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MANUFACTURING & ENGINEERING MATERIALS

2 METAL CASTING

1. CASTING

Casting is one of the oldest manufacturing processes, and even today is the first step in
manufacturing most products.
“Casting is a process in which the liquid molten metal is poured into the mould cavity whose
shape is same as that of the casting to be produced, & allowed it to solidify and after
solidification the casting can be taken out by breaking the mould.
1.1. PATTERN AND MOULD
• A Pattern is the replica of the part to be cast (to be produced) and is used to prepare
the mould cavity. Patterns are made of either wood or metal.
• A Mould is an assembly of two or more metal blocks, or bonded refractory particles
(sand) consisting of a primary cavity.
The mould cavity holds the liquid material. The mould also contains secondary cavities
for pouring and channelling the liquid material into the cavity and to act as a reservoir, if
necessary.
Casting process requires a knowledge in the following areas:
(i) Preparation of moulds and patterns
(ii) Melting and pouring of the liquefied metal.
(iii) Solidification and further cooling to room temperature.
(iv) Defects and inspection.
The suitability of the casting operation for a given material depends on
(i) the melting temperature of the job and the mould materials,
(ii) the solubility of and the chemical reaction between the job and the mould materials,
(iii) the solubility of the atmosphere in the material at different temperatures to be
encountered in the casting operation.
(iv) the thermal properties such as conductivity and coefficient of linear expansion of both
the mould and job materials.
Flask is a four-sided frame in which a sand mould is made is referred to as a flask.
1.2. Advantages of Casting:
1. Molten metal can flow into any small section in the mould cavity, hence any intricate
shape can be produced.

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2. Practically any material can be casted.
3. Tools required are very simple and inexpensive.
4. Ideal method for producing small quantities.
5. Due to same cooling rate from all directions, the uniform mechanical properties can be
obtained.
6. Any size of the casting can be produced like upto 200tonnes.
7. Heavy equipment like machine beds, ship's propeller etc. can be cast easily in the
required size rather than fabricating them by joining several smaller pieces.
1.3. Limitations:
1. With normal sand-casting process the dimensional accuracies and surface finish is poor.
2. Defects are inevitable.
3. Sand casting is labour intensive.

2. STEPS INVOLVED IN CASTING

The sequence of steps involved in casting are:

1. Preparation of pattern
2. Preparation of mould
3. The Gating system (Design)
4. Design of Riser
5. Melt the metal whose casting is to be formed
6. Pouring of molten metal
7. Solidification
8. Fettling
9. Finishing
10. Testing

3. PATTERN MAKING

Pattern is the replica of object to be produced with some modification and modification is in
the form of allowances.
Pattern Allowance - A pattern is always slightly different from the final job to be Produced.
This difference in dimensions is referred as the pattern allowance.
Pattern size = casting size ± allowances.
Different types of Pattern Allowance:
3.1. Shrinkage Allowance
Shrinkage allowance is a positive allowance which is provided to take care of the
contractions of a casting.

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The total contraction of a casting takes place in three stages.
1. Liquid Shrinkage – it is the shrinkage of molten metal when it cools from pouring
temperature to freezing temperature and phase of molten metal remains liquid.
2. Solidification Shrinkage – it is the shrinkage of molten metal when the phase of the
molten metal changes from liquid to solid.
3. Solid Shrinkage – it is the Shrinkage associated when the temperature of solid
casting changes from freezing temperature to room temperature.
Note:
The first two will be taken care by providing riser during casting. But the third will be
provided as a shrinkage allowance in the pattern. Thus, Shrinkage allowance provided on
the pattern to compensate the solid shrinkages taking place during the cooling of the
material from freezing temp to room temp as a solid.
Let a molten metal is having the pouring ‘TP’, Freezing Temperature ‘TF’ & room
temperature is ‘To’

Fig.1: Cooling curve for a pure metal during casting.

Then, Solid shrinkage is given by:
Solid shrinkage, δL = LαT = Lα ( TF − TO ) = Shrinkage allowance to be provided on

pattern
Where, L =Change in dimension
L = Initial dimension of component
α = Coefficient of thermal expansion

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Thus, Amount of shrinkage allowance depends on the linear coefficient of thermal
expansion of the material. The higher the value of this coefficient, the more the value of
shrinkage allowance.

Metals Solid shrinkage in mm (per 1000mm)

Invar, Bismuth 0

White Metal 5

Cast iron 10

Aluminium 13

Copper 17

Steel 20

Brass 23
• From the table, we can see that solid shrinkage is maximum for Brass.
• Out of all the metals used in casting process, Aluminium have highest liquid shrinkage
of about 6% whereas total shrinkage i.e., liquid as well as solid shrinkage both together
will be highest in case of steels.
3.2. Machining Allowance:
• Usually, A cast surface is too rough to be used in the same way as the surface of the
final product. As a result, operations are required to produce the finished surface.
• The excess in the dimensions of the casting (consequently in the dimensions of the
pattern) over those of the final job to take care of the machining to produce good
surface finish is called the machining allowance.

Fig.2: Showing casted product and extra allowance given

• It is specified by 'x' mm/side.
• Since while producing internal surface (i.e. hole), we have to reduce the dimension of
the surface. Thus, the allowances provided for internal surface should be negative.
• The amount of machining allowance provided would depend on the metal cast, the
type of moulding used, the class of accuracy required on the surface and the
complexity of surface details.
• Typical machining allowances for sand castings range between 1.5 mm and 3 mm

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3.3. Draft or Taper Allowance:
• When a pattern is withdrawn from the cavity there is possibility that pattern may
damage the surface the cavity produced due to continuous contact between pattern
and wall of mould cavity. So, to avoid this, the surface parallel to the direction of
withdrawal of the pattern from the mould cavity is tapered little bit.
• This process of inclining the surface in the direction of the withdrawal of pattern from
the cavity called drafting or tapering the angle of inclination is known as draft or taper
allowance.
• A draft facilitates easy withdrawal of the pattern. The value of the draft is between
1
2  and 2 .

Fig.3: Effect of draft on pattern (External)withdrawing

Fig.4: Effect of draft on pattern (Internal) withdrawing

• In an internal surface, since it surrounded by moulding sand from two sides thus it is
having more area of contact so more tapering is required for internal surface as
compare to external surface.
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• In general, 5° to 8° draft is given for internal surfaces and 2  and 2 is given for
external surfaces.
Note:
• In casting process if the pattern is made by using wax, mercury or polystyrene then
no draft allowance is required.

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3.4. Shake Allowance:

• When a material comes into contact with another material then a bond is developed

between those materials. If both the materials are same, then the bond is known as

cohesive bond and if the materials are different then the bond is known as adhesive

bond. So, in case of casting an adhesive bond is persist between moulding sand and

pattern and pattern will try to stick on the moulding sand which can damage the

surface of mould.

• Thus, before pulling out the pattern, shaking of pattern is done for easy removal which

will increase the size of the mould & consequently will increase size of the casting.

• So, allowance provided on the pattern to compensate this increase in size of the casting

is known as shaking Allowance.

Fig.5: shake allowance

• It purely depends on the skill of mould making person.

• since it increases the size of the casting, thus make to accurate pattern size to be

reduces so it is taken as negative allowance.

• If the pattern is made by using the materials like wax, mercury, polystyrene as pattern

material, no shake allowance is provided.

Note:

Shake allowance is necessary while draft is optional. [in small height object].

3.5. Distortion Allowances:

• In casting, molten metals is use which is having very high temperature. Thus, when a

thin or U or V shape castings are produced then they are prone to distortion. Due to

this, shape of the casting changes as shown in the Fig.6.

• “To avoid the distortion, the shape of pattern itself should be given a distortion of equal

amount in the opposite direction of the likely distortion direction so that final product

will come in true shape known as distortion allowance”.

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Fig.6: distortion allowance

L
• This value depends will depends upon ratio.
t

4. TYPES OF PATTERNS

The type of pattern used depends upon the design of casting, complexity of shape, number of
castings required, moulding process, surface finish and accuracy.
Different types of patterns are:
(i). Solid or single piece pattern:
• It is the simplest pattern in which pattern is made up of single piece and it does not contain
any attached part.
• In this type of pattern, simple shape and withdrawn for very easily shape can produced from
the mould.
• The solid pattern placed in the drag position.
• It is used for making a flat surface like as gear blanks, square blocks etc.

Fig.7: Solid or single piece pattern

(ii). Split Pattern or Two-Piece Pattern
• This is the most widely used type of pattern for intricate castings.
• When the contour of the casting makes its withdrawal from the mould difficult or when the
depth of the casting is too high, then the pattern is split into two parts so that one part is in
the drag (lower part) and the other in the cope (upper part).
• The split surface of the pattern is same as the parting plane of the mould. The two halves
of the pattern should be aligned properly by making use of the dowel pins which are fitted
to the cope half. These dowel pins match with the precisely made holes in the drag half of
the pattern and thus align the two halves properly as shown in Fig.8.

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Fig.8: Split pattern

(iii). Match plate pattern:
• This pattern is made in two halves mounted on both sides of a match plate (of wood or
metal) confirming to the contour of the parting surface.
• The match plate is accurately placed between the cope and the drag flasks by means of
locating pins. For small castings, several patterns can be mounted on the same match plate
thus same time number of products can be produced.
• Thus, Production efficiency and dimensional accuracy is improved by this method.

Fig.9. Match plate pattern

(iv). Cope and drag pawn:
• The cope and drag halves of a split pattern are separately mounted on two match plates.
Thus, the cope and the drag flasks are made separately and brought together (with accurate
relative location) to produce the complete mould.

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Fig.10: Cope and drag pattern

• While designing a pattern, the parting line should he chosen so as, the smallest portion of
the pattern in the cope.
• As the moulding sand has greater strength the compression than in tension, the heavier
sections of the pattern should be included in the drag. The possible defects due to loosen
sand in the mould are more frequent in the cope half.
(v). Gated Pattern
• This is simply one or more than one loose pattern with attached gates and runners and
provides a channel through which the molten metal can flow from the pouring sprue to the
mould cavity.
(vi). Loose Piece Pattern
• It is frequently the case that parts of the pattern will overhang so that the pattern cannot
be removed from the sand in any direction, even if parted.
• In such cases the overhanging parts are fastened loosely to the main part of the pattern
by wires or wooden pins. That fastened part is known as loose piece & these types of pattern
is known as loose piece pattern.

Fig.11: Loose piece pattern

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(vii). Sweep pattern
• Sweep patten is used to generate surfaces of revolution in large castings, and to prepare
moulds & it is done by sweeping the complete casting by means of a plane.
• Here "sweep" refers to the section that rotate about an edge to yield circular sections.
• These are used for generating large shapes which are axi- symmetrical or prismatic in nature
such as bell shaped or cylindrical as shown in Fig.12.
• This type of pattern is particularly suitable for very large castings such as bells for
ornamental purposes used.

Fig.12: Sweep Pattern

(viii). Follow board pattern
• It is used for those castings where there are some portions which are structurally weak and
if not supported properly are likely to break under the force of ramming.

Fig.13: Follow board pattern

(ix). Skeleton Pattern
• This consists of a simple wooden frame outlining the shape of the casting.
• It is used for building the final pattern by packing sand around the skeleton. After packing
the sand, the desired form is obtained with the help of a stickler as shown in Fig.14.
• The type of skeleton to be made is dependent upon the geometry of the work piece.
• This type of pattern is useful generally for very large castings required in small quantities
where large expense on complete wooden pattern is not justified.

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Fig.14: Skeleton pattern

5. PROPERTIES OF PATTERN MATERIAL

1. It should not absorb the moisture

2. Low density
3. Good in surface finish
4. Easiness in fabrication
5. Cheap
The most commonly used pattern materials are:
(i). Wood:
• It is easily available, low weight, easily shaped and relatively cheap but will absorb moisture.
• For very large castings, wood may be only practical pattern material.
• The most common wood for patterns is teak wood and mahogany.
(ii). Metals:
• Metal which are most commonly used for patterns are Aluminium and white metal.
• These are light, easily workable and are corrosion resistant. Also, white metal has very small
shrinkage.
• In general, Metals patterns are used when the number of the unit to be produced is high.
(Mass production)
(iii). Plastics:
• They possess all good properties which are required for a pattern material. Hence plastic
patterns are the most commonly used materials in industry.
Ex: epoxy resin (thermosetting), PVC, Nylon, Cellulose and polystyrene etc.
• If the polystyrene is used as pattern it will get evaporated means the pattern is removed in
the form of vapours.
(iv). Wax:
• The wax is taken as a special pattern material because it is very light material. If wax pattern
is used along with green sand mould then rat tail defects are formed.

Fig.15: Rat-Tail
Ex: The components produced by wax patterns are gold and silver ornaments, turbine
blades, handicrafts, wave guides of radar system, shuttle eye for weaving etc.

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(v). Mercury:
• It is used as a pattern material for producing very special casting having very small size with
excellent surface finish without machining.
5.1. MOULD MAKING
• Moulding sand will be used for manufacturing of the mould. Moulding sand is the freshly
prepared refractory material used for making the mould cavity.
• It is a mixture of silica, clay and moisture in appropriate proportions to get the desired
results and it surrounds the pattern while making the mould.
The main ingredients of any moulding sand are:
(i). Silica grains (Si02)
(ii). Clay as binder
(iii). Moisture to activate the clay and provide plasticity
Besides, some other materials are also added to these to enhance the specific
properties of moulding sands.
1. Silica sand particles (75-80%):
• The sand which forms the major portion of the moulding sand is essentially silica grains,
the rest being the other oxides of various element such as alumina, sodium (Na2O + K2O)
and magnesium oxide (MgO) & these impurities should be minimised to about 2% since
they affect the fusion point of the silica sands.
• The main source of moulding sand is the river sand which is used with or without
washing.
• In the river sand, all sizes and shapes of grains are mixed. The sand grains may vary
in size from a few micrometres to a few millimetres. Shape of the grains may be round,
sub-angular, angular and very angular. The size and shapes of these sand grains greatly
affect the properties of the moulding sands
Various type of sand and their composition
(i). Zircon sand (zirconium silicate, ZrSiO 4) composition:
• ZrO2 = 66.25%, SiO2 = 30.96%, Al2O3 = 1.92%, Fe2O3 = 0.74% and traces of other
oxides.
• It has a fusion point of about 2400°C and also a low coefficient of thermal expansion.
• The other advantages are high thermal conductivity & high density & requires very small
amount of binder (about 3%).
(ii). Chromite sand composition:
• Cr2O3 = 44%, Fe2O3 = 28%, SiO2 = 2.5%, CaO = 0.5%, and Al2O3 + MgO = 25%.
• The fusion point is about 1800°C. It also requires a very small amount of binder (about
3%) & used to manufacture heavy steel castings requiring better surface finish.
(iii). Olivine sand composition:
• Fosterite (Mg2SiO4) and Fayalite (Fe2SiO4).
• It is very versatile sand and the same mixture can be used for a range of steels.

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2. Clay (15-20%)
• Clays are the most generally used binding agents mixed with the moulding sands to
provide the strength because of their low cost and wider utility.
• The most popular clay types used are:
Kaolinite or fire clay (Al2O3.2SiO2.2H2O) and
Bentonite (Al2O3.4SiO2.H2O.nH2O)
• The clays besides these basic constituents may also contain some mixtures of lime,
alkalis and other oxides which tend to reduce their refractoriness.
• Kaolinite has a melting point about 1750°C and Bentonite has inciting temperature
range of 1250 to 1300°C. Of the two, bentonite can absorb more water which increases
its bonding power.
• The clay chosen for moulding sand should give it the requisite strength for the given
application taking into account the metal being cast and thickness of the casting.
• Normally, the river sand contains a large amount of clay and therefore can be directly
used.
3. Water:
• Clay is activated by water so that it develops the necessary plasticity and strength. The
amount of water used should be properly controlled. This is because a part of the water
absorbed by clay helps in bonding while the remainder within the limit helps in improving
the plasticity, but if the percentage of water increases more than specified limit than it
would decrease the strength.
• The normal percentages of water used are from 2 to 8.
Note:
• The optimum moisture (or water) content in the moulding sand is 7-8%.
• If moisture content is less than 8%, then binding material will not be able to activate
itself and the sand particles will not be able to bind together.
• If moisture content is more than 8%, then excess water will be filled in the voids present
between sand particles and it will reduce the permeability of the moulding sand.
Note.:
Besides these three main ingredients, many other materials also may be added to
enhance the specific properties.
• Cereal binder up to 2% increases the strength.
• By-product in coke making up to 3% would improve the hot strength
• Saw dust: up to 2% improves the collapsibility by slowly burning and increase the

permeability.

• Starch or dextrin: Used for increasing strength and resistance for deformation of

moulding sand.

• Coal Dust: It is basically used for providing better surface finish to the castings.

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4. Sands and its type

(i). Green sand:

• The material for a green sand mould is a mixture of sand, clay, water, and some organic

additives, e.g., wood flour, dextrin, and sea coal.

• The percentage of these ingredients on weight basis is approximately 70-85% sand,

10-20% clay, 3 – 6% water, and 1-6% additives.

• This ratio may vary slightly depending on whether the casting is ferrous or non-ferrous.

(ii). Loam sand:

• Green or dry sand with at least 50% clay and dries hard. It also contains fire clay.

• It has 18 to 20% moisture and produces good surface finish.

(iii). Parting sand:

• It is used for separating the moulds from adhering to each other by spreading a fine

sharp dry sand called parting sand.

• It also can be used to keep green sand from sticking to the pattern. It is the clean clay

free silica sand.

(iv). Baking sand:

• It consists of refractory material and it is made of used sand or burnt sand.

(v). Facing sand:

• It is the Carboneous material sprinkled on the inner surfaces of the moulding cavity for

obtaining better surface finish.

(vi). CO2 sand:

• In this sand in-place of clay if sodium silicate is used, called as CO 2 sand. When the CO2

is supplied into this sand, CO2 will chemically react with sodium silicate and forms silica

gel, which on drying will produces high strength moulds without ramming.

(vii). Dry sand: Silica sand + clay +sodium silicate

Fig.16: effect of moisture content on properties of moulding sand

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6. PROPERTIES OF MOULDING SAND

(i). Permeability:
• The moulding sand should be sufficiently porous so that the gases which have been absorbed
by the metal in the furnace, air absorbed from the atmosphere, steam and other gases that
are generated by the moulding and core sands is allowed to escape from the mould. This
capability of the moulding sand is termed as permeability.
• Thus, “It is the ability of moulding sand to allow the air to escape”. If these gases are not
allowed to escape from the mould, they would be trapped inside the casting causing defects.
• It depends on size and shape of grains, moisture content and degree of compaction.
• Mathematically it is expressed as “the gas flow rate through the specimen under a specified
pressure difference across it”.
Permeability test is used for determining the Porosity property of moulding sand is denoted by
permeability number. The permeability number is given by:
VH
Pn =
PAT
Where,
V= volume of air in cm3,
H = height of the sand specimen in cm
P = air pressure, gm/cm2
A = Cross section area of sand specimen in cm 2
T = time in minutes,
• As pe the American foundry society (AFS) standard or ASTM standard, the standard test
conditions are
D = H = 5.08 cm = 2inch
V = 2000 cc
P = gauge pressure=10 (g/cm2)
T= time in minutes
VH 2000  5.08
Pn = =
PAT 
10   5.082  T
4
50.127
Pn =
T
Where, T is in minute
Higher the permeability number, higher the porosity property of a moulding sand.
Factors affecting the porosity property of a moulding sand are:
(i). Sand particle size.
(ii). Percentage of clay
(iii). Ramming force.
(iv). Adding saw dust and wood powder
(v). Providing venting or vent holes

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(ii). Strength:
• Measurement of strength of moulding sands can be carried out on the universal sand strength
testing machine.
• The strength can be measured in compression, shear, and tension.
(iii). Green Strength:
• The moulding sand that contains moisture is termed as green sand.
• The green sand should have enough strength so that the constructed mould retains its shape.
(iv). Dry Strength:
• When the moisture in the moulding sand is completely removed, it is called dry sand.
• When molten metal is poured into a mould, moisture in the sand immediately evaporates due
to the heat in the molten metal and the sand around the mould cavity is quickly converted into
dry sand. At this stage, it should retain the mould cavity and at the same time withstand the
metallostatic forces.
• The tests similar to the above can also be carried with the standard specimens dried between
105° to 110°C for 2 hours.
(v). Hot Strength:
• After all the moisture is eliminated, the sand would reach a high temperature when the metal
in the mould is still in the liquid state.
• The strength of the sand that is required to hold the shape of the mould cavity is called hot
strength.
(vi). Green Compression Strength:
• Green compression strength or simply green strength generally refers to the stress required
to rupture the sand specimen under compressive loading.
• The green strength of sands is generally in the range of 30 to 160 kPa.
(vii). Green Shear Strength:
• With a sand sample similar to the green compressive strength test, shearing of the sand
sample is done.
• The stress required to shear the specimen along the axis is then represented as the green
shear strength.
• The green shear strengths may vary from 10 to 50 kPa.
(viii). Refractoriness:
• The ability of withstanding higher temperature of the molten metal so that it does not cause
fusion i.e. without losing its strength and hardness is called refractoriness.
(ix). Mould Hardness
• When molten metal is filled in the mould cavity, it comes into contact with the moulding sand
die to friction will try to take away the sand with it.
• The ability of moulding sand to resist this phenomenon is known as hardness.
• Hardness is opposite to permeability. Higher the permeability number lower will be the
hardness and vice versa.

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• Hardness number of moulding sands varies between 0 to 100. When no penetration occurs,
then it is a mould hardness of 100 and when it sinks completely, the reading is zero indicating
a very soft mould. For a good moulding sand, hardness must be lie in between 60 -80.
(x). Cohesiveness:
• The ability to form bond between same material particles is called as cohesiveness.
(xi). Adhesiveness:
• The ability of bond formation of sand particles with other materials is called adhesiveness.
(xii). Collapsibility:
• Collapsibility is the property of material due to which, it does not provide any resistance during
the contraction of the solidified casting.
• Low collapsibility result in cracks in the casting due to resistance provided by the moulding
sand.
(xiii). Flowability:
• The ability of flowing of moulding sand into each and every corner of the mould is called
flowability.

7. METHODS OF MOULD MAKING

(i). Hand moulding: If force required for Ramming or compressing the moulding sand is
obtained by human hand, it is called as hand moulding
Advantages:
• It is a cheaper methodology
• Complex shape of casting can be produced easily without any damages to the pattern.
2. Machine moulding
If machine is used for producing required ramming or compressive force, it is called machine
moulding.
(i). Jolting:
• In jolting operation, sand filled mould is raised to certain amount of height and it is allowed
to fall freely on to the ground so that the reaction load produced by the ground will be used for
ramming.

Fig.17: Jolting
• With jolting the bottom part of mould will get higher strength and hardness but the top of the
mould will get lower strength. Hence only jolting may not be used.

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(ii). Squeezing:
• Here the force is applied using mechanical or hydraulic press on to the squeeze plate. The
plate is squeezing to the moulding sand for ramming or compressing of moulding sand.

Fig.18: squeezing machine

• Because force may not be transmitted up to the bottom of the mould, there is variation in
strength of mould.
(iii). Jolting & squeezing:
• By combining jolting and squeezing operation it is possible to get higher strength and
hardness at top and bottom of mould. In this, Jolting is done 1st and squeezing is done later.
• If the height of mould is higher, the top and bottom of mould will get higher strength and
middle of mould will get lower strength.
(iv). Sand slinging:
• Here sand is thrown into flask rapidly with great force by using some machine and it develops
uniformly high mould hardness and uniform ramming. The only disadvantage of this its higher
initial cost.

Fig.19: Sand slinging

8. CORE MAKING

core is used for making cavities and hollow projections which cannot normally be produced by
the pattern alone.
8.1. Types of cores:
Based on the type of sand used, the cores are essentially of two types.
(i). Green sand cores:
• Green sand cores are those which are obtained by the pattern itself during moulding.
Though this is the most economical way of preparing core, the green sand being low in
strength cannot be used for fairly deep holes.
• Also, a large amount of draft is to be provided (due to presence of moisture) so that
the pattern can be withdrawn.

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• This is used only for those types of cavities which permit the withdrawal of the pattern
as in Fig.20.

Fig.20: Green sand core

(ii). Dry sand cores:
• Dry sand cores are those which are made by means of special core sands in a separate
core box, baked and then placed in the mould before pouring. Some of the typos of core
used in various situations are presented in Fig.21.

Fig.21: Dry sand cores

8.2. Properties which must be possessed by core material
• It should be non-metal
• It should be free from moisture
• Core material should have high strength.
• It should have high collapsibility
• To satisfy the above four properties of core material, the commonly used material for
manufacturing of core is dry sand with CO, bonding
• Whatever high strength of core is obtained due to formation of silica gel and drying
will not be lasting for longer period.
• Cores are normally made by CO2 moulding
8.3. Core print:
• Recess provided in the mould for locating, positioning and supporting of cores is called
core print.
• The core prints are provided so that the cores are securely and correctly positioned in
the mould cavity.

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• The design of core prints is such as to take care of the weight of the core before pouring
and the upward metallostatic pressure of the molten metal after pouring.
• The core prints should also ensure that the core is not shifted during the entry of the
metal into the mould cavity.
• The main force acting on the Core, when metal is poured into the mould cavity, is due
to buoyancy.
Let ρm = density of molten metal
ρ = density of core material
V = volume of core

Fig.22: buoyancy force on core

Net buoyancy force acting on the core = Weight of liquid displaced due to projected
portion —total weight of core

F = ρmVg − ρVg

F = Vg(ρm − ρ)
The above equation would be valid for horizontal cylinder.
But for vertical cores as those shown in Fig.23.

Fig.23: Vertical core

 2
F= (D1 − D2 )Hm − V
4
Where, V= total volume of the core in the mould.

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Note.:
• In order to keep the core in position, it is empirically suggested that core print will be
able to support a load of 3.5 N/cm2 of surface area.
P ≤ 3.50Ac
Where, Ac = core print area, cm2

9. CHAPLET

• Chaplets are metallic supports often kept inside the mould cavity to support the cores. These
are of the same composition as that of the pouring metal so that the molten metal would
provide enough heat to completely melt them and thus fuse with it during solidification.
• Chaplets is used to support the core when the force applied by the molten metal is very
high.
Thus, load supported by chaplets = Excess load = P – 3.5Ac
• In chaplets, there is a possibility of moisture content which ends up as blow holes. Thus,
before placing it inside the core proper cleaning is mandatory.
• Chaplets also provides the directional solidification as an additional functionality.

10. CHILLS

• Chills are metallic objects which am placed in the mould to increase the cooling rate of
castings to provide uniform or desired cooling rate.
• Metallic chills are used in order to provide progressive solidification or to avoid the shrinkage
cavities.
• Chills are essentially large heat sinks. Whenever it is not possible to provide a riser for a
part of the casting which is heavy, a chill is placed close to that part as shown in Fig.24, so
that more heat is quickly absorbed by the chill from the larger mass, making the cooling
rate equal to that of the thin sections. Thus, this does not permit the formation of a shrinkage
cavity.

Fig.24: Chill
• By providing chill, the heat transfer can be directed in a such a way that the solidification of
molten metal will take place according to the required methodology called as directional
solidification.

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11. PADS OR PADDING

• At the corner, due to improper ramming, there is always chances of erosion of sand When
the molten metal is filled in mould cavity. To avoid this, some objects are provided at the
corner to support the mould which is known as pad & this process is known as padding.
• They can be made up metal or non-Metal, if it is metal, it should have high melting point
and if is non-metal then it should be made by ceramics.

Fig.25: Padding

12. FLUIDITY OF MOLTEN METAL

• The molten metal flow characteristics are often described by the term fluidity, a measure of
the capability of a metal to flow into and fill the mould before freezing.
• Fluidity is the inverse of viscosity. As viscosity increases, fluidity decreases.
Spiral Fluidity Test:
• It is a test to measure the fluidity of the molten metal. A spiral tube is used in which fluidity
is indicated by the length of the solidified metal in the spiral channel.
• A longer cast spiral means greater fluidity of the molten metal.
Factors affecting fluidity:
(i). Pouring Temperature:
• A higher pouring temperature relative to the freezing point of the metal increases the time
it remains in the liquid state, allowing it to flow further before freezing.
• Thus, higher pouring temperature increase the fluidity of the molten metal. Higher pouring
temperature accelerate certain casting problems such as oxide formation, gas porosity, thus
pouring temperature does not be more than a certain degree.

Fig.26: Spiral mould test for fluidity

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Properties increases  Effect on Fluidity

Pouring temperature 
Surface finish of the mould 
Viscosity 
Density 
Surface tension 
Moisture content 

13. GATING SYSTEM

In the casting process, to avoid the oxidation, the molten metal is always poured in to casting
cavity using a system called as gating system. It consists of four basic elements
1. Pouring basin
2. Sprue
3. Runner
4. Ingate
• A good gating design ensures distribution of the metal in the mould cavity at a proper rate
without excessive temperature loss, turbulence, and entrapping gases and slags.
• If the liquid metal is poured very slowly, Then the time taken to fill up the mould is rather
long and the solidification even before the mould has been completely tilled up. This can be
avoided by using too much superheat, but then gas solubility may cause a problem.
• On the other hand, if the liquid Metal impinges on the mould cavity with too high a velocity,
the mould surface may be eroded. Thus, a compromise has to be made in arriving at an
optimum velocity. This is done by a good gating design.

Fig.27: Complete gating system with drag and cope

Characteristic of idle gating system:
• The time taken for pouring should be minimum.
• Laminar flow of material is preferable.
• The impurities shall be separated.

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• Avoid the sand erosion.
• No aspiration effect should take place.
• No partial flow is allowed.

Fig.28: Gating design to prevent impurities.

The common items employed in a gating design to prevent impurities in the casting are as
follows.
(i). Pouring Basin:
• It is work as a reservoir for molten metal. Thus, the molten metal entering into cavity will
be having low velocity as compare to direct entry.
• This also reduces the eroding force of the liquid metal stream coming directly from the
furnace. A constant pouring head can also he maintained by using a pouring basin.
• A thin plate made by using same material as that of casting is placed at the end of pouring
basin so that molten metal is collected over plate and when it is melting it ensures the full
flow of molten metal.

Fig.29: Pouring Basin

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(ii). Sprue:
• It is a connecting passage between pouring basin and runner.
• It is always vertical with straight tapered circular cross-section. The height of sprue is mainly
responsible for producing required velocity of molten metal into the gating system.
• Straight tapered sprue is selected to avoid the air-aspiration effect in the gating system
(iii). Strainer:
• It is acting as filter for separating the impurities present in molten metal.
• It is made by using ceramic material with high porosity

(iv). Splash core:

• A ceramic splash core placed at the end of the sprue ensure that high density impurities get
settle down. It also reduces the eroding force of the liquid metal stream.
(v). Runner:
• It is a connecting passage between bottom of sprue and ingate.
• It is always horizontal with uniform trapezoidal cross-section.
• It is mainly used for minimizing the sand erosion in casting process.

(vi). Skim bob:

• It is a semi-circular cut in a horizontal gate to prevent heavier and lighter impurities from
entering the mould.
(vii). Ingate:
• It is the last point of gating system from where the molten metal enters into the casting
cavity.
• It is also horizontal and have uniform trapezoidal C.S.
• Ag = C.S area of ingate

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13.1. Gating Ratio:
• The gating ratio refer to the proportion of the cross-sectional areas between the sprue,
runner and in-gates, and is generally denoted as sprue area, runner area, and ingate
area.
Gating ratio = Ratio of (sprue area: runner area: ingate area)
Gating ratio = AS: AR: AG
Where:
AS = sprue area
AR = runner area
AG = ingate area
Casting Yield
• All the metal that is used while pouring is not finally ending up as it is casting. There
will be so many losses in the melting. Also, there is a possibility that some casting; may
be rejected because of the presence of various defects.
• Hence, the casting yield is the proportion of the actual casting mass, M, to the mass
of metal poured into the mould m, expressed as a percentage as follows.
M
Casting yield =  100
m
Since, mass = volume × density
V
So, Casting yield =  100
v
• Higher the casting yield, higher is the economics of the foundry practice. It is,
therefore, desirable to give consideration to maximising the casting yield, at the design
stage.
Choke Area:
• It is main control area which meters the metal flow into the mould cavity, so that the
mould is filled within the calculated pouring time.
• It is also the minimum area in whole gating system (sprue area, runner area, ingate
area).
• Normally, the choke area happens to be at the bottom of the sprue and hence, the
first element to be designed in the gating system is the sprue size and its proportions.
• The main advantage in having True bottom as the choke area is that proper flow
characteristics are established early in the mould.
The choke area can he calculated using Bernoulli's equation as:
m
A=
Cd t f 2gH
where A = choke area, in mm2
m = casting mass, kg
tf = pouring time, s

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ρ = mass density of the molten metal, kg/mm3
g = acceleration due to gravity, mm/s2
H = effective metal head (sprue height), mm
Cd = efficiency factor which is a function of the gating system used
13.2. Types of Gating system and Mould filling time
The classification of Gating system is given as:
1. Based on choke area metal in pouring basin
(a). Non pressurized gating system:
• A non-pressurised gating system has choke area (minimum area) at the bottom of the
sprue base and have total runner area and in-gate areas higher than the sprue area.
• In this system, there is no pressure existing in the metal-flow system and thus it helps
to reduce turbulence. Due to low turbulence oxides formation will not takes place thus
we can cast Non-Ferrous alloys such as aluminium and magnesium alloys.
• The gating ratio of a typical example are
Sprue: Runner: In-gate∷ 1:4:4, 1:2:2, 1:2:4, 0.5:1.5:1
The disadvantages of Non-pressurised gating system:
(i). The system needs to be carefully designed to see that all parts flow full otherwise
the air aspiration will take place.
(ii). Casting yield gets reduced because of the large metal involved in the runners and
gates.
volume of cavity
casting yield =
total Volume of molten metal enteringint o cavity

volume of cavity
casting yield =
volume of cavity + volume of gating element

(b). Pressurized gating system:

• In case of a pressurised gating system, normally the in-gate area is the smallest, thus
maintaining a back pressure throughout the gating system.
• Because of this back pressure in the gating system generally flows fully, thus overall
pressure in the gating system is higher than the atmospheric pressure, thus there is no
chance of Aspiration effect.
• The back pressure also increases the turbulent thus metal oxides formation will take
place, Thus, not used for light alloys but can be advantageously used for Ferrous
castings.
• These systems generally provide a higher casting yield since the volume of metal used
up in the runners and gates is reduced.
Gating ratio of a typical pressurised gating system is
sprue: Runner: In gate :: 1:2:1, 4:2:1, 2:2:1, 2:1:0.5

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2. Based on the position of ingate
(a). Top gating system:
In top gating, the liquid metal is poured directly from the sprue to mould cavity with
atmospheric pressure at the base.

Fig.30: Top Gating System

Pouring time calculation:
Applying the Bernoulli’s equation between 1 & 3

P1 V2 P V2
+ 1 + z1 = 3 + 3 + z3
ρg 2g ρg 2g
P1 = P3 = Patm
V1 ≪≪< V3
Z1 = 0, Z3 = -ht

Patm 02 P V2
+ + 0 = atm + 3 + −ht
ρg 2g ρg 2g

V32
= ht
2g

V3 = 2gh t …………… (1)

Pouring or filling time

Total volume of cavity = area of actual entry × Velocity
Vmould
tf =
A g  vg

V
tf =
A g  vg

Vmould = Amould  hmould

Am  hm
tf = ……………… (2)
Ag  2ght

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Features of top gating system:
• Pouring time is lowest with top gating system.
• Manufacturing of the top gating system is easier.
• Cannot be used for filling of cavities with loose sand moulds.
• The top gating system is used up to 200 mm high castings to avoid the sand erosion.
Aspiration effect
• For a mould made of a permeable material, care should be taken to ensure that the
pressure anywhere in the liquid metal stream does not fall below the atmospheric
pressure.
• Otherwise, air from the atmosphere may enter into the molten metal stream,
producing porous castings. This is known as the aspiration effect.
For top gating:

V3 = 2ght

Applying the Bernoulli’s equation between 2 & 3:

P2 V2 P V2
+ 2 + z 2 = 3 + 3 + z3
ρg 2g ρg 2g

Since cross section is same in between 2 & 3:

Thus, V2 = V3
P1 = Patm, Z3 =-ht, Z2 = -hc

P2 V2 P V2
+ 2 + −hc = atm + 3 + −ht
ρg 2g ρg 2g

P2 P
= atm + hc − ht
ρg ρg

Since, ht > hc
Thus, P2 will be less that Patm which is not possible. So, to avoid negative pressure at
point 2 (to ensure positive anywhere in the liquid column).
In limiting case, P2 = patm

Patm V22 P V2
+ + −hc = atm + 3 + −ht
ρg 2g ρg 2g

V22 V2
+ −hc = 3 + −ht
2g 2g

V3 = 2ght

V22 2ght
+ −hc = − ht
2g 2g

V22
= hc
2g

V2 = 2ghc

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V3 2ght V3 ht
=  =
V2 2ghc V2 hc

From the continuity equation

A2V2 = A3V3

A2 V A ht
= 3  2 =
A3 V2 A3 hc

1
A
h
Thus, ideally, the sprue profile parabolic as shown by the solid lines in the fig.31 but is
difficult to design thus, a straight tapered sprue (shown by the dashed lines) is
preferred.

Fig.31: Ideal and actual shape of the sprue

(b). Bottom gating system:
• In bottom gating, on the other hand, the. liquid metal is filled in mould from bottom
to top, thus avoiding the splashing and oxidation

Fig.32: Bottom Gating System

• As, Velocity of the liquid metal in the cavity is changing, there will be no Turbulence
and splashing.
• It can be used for casting of Non-Ferrous materials.

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Expression for time required for filling by Bottom Gating system is given by:
H= h1 +h2 = total height above ingate
tf = time of filling for bottom gating

Fig.33: Bottom gating system

Applying Bernoulli’s equation between (1) and (3):

P1 V2 P V2
+ 1 + z1 = 3 + 3 + z3
ρg 2g ρg 2g

P1 = Patm = 0 (Gauge pressure)

and V1 ≪<V3
Z1 = 0, Z3 = -ht

P3 V32
0+0+0 = + + −ht
g 2g

P3 V32
ht = +
g 2g

Now, P3 = P3

So P3 = ρgh
P3
=h
ρg

V32
So, ht = h +
2g

V3 = Vg = 2g(ht − h)

V3 is the velocity of a jet discharging against a static head, making the effective head as
(ht – h).
Now, for the instant shown, let the metal level in the mould move up through a height
‘dh’ in a time interval dt.
Am and Ag are the cross-sectional areas of the mould and the gate, respectively. Then,
Amdh = Agvg dt
V3 = Vg = 2g(ht − h)

1 dh Ag
= dt
2g ht − h Am

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At t = 0, h = 0 and at t = tf (filling time), h = hm
hm 1 dh tf Ag
0
2g ht − h
= 0 Am
dt

hm
 
1  ht − h  Ag
  = (t f − 0)
2g  − 1 + 1  Am
 2  0

Ag
2
2g
( ht − hm − ht = ) Am
(t f − 0)

Am 1 
tf = 2 h t − ht − hm 
Ag 2g  

Special case:
If height of the mould(hm) is equal to total height (ht).
Am 1 
tf = 2 h t − ht − hm 
Ag 2g  

h t = hm
Am 1 
tf = 2 h t − ht − ht 
Ag 2g  

Am ht Am hm
tf = 2 =2
Ag 2g Ag 2g

Am hm hm Am hm
tf = 2  =2
Ag 2g hm Ag 2ghm

Am hm
tf = 2
Ag 2ght

Vm
tf = 2
A g 2ght

( tf )Bottom = 2  ( t f )Top

• The limitations of bottom gating system are

(i). Pouring time is greater than top gating system.
(ii). Cutting of bottom gating system is difficult.
(c). Parting Gating System:
• Gate is provided along the parting line such that liquid metal can be enter into the
cavity, below the parting line by assuming top gate & above the parting line cavity can
be filled by assuming bottom gate.
• It is most commonly used type of gate. Such that liquid metal can be filled into the
cavity within a given time without causing turbulence & splashing of the liquid metal.

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Fig.34: Parting Gating System

• Pouring time = T1 + T2

Pouring time with parting gate is greater than the top gating system and less than

bottom gating system.

(d). Step gating system

• To fill the molten liquid metal into very large size mould cavities, number of gates are

provided vertically in the bottom of step. Such that liquid metal can be enter into cavity

gradually within a given time without causing turbulence & splashing of liquid metal.

Fig.35: Step Gate

• Flow rate in step gating system = n x flow rate with given gating system, hence the

time taken will be reduced to "1/n" times. (n= number of gates)

Example: A mould having dimensions 100mm x 90mm x 20mm is filled with molten

metal through a gate with height 'h' and C.S area A, the mould filling time is t 1. The

height is now quadrupled and the cross-sectional area is halved. The corresponding

filling time is t2. The ratio t2/t1 is _______.

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Sol:
Vol
filling time, t1 = ,
A  2gh
Vol
t2 =
A
 2  g  4h
2
 Vol 
A 

 t2   2  2g4h 
 = 
 t1   Vol 
 
 A  2gh 
 
 h   1 
  =   =1
 0.5 4h   0.5 4 
Example. Two gating designs for a mould of 50 cm x 25 cm x 15 cm are shown in Fig.
2.7. The cross-sectional area of the gate is 5 cm2. Determine: the filling time for both
the designs.

Solution.
Fig. (a) Since ht = 15 cm

Thus, V3 = 2  981  15 cm/sec = 171.6 cm/sec.

The volume of the mould is:

V = 50 × 25 × 15 cm3
The cross-sectional area of the gate is: Ag = 5 cm2
Thus, mould filling time:
50  25  15
tf = sec = 21.86 sec
5  171.86
Fig. (b): Here, (h)= 15cm
Mould height (hm) = 15cm
Mould area (Am) = 50 x 25 cm2, and
Gate area (Ag) = 5cm2

50  25 2
tf = 15 sec = 43.71sec
5 981
It should be noted that in Fig. (b), the time taken is double of that in fig (a). We can
easily verify that this will always be so if hm = h1.

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Example. Schematic diagram of pouring basin anu sprue of a gating system is shown
in the Figure. Depth of molten metal in the pouring basin is 100 mm and the height of
the sprue is 1,500 mm.

Considering the cross-section of the sprue is circular, the ratio d 1:d2 to avoid aspiration
is
Solution.
Given that ht = 1500 + 100 = 1600 mm
h2 = 1500 mm
To avoid aspiration effect:
A1 / A2 = [(ht – h2)/ ht ]0.5
A1 / A2 = [(1600 – 1500) / 1600]0.5
0.5
d12  1 
=
d22  16 

d12  1 
=
d22  4 
d1 1
=
d2 2
Example. A rectangular block of steel C.S 100 mm x 150 mm and 250 mm height is to
be cast without any riser. The block is moulded entirely in the drag of a green sand flask
& is top gated. The cope of the flask is 200 mm height & the height of the metal during
pouring is 100 mm above the cope level. A tapered sprue is employed & the gating ratio
is 1 :4:2. assume no energy losses in the system, The time taken (in seconds) to fill the
casting cavity, if the tapered sprues with 500 mm2 exit area is used.
Solution.
Volume of casting (Vc) = 100 × 150 × 250 = 3750000 mm3
Cavity is kept in drag and is top gated
Height of cope = 200 mm

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Height of molten metal in pouring basin = 100mm
Gating ratio = 1:4:2
Sprue cross-section area (As) = 500 mm2
Min. CS area = Ac = 500 mm2
ht = 200 + 100 = 300 mm

Thus, Vc = 2ght = 2  9810  300 = 2426 mm2 /s

Vol 3750000
Pouring time = = = 3.09 sec
AC  V 500  2426

14. COOLING AND SOLIDIFICATION TIME

• After pouring into the mould, the molten metal cools mid solidifies. Issues associated with
solidification include the time for a metal to freeze, shrinkage, directional solidification, and
riser design.
14.1. Solidification of metals
• Solidification involves the transformation of the molten metal back into the solid state.
The solidification process differs depending on whether the metal is a pure element or
an alloy.
Pure Metals
• A pure metal solidifies at a constant temperature equal to its freezing point, which is
the same as its melting point.
• The process occurs over time as shown in the plot of Fig.36, called a cooling curve.
• The actual freezing takes time, called the local solidification time in casting, during
which the metal's latent heat of fusion is released into the surrounding mould. The total
solidification time is the time taken between pouring and complete solidification.

Fig.36: Cooling curve for a pure metal during casting.

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• Because of the chilling action of the mould wall, a thin skin of solid metal is initially
formed at the interface immediately after pouring and thickness of the skin increases to
form a shell around the molten metal as solidification progresses inward toward the
center of the cavity.
• The rate at which freezing proceeds depends on heat transfer into the mould, as well
as the thermal properties of the metal.
• As the solidification starts on the mould wall and is rapidly cooled by the extraction of
heat through the mould wall. Thus, this rapid cooling action causes the grains in the
skin to be fine and randomly oriented.
• As cooling continues further grain formation and growth occur in a direction away from
the heat transfer. Now, the grains grow inwardly as needles or spines of solid metal. As
these spines enlarge, lateral branches form, and as these branches grow, further
branches form at right angles to the first branches. This type of grain growth is
referred to as dendritic growth, and it occurs not only in the freezing of pure
metals but alloys as well.
• These treelike structures are gradually filled-in during freezing, as additional metal is
continually deposited onto the dendrites until complete solidification has occurred. The
grains resulting from this dendritic growth take on a preferred orientation, tending to
be coarse, columnar grains aligned toward the center of the casting. The resulting grain
formation is illustrated in Fig.37.

Fig.37: Characteristic grain structure in a casting of a pure metal

Alloys:
• Most alloys freeze over a temperature range rather than at a single temperature. The
exact range depends on the alloy system and the particular composition.
• Solidification of an alloy can be explained with reference to Fig.38, which shows the
phase diagram for a particular alloy system and the cooling curve for a given
composition. As temperature drops, freezing begins at the temperature indicated by the
liquidus and is completed when the solidus is reached.

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• The start of freezing is like that of the pure metal. A thin skin is formed at the mould

wall due to the large temperature gradient at this surface. Freezing then progresses as

before through the formation of dendrites that grow away from the walls. However,

owing to the temperature spread between the liquidus and solidus, the nature of the

dendritic growth is such that an advancing zone is formed in which both liquid and solid

metal coexist.

Fig.38: (a) Phase diagram for a copper—nickel alloy system and (b)

associated cooling curve for a 50%Ni--50%Cu composition during casting.

• The solid portions are the dendrite structures that have formed sufficiently to trap

small islands of liquid metal in the matrix. This solid-liquid region has a soft consistency

that has motivated its name as the mushy zone. Depending on the conditions of

freezing, the mushy zone can be relatively narrow, or it can exist throughout most of

the casting.

• Another factor complicating solidification of alloys is that the composition of the

dendrites as they start to form favors the metal with the higher melting point.

• As freezing continues and the dendrites grow, there develops an imbalance in

composition between the metal that has solidified and the remaining molten

metal. This composition imbalance is finally manifested in the completed casting in the

form of segregation of the elements.

The segregation is of two types, microscopic and macroscopic.

At the microscopic level: The chemical composition varies throughout each individual

grain.

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Fig.39: Characteristic grain structure in an alloy casting, showing segregation

of alloying components in the center of casting

At the macroscopic level: The chemical composition varies throughout the entire
casting. Since the regions of the casting that freeze first (at the outside near the mould
walls) are richer in one component than the other, the remaining molten alloy is
deprived of that component by the time freezing occurs at the interior. Thus, there is a
general segregation through the cross section of the casting, sometimes called Ingot
segregation.
14.2. Solidification time:
• “Solidification time is the time required for the casting to solidify after pouring”. This
time is dependent on the size and shape of the casting by an empirical relationship
known as Chvorinov's rule, which states:
n
 V 
tS = k  
 SA 

Where,
ts = total solidification time
k = mould constant (or) solidification factor
V = volume of the casting,
SA = surface area of the casting,
n is an exponent usually taken to have a value = 2
2
 V 
tS = k  
 SA 

V Volume of casting
Modulus = =
SA Surface area

tS = k (M)
2

2
 V 
TS   
 SA 

• Chvorinov's rule indicates that a casting with a higher volume-to-surface area ratio will
cool and solidify more slowly than one with a lower ratio.

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15. RISER DESIGN

• Most foundry alloys shrink during solidification. As a result of this volumetric shrinkage
during solidification, voids are likely to form in the castings, as shown in Fig.40, unless
additional molten metal is fed.
• Let us consider the mould of a cube which is completely filled with liquid metal (a). As time
progresses, the metal starts losing heat through all sides and as a result starts freezing from
all sides equally trapping the liquid metal inside, (b). But further solidification and
subsequent volumetric shrinkage and the metal contraction due to change in temperature
causes formation of a void (c). The solidification when complete, finally results in the
shrinkage cavity (d).

Fig.40: Solidification of cube casting

• “Hence, a reservoir of molten metal is to be maintained from which the metal can flow into
the casting when the need shrinkage is taking place. These reservoirs are called risers”.
• To perform its function of feeding molten metal to the main cavity, the metal in the riser
must remain in the liquid phase longer than the casting. In other words, the solidification
time for the riser must exceed the solidification time for the main casting. Since the mould
conditions for both riser and casting are the same, their mould constants will be equal.
• By designing the riser to have a larger volume-to-area ratio, the main casting will most likely
solidify first, and the effects of shrinkage will be minimized.
Thus,
(i). The metal in the riser should solidify in the end.
(ii). The riser volume should be sufficient for compensating the shrinkage in the casting.
Condition to Design the Riser

1. VR  3VSc … [Necessary condition] i.e. Volume of riser should be at least 3 times the shrinkage

volume of castings.

2. TSRiser  TS [Sufficient condition] The solidification time of molten metal in the riser must
cavity

be at least equal to the solidification time of molten metal in the casting cavity.

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15.1. Types of Riser
(1). Side Riser

Fig.41: Side Riser

Total surface area which is exposed to heat transfer

 2
SA = 2  d  + dh
4 
• Side riser is used if depth of cavity is more.
(2). Top riser:
• Top riser is more effective when compared to side riser because it is having less surface
area. Side risers are preferred if depth of the cavity is maximum.

Fig.42: Top Riser

Heat transfer surface area
 2
SA= d + dh
4
• The top riser is the most conventional and convenient to make but the position where
it can be placed is limited. The top being open losses heat to the atmosphere by radiation
and convection. To reduce this, often insulation is provided on the top such as plaster
of Paris, asbestos sheet, etc.
15.2. Location of riser:
During casting process, the location of riser takes place on two conditions.
(a). During casting of a uniform cross section casting, the riser is located at topmost
point of casting cavity
(b). During casting of non-uniform cross section castings, the riser is located near to
the thickest portion than the thinnest portion.
• One central riser is satisfactory if the maximum feeding distance is less than 4.5 times
the plate thickness. The feeding distance should be measured from the edge of the riser,
as explained in Fig.43 (a). It should be noted that, of the total distance 4.5t the riser
gradient prevails up to a distance 2t, whereas the end wall gradient prevails in the

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remaining distance 2.5t. Thus, the maximum distance between the edges of two
consecutive risers is 4t and not 9t [Fig.43 (b)].

Fig.43: Placement of risers.

15.3. Shape of riser:
2
 V 
ts = K  
 SA 
• The best preferable shape of the riser is cylindrical shape of riser and next preferable
is spherical. But never prefer to use cubical or rectangular shape of riser.
Cylindrical Riser:
Optimum condition to get minimum surface area or maximum solidification time in case
of cylindrical riser.
Side Riser:
 2
Surface area: SA = 2  d + dh
4
 2 4V
Volume: V = dhh=
4 d2
 2 4V
A= d + d  2
2 d
 2 4V
A= d +
2 d
For maximum solidification time, area should be minimum.
dA  4V 
Thus, = 2  d − 2 = 0  V = d3
d(d) 2 d 4

d3 d2h
=
4 4
h=d

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For optimum condition, diameter of riser must be equal to the height of the cylinder.
 2
2 d + d  d
SA 4
=
V  2
d d
4

3d2
SA
= 2
V  3
d
4
SA 6 V d
=  =
V d SA 6
Top Riser:
 2
Surface Area: SA = d + dh
4
 2 4V
V= dhh=
4 d2

 2 4V
SA = d + d  2
4 d

 2 4V
A= d +
4 d

dA
=0
d(d)

 4V 
d − 2 = 0  V = d3
2 d 8

 3 
V= d = d2h
8 4

d
h=
2
Ratio of surface area to volume:
 2 d
d + d 
SA 4 2
=
V  2 d
d 
4 2
2 2
d
SA
= d
V  3
d
8
SA 6 V d
=  =
V d SA 6

A 6
Side Riser h=d V = d
 
A 6
Top Riser h =d/2 V = d
 
Where d & h are diameter and height of the riser respectively.

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Blind Riser
Blind risers are those risers which are completely concealed inside the mould cavity.
Since, the blind riser is completely surrounded by the moulding sand, would lose heat
slowly and thus would be more effective. Also, it can be located more conveniently than
an open riser.

16. MODULUS

• It is the ratio of volume to surface area of any casting. This formula is applicable for risers
also.

V
Modulus: M =
SA
1. For spherical riser
4
V = R 3 & SA = 4R 2
3
4 3
R
V R D
M= = 3 2 = =
SA 4R 3 6
2. For Top cylindrical riser
 2 
V= D h & SA = D 2 + Dh
4 4
 2
Dh
V 4 Dh
M= = =
SA  2 D + 4h
D + Dh
4
For top riser: D=2h
D
D
2 D
M= =
D 6
D+ 4
2
3. For side cylindrical riser
 2 
V= D h & SA = 2  D 2 + Dh
4 4
 2
Dh
V 4 Dh
M= = =
SA  2D + 4h
2  D2 + Dh
4
For side riser, D = h
DD D
M= =
2D + 4  D 6
D
M= ( D=h)
6

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17. METHODS OF RISER DESIGN

There are four methods of the riser design:

1. Caine’s Method:
• This method can be used to calculate the dimensions of the Riser for the simple shape of the
castings.
• Using the graph already designed dimensions of the riser can be cross checked.

Fig.44: Modulus vs. freezing ratio

 SA 
 V 
 C
Freezing ratio (FR) =
 SA 
 V 
 R

FR  1
a
X= +C
Y −b

Where,
X = FR
VR
Y= , & a, b, c are constants.
VC

Example - A cast steel slab of dimension 30 × 20 × 5 cm is poured horizontally using a side

riser. The riser is cylindrical in shape with diameter and height, both equal to D. The freezing
ratio of the mould is __________.
Solution.
Casting size = 30 × 20 5 5 cm
Riser: D = H
 V  30  20  5 3000
  = = = 1.76
 A S C 2(30  20 + 20  5  30) 1700

 V  D
  =
 S R 6
A

(V / A S )R  D/6 
Freezing ratio = =
( V / AS )C  30 / 17 

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 D  17  8D
Freezing ratio (FR) =   = 0.095D  75
 30  6 
2. Modulus Method:
2
 V 
As we know that: tS   
 SA 

Solidification time of riser should be greater than the solidification time cavity.

tR  tC
V
M=
SA

 M R2  M 2C

 MR  MC

According to standard condition:

Modulus of riser is taken as 20% higher than modulus of casting.
M R = 1.2 M C
D
 = 1.2MC
6
 D = 7.2MC

3. Shrinkage Volume consideration Method:

If percentage of shrinkage of metal is given.
Volume of riser = 3 x shrinkage volume and calculate the D & h of riser. Then check again so
that
(t s )riser  ( t s )casting

• If the above condition satisfies, the dimensions of riser are final.

• If the above equation is not satisfied, then Assume that
(t s )riser = ( t s )casting and determine the size of riser.

4. Shape factor method:

• It is also known as Modified caine’s method (Naval Research laboratory method).
• In case of castings, shape factor is defined as the ratio of sum of the largest length and largest
breath to the largest thickness of the casting.
L+W
shape factor ,(S.F.) =
t
Shape factor for various geometries
1. For Plate (L × w × t):
L+w
SF =
t
Where:
L = length of the plate
t = thickness of plate
w = width of the plate

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2. For Cube (a × a × a):

a+a
SF = =2
a

3. For Sphere (of Diameter D):

D+D
SF = =2
D

4. For Solid cylinder (Diameter D and Height H)

D+H
SF =
D

5. for thin cylinder


L+ (D + Di )
2 o
SF =
(Do − Di )
2

Where:

Do = outer diameter of the cylinder

Di = inner diameter of the cylinder

Methods to increase the performance of riser:

(1). By providing insulating material on the top surface and around the circumference of riser

heat transfer losses can be minimized.

(2). By providing exothermic material on the top surface of riser due to exothermic reaction

heat will be produced and this will supply to the solid metal to increase the solidification time.

{Thermal maximum, Graphite}

(3). Use optimum conditions in riser design.

(4). Use blind Risers.

Example: The shape factor for a casting in the form of an annular cylinder of outside diameter

30cm, inside diameter 20cm and height 30cm will be _________.

Sol:

L+W
In casting, shape factor = = S.F
t

L = height = 30 cm

30 + 20
W = D = 
2

=   25 = 78.5

30 − 20
t= =5
2

30 + 78.5
S.F = = 21.7
5

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18. SPECIAL CASTING PROCESSES

18.1. Expendable mould casting processes:

As versatile as sand costing is, there are other casting processes that have been
developed to meet special needs. The differences between these methods are in the
composition of the mould material, or the manner in which the mould is made, or in the
way the pattern is made.
18.1.1. Shell moulding
• Shell moulding is a casting process in which the mould is a thin shell (typically 9 mm)
made of sand held together by a thermosetting resin binder.
• The thickness of the shell can be determined accurately by controlling the time that
the pattern is in contact with the mould.
• The process is described and illustrated in Fig.45.
Steps:
(1). A match-plate or cop and drag metal pattern is heated and placed over a box
containing sand mixed with thermosetting resin (usually 2.5 to 4% of phenol-
formaldehyde).

Fig.45: Steps in shell-moulding

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(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 to complete curing.

(5). Shell mould is stripped from the pattern.

(6). Two halves of the shell mould are assembled, supported by sand or metal shot in a

box, and pouring is accomplished. The finished casting with sprue removed is shown in (7).

Advantages:

• The surface of the shell-mould cavity is smoother than a conventional green-sand

mould, and this smoothness permits easier flow of molten metal during pouring and

better surface finish on the final casting. Finishes of 2.5 μm can be obtained.

• Good dimensional accuracy is also achieved, with tolerances of ± 0.25 mm possible on

small-to-medium-sized parts.

• The good finish and accuracy often are good and there is no need of further machining.

• Collapsibility of the mould is generally sufficient to avoid tearing and cracking of the

casting.

• Shell moulding can be mechanized for mass production and is very economical for

large quantities.

Disadvantages of shell moulding:

• It includes a more expensive metal pattern than the corresponding pattern for green-

sand moulding. This makes shell moulding difficult to justify for small quantities of parts.

• It seems particularly suited to steel castings of less than 20 lb.

Applications:

• Shell-moulding applications include small mechanical parts requiring high precision,

such as gear housings, cylinder heads, and connecting rods.

• The process also is used widely in producing high-precision moulding cores.

18.1.2: Vacuum Moulding:

• Vacuum moulding, also called the V-process, was developed in Japan around 1970.

• It uses a sand mould held together by vacuum pressure rather than by a chemical

binder. Accordingly, the term vacuum in this process refers to the making of the mould

rather than the casting operation itself.

• The steps of the process are explained in Fig.46.

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Fig.46: Steps in vacuum moulding

(1). A thin sheet of preheated plastic is drawn over a match-plate or cope and drag
pattern by vacuum—the pattern has small vent holes to facilitate vacuum forming.
(2). A specially designed flask is placed over the pattern plate and filled with sand, and
a sprue and pouring cup are formed in the sand.
(3). Another thin plastic sheet is placed over the flask, and a vacuum is drawn that
causes the sand grains to be held together, forming a rigid mould.
(4). The vacuum on the mould pattern is released to permit the pattern to be stripped
from the mould.
(5). This mould is assembled with its matching half to form the cope and drag, and with

vacuum maintained on both halves, pouring is accomplished. The plastic sheet quickly

burns away on contacting the molten metal. After solidification, nearly all the sand can

be recovered for reuse.

• Because no hinders are used, the sand is readily recovered in vacuum moulding.

• Also, the sand does not require extensive mechanical reconditioning normally done

when binders are used in the moulding sand. Since no water is mixed with the sand,

moisture-related defects are absent from the product.

• Disadvantages of the V-process are that it is relatively slow and not readily adaptable

to mechanization.

• It is suitable particularly for thin walled (0.75 mm) complex shapes with uniform

properties. Typical parts made are superalloy gas-turbine components with walls as thin

as 0.5 mm.

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18.1.3: Expanded Polystyrene process

• Evaporative-pattern and investment casting are sometimes referred to as expendable

pattern casting processes or expendable mould-expendable pattern processes. They are

unique in that a mould and a pattern must be produced for each casting, whereas the

patterns in the processes described in the preceding section are reusable.

• The expanded polystyrene casting process uses a mould of sand packed around a

polystyrene foam pattern that vaporizes when the molten metal is poured into the

mould.

• The process and variations of it are known by other names, including lost-foam

process, lost-pattern process, evaporative foam process, and full-mould process (the

last being a trade name).

• The foam pattern includes the sprue, risers, and gating system, and it may also contain

internal cores (if needed). Thus, eliminating the need for a separate core in the mould.

Also, since the foam pattern itself becomes the cavity in the mould, considerations of

draft and parting lines can be ignored.

• The mould does not have to he opened into cope and drag sections.

The sequence in this casting process:

The sequences of processes has been illustrated and described in Fig.47.

Fig.47: Expanded polystyrene casting process

(1). pattern of polystyrene is coated with refractory compound.
(2). Foam pattern is placed in mould box, and sand is compacted around the pattern.
(3). Molten metal is poured into the portion of the pattern that forms the pouring cup
and sprue. As the metal enters the mould, the polystyrene foam is vaporized ahead of
the advancing liquid, thus allowing the resulting mould cavity to be filled.

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• Various methods for making the pattern can he used, depending on the quantities of
castings to be produced. For one-of-a-kind castings, the foam is manually cut from large
strips and assembled to form the pattern.
• For large production runs, an automated moulding operation can be set up to mould
the patterns prior to making the moulds for casting.
• The pattern is normally coated with a refractory compound to provide a smoother
surface on the pattern and to improve its high temperature resistance.
• Moulding sands usually include bonding agents. However, dry sand is used in certain
processes in this group, which aids recovery and reuse.
Advantages:
• A significant advantage for this process is that the pattern need not be removed from
the mould. This simplifies and expedites mould making.
• In a conventional green sand mould, two halves are required with proper parting lines,
draft allowances must be provided in the mould design, cores must he inserted, and the
gating and riser system must be added. With the expanded polystyrene process, these
steps are built into the pattern itself.
Disadvantages:
• A new pattern is needed for every casting, so the economics of the expanded
polystyrene casting process depend largely on the cost of producing the patterns.
Applications:
• Typical applications are cylinder heads, engine blocks, crankshafts, brake components,
manifolds, and machine bases.
• The process has been applied to mass produce castings for automobiles engines.
• Automated production systems are installed to mould the polystyrene foam patterns
for these applications.
18.1.4. Investment casting
• In investment casting, a pattern made of wax is coated with a refractory material to
make the mould, after which the wax is melted away prior to pouring the molten metal.
• The term Investment comes from one of the less familiar definitions of the word Invest,
which is "to cover completely," this referring to the coating of the refractory material
around the wax pattern.
• It is a precision casting process, because it is capable of making castings of high
accuracy and intricate detail. It is also known as the lost-wax process because the wax
pattern is lost from the mould prior to casting.
• Since the wax pattern is melted off after the refractory mould is made, a separate
pattern must be made for every casting.
• Pattern production is usually accomplished by a moulding operation—pouring or
injecting the hot wax into a master die that has been designed with proper allowances
for shrinkage of both wax and subsequent metal casting.
• In cases where the part geometry is complicated, several separate wax pieces must
be joined to make the pattern.

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• In high-production operations, several patterns are attached to a sprue, also made of
wax, to form a pattern tree; this is the geometry that will be cast out of metal.
Steps in investment casting: The steps are described in Fig.48.
(1). Wax patterns are produced.
(2). Several patterns are attached to a sprue to form a pattern tree.
(3). The pattern tree is coated with a thin layer of refractory material.
(4). The full mould is formed by covering the coated tree. with sufficient refractory
material make it rigid.
(5). the mould is held in an inverted position and heated to melt the wax and permit it
to drip out of the cavity.

Fig.48: Steps in investment casting

(6). The mould is preheated to a high temperature, which ensures that all contaminants
are eliminated from the mould; it also permits the liquid metal to flow more easily into
the detailed cavity; the molten metal is poured; it solidifies; arid

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(7). The mould is broken away from the finished casting. Parts are separated from the
sprue.
• Coating with refractory (step 3) is usually accomplished by dipping the pattern tree
into a slurry of very fine-grained silica or other refractory (almost in powder form) mixed
with plaster to bond the mould into shape. The small grain size of the refractory material
provides a smooth surface and captures the intricate details of the wax pattern. The
final mould (step 4) is accomplished by repeatedly dipping the tree into the refractory
slurry or by gently packing the refractory around the tree in a container. The mould is
allowed to air dry for about 8 hours to harden the binder.
Advantages of investment casting:
(1). parts of great complexity and intricacy can be cast
(2). Close dimensional control—tolerances of ±0.075 mm, are possible.
(3). Good surface finish is possible.
(4). The wax can usually he recovered for reuse; and
(5). Additional machining is not normally required—this is a net shape process.
Disadvantage:
• Because many steps are involved in this casting operation, it is a relatively expensive
process.
• Investment castings are normally small in size, although parts with complex
geometries weighing up to 75 lb have been successfully cast.
Applications:
• All types of metals. including steels, stainless steels, and other high temperature
alloys, can be investment cast.
• Examples of parts include complex machinery parts, blades. and other components for

turbine engines, jewelry, and dental fixtures.

18.1.5. Plaster mould and ceramic mould casting
• Plaster-mould casting is similar to sand casting except that the mould is made of

plaster of Paris (gypsum —CaSO4.2H2O) instead of sand.

• Additives such as talc and silica flour arc mixed with the plaster to control contraction
and setting time, reduce cracking, and increase strength.
• To make the mould, the plaster mixture combined with water is poured over a plastic
or metal pattern in a flask and allowed to set.
• Wood patterns are generally unsatisfactory due to the extended contact with water in
the plaster. The fluid consistency permits the plaster mixture to readily flow around the
pattern, capturing its details and surface finish. Thus, the cast product in plaster
moulding is noted for these attributes.

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Disadvantages:
• Curing of the plaster mould is one of the disadvantages of this process, at least in high
production. The mould must set for about 20 minutes before the pattern is stripped.
The mould is then baked for several hours to remove moisture.
• Even with the baking, not all of the moisture content is removed from the plaster. The
dilemma faced by laundrymen is that mould strength is lost when the plaster becomes
too dehydrated and yet moisture content can cause casting defects in the product. A
balance must be achieved between these undesirable alternatives.
• Another dis-advantage with the plaster mould is that it is not permeable, thus limiting
escape of gases from the mould cavity. This problem can be solved in a number of ways:
(1). Evacuating air from the mould cavity before pouring.
(2). Aerating the plaster slurry prior to mould making so that the resulting hard plaster
contains finely dispersed voids; and
(3). Using a special mould composition and treatment known as the Antioch process.
This process involves using about 50% sand mixed with the plaster, heating the mould
in an autoclave (an oven that uses superheated steam under pressure), and then drying.
The resulting mould has considerably greater permeability than a conventional plaster
mould.
• Plaster moulds cannot withstand the same high temperatures as sand moulds. They
are therefore limited to the casting of lower-melting-point alloys such as aluminum,
magnesium. and some copper-base alloys.

Applications:
• It includes metal moulds for plastic and rubber moulding, pump and turbine impellers,
and other parts of relatively intricate geometry.
• Casting sizes range from about 20 g to more than 100 kg. Parts weighing less than
about 10 kg (20 lb) are most common.
• Advantages of plaster moulding for these applications are good surface finish and
dimensional accuracy and the capability to make thin cross sections in the casting.
18.1.6. Ceramic-mould casting:
• It is similar to plaster-mould casting, except that the mould is made of refractory
ceramic materials that can withstand higher temperatures than plaster.
• Thus, ceramic moulding can he used to cast steels, cast irons, and other high-
temperature alloys. Its applications (relatively intricate parts) are like those of plaster
mould casting except for the metals cast.
• Its advantages (good accuracy and finish) are also similar.

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18.2. Permanent-Mould Casting Processes
• The economic disadvantage of any of the expendable mould processes is that a new
mould is required for every casting. In permanent-mould casting, the mould is reused
many times.
• In this section. permanent-mould casting is treated as the basic process in the group
of casting processes that all use reusable metal moulds. Other members of the group
include die casting and centrifugal casting.
The basic permanent-mould process
• Permanent-mould casting uses a metal mould constructed of two sections that arc
designed for easy, precise opening and closing. These moulds are commonly made of
steel or cast iron.
• The cavity, with gating system included, is machined into the two halves to provide
accurate dimensions and good surface finish.
• Metals commonly cast in permanent moulds include aluminum, magnesium, copper-
base alloys, and cast iron. However, cast iron requires a high pouring temperature,
1250°C to 1500°C, which takes a heavy toll on mould life. The very high pouring
temperatures of steel make permanent moulds unsuitable for this metal unless the
mould is made of refractory material.
• Cores can be used in permanent moulds to form interior surfaces in the cast product.
The cores can be made of metal, but either their shape must allow for removal from the
casting or they must be mechanically collapsible to permit removal. If withdrawal of a
metal core would be difficult or impossible, sand cores can be used, in which case the
casting process is often referred to as semi-permanent-mould casting.
Steps in the basic permanent mould casting process:
The basic steps are described in Fig.49.

Fig.49: Steps in permanent-mould casting

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(1). Mould is preheated and coated.
(2). Cores (if used) are inserted, and mould is closed.
(3). Molten metal is poured into the mould.
(4). Mould is opened.
(5). Finished part is shown in figure.
• In preparation for casting, the mould is first preheated, and one or more coatings are

sprayed on the cavity. Preheating facilitates metal flow through the gating system and
into the cavity.
• The coatings aid heat dissipation and lubricate the mould sur-faces for easier
separation of the cast product.
• After pouring, as soon as the metal solidifies, the mould is opened, and the casting is

removed. Unlike expendable moulds, permanent moulds do not collapse, so the mould
must be opened before appreciable cooling contraction occurs to prevent cracks from
developing in the casting.
Advantages:
• Advantages of permanent-mould casting include good surface finish and close

dimensional control, as previously indicated.

• In addition, more rapid solidification caused by the metal mould results in a finer grain

structure, so stronger castings are produced.

Limitations:
• The process is generally limited to metals of lower melting points.
• Other limitations include simple part geometries compared to sand casting (because

of the need to open the mould), and the expense of the mould.
• Because mould cost is substantial, the process is best suited to high-volume production
and can be automated accordingly.
Applications:
• Typical parts include automotive pistons. pump bodies, and certain castings for aircraft
and missiles.
Variations of permanent-mould casting
• Several casting processes are quite like the basic permanent-mould method. These

include:
(1). Slush casting.
(2). Low-pressure casting
(3). Vacuum permanent-mould casting.

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18.2.1. Slush casting:
• In the basic permanent mould casting process and in slush casting, the flow of metal
into the mould cavity is caused by gravity.
• Slush Casting Slush casting is a permanent mould process in which a hollow casting is
formed by inverting the mould after partial freezing at the surface to drain out the liquid
metal in the center.
• Solidification begins at the mould walls because they are relatively cool, and it
progresses over time toward the middle of the casting.
• Thickness of the shell is controlled by the length of time allowed before draining.
Applications:
• Slush casting is used to make statues, lamp pedestals, and toys out of low-melting-
point metals such as zinc and tin.
• In these Items, the exterior appearance is important, but the strength and interior
geometry of the casting are minor considerations.
18.2.2. Low-Pressure Casting:
• In low-pressure casting, the liquid metal is forced into the cavity under low pressure—
approximately 0.1 MPa from beneath so that the flow is upward, as illustrated in Fig.50.

Fig.50: Low-pressure casting. The diagram shows how air pressure is used to
force the molten metal in the ladle upward into the mould cavity.
• Pressure is maintained until the casting has solidified.
• The advantage of this approach over traditional pouring is that clean molten metal
from the center of the ladle is introduced into the mould, rather than metal that has
been exposed to air.
• Gas porosity and oxidation defects are thereby minimized, and mechanical properties
are improved.

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18.2.3. Vacuum Permanent-Mould Casting
• Not to be confused with vacuum moulding, this process is a variation of low-pressure
casting in which a vacuum is used to draw the molten metal into the mould cavity.
• The general configuration or the vacuum permanent-mould casting process is like the
low-pressure casting operation. The difference is that reduced air pressure from the
vacuum in the mould is used to draw the liquid metal into the cavity, rather than forcing
it by positive air pressure from below.
• There are several benefits, of the vacuum technique relative to low-pressure casting:
air porosity, and, related defects are reduced, and greater strength is given to the cast
product.
18.2.4. Die casting
• Die casting is a permanent-mould casting process in which the molten metal is injected
into the mould cavity under high pressure. Typical pressures are 7 to 350 MPa.
• The pressure is maintained during solidification, after which the mould is opened, and
the part is removed.
• Moulds in this casting operation are called dies, hence the name die casting. The use
of high pressure to force the metal into the die cavity is the most notable feature that
distinguishes this process from others in the permanent mould category.
• Die casting operations are carried out in special die-casting machines which are
designed to hold and accurately close the two halves of the mould and keep them closed
while the liquid metal is forced into the cavity. The general configuration is shown in
Fig.51 There are two main types of die-casting machines:
(1). Hot chamber die casting
(2). Cold chamber die casting
• Both are differentiated by how the molten metal is injected into the cavity.

Fig.51: General configuration of a (cold chamber) die-casting machine.

Hot chamber die casting:
• In hot-chamber machines, the metal is melted in a container attached to the machine,
and a piston is used to inject the liquid metal under high pressure into the die. Typical
injection pressures are 7 to 35 MPa.

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• The casting cycle is summarized in Fig.52.

Fig.52: Hot chamber die casting

Cycle in hot-chamber casting:
(1). with die closed and plunger withdrawn, molten metal flows into the chamber.
(2). Plunger forces metal in chamber to flow into die, maintaining pressure during
cooling and solidification.
(3). Plunger is withdrawn, die is opened, and solidified part is ejected. Finished part is
shown in (4).
• Production rates up to 500 parts per hour are not uncommon.
• Hot chamber die casting imposes a special hardship on the injection system because
much of it is submerged in the molten metal.
• The process is therefore limited in its applications to low-melting-point metals
that do not chemically attack the plunger and other mechanical components. The
metals include zinc, tin, lead, and sometimes magnesium.
Cold chamber die casting:
(1). With die closed and ram withdrawn, molten metal is poured into the chamber.
(2). Ram forces metal to flow into die, maintaining pressure during cooling and
solidification; and
(3). Ram is withdrawn, die is opened, and part is ejected. (Gating system is simplified.)

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Fig.53: Cycle in cold-chamber casting

• In cold-chamber die-casting machines, molten metal is poured into an unheated
chamber from an external melting container, and a piston is used to inject the metal
under high pressure into the die cavity.
• Injection pressures used in these machines are typically 14 to 140 MPa. The
production cycle is explained in Fig.53.
• Compared to hot-chamber machines, cycle rates are not usually as fast because of the

need to ladle the liquid metal into the chamber from an external source. Nevertheless,

this casting process is a high production operation.

• Cold-chamber machines are typically used for casting aluminum, brass, and

magnesium alloys. Low-melting-point alloys (zinc, tin, lead) can also be cast on cold-

chamber machines, but the advantages of the hot-chamber process usually favor its use

on these metals.

• Moulds used in die-casting operations are usually made of tool steel, mould steel, or

managing steel. Tungsten and molybdenum with good refractory qualities are also being

used especially in attempts to die cast steel and cast iron.

• Dies can be single-cavity or multiple-cavity. Ejector pins are required to remove the

part from the die when it opens as shown in the diagrams. These pins push the part

away from the mould surface so that it can be removed.

• Lubricants must also he sprayed into the cavities to prevent sticking.

• Because the die materials have no natural porosity and the molten metal rapidly flows

Into the die during injection, venting holes and passageways must he built into the dies

at the parting line to evacuate the air and gases in the cavity. The vents are quite small:

yet they fill with metal during injection. This metal must later be trimmed from the part.

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• Also, formation of flash is common in die casting, in which the liquid metal under high
pressure squeezes into the small space between the die halves at the parting line or into
the clearances around the cores and ejector pins. This flash must he trimmed from the
casting, along with the sprue and gating system.
Advantages of die casting:
(1). high production rates possible:
(2). economical for large production quantities
(3). close tolerances possible, on the order of ±0.076 mm for small parts.
(4). Good surface finish
(5). thin sections are possible, down to about 0.5 mm.
(6). rapid cooling provides small grain size and good strength to the casting. Limitation
of this process:
• In addition to the metals cast, is the shape restriction. The part geometry must allow
for removal from the die cavity.
18.2.5. Squeeze casting and semi-solid metal casting
These are two processes that are often associated with die casting.
Squeeze casting:
• It is a combination of casting and forging in which a (molten metal is poured into a
preheated lower die, and the upper die is closed to create the mould cavity after
solidification begins.
• This differs from the usual permanent-mould casting process in which the die halves
are closed prior to pouring or injection. Owing to the hybrid nature of the process, it is
also known as liquid-metal forging.
• The pressure applied by the upper die in squeeze casting causes the metal to
completely fill the cavity, resulting in good surface finish and low shrinkage.
• The required pressures arc significantly less than in forging of a solid metal billet and
much liner surface detail can be imparted by the die than in forging.
• Squeeze casting can be used for both ferrous and non-ferrous alloys, but
aluminum and magnesium alloys are the most common due to their lower melting
temperatures. Automotive parts are a common application.
Semi-solid metal casting:
• It is a family of net-shape and near net-shape processes performed on metal alloys at
temperatures between the liquidus and solidus. Thus, the alloy is a mixture of solid and
molten metals during casting, like a slurry: it is in the mushy state.
• In order to flow properly, the mixture must consist of solid metal globules in a
liquid rather than the more typical dendritic solid shapes that form during freezing of a
molten metal. This is achieved by force-fully stirring the slurry to prevent dendrite
formation and instead encourage the spherical shapes, which in turn reduces the
viscosity of the work metal.

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Advantages of semi-solid metal casting: It include the following advantages:
(1). complex part geometries.
(2). Thin walls in parts.
(3). Close tolerances.
(4). zero or low porosity, resulting in high strength of the casting.
• There are several forms of semi-solid metal casting. When applied to aluminum, the
terms Thixo-casting and Rheo-casting are used.
(a). Thixocasting:
• The prefix in thixocasting is derived from the word thixotropy, which refers to the
decrease in viscosity of some fluid-like materials when agitated.
• In thixocasting, the starting work material is a pre-cast billet that has a non-dendritic
microstructure; this is heated into the semi-solid temperature range and injected into a
mould cavity using die casting equipment.
• When applied to magnesium, the term is thixomoulding, which utilizes equipment like
an injection-moulding machine. Magnesium alloy granules are fed into a barrel and
propelled forward by a rotating screw as they are heated into the semi-solid temperature
range. The required globular form of the solid phase is accomplished by the mixing
action of the rotating screw. The slurry is then injected into the mould cavity by a linear
forward movement, of the screw.
(b). Rheocasting:
• The prefix in rheocasting comes from rheology, the science that relates deformation
and flow of materials.
• In rheocasting, a semi-solid slurry is injected into the mould cavity by a die casting
machine, very much like conventional die casting. The difference is that the starting
metal in rheocasting is at a temperature between the solidus and liquid as rather than
above the liquidus and the mushy mixture is agitated to prevent dendrite formation.
18.3. Centrifugal casting
• Centrifugal casting refers to several casting methods in which the mould is rotated at
high speed so that centrifugal force distributes the molten metal to the outer regions of
the die cavity. The group includes:
(1). True centrifugal casting
(2). Semi centrifugal casting
(3). Centrifuge casting.
18.3.1: True Centrifugal Casting
• In true centrifugal casting, molten metal is poured into a rotating mould to produce a
tubular part.
• Examples of parts made by this process include pipes, tubes, bushings, and rings. One
possible setup is illustrated in Fig.54.
• Molten metal is poured into a horizontal rotating mould at one end. In some operations,
mould rotation commences after pouring has occurred rather than beforehand.

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• The high-speed rotation results in centrifugal forces that cause the metal to take the
shape of the mould cavity. Thus, the outside shape of the casting can be round,
octagonal, hexagonal, and so on. However, the inside shape of the casting is
(theoretically) perfectly round, due to the radially symmetric forces at work.
• Orientation of the axis of mould rotation can be either horizontal or vertical, the former
being more common. The question is, how fast must the mould be rotated for the
process to work successfully.

Fig.54: setup for true centrifugal casting

Consider horizontal centrifugal casting first. Centrifugal force is defined by this physics
equation:

mv2
F= …………… (1)
R
where F = force, N
m = mass, kg
v = velocity, m/s
and R = in-side radius of the mould, m
The force of gravity is its weight W = mg, where W is given in kg, and g = acceleration
of gravity, 9.8 m/s2. The so-called G-factor GF is the ratio of centrifugal force divided
by weight:

F mv2 v2
GF = = = …………… (2)
W Rmg Rg
Since Velocity v can be expressed as:
v = 2πRN/60 = πRN/30, where the constant 60 converts seconds to minutes; so that N
= rotational speed, rev/min. Substituting this expression into Equation (2):
2 2
 RN   N 
 30  R 
GF =   =  30  …………….. (3)
Rg g
Rearranging this to solve for rotational speed N and using diameter D rather than radius
in the resulting equation.

0 gGF 30 2gGF
N= = ……………….. (4)
 R  D
where D = inside diameter of the mould (in m).

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• If the G-factor is too low in centrifugal casting, the liquid metal will not remain forced
against the mould wall during the upper half of the circular path but will "rain" inside
the cavity. Slipping occurs between the molten metal and the mould wall, which means
that the rotational speed of the metal is less than that of the mould.
• On an empirical basis. values of GF = 60 to 80 are found to be appropriates for
horizontal centrifugal casting pi, although this depends to some extent on the metal
being cast.
Example: A true centrifugal casting operation is to be performed horizontally to make
copper tube sections with OD = 25 cm and ID = 22.5 cm. What rotational speed is
required if a G-factor of 65 is used to cast the tubing?
Sol.
The inside diameter of the mould D = OD of the casting = 25 cm = 025 m. The required
rotational speed can be computed from Equation as follows:

30 2(9.8)(6.5)
N= = 681.7 rev / min
 0.25
18.3.2: Vertical centrifugal casting:
• In vertical centrifugal casting, the effect of gravity acting on the liquid metal causes
the casting wall to be thicker at the base than at the top.
• The inside profile of the casting wall takes on a parabolic shape. The difference in inside
radius between top and bottom is related to speed of rotation as follows:

30 2gL
N= ………(5)
 R t − Rb2
2

Where L = vertical length of the casting (in m), R t = inside radius at the top of the
casting (in m) and Rb = inside radius at the bottom of the casting (in m). Equation (5)
can be used to determine the required rotational speed for vertical centrifugal casting,
given specifications on the inside radii at top and bottom. One can see from the formula
that for Rt to equal Rb, the speed of rotation N would have to be infinite, which is
impossible of course. As a practical matter, part lengths made by vertical centrifugal
casting are usually no more than about twice their diameters.
• This is quite satisfactory for bushings and other parts that have large diameters
relatively to their lengths, especially if machining will be used to accurately size the
inside diameter.
• Castings made by true centrifugal casting are characterized by high density, especially
in the outer regions of the part where F is greatest.
• Solidification shrinkage at the exterior of the cast tube is not a factor, because the
centrifugal force continually reallocates molten metal toward the mould wall during
freezing. Any impurities in the casting tend to be on the inner wall and can be removed
by machining if necessary.

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18.3.3. Semi centrifugal Casting
• In this method, centrifugal force is used to produce solid castings, as in fig. rather than
tubular parts.
• The rotation speed in semi centrifugal casting is usually set so that G factors of around
15 are obtained, and the moulds are designed with risers at the center to supply feed
metal.
• Density of metal in the final casting is greater in the outer sections than at the center
of rotation.
• The process is often used on parts in which the center of the casting is machined away,
thus eliminating the portion of the casting where the quality is lowest. Wheels and
pulleys are examples of castings that can be made by this process. Expendable moulds
are often used in semi centrifugal casting, as suggested by the illustration of the
process.

Fig.55: Semi centrifugal casting

18.3.4. Centrifuge Casting
• In centrifuge casting, Fig.56, the mould is designed with part cavities located away
from the axis of rotation, so that the molten metal poured into the mould is distributed
to these cavities by centrifugal force.
• The process is used for smaller parts, and radial symmetry of the part is not a
requirement as it is for the other two centrifugal casting methods.

Fig.56: (a) Centrifuge casting-centrifugal force causes metal to flow to the

mould cavities away from the axis of rotation and (b) the casting

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19. CASTING QUALITY

There are numerous opportunities for things to go wrong in a casting operation. resulting in
quality defects in the cast product. In this section, the common defects that occur in casting
are listed, and the inspection procedures to detect them are indicated.
Casting Defects
Some defects arc common to any and all casting processes. These defects are illustrated in
Fig.57 and briefly described in the following:
(a). Misruns: which are castings that solidify before completely filling the mould cavity. Typical
causes include
(1). Fluidity of the molten metal is insufficient.
(2). Pouring temperature is too low
(3). Pouring is done too slowly
(4). Cross section of the mould cavity is too thin.
(b). Cold Shuts: which occur when two portions of the metal flow together but there is a lack
of fusion between them due to premature freezing. Its causes are like those of a misrun.
(c). Cold shots which result from splattering during pouring, causing the formation of solid
globules of metal that become entrapped in the casting. Pouring procedures and gating system
designs that avoid splattering can prevent this defect.
(d). Shrinkage cavity is a depression in the surface or an internal void in the casting, caused
by solidification shrinkage that restricts the amount of molten metal available in the last region
to freeze. It often occurs near the top of the casting; in which case it is referred to as a "pipe."
See Fig. 57 (3). The problem can often be solved by proper riser design.
(e). Micro porosity consists of a network of small voids distributed throughout the casting
caused by localized solidification shrinkage of the final molten metal in the dendritic structure.
The defect is usually associated with alloys, because of the protracted manner in which freezing
occurs in these metals.

Fig.57: Some common defects in castings

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(f). Hot tearing, also called hot cracking, occurs when the casting is restrained from
contraction by an unyielding mould during the final stages of solidification or early stages of
cooling after solidification. The defect is manifested as a separation of the metal (hence, the
terms tearing and cracking) at a point of high tensile stress caused by the metal's inability to
shrink naturally. In sand casting and other expendable-mould processes, it is prevented by
compounding the mould to be collapsible. In permanent-mould processes, hot tearing is
reduced by removing the part from the mould immediately after solidification.
Some defects are related to the use of sand moulds, and therefore they occur only in sand
castings. To a lesser degree, other expendable mould processes are also susceptible to these
problems. Defects found primarily in sand castings are shown in Fig.58 and described here:

Fig.58: Common defects in sand castings

(a). Sand blow is a defect consisting of a balloon-shaped gas cavity caused by release of
mould gases during pouring. It occurs at or below the casting surface near the top of the
casting. Low permeability, poor venting, and high moisture content of the sand mould are the
usual causes.
(b). Pinholes also caused by release of gases during pouring, consist of many small gas
cavities formed at or slightly below the surface of the casting.
(c). Sand wash which is an irregularity in the surface of the casting that results from erosion
of the sand mould during pouring, and the contour of the erosion is formed in the surface of
the final cast part.
(d). Scabs are rough areas on the surface of the casting due to encrustations of sand and
metal. It is caused by portions of the mould surface flaking off during solidification and
becoming imbedded in the casting surface.

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(e). Penetration refers to a surface defect that occurs when the fluidity of the liquid metal is
high, and it penetrates into the sand mould or sand core. Upon freezing. the casting surface
consists of a mixture of sand grains and metal. Harder packing of the sand mould helps to
alleviate this condition.
(f). Mould shift refers to a defect caused by a sidewise displacement of the mould cope
relative to the drag, the result of which is a step in the cast product at the parting line.
(g). Core shift is similar to mould shift, but it is the core that is displaced, and the dis-
placement is usually vertical. Core shift and mould shift are caused by buoyancy of the
molten metal.
(h). Mould crack occurs when mould strength is insufficient, and a crack develops, into
which liquid metal can seep to form a "fin" on the final casting.

20. FOUNDRY PRACTICE

• In all casting processes, the metal must be heated to the molten state to be poured or
otherwise forced into the mould.
• Heating and melting are accomplished in a furnace. This section covers the various types
of furnaces used in foundries and the pouring practices for delivering the molten metal from
furnace to mould.
Furnaces
The types of furnaces most commonly used in foundries are
(1). Cupolas.
(2). Direct fuel-fired furnaces.
(3). Crucible furnaces,
(4). Electric-arc furnaces
(5). Induction furnaces
Selection of the most appropriate furnace type depends on factors such as the casting alloy,
its melting and pouring temperatures, capacity requirements of the furnace, costs of
investment, operation, and maintenance, and environmental pollution considerations.
20.1. Cupolas
• A cupola is a vertical cylindrical furnace equipped with a tapping spout near its base.
• Cupolas are used only for melting cast irons, and although other furnaces are also
used. The largest tonnage of cast iron is melted in cupolas.
• General construction and operating features of the cupola are illustrated in Fig.59. It
consists of a large shell of steel plate lined with refractory.
• The "charge." consisting of iron, coke, flux, and possible alloying elements, is loaded
through a charging door located less than halfway up the height of the cupola.
• The iron is usually a mixture of pig iron and scrap (including risers, runners, and
sprees left over from previous castings). Coke is the fuel used to heat the furnace.

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• Forced air is introduced through openings near the bottom of the shell for combustion
of the coke. The flux is a basic compound such as limestone that reacts with coke
ash and other impurities to form slag.

Fig.59: Cupola used for melting cast iron

• Furnace shown is typical for a small foundry and omits details of emissions control
system required in a modern cupola.
• The slag serves to cover the melt, protecting it from reaction with the environment
inside the cupola and reducing heat loss.
• As the mixture is heated and melting of the iron occurs, the furnace is periodically
tapped to provide liquid metal for the pour.
20.2. Direct Fuel-Fired Furnaces
• A direct fuel-fired furnace contains a small open hearth, in which the metal charge is
heated by fuel burners located on the side of the furnace.

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• The roof of the furnace assists the heating action by reflecting the flame down against
the charge. Typically, fuel is natural gas, and the combustion products exit the
furnace through a stack.
• At the bottom of the hearth is a tap hole to release the molten metal. Direct fuel-
fired furnaces are generally used in casting for melting nonferrous metals such as
copper-base alloys and aluminum.
20.3. Crucible Furnaces
• These furnaces melt the metal without direct contact with a burning fuel mixture. For
this reason, they are sometimes called Indirect fuel-fired furnaces. Three types of
crucible furnaces are used in foundries:
(a). Lift-out type
(b). Stationary, and
(c). Tilting
• They all utilize a container (the crucible) made out of a suitable refractory material
(e.g., a clay-graphite mixture) or high-temperature steel alloy to hold the charge.
• In the lift-out crucible furnace, the crucible is placed in a furnace and heated
sufficiently to melt the metal charge.
• Oil, gas, or powdered coal are typical fuels for these furnaces.
• When the metal is melted, the crucible is lifted out of the furnace and used as a pouring
ladle. The other two types, sometimes referred to as pot furnaces, have the heating
furnace and container as one integral unit.
• In the stationary pot furnace, the furnace is stationary, and the molten metal is ladled
out of the container.
• In the tilting-pot furnace, the entire assembly can be tilted for pouring.
• Crucible furnaces are used for nonferrous metals such as bronze, brass, and
alloys of zinc and aluminum. Furnace capacities are generally limited to several hundred
pounds.
20.4. Electric-Arc Furnaces
• In this furnace type, the charge is melted by heat generated from an electric arc.
• Power consumption is high, but electric-arc furnaces can be designed for high melting
capacity (23,000-45,000 kg/hr), and they are used primarily for casting steel.

20.5. Induction Furnaces

• An induction furnace uses alternating current passing through a coil to develop a
magnetic field in the metal, and the resulting induced current causes rapid heating and
melting of the metal. Features of an induction furnace for foundry operations are
illustrated in Fig.

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Fig.60: Induction furnace

• The electromagnetic force field causes a mixing action to occur in the liquid metal.
Also, since the metal does not come in direct contact with the heating elements, the
environment in which melting takes place can be closely controlled.
• All of this results in molten metals of high quality and purity, and induction furnaces
are used for nearly any casting alloy when these requirements are important.
• Melting steel, cast iron, and aluminium alloys are common applications in foundry
work.

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PRACTICE QUESTIONS

1. In a particular mould design, the down sprue has an area of cross section 6.45 cm where the
pouring basin leads into sprue. The sprue is 20 cm long. The required metal flow rate at the top
section of the sprue is 820 cm3/sec. Determine the pouring height necessary above the sprue
top. Also determine the area of the cross section of the sprue at its bottom to avoid aspiration
of the liquid metal.
Ans. 8.246 cm and 3.52 cm2
2. In the casting of steel under certain mould conditions, the mould constant in Chvorinov's rule is
known to be 4.0 min/cm2, based on previous experience. The casting is a flat plate whose length
= 30 cm, width = 10 cm, and thickness = 20 mm. Determine how long it will take for the casting
to solidify.
Ans. 2.49 min
3. In casting experiments performed using a certain alloy and type of sand mould, it took 155 sec
for a cube-shaped casting to solidify. The cube was 50 mm on a side.
(a). Determine the value of the mould constant in Chvorinov's rule.
(b). If the same alloy and mould type were used, find the total solidification time for a cylindrical
casting in which the diameter = 30 mm and length = 50 mm
Ans. 2.231 s/mm2, 1.24 min
4. A riser in the shape of a sphere is to be designed for a sand-casting mould. The casting is a
rectangular plate, with length = 200 mm, width = 100 mm, and thickness = 18 mm. If the total
solidification time of the casting itself is known to be 3.5 min, determine the diameter of the
riser so that i will take 25% longer for the riser to solidify.
Ans. 47.5 mm
5. Molten aluminum was poured in a sand mould and the thickness of the solid skin formed after
20 seconds and 50 seconds were found to be 3 mm and 4.5 mm respectively. What would be
the thickness of the solid skin at the end of 100 seconds after pouring?
Ans. 6.16 mm
6. It takes 35 s to pass 2000 cm of air at a pressure of 6 g/cm through a standard sand sample.
3 2

If the permeability number is 152, calculate the height of the sand sample. Examine the
adequacy of the moisture content in the sand sample based on its height.
Ans. 5.391 cm
7. A foundry is producing gray—iron blocks 300 x 150 x 100 mm. The parting line is at the midpoint
of the height of the block so that there the mould cavity extends 50 mm into both the cope and
the drag. The flasks are 500 x 300 x 125 mm, so the combined height of the cope and drag is
250 mm. The mould is poured with molten gray—iron which weight 0.0597 N/cm 3 and the
compacted sand weighs 0.0157 N/cm3. What would be the total lifting force in Newtons tending
to separate the cope from the drag because of the metallo-static pressure within the mould?
Ans. 50 N

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8. Two castings are moulded in green sand. They differ in weight by a factor of 3.8 but they are
both cubes. An experiment has shown that the lighter casting solidifies in 8.7 minutes. How
much time would you estimate that it would take for the larger casting to solidify.
Ans. 21.18 min
9. A casting of size 100 mm x 100 mm x 50 mm is required. Assume volume shrinkage of casting
as 2.6%. If the height of the riser is 80 mm and the riser volume desired is 4 times the shrinkage
in casting, the appropriate riser diameter (in mm) will be ________.
Ans. 57.52
10. The shape factor for a casting in the form of an annular cylinder of outside diameter 30 cm,
inside diameter 20 cm and height 30 cm (correction factor k = 1) will be __________.
Ans. 21.77
11. A mould of dimension 60 cm x 30 cm x14 cm is to be filled by liquid metal using the bottom
pouring method. The liquid metal height in the vertical column is 14 cm and the area of the gate
is 6 cm2. Find the time taken to fill the mould ________.
Ans. 50.68 sec
12. A down sprue of 180 mm length has a diameter of 20 mm at its top end. The liquid metal in
pouring cup is maintained up to 60 mm height. diameter of the down sprue at its lower end to
avoid aspiration will be ___________.
Ans. 14.14 mm
13. The height of the down sprue is 175 mm and its cross-section area at 1 the base is 200 mm 2.
The cross-section area of the horizontal runner is also 200 mm 2. Assuming no losses, the time
(second) required to fill a mould cavity of volume 10 6 mm3 (take g = 10 m/s2) will be
___________.
Ans. 2.67
14. True centrifugal casting is per-formed horizontally to make copper tube sections. The tubes have
a length = 1.0 m, diameter = 0.25 m, and wall thickness 15 mm. (a) If the rotational speed of
the pipe 700 rev/min, determine the G-factor on the molten metal. (b) What volume of molten
metal must be poured into the mould to make the casting if solidification shrinkage and solid
thermal contraction for copper are 4.5% and = 7.5%, respectively (both are volumetric
contractions).
Ans. (a). GF=68.54 (b). 0.01254 m3
15. A riser in the shape of a sphere is to be designed for a sand-casting mould. The casting is a
rectangular plate, with length = 200 mm, width = 100 mm, and thickness = 18 mm. If the total
solidification time of the casting itself is known to be 3.5 min, determine the diameter of the
riser so that it will take 25% longer for the riser to solidify.
Ans. 47.5 mm

****

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