 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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Core Used for making cavities and hollow projections.

All sides of core are surrounded by the molten metaland 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.

Dry 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.

Contd…

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.

Permeability Number: The rate of flow of air passingthrough a standard specimen under a standard pressure istermed as permeability number.

The standard permeability test is to measure timetaken by a 2000 cu cm of air at a pressure typically of980 Pa (10 g/cm2), to pass through a standard sandspecimen confined in a specimen tube. The standardspecimen size is 50.8 mm in diameter and a length of50.8 mm.

Then, the permeability number, R is obtained by

Where V= volume of air = 2000 cm3

H = height of the sand specimen = 5.08 cmp = air pressure, g/cm2

A = cross sectional area of sand specimen = 20.268 cm2

T = time in minutes for the complete air to pass through

Inserting the above standard values into the expression, we get

VHRpAT

501.28.

Rp T

Calculate the permeability number of sand if it takes 1 min 25 s to pass 2000 cm3 of air at a pressure of5 g/cm2 through the standard sample.

25.0 /1min 25 1.417 min

501.28 70.755 1.417

p g cmT s

R

Collapsibility: At the time of cooling, casting shrinks, and unless the core has good collapsibility (ability to decrease in size) it is likely to provide resistance against shrinkage and thus can cause hot tears.

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Friability: The ability to crumble should be a very

important consideration at the time of removal.

Smoothness: Surface of the core should be smooth

for good finish to the casting.

Low Gas Emission

Core Sands Used 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%).

Carbon Dioxide Moulding Sodium 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).

Contd…

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.

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 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 Composition Sand: Ordinary silica Sand (SiO2), zircon, or olivine

sands.

Clay: Acts as 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.

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Other Additives Cereal binder up to 2% increases the strength.

Pitch if used up to 3% would improve the hot

strength.

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.

Moulding Sand Properties Porosity or Permeability: Permeability or porosity of

the moulding sand is the measure of its ability topermit air to f low through it.

Strength: It is defined as the property of holdingtogether 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.

Contd…

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 and 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 Sands Facing sand: The small amount of carbonaceous

material 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 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.

IES-2008

Small amount of carbonaceous material sprinkled

on the inner surface of mould cavity is called

(a) Backing sand

(b) Facing sand

(c) Green sand

(d) Dry sand

Grain size number ASTM (American Society for Testing and Materials)

grain size number, defined asN

Where 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 a few massive grains; highnumbers refer to many small grains.

-n 12

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IES-2002In 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 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

Gating System

Contd…

Gating System Pouring basin: A small funnel shaped cavity at the

top of the mould into which the molten metal ispoured.

Sprue: The passage through which the molten metal,from the pouring basin, reaches the mould cavity. Inmany cases it controls the f low of metal into themould.

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

Contd…

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.

Types of Gate or In-gateTop gate: Causes turbulence in the mould cavity, it is prone

to form dross, favourable temperature gradient towards the

gate, only for ferrous alloys.

Bottom gate: No mould erosion, used for very deep moulds,

higher pouring time, Causes unfavourable temperature

gradients.Parting Gate: most widely used gate, easiest and mosteconomical in preparation.Step Gate: Used for heavy and large castings, size of ingatesare normally increased from top to bottom.

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

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

unequal volume of f low 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 :

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

List –I List –IIA. Top gate 1. Heavy and large castings

B. Bottom gate 2. Most widely used and economical

C. Parting gate 3. Turbulence

D. Step gate 4. Unfavourable temperature gradient

IES-1998A sand casting mouldassembly is shown inthe above figure. Theelements marked Aand B are respectively(a) Sprue and riser(b) Ingate and riser(c) Drag and runner(d) Riser and runner

The goals for the gating system To minimize turbulence to avoid trapping gasses into

the mold To get enough metal into the mold cavity before the

metal starts to solidify To avoid shrinkage Establish the best possible temperature gradient in the

solidifying 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-1998Which 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 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

Types of Gating Systems

The gating systems are of two types:

Pressurized gating system

Un-pressurized gating system

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Pressurized Gating System The total cross sectional area decreases towards the

mold cavity Back pressure is maintained by the restrictions in the

metal f low Flow of liquid (volume) is almost equal from all gates Back pressure helps in reducing the aspiration as the

sprue always runs full Because of the restrictions the metal f lows at high

velocity leading to more turbulence and chances ofmold erosion.

Un-Pressurized Gating System The total cross sectional area increases towards the

mold cavity

Restriction only at the bottom of sprue

Flow of liquid (volume) is different from all gates

Aspiration in the gating system as the system never

runs full

Less turbulence.

Sprue Design Sprue: Sprue is the channel through which the molten

metal 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-pressurearea 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).

Contd…

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

Contd…

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

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Gating ratio Gating 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

f low.

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

oxidation defects.

IES-2003A 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. 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

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

Risers and Riser Design Risers are added reservoirs designed to feed liquid

metal to the solidifying casting as a means ofcompensating for solidification shrinkage.

To perform this function, the risers must solidify afterthe 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.

IES-1994Assertion (A): In a mould, a riser is designed and placedso that the riser will solidify after the casting has solidified.Reason (R): A riser is a reservoir of molten metal whichwill supply molten metal where a shrinkage cavity wouldhave occurred.(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

Chvorinov’s rule Total 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

2

2

V D H / 4DA DH 2 4

For cylinder of diameter D and height H

or

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

Where K is a constant

2

2

( )

( )

( )

( )

Va t kAAb t kV

Ac t kV

Vd t kA

IES-1998A spherical drop of molten metal of radius 2 mm

was found to solidify in 10 seconds. A similar drop

of radius 4 mm would solidify in

(a) 14.14 seconds

(b) 20 seconds

(c) 28.30 seconds

(d) 40 seconds

IES-2006According to Chvorinov's equation, the

solidification time of a casting is proportional to:

(a) v2

(b) v

(c) 1/v

(d) 1/v2

Where, v = volume of casting

IES - 2012The ratio of surface area of volume for a unit volume of

riser is minimum in case of

(a) Cylindrical riser

(b) Spherical riser

(c) Hemispherical riser

(d) Cuboids riser

Modulus Method It 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…

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22

4D D

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

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

Casting

Riser

AV

XA

V

Chills External 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 oftenbe 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

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.

Cupola Cupola has been the most widely used furnace for

melting cast iron. In hot blast cupola, the f lue gases are used to preheat the

air blast to the cupola so that the temperature in thefurnace is considerably higher than that in aconventional cupola. Coke is fuel and Lime stone(CaCO3) is mostly used flux.

Cost of melting low. Main disadvantages of cupola is that it is not possible to

produce iron below 2.8% carbon. Steel can be also prepared in cupola by employing

duplexing and triplexing operations.

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IES-1997Assertion (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 aconventional 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

IES - 2012Statement (I): Cupola furnace is not employed formelting steel in foundryStatement (II): The temperatures generated within acupola are not adequate for melting Steel(a) Both Statement (I) and Statement (II) areindividually true and Statement (II) is the correctexplanation of Statement (I)(b) Both Statement (I) and Statement (II) areindividually true but Statement (II) is not the correctexplanation of Statement (I)(c) Statement (I) is true but Statement (II) is false(d) Statement (I) is false but Statement (II) 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 Furnace Smaller foundries generally prefer the crucible furnace. The crucible is generally heated by electric resistance

or gas f lame.

Induction Furnace The induction furnaces are used for all types of

materials, 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.

Ladles Two 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) Bottom well(iii) Skim bob

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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 GateHm = Gate height

Time taken to fill the mould with bottom gate

Ag m

A.HtA 2gh

B m mg

2At h h HA 2g

Expression for choke area

Where m = mass of the casting, kg = 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

2mCA mmcρt 2gH

ρ

Contd…

H=h- for bottom gate

=h- for parting line gate

top gate parting line gate bottom gate

mh2

2c

m

h2h

Top galemh

Parting line gate

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.

Gas Defects A condition existing in a casting caused by the

trapping of gas in the molten metal or by mold gasesevolved during the pouring of the casting.

The defects in this category can be classified intoblowholes 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 Cavities These 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 theappropriate places in the mold.

Sprues may be too thin, too long or not attached in theproper location, causing shrinkage cavities.

It is recommended to use thick sprues to avoid shrinkagecavities.

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Molding Material Defects

Cuts and washes,

Scab

Metal penetration,

Fusion, and

Swell

Cut and washes These appear as rough spots and areas of excess metal, and

are 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 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.

Scab This defect occurs when a portion of the face of a mould

lifts or breaks down and the recess thus made is filled bymetal.

When the metal is poured into the cavity, gas may bedisengaged with such violence as to break up the sand,which is then washed away and the resulting cavity filledwith metal.

The reasons can be: - too fine sand, low permeability ofsand, high moisture content of sand and uneven mouldramming.

Metal penetration When 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 sandgrains more the metal penetration.

Fusion This 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 inf luence of metallostatic forces, the mold

wall 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

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Pouring Metal DefectsThe likely defects in this category are Mis-runs and Cold 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.

Contd…

The mis-run and cold shut defects are caused either bya lower f luidity 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 themetal.

GATE-2009Two 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) Scab

Mold ShiftThe mold shift defect occurs when cope and drag

or molding boxes have not been properly aligned.

IES-2001

Scab is a

(a) Sand casting defect

(b) Machining defect

(c) Welding defect

(d) Forging defect

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 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

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IES-2005In gating system design, which one of thefollowing is the correct sequence in which chokearea, pouring time, pouring basin and sprue sizesare 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

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(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(c) Crucible furnace(d) Pot furnace

Cast Aluminium Code Four digit identification system First digit indicates alloy group

1 – Aluminium, 99% or more2 – copper3 – Silicon, with copper and/or magnesium4 – silicon5 – magnesium6 – not used7 – zinc8 – tin9 – other elements

Cast Aluminium Code Contd..

Second two digits identify the aluminium alloy orindicate the aluminium purity.

The last digit is separating from the other three by adecimal point and indicates the product form; that is,castings or ingots

A modification of the original alloy is indicated by aserial letter before the numerical designation.

Alloy A514.0 indicates an aluminium alloy casting withmagnesium as the principal alloy. One modification tothe original alloy has made, as indicated by the letter A.

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

Ans. (d)