LECTURE #01
Introduction
As a field of study in the modern context, manufacturing can be defined two ways, one
technologic and the other economic.
Technologically, manufacturing is the application of physical and chemical processes to
alter the geometry, properties, and/or appearance of a given starting material to make
parts or products;
Manufacturing also includes assembly of multiple parts to make products. The processes
to accomplish manufacturing involve a combination of machinery, tools, power, and
labor Manufacturing is almost always carried out as a sequence of operations. Each
operation brings the material closer to the desired final state.
Economically, manufacturing is the transformation of materials into items of greater
value by means of one or more processing and/or assembly operations. The key point is
that manufacturing adds value to the material by changing its shape or properties, or by
combining it with other materials that have been similarly altered.
Module 1: Metal casting
The material has been made more valuable through the manufacturing operations
performed on it. When iron ore is converted into steel, value is added. When sand is
transformed into glass, value is added. When petroleum is refined into plastic, value is
added. And when plastic is molded into the complex geometry of a patio chair, it is made
even more valuable.
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More than one processing operation is usually required to transform the starting material
into final form. The operations are performed in the particular sequence required to
achieve the geometry and condition defined by the design specification. Three categories
of processing operations are distinguished: (1) shaping operations, (2) property-enhancing
operations, and (3) surface processing operations.
Shaping operations alter the geometry of the starting work material by various methods.
Common shaping processes include casting, forging, and machining.
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Classification of manufacturing processes
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Property-enhancing operations add value to the material by improving its physical
properties without changing its shape. Heat treatment is the most common example.
Surface processing operations are performed to clean, treat, coat, or deposit material
onto the exterior surface of the work.
Shaping Processes:
Most shape processing operations apply heat, mechanical force, or a combination of
these to effect a change in geometry of the work material. There are various ways to
classify the shaping processes. The classification used in this book is based on the state
of the starting material, by which we have four categories:
(1) Solidification processes, in which the starting material is a heated liquid or semi
fluid that cools and solidifies to form the part geometry;
(2) Particulate processing, in which the starting material is a powder, and the
powders are formed and heated into the desired geometry;
(3) Deformation processes, in which the starting material is a ductile solid
(commonly metal) that is deformed to shape the part; and
(4) Material removal processes, in which the starting material is a solid (ductile or
brittle), from which material is removed so that the resulting part has the desired
geometry.
Module 1: Metal casting
In the first category, the starting material is heated sufficiently to transform it into a
liquid or highly plastic (semi fluid) state. Nearly all materials can be processed in this way.
Metals, ceramic glasses, and plastics can all be heated to sufficiently high temperatures to
convert them into liquids. With the material in a liquid or semi fluid form, it can be poured
or otherwise forced to flow into a mold cavity and allowed to solidify, thus taking a solid
shape that is the same as the cavity. Most processes that operate this way are called
casting or molding. Casting is the name used for metals, and molding is the common term
used for plastics.
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In particulate processing, the starting materials are powders of metals or ceramics.
Although these two materials are quite different, the processes to shape them in
particulate processing are quite similar. The common technique involves pressing and
sintering, in which the powders are first squeezed into a die cavity under high pressure
and then heated to bond the individual particles together.
Material removal processes are operations that remove excess material from the starting
workpiece so that the resulting shape is the desired geometry. The most important
processes in this category are machining operations such as turning, drilling, and milling.
These cutting operations are most commonly applied to solid metals, performed using
cutting tools that are harder and stronger than the work metal. Grinding is another
common process in this category.
Other material removal processes are known as nontraditional processes because they
use lasers, electron beams, chemical erosion, electric discharges, and electrochemical
energy to remove material rather than cutting or grinding tools.
Module 1: Metal casting
In deformation processes, the starting work part is shaped by the application of forces
that exceed the yield strength of the material. For the material to be formed in this way,
it must be sufficiently ductile to avoid fracture during deformation. To increase ductility
(and for other reasons), the work material is often heated before forming to a
temperature below the melting point. Deformation processes are associated most
closely with metal working and include operations such as forging and extrusion.
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It is desirable to minimize waste and scrap in converting a starting work part into its
subsequent geometry. Certain shaping processes are more efficient than others in terms
of material conservation. Material removal processes (e.g., machining) tend to be
wasteful of material, simply by the way they work. The material removed from the
starting shape is waste, at least in terms of the unit operation. Other processes, such as
certain casting and molding operations, often convert close to 100% of the starting
material into final product.
Property-Enhancing Processes
The second major type of part processing is performed to improve mechanical or physical
properties of the work material. These processes do not alter the shape of the part,
except unintentionally in some cases. The most important property-enhancing processes
involve heat treatments, which include various annealing and strengthening processes for
metals and glasses. Sintering of powdered metals and ceramics is also a heat treatment
that strengthens a pressed powder metal work part.
Surface Processing
Surface processing operations include (1) cleaning, (2) surface treatments, and (3) coating
and thin film deposition processes.
Cleaning includes both chemical and mechanical processes to remove dirt, oil, and
other contaminants from the surface.
Coating and thin film deposition processes apply a coating of material to the
exterior surface of the work part. Common coating processes include
electroplating, anodizing of aluminum, organic coating (call it painting), and
porcelain enameling. Thin film deposition processes include physical vapor
deposition and chemical vapor deposition to form extremely thin coatings of
various substances.
Module 1: Metal casting
Surface treatments include mechanical working such as shot peening and sand
blasting, and physical processes such as diffusion and ion implantation.
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LECTURE #02
In this module, we consider those manufacturing processes (shaping process) in which
the starting work material is either a liquid or is in a highly plastic condition, and a part is
created through solidification of the material. Casting and molding processes dominate
this category of shaping operations. The solidification processes can be classified
according to the engineering material that is processed: (1) metals, (2) ceramics,
specifically glasses, and (3) polymers and polymer matrix composites (PMCs).
Casting is a process in which molten metal flows by gravity or other force into a mold
where it solidifies in the shape of the mold cavity. The term casting is also applied to the
part that is made by this process. It is one of the oldest shaping processes, dating back
6000 years. The principle of casting seems simple: melt the metal, pour it into a mold,
and let it cool and solidify; yet there are many factors and variables that must be
considered in order to accomplish a successful casting operation.
CASTING PROCESS:
Discussion of casting logically begins with the mold. The mold contains a cavity known as
mold cavity, whose geometry determines the shape of the cast part. The actual size and
shape of the cavity must be slightly oversized to allow for shrinkage that occurs in the
metal during solidification and cooling. Different metals undergo different amounts of
shrinkage, so the mold cavity must be designed for the particular metal to be cast if
dimensional accuracy is critical. Molds are made of a variety of materials, including sand,
plaster, ceramic, and metal. The various casting processes are often classified according
to these different types of molds.
To accomplish a casting operation, the metal is first heated to a temperature high
enough to completely transform it into a liquid state. It is then poured, or otherwise
directed, into the cavity of the mold.
Module 1: Metal casting
In an open mold, Figure (a), the liquid metal is simply poured until it fills the open cavity.
In a closed mold, Figure (b), a passageway, called the gating system, is provided to permit
the molten metal to flow from outside the mold into the cavity. The closed mold is by far
the more important category in production casting operations.
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As soon as the molten metal is in the mold, it begins to cool. When the temperature
drops sufficiently (e.g., to the freezing point for a pure metal), solidification begins.
Solidification involves a change of phase of the metal. During the solidification process, a
series of events takes place. These events greatly affect the size, shape, uniformity and
chemical composition of the grains. Time is required to complete the phase change, and
considerable heat is given up in the process. It is during this step in the process that the
metal assumes the solid shape of the mold cavity and many of the properties and
characteristics of the casting are established.
Once the casting has cooled sufficiently, it is removed from the mold. Depending on the
casting method and metal used, further processing may be required. This may include
trimming the excess metal from the actual cast part, cleaning the surface, inspecting the
product, and heat treatment to enhance properties. In addition, machining may be
required to achieve closer tolerances on certain part features and to remove the cast
surface.
Depending upon the extraction of cast product, the casting process in divided into
expendable-mold casting and permanent-mold casting.
A permanent mold is one that can be used over and over to produce many
castings. It is made of metal (or, less commonly, a ceramic refractory material) that can
withstand the high temperatures of the casting operation. In permanent-mold casting,
the mold consists of two (or more) sections that can be opened to permit removal of the
finished part. Die casting is the most familiar process in this group. More intricate casting
geometries are generally possible with the expendable-mold processes. Part shapes in
Module 1: Metal casting
An expendable mold means that the mold in which the molten metal solidifies
must be destroyed in order to remove the casting. These molds are made out of sand,
plaster, or similar materials, whose form is maintained by using binders of various kinds.
Sand casting is the most prominent example of the expendable-mold processes. In sand
casting, the liquid metal is poured into a mold made of sand. After the metal hardens, the
mold must be sacrificed in order to recover the casting.
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the permanent-mold processes are limited by the need to open the mold. On the other
hand, some of the permanent mold processes have certain economic advantages in high
production operations.
SAND CASTING MOLD:
Sand casting is by far the most important casting process. A sand-casting mold will be
used to describe the basic features of a mold. Many of these features and terms are
common to the molds used in other casting processes. Figure (b) shows the crosssectional view of a typical sand-casting mold, indicating some of the terminology. The
mold consists of two halves: cope and drag. The cope is the upper half of the mold, and
the drag is the bottom half. These two mold parts are contained in a box, called a flask,
which is also divided into two halves, one for the cope and the other for the drag. The
two halves of the mold separate at the parting line.
In sand casting (and other expendable-mold processes) the mold cavity is formed by
means of a pattern, which is made of wood, metal, plastic, or other material and has the
shape of the part to be cast. The cavity is formed by packing sand around the pattern,
about half each in the cope and drag, so that when the pattern is removed, the remaining
void has the desired shape of the cast part. The pattern is usually made oversized to allow
for shrinkage of the metal as it solidifies and cools. The sand for the mold is moist and
contains a binder to maintain its shape.
The cavity in the mold provides the external surfaces of the cast part. In addition, a
casting may have internal surfaces. These surfaces are determined by means of a core, a
form placed inside the mold cavity to define the interior geometry of the part. In sand
casting, cores are generally made of sand, although other materials can be used, such as
metals, plaster, and ceramics.
In addition to the gating system, any casting in which shrinkage is significant requires a
riser connected to the main cavity. The riser is a reservoir in the mold that serves as a
source of liquid metal for the casting to compensate for shrinkage during solidification.
The riser must be designed to freeze after the main casting in order to satisfy its function.
As the metal flows into the mold, the air that previously occupied the cavity, as well as
Module 1: Metal casting
The gating system in a casting mold is the channel, or network of channels, by which
molten metal flows into the cavity from outside the mold. As shown in the figure, the
gating system typically consists of a downsprue (also called simply the sprue), through
which the metal enters a runner that leads into the main cavity. At the top of the
downsprue, a pouring cup is often used to minimize splash and turbulence as the metal
flows into the downsprue. It is shown in our diagram as a simple cone-shaped funnel.
Some pouring cups are designed in the shape of a bowl, with an open channel leading to
the downsprue.
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Module 1: Metal casting
hot gases formed by reactions of the molten metal, must be evacuated so that the metal
will completely fill the empty space. In sand casting, for example, the natural porosity of
the sand mold permits the air and gases to escape through the walls of the cavity. In
permanent metal molds, small vent holes are drilled into the mold or machined into the
parting line to permit removal of air and gases.
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LECTURE #03
MOLDING SAND:
The general sources of receiving molding sands are the beds of sea, rivers, lakes,
granulular elements of rocks, and deserts. The common sources of molding sands
available in India are as follows:
1 Batala sand ( Punjab)
2 Ganges sand (Uttar Pradesh)
3 Oyaria sand (Bihar)
4 Damodar and Barakar sands (Bengal- Bihar Border)
5 Londha sand (Bombay)
6 Gigatamannu sand (Andhra Pradesh) and
7 Avadi and Veeriyambakam sand (Madras)
Molding sands may be of two types namely natural or synthetic. Natural molding sands
contain sufficient binder. Whereas synthetic molding sands are prepared artificially using
basic sand molding constituents (silica sand in 88-92%, binder 6-12%, water or moisture
content 3-6%) and other additives in proper proportion by weight with perfect mixing and
mulling in suitable equipments.
The main constituents of molding sand involve silica sand, binder, moisture content and
additives.
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Silica sand forms the main constituent of molding sand having enough
refractoriness which can impart strength, stability and permeability to molding
and core sand.
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The silica sand can be specified according to the size (small, medium and large
silica sand grain) and the shape (angular, sub-angular and rounded).
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But along with silica small amounts of iron oxide, alumina, lime stone, magnesia,
soda and potash are present as impurities.
Module 1: Metal casting
Silica sand (88%-92%)
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Binder (6%-12%)
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In general, the binders can be either inorganic or organic substance.
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The inorganic group includes clay, sodium silicate etc.
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In foundry shop, the clay acts as binder which may be Kaolonite, Ball Clay, Fire Clay,
Limonite, Fuller’s earth and Bentonite.
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Organic group binders are dextrin, molasses, cereal binders, linseed oil and resins
like phenol formaldehyde, urea formaldehyde etc.
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Among all the above binders, the bentonite variety of clay is the most common.
Moisture (3%-6%)
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The effect of clay and water decreases permeability with increasing clay and
moisture content.
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The green compressive strength first increases with the increase in clay content,
but after a certain value, it starts decreasing.
Additives
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Coal dust
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Corn flour
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Dextrin
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Sea coal
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Pitch
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Wood flour
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Silica flour
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Some common used additives for enhancing the properties of molding and core sands
are discussed as under.
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TYPES OF MOLDING SAND:
Green sand
Green sand is also known as tempered or natural sand which is a just prepared mixture of
silica sand with 18 to 30 percent clay, having moisture content from 6 to 8%. The clay and
water furnish the bond for green sand. It is fine, soft, light, and porous. Green sand is
damp, when squeezed in the hand and it retains the shape and the impression to give to it
under pressure. Molds prepared by this sand are not requiring backing and hence are
known as green sand molds. This sand is easily available and it possesses low cost. It is
commonly employed for production of ferrous and non-ferrous castings.
Dry sand
Green sand that has been dried or baked in suitable oven after the making mold and
cores is called dry sand. It possesses more strength, rigidity and thermal stability. It is
mainly suitable for larger castings. Mold prepared in this sand are known as dry sand
molds.
Loam sand
Loam is mixture of sand and clay with water to a thin plastic paste. Loam sand possesses
high clay as much as 30-50% and 18% water. Patterns are not used for loam molding and
shape is given to mold by sweeps. This is particularly employed for loam molding used for
large grey iron castings.
Facing sand
Backing sand
Backing sand or floor sand is used to back up the facing sand and is used to fill the whole
volume of the molding flask. Used molding sand is mainly employed for this purpose. The
backing sand is sometimes called black sand because that old, repeatedly used molding
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Facing sand is just prepared and forms the face of the mould. It is directly next to the
surface of the pattern and it comes into contact molten metal when the mould is poured.
Initial coating around the pattern and hence for mold surface is given by this sand. This
sand is subjected severest conditions and must possess, therefore, high strength
refractoriness. It is made of silica sand and clay, without the use of used sand. Different
forms of carbon are used to prevent the metal burning into the sand. A facing sand
mixture for green sand of cast iron may consist of 25% fresh and specially prepared and
5% sea coal. They are sometimes mixed with 6-15 times as much fine molding sand to
make facings. The layer of facing sand in a mold usually ranges from 22-28 mm. From 10
to 15% of the whole amount of molding sand is the facing sand.
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sand is black in color due to addition of coal dust and burning on coming in contact with
the molten metal.
System sand
In mechanized foundries where machine molding is employed. A so-called system sand is
used to fill the whole molding flask. In mechanical sand preparation and handling units,
no facing sand is used. The used sand is cleaned and re-activated by the addition of water
and special additives. This is known as system sand. Since the whole mold is made of this
system sand, the properties such as strength, permeability and refractoriness of the
molding sand must be higher than those of backing sand.
Parting sand
Parting sand without binder and moisture is used to keep the green sand not to stick to
the pattern and also to allow the sand on the parting surface the cope and drag to
separate without clinging. This is clean clay-free silica sand which serves the same
purpose as parting dust.
Core sand
Core sand is used for making cores and it is sometimes also known as oil sand. This is
highly rich silica sand mixed with oil binders such as core oil which composed of linseed
oil, resin, light mineral oil and other bind materials. Pitch or flours and water may also be
used in large cores for the sake of economy.
PROPERTIES OF MOLDING SAND:
Refractoriness is defined as the ability of molding sand to withstand high temperatures
without breaking down or fusing thus facilitating to get sound casting. It is a highly
important characteristic of molding sands. Molding sand with poor refractoriness may
burn on to the casting surface and no smooth casting surface can be obtained.
Refractoriness can only be increased to a limited extent. The degree of refractoriness
depends on the SiO2 i.e. quartz content, and the shape and grain size of the particle. The
higher the SiO2 content and the rougher the grain volumetric composition the higher is
the refractoriness of the molding sand and core sand.
Refractoriness is measured by the sinter point (The temperature at which a molding
material begins to adhere to a casting is known as sintering point) of the sand rather than
its melting point.
Module 1: Metal casting
Refractoriness
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Permeability
It is also termed as porosity of the molding sand in order to allow the escape of any air,
gases or moisture present or generated in the mould when the molten metal is poured
into it. All these gaseous generated during pouring and solidification process must
escape otherwise the casting becomes defective. Permeability is a function of grain size,
grain shape, and moisture and clay contents in the molding sand. The extent of ramming of
the sand directly affects the permeability of the mould. Permeability of mold can be
further increased by venting using vent rods
Cohesiveness
It is property of molding sand by virtue which the sand grain particles interact and attract
each other within the molding sand. Thus, the binding capability of the molding sand gets
enhanced to increase the green, dry and hot strength property of molding and core sand.
Green strength
The green sand after water has been mixed into it, must have sufficient strength and
toughness to permit the making and handling of the mould. For this, the sand grains must
be adhesive, i.e. they must be capable of attaching themselves to another body and
therefore, and sand grains having high adhesiveness will cling to the sides of the molding
box. Also, the sand grains must have the property known as cohesiveness i.e. ability of the
sand grains to stick to one another. By virtue of this property, the pattern can be taken
out from the mould without breaking the mould and also the erosion of mould wall
surfaces does not occur during the flow of molten metal. The green strength also
depends upon the grain shape and size, amount and type of clay and the moisture content.
Dry strength/thermal stability
As soon as the molten metal is poured into the mould, the moisture in the sand layer
adjacent to the hot metal gets evaporated and this dry sand layer must have sufficient
strength to its shape in order to avoid erosion of mould wall during the flow of molten
metal. The dry strength also prevents the enlargement of mould cavity cause by the
metallostatic pressure of the liquid metal.
It is the ability of the sand to get compacted and behave like a fluid. It will flow uniformly
to all portions of pattern when rammed and distribute the ramming pressure evenly all
around in all directions. Generally sand particles resist moving around corners or
projections. In general, flowability increases with decrease in green strength, an, decrease
in grain size. The flowability also varies with moisture and clay content.
Module 1: Metal casting
Flowability or plasticity
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Adhesiveness
It is property of molding sand to get stick or adhere with foreign material such sticking of
molding sand with inner wall of molding box
Collapsibility
After the molten metal in the mold gets solidified, the sand mould must be collapsible so
that free contraction of the metal occurs and this would naturally avoid the tearing or
cracking of the contracting metal. In absence of this property the contraction of the
metal is hindered by the mold and thus results in tears and cracks in the casting. This
property is highly desired in cores
Reusability
Since large quantities of sand are used in a foundry it is very important that the sand be
reusable otherwise apart from cost it will create disposal problems
PATTERN AND CORE
Module 1: Metal casting
Refer entire chapter of pattern and core in “introduction-to-basic-manufacturingprocesses-and-workshop-technology”
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GATING SYSTEM IN MOLD
1. Pouring basin
It is the conical hollow element or tapered hollow vertical portion of the gating system
which helps to feed the molten metal initially through the path of gating system to mold
cavity. It may be made out of core sand or it may be cut in cope portion of the sand mold.
It makes easier for the ladle operator to direct the flow of molten metal from crucible to
pouring basin and sprue. It helps in maintaining the required rate of liquid metal flow. It
reduces turbulence and vertexing at the sprue entrance. It also helps in separating dross,
slag and foreign element etc. from molten metal before it enters the sprue.
2. Sprue
It is a vertical passage made generally in the cope using tapered sprue pin. It is connected
at bottom of pouring basin. It is tapered with its bigger end at to receive the molten
metal the smaller end is connected to the runner. It helps to feed molten metal without
turbulence to the runner which in turn reaches the mold cavity through gate. It some
times possesses skim bob at its lower end. The main purpose of skim bob is to collect
impurities from molten metal and it does not allow them to reach the mold cavity
through runner and gate.
It is a small passage or channel being cut by gate cutter which connect runner with the
mould cavity and through which molten metal flows to fill the mould cavity. It feeds the
liquid metal to the casting at the rate consistent with the rate of solidification.
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3. Gate
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5. Runner
It is a channel which connects the sprue to the gate for avoiding turbulence and gas
entrapment.
6. Riser
It is a passage in molding sand made in the cope portion of the mold. Molten metal rises
in it after filling the mould cavity completely. The molten metal in the riser compensates
the shrinkage during solidification of the casting thus avoiding the shrinkage defect in
the casting. It also permits the escape of air and mould gases. It promotes directional
solidification too and helps in bringing the soundness in the casting.
7. Chaplets
Chaplets are metal distance pieces inserted in a mould either to prevent shifting of mould
or locate core surfaces. The distances pieces in form of chaplets are made of parent
metal of which the casting is. These are placed in mould cavity suitably which positions
core and to give extra support to core and mould surfaces. Its main objective is to impart
good alignment of mould and core surfaces and to achieve directional solidification.
When the molten metal is poured in the mould cavity, the chaplet melts and fuses itself
along with molten metal during solidification and thus forms a part of the cast material.
FACTORS CONTROLING GATING DESIGN
The following factors must be considered while designing gating system.
(i) Sharp corners and abrupt changes in at any section or portion in gating system should
be avoided for suppressing turbulence and gas entrapment. Suitable relationship must
exist between different cross-sectional areas of gating systems.
(ii) The most important characteristics of gating system besides sprue are the shape,
location and dimensions of runners and type of flow. It is also important to determine
the position at which the molten metal enters the mould cavity.
(iii) Gating ratio should reveal that the total cross-section of sprue, runner and gate
decreases towards the mold cavity which provides a choke effect.
(v) Developing the various cross sections of gating system to nullify the effect of
turbulence or momentum of molten metal.
(vi) Streamlining or removing sharp corners at any junctions by providing generous
radius, tapering the sprue, providing radius at sprue entrance and exit and providing a
basin instead pouring cup etc.
Module 1: Metal casting
(iv) Bending of runner if any should be kept away from mold cavity.
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RISER IN MOLD
Metals and their alloys shrink as they cool or solidify and hence may create a partial
vacuum within the casting which leads to casting defect known as shrinkage or void. The
primary function of riser as attached with the mould is to feed molten metal to
accommodate shrinkage occurring during solidification of the casting. As shrinkage is
very common casting defect in casting and hence it should be avoided by allowing
molten metal to rise in riser after filling the mould cavity completely and supplying the
molten metal to further feed the void occurred during solidification of the casting
because of shrinkage. Riser also permits the escape of evolved air and mold gases as the
mold cavity is being filled with the molten metal. It also indicates to the foundry man
whether mold cavity has been filled completely or not. The suitable design of riser also
helps to promote the directional solidification and hence helps in production of desired
sound casting.
Considerations for Designing Riser
While designing risers the following considerations must always be taken into account.
(A) Freezing time
1 For producing sound casting, the molten metal must be fed to the mold cavity till it
solidifies completely. This can be achieved when molten metal in riser should freeze at
slower rate than the casting.
2 Freezing time of molten metal should be more for risers than casting.
(B) Feeding range
1. When large castings are produced in complicated size, then more than one riser are
employed to feed molten metal depending upon the effective freezing range of each
riser.
2. Casting should be divided into different zones so that each zone can be feed by a
separate riser.
4. Riser should maintain proper temperature gradients for continuous feeding
throughout freezing or solidifying.
Module 1: Metal casting
3. Risers should be attached to that heavy section which generally solidifies last in the
casting.
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(C) Feed Volume Capacity
1 Riser should have sufficient volume to feed the mold cavity till the solidification of the
entire casting so as to compensate the volume shrinkage or contraction of the solidifying
metal.
2 The metal is always kept in molten state at all the times in risers during freezing of
casting. This can be achieved by using exothermic compounds and electric arc feeding
arrangement. Thus it results for small riser size and high casting yield.
3 It is very important to note that volume feed capacity riser should be based upon
freezing time and freezing demand. Riser system is designed using full considerations on
the shape, size and the position or location of the riser in the mold.
SOLIDIFICATION TIME
Whether the casting is pure metal or alloy, solidification takes time. The total
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:
Where TTS= total solidification time, min;
V=volume of the casting, cm3;
A=surface area of the casting, cm2;
n is an exponent usually taken to have a value =2; and
Given that n = 2, the units of Cm are min/cm2, and its value depends on the particular
conditions of the casting operation, including mold material (e.g., specific heat, thermal
conductivity), thermal properties of the cast metal (e.g., heat of fusion, specific heat,
thermal conductivity), and pouring temperature relative to the melting point of the
metal. The value of Cm for a given casting operation can be based on experimental data
from previous operations carried out using the same mold material, metal, and pouring
temperature, even though the shape of the part may be quite different.
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Cm is the mold constant.
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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. This principle is put to good use
in designing the riser in a mold. 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 TTS for the riser must exceed the TTS for the main casting. Since the
mold conditions for both riser and casting are the same, their mold constants will be
equal. By designing the riser to have a larger volume-to-area ratio, we can be fairly sure
that the main casting solidifies first and that the effects of shrinkage are minimized.
Before considering how the riser might be designed using Chvorinov’s rule, let us
consider the topic of shrinkage, which is the reason why risers are needed.
CASTING DEFECTS
There are numerous opportunities for things to go wrong in a casting operation,
resulting in quality defects in the cast product. In this section, we compile a list of the
common defects that occur in casting, and we indicate the inspection procedures to
detect them. Casting Defects Some defects are common to any and all casting processes.
These defects are illustrated in Figure and briefly described in the following:
(a) Misruns, which are castings that solidify before completely filling the mold cavity.
Typical causes include (1) fluidity of the molten metal is insufficient, (2) pouring
temperature is too low, (3) pouring is done too slowly, and/or (4) cross-section of the
mold 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 similar to those of
a misrun.
(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.’’ The problem can often be solved by proper riser design.
(e) Microporosity 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.
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(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.
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(f) Hot tearing, also called hot cracking, occurs when the casting is restrained from
contraction by an unyielding mold 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-mold
processes, it is prevented by compounding the mold to be collapsible. In permanentmold processes, hot tearing is reduced by removing the part from the mold immediately
after solidification.
Some defects are related to the use of sand molds, and therefore they occur only in sand
castings. To a lesser degree, other expendable-mold processes are also susceptible to
these problems. Defects found primarily in sand castings are shown in Figure 11.23 and
described here:
(a) Sand blow is a defect consisting of a balloon-shaped gas cavity caused by release of
mold 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 mold are
the usual causes.
(c) Sand wash, which is an irregularity in the surface of the casting that results from
erosion of the sand mold 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 mold surface flaking off during solidification and
becoming imbedded in the casting surface.
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(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.
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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 mold or sand core. Upon freezing, the casting
surface consists of a mixture of sand grains and metal. Harder packing of the sand mold
helps to alleviate this condition.
(f) Mold shift refers to a defect caused by a sidewise displacement of the mold 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 mold shift, but it is the core that is displaced, and the
displacement is usually vertical. Core shift and mold shift are caused by buoyancy of the
molten metal.
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(h) Mold crack occurs when mold strength is insufficient, and a crack develops, into
which liquid metal can seep to form a ‘‘fin’’ on the final casting.
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SHELL MOLD CASTING
Shell mold casting process is recent invention in casting techniques for mass production
and smooth surface finish. It was originated in Germany during Second World War. It is
also called as Carning or C process. It consists of making a mold that possesses two or
more thin shells (shell line parts, which are moderately hard and smooth with a texture
consisting of thermosetting resin bonded sands. The shells are 0.3 to 0.6 mm thick and
can be handled and stored. Shell molds are made so that machining parts fit togethereasily. They are held using clamps or adhesive and metal is poured either in a vertical or
horizontal position. They are supported using rocks or mass of bulky permeable material.
Thermosetting resin, dry powder and sand are mixed thoroughly in a muller.
Complete shell molding casting processes is carried in four stages as shown in Fig. 13.4. In
this process a pattern is placed on a metal plate and it is then coated with a mixture of
fine sand and Phenol-resin (20:1). The pattern is heated first and silicon grease is then
sprayed on the heated metal pattern for easy separation. The pattern is heated to 205 to
230°C and covered with resin bounded sand. After 30 seconds, a hard layer of sand is
formed over pattern. Pattern and shell are heated and treated in an oven at 315°C for 60
secs., Phenol resin is allowed to set to a specific thickness. So the layer of about 4 to 10
mm in thickness is stuck on the pattern and the loose material is then removed from the
pattern. Then shell is ready to strip from the pattern. A plate pattern is made in two or
more pieces and similarly core is made by same technique. The shells are clamped and
usually embedded in gravel, coarse sand or metal shot. Then mold is ready for pouring.
The shell so formed has the shape of pattern formed of cavity or projection in the shell.
In case of unsymmetrical shapes, two patterns are prepared so that two shell are
produced which are joined to form proper cavity.
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Internal cavity can be formed by placing a core. Hot pattern and box is containing a
mixture of sand and resin. Pattern and box inverted and kept in this position for some
time. Now box and pattern are brought to original position. A shell of resin-bonded sand
sticks to the pattern and the rest falls. Shell separates from the pattern with the help of
ejector pins. It is a suitable process for casting thin walled articles. The cast shapes are
uniform and their dimensions are within close limit of tolerance ± 0.002 mm and it is
suitable for precise duplication of exact parts. It has various advantages which are as
follows. There are some advantages and disadvantages of this process which are given as
under.
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Advantages
The main advantages of shell molding are:
(i) High suitable for thin sections like petrol engine cylinder.
(ii) Excellent surface finish.
(iii) Good dimensional accuracy of order of 0.002 to 0.003 mm.
(iv) Negligible machining and cleaning cost.
(v) Occupies less floor space.
(vi) Skill-ness required is less.
(viii) Better quality of casting assured.
(ix) Mass production.
(x) It allows for greater detail and less draft.
(xi) Unskilled labor can be employed.
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(vii) Moulds formed by this process can be stored until required.
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(xii) Future of shell molding process is very bright.
Disadvantages
The main disadvantages of shell molding are:
1. Higher pattern cost.
2. Higher resin cost.
3. Not economical for small runs.
4. Dust-extraction problem.
5. Complicated jobs and jobs of various sizes cannot be easily shell molded.
6. Specialized equipment is required.
7. Resin binder is an expensive material.
8. Limited for small size.
VACUUM MOLD CASTING
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Vacuum molding, also called the V-process, was developed in Japan around 1970. It uses a
sand mold held together by vacuum pressure rather than by a chemical binder.
Accordingly, the term vacuum in this process refers to the making of the mold rather
than the casting operation itself. The steps of the process are explained in Figure below.
Because no binders are used, the sand is readily recovered in vacuum molding. Also, the
sand does not require extensive mechanical reconditioning normally done when binders
are used in the molding 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.
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INVESTMENT CASTING
Steps in investment casting are described in Figure 11.8. Since the wax pattern is melted
off after the refractory mold is made, a separate pattern must be made for every casting.
Pattern production is usually accomplished by a molding 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. In high
production operations, several patterns are attached to a sprue, also made of wax, to
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In investment casting, a pattern made of wax is coated with a refractory material to
make the mold, 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. The process dates back to ancient Egypt
and is also known as the lost-wax process, because the wax pattern is lost from the mold
prior to casting.
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form a pattern tree; this is the geometry that will be cast out of metal. 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 mold 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 mold (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 mold is allowed to air dry for
about 8 hours to harden the binder.
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PRODUCT DESIGN CONSIDERATIONS
If casting is selected by the product designer as the primary manufacturing process for a
particular component, then certain guidelines should be followed to facilitate production
of the part and avoid many of the defects enumerated in Section 11.5. Some of the
important guidelines and considerations for casting are presented here.
Geometric simplicity: Although casting is a process that can be used to produce complex
part geometries, simplifying the part design will improve its castability. Avoiding
unnecessary complexities simplifies mold making, reduces the need for cores, and
improves the strength of the casting.
Corners: Sharp corners and angles should be avoided, because they are sources of stress
concentrations and may cause hot tearing and cracks in the casting. Generous fillets
should be designed on inside corners, and sharp edges should be blended.
Section thicknesses: Section thicknesses should be uniform in order to avoid shrinkage
cavities. Thicker sections create hot spots in the casting, because greater volume
requires more time for solidification and cooling. These are likely locations of shrinkage
cavities. Figure 11.24 illustrates the problem and offers some possible solutions.
Draft: Part sections that project into the mold should have a draft or taper, as defined in
Figure 11.25. In expendable-mold casting, the purpose of this draft is to facilitate removal
of the pattern from the mold. In permanent-mold casting, its purpose is to aid in removal
of the part from the mold. Similar tapers should be allowed if solid cores are used in the
casting process. The required draft need only be about 1_ for sand casting and 2_ to 3_
for permanent-mold processes.
Use of cores: Minor changes in part design can reduce the need for coring, as shown in
Figure 11.25.
Surface finish: Typical surface roughness achieved in sand casting is around 6 mm (250 min). Similarly poor finishes are obtained in shell molding, while plaster-mold and
investment casting produce much better roughness values: 0.75 mm (30 m-in) Among
the permanent-mold processes, die casting is noted for good surface finishes at around 1
mm (40 m-in).
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Dimensional tolerances: There are significant differences in the dimensional accuracies
that can be achieved in castings, depending on which process is used. Table 11.2 provides
a compilation of typical part tolerances for various casting processes and metals.
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Machining allowances: Tolerances achievable in many casting processes are insufficient
to meet functional needs in many applications. Sand casting is the most prominent
example of this deficiency. In these cases, portions of the casting must be machined to
the required dimensions. Almost all sand castings must be machined to some extent in
order for the part to be made functional. Therefore, additional material, called the
machining allowance, is left on the casting for machining those surfaces where
necessary. Typical machining allowances for sand castings range between 1.5 mm and 3
mm (0.06 in and 0.12 in).
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THE BASIC PERMANENT-MOLD PROCESS
Permanent-mold casting uses a metal mold constructed of two sections that are designed
for easy, precise opening and closing. These molds 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 molds
include aluminum, magnesium, copper-base alloys, and cast iron. However, cast iron
requires a high pouring temperature, 12500C to 15000C, which takes a heavy toll on mold
life. The very high pouring temperatures of steel make permanent molds unsuitable for
this metal, unless the mold is made of refractory material.
Cores can be used in permanent molds 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-mold casting.
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Steps in the basic permanent-mold casting process are described in Figure 11.10. In
preparation for casting, the mold 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 mold surfaces for easier
separation of the cast product. After pouring, as soon as the metal solidifies, the mold is
opened and the casting is removed. Unlike expendable molds, permanent molds do not
collapse, so the mold must be opened before appreciable cooling contraction occurs in
order to prevent cracks from developing in the casting.
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Advantages of permanent-mold casting include good surface finish and close dimensional
control, as previously indicated. In addition, more rapid solidification caused by the metal
mold results in a finer grain structure, so stronger castings are produced. 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 mold), and the
expense of the mold. Because mold cost is substantial, the process is best suited to highvolume production and can be automated accordingly. Typical parts include automotive
pistons, pump bodies, and certain castings for aircraft and missiles.
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DIE CASTING
Die casting is a permanent-mold casting process in which the molten metal is injected
into the mold cavity under high pressure. Typical pressures are 7 to 350 MPa. The pressure
is maintained during solidification, after which the mold is opened and the part is
removed. Molds in this casting operation are called dies; hence the name is 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-mold 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 mold, and keep them closed while the liquid metal
is forced into the cavity. The general configuration is shown in Figure 11.12.There are two
main types of die casting machines: (1) hot-chamber and (2) cold-chamber, differentiated
by how the molten metal is injected into the cavity.
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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. The casting cycle is summarized in Figure 11.13.
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.
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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 Figure 11.14.
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 hotchamber process usually favor its use on these metals.
Molds used in die casting operations are usually made of tool steel, mold steel, or
maraging 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. Single-cavity dies are shown in Figures 11.13 and 11.14. Ejector pins are
required to remove the part from the die when it opens, as in our diagrams. These pins
push the part away from the mold surface so that it can be removed. Lubricants must
also be 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 be 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. 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 be trimmed from the casting, along with the sprue and
gating system.
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Advantages of die casting include (1) high production rates possible; (2) economical for
large production quantities; (3) close tolerances possible, on the order of 0.076mm for
small parts; (4) good surface finish; (5) thin sections are possible, down to about 0.5mm;
and (6) rapid cooling provides small grain size and good strength to the casting. The
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.
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