Casting

Materials Processing
• Materials processing is the science and technology by
which a material is converted into a useful shape with a
structure and properties that are optimized for the service
environment.
• Materials processing by hand is as old as civilization;
mechanization began with the Industrial Revolution of the
18th century, and in the early 19th century the basic
machines for forming, shaping, and cutting were
developed, principally in England. Since then, materialsprocessing methods, techniques, and machinery have
grown in variety and number.
• Numerous processes and operations can be involved in the
manufacture of products and components and many of
them are associated with the production of a desired
shape.

Categories of manufacturing
processes:
• The processes used to convert raw materials
into finished products perform one or both of
two major functions: first, they form the
material into the desired shape; second, they
alter or improve the properties of the
material. Manufacturing processes can be
categorized on the basis of liquid and solid
states.

Casting Process
• The processing of materials in liquid state is
commonly known as casting, in which the
molten material (mostly metals) is converted
into a desired shape by pouring the material
into the mold. Cast products can have
extremely complex shapes but also posses
structures that are produced by solidification
and are therefore, subject to such defects as
shrinkage and porosity.

Mechanical manufacturing processes
• Forging, rolling, extrusion, wire drawing, swaging, roll
forming, deep drawing are the mechanical manufacturing
processes, are performed in solid state. These are
performed on the basis of tendency to deform which can
be referred as deformation processes, using temperature
dependence. Deformation processes exploit the ductility of
certain materials mostly metals and produce the desired
shape by mechanically moving or rearranging the solid
though a phenomenon known as plasticity. These processes
can have high rates of production, but generally require
powerful equipments and dedicated tools or dies. Their
brief description is as follow:
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Forging is a manufacturing
process involving the shaping of metal
using localized compressive forces.
Forging is often classified according to
the temperature at which it is
performed: "cold," "warm," or "hot"
forging or at room temperature. Warm
or hot forgings are performed for the
less ductile materials. Highly ductile
can be forged at room temperature. It
is usually hot working process.

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Forging

Rolling

Rolling is a metal forming process in
which metal piece is passed through a
pair of rolls to lessen the crossectional
area or thickness to get the required
shape. Rolling is classified according to
the temperature of the metal rolled. If
the temperature of the metal is above
its recrystallization temperature, then
the process is termed as hot rolling. If
the temperature of the metal is below
its recrystallization temperature, the
process is termed as cold rolling. It is
usually hot working process.

– Extrusion

• Extrusion is a process used to
manufacture objects of a
fixed cross-sectional profile. A
material is forced through
a die of the desired crosssection. It is also classified as
warm, hot and cold extrusions
depending upon the material
nature. It is usually hot
working process.
– Wire drawing

• Wire drawing is a
manufacturing process in
which a metal piece is pulled
through a die orifice by
means of a tensile force is
applied from the outside. It is
usually cold working process.

Machining Processes
• Machining is a term used to describe a variety of
material removal processes in which a cutting tool
removes unwanted material from a work piece to
produce the desired shape. It can be done at room
temperature. The removal processes are capable of
outstanding dimensional precision, but produce scrap
when material is cut away to produce the desired
shapes.
• Basically it is used to finish the component, to make
the accurate size. Punching, blanking and piercing,
stamping are the well known machining processes.
Their brief description is as follow:
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Punching is a forming process that
uses a punch press to force a tool,
called a punch, through the work
piece to create a hole via shearing.
The punch often passes through the
work into a die. Punching is often the
cheapest method for creating holes in
sheet metal in medium to high
production volumes.
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Stamping

It is a type of forging.
Stamping includes a variety of sheetmetal forming manufacturing
processes, such using a machine
press as stamping press, This could be
a single stage operation where every
stroke of the press produce the
desired form on the sheet metal part,
or could occur through a series of
stages. The process is usually carried
out on sheet metal.
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Punching

Blanking and piercing

Blanking and piercing are shearing pro
cesses in which a punch and die are
used to modify webs. The tooling and
processes are the same between the
two, only the terminology is different:
in blanking the punched out piece is
used and called a blank; in piercing
the punched out piece is scrap.

Consolidation or joining processes
• It is a technique of manufacturing. This process is
used to join the different components to join
together to get a single article according to the
requirement. Complex products can often be
assembled from simple shapes but the joint areas
are often affected by the joining process and may
possess characteristics different from the original
material. Welding, Brazing, soldering and
adhesive bonding and mechanical joining are the
well known techniques.
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– Welding

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It is a fabrication that joins
materials,
usually metals or thermoplastics,
by causing coalescence (to join
into a single mass). It is mostly
used for the assembling purposes.
– Brazing

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Brazing is a metal-joining process
whereby a filler metal is heated
above and distributed between
two or more close-fitting parts
by capillary action (the ability of a
liquid to flow against gravity
where liquid spontaneously rises
in a narrow space such as a thin
tube).
– Soldering

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Soldering is a process in which
two or more metal items are
joined together by melting and
flowing a filler metal into the
joint, the filler metal having a
lower melting point than the work
piece.

– Adhesive bonding

• Adhesive bonding is used to fasten
two surfaces together, usually
producing a smooth bond. This
joining technique involves glues,
epoxies, or various plastic agents
that bond by evaporation of a
solvent or by curing a bonding
agent with heat, pressure, or time.
– Mechanical joining

• Mechanical Joining is a process for
joining parts through mechanical
methods, which often involve
threaded holes. Joining parts using
screws or nuts and bolts are
common examples of mechanical
joining.
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Powder Metallurgy
• It is a technique of manufacturing components in which we perform
working on the powder of metals. That powder metal is converted
into the required component. High accuracy is achieved in this
process. It is used only for small component manufacturing. In this
process, no finishing and machining is required.
– Steps:

• Powdering of metal
• Selection of binder
• Mixing or blending with other powders to make homogeneous
mixture.

• Compacting (the process of shaping metal powder in a die
through the application of high pressures.)
• Sintering (it is an agglomeration process in which recrystallization of
the mineral is achieved. The process of sintering is used to remove
the porosity and increase the strength of materials. In this, the
material does not melt.)
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Casting Technology
Casting is a manufacturing
process where a solid is
melted, heated to proper
temperature (sometimes
treated to modify its
chemical composition),
and is then poured into a
cavity or mold, which
contains it in the proper
shape during
solidification. Thus, in a
single step, simple or
complex shapes can be
made from any metal
that can be melted. The
resulting product can
have virtually any
configuration the
designer desires.

Types of Casting processes
• The casting process is subdivided into two main
categories: expendable and non-expendable casting. It
is further broken down by the mold material, such as
sand or metal, and pouring method, such as gravity,
vacuum, or low pressure.
• Expendable mold casting:
• Expendable mold casting is a generic classification that
includes sand, plastic, shell, plaster, and investment
(lost-wax technique) moldings. This method of mold
casting involves the use of temporary, non-reusable
molds. Sand casting, Plaste mold casting, Shell
molding, Investment casting, Evaporative-pattern
casting are the types of expandable mold casting.

Non-expendable mold casting
Non-expendable mold casting differs from
expendable processes, in that the mold need
not be reformed after each production cycle.
This technique includes at least four different
methods: permanent, die, centrifugal, and
continuous casting.

Advantages of Casting process:
There is no limitation of producing
castings of any size or weight.
This advantage is not in other
manufacturing processes.
Complicated shape castings can be
produced. Complicated patterns
are key to complex shapes.
Hollow castings can be produced.
Any composition of materials can
be used for producing required
casting. Any composition can be
achieved. Mechanical
manufacturing processes can do
this job.
Some casting processes are net
shape; other is near net shape.

Dimensional modification is achieved.
Dimensional accuracy is achieved. Machining is
not required but surface finishing is required.
In sand casting the finishing is required and
machining also but for investment casting,
machining is not required due to high
accuracy but finishing can be required.
Metal casting is a process highly adaptable to
the requirements of mass production. Large
numbers of a given casting may be produced
very rapidly.

• Weight saving is possible. Component made with
casting process is lighter than the component made
with other machining processes.
• Casting can be made with hair like precision or
accuracy provided proper molding and casting
technique is employed.
• Only castings have the advantage of fibrous structure.
Casting leaves component with its solid fibrous
structure which inherit great compressive strength. So,
component subjected to compressive strength are
made with casting.
• In soft material casting, melting is not always
necessary, in some cases only pressing deformation is
enough for soft material that is entered into the cavity.
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Metallurgical Advantages of Casting of Metals:

• This process can give any required micro structure to obtain the
required mechanical properties. We can change the microstructure
by changing the grain boundaries.
• Grain size control is possible in casting process by controlling the
rate of solidification. High rate of solidification gives fine size grain
which has high strength as compared to coarse size that is achieved
at low rates of solidification. During the solidification, the
uniformity of crystallization gives strength. So mechanical
properties are controlled by the solidification rate.
• If we perform sintering of the casting, high dense structure can be
achieved.
• Strength and lightness in certain light metal alloys, which can be
produced only as castings.
• Casting provides versatility. Wide range of properties can be
attained by adjusting percentage of alloying elements.
• Casting is one of cheapest method for mass production.
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Limitations of Casting process:
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Though casting is cheapest for Mass Production, it becomes non economical
for JOB production.
Sand casting leaves rough surface which needs machining in most of cases. It adds
up the cost in production.

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Cast products are superior for compressive loads but they are very poor in tensile
or shock loads. They are brittle.

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In sand casting, porosity is achieved.

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To manufacture small sized castings with high accuracy, good machining and
finishing; the type of casting chosen raise to go expensive.

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It is almost impossible to design a part that cannot be cast by one or more of the
commercial casting processes. But it will be the skill of a manufacturer to adopt
such a casting process to get
a) good results and
b) lowest cost expenditures.
The various casting process are distinguished primarily by the mold material
(whether sand, metal or other material) and pouring method (gravity, vacuum, low
pressure or high pressure). All share the requirement tht the material should
solidify in a manner that will maximize the properties and avoid the formation of
defects.

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• Basic Requirements of Casting processes
• (For both expendable and non-expendable)
1- Mold Production
• The material for making mold is sand, wax, ceramic materials and
metallic materials. Metallic materials are used for specifically for
non-expendable use mold.
• The mold cavity having the desired shape and size must be
produced with due allowance for shrinkage of solidifying material.
Grey cast iron is used that doesn’t shrink due to the presence of the
atoms of carbon atoms fibers at their place and has the ability to
resist high pressure and to resist the vibration hazard effects while
white cast iron shrinks.
• Strength of casting is very important. Engine blocks are made of
steel alloys to resist vibration. If simply steel is used, then it will be
deformed earlier. The alignment is disturbed.
• Either attempt single-use molds or double-use molds, but keep in
front the economical factor. The more economical single-use molds
are usually preferred for the production of smaller quantities.
• A sand mold is shown in the figure below:

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Production of mold must have following parts:
The flask is the rigid metal or wood frame that holds the molding aggregate.
Cope is the name given to the top half of the flak or mold.
Drag refers to the bottom half of the flask or mold.
Core is a sand shaped that is used to produce the hollow castings.
Core print is added to the pattern, core or mold and is used to locate and support
the core within the mold.
The molten metal and core combine to produce the Mold Cavity, the shaped hole
into which the molten metal is poured and solidified to produse desried casting.
Riser is made for the information of comlete pouring of metal in the mold.
The Runner gives a flow to molten metal to mold cavity to compensate the
shrinkage.
Gating System (pouring cup+sprue+runner)) is the network of connected channels
used to deliver the molten metal to the mold cavity.
Metal travels down a sprue is the vertical portion of the gating system.
Pouring cup is the portion of gating system that initially recieves the molten metal
and control its delievery to the rest of the mold.
Vents for the escape of gases.
The parting line is the interface that separate cope and drag in the flask.
Draft is the on the pattern or casting that permits to be withdrawn from the mold.
Casting is used to describe both the process and the product.

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2- Melting Process
A melting process must be capable of providing
molten material at the proper temperature and in
the desired quantity of acceptable quality and at
reasonable cost.
To get the good quality, we have to decrease the
tendency of metal to react with oxygen, to avoid
the formation of oxide (corrosion). Because if
metal converts into oxide, then the melting of
pure metal will not be achieved. In this case we
will get the molten oxidezed metal that will make
the casting not fair. Non ferrous metals have high
tendency of forming oxides than the ferrous
metals. So avoid oxidation. For this purpose flux
is added in metal during the metal process. The
covering flux is added to avoid oxidation. This flux
flows on the molten metal to provide the barrier
to the contact of air with metal. For Al, chloride
flux is used mostly. Since iron has low tendency to
react with air, so in this case, the covering flux is
not urgently required but the use of flux is better.
Cleaning fluxes are added in the molten metal to
separate out the slag to avoid the porosity and
weakness in the strength of mold for casting.
To remove the air bubbles from the molten
metal, it is known as de-oxidation and for this
purpose agents used are known as de-oxidizers.
For alloying elements, Ferro alloys are used as deoxidizers that give the de-oxidation, as well as
alloying addition. Ferro alloys (M.P ranges 130014000C) are the alloys of ferrous and non ferrous
e.g. Fe-W, Fe-Si, Fe-Mo. If we don’t require the
addition of alloying elements, then Al is added as
de oxidizer.

3- Pouring Tech It is a technique by which the
moulds are filled in with molten metal.
There must be provisions for air or gases inside
the mould to come out when the molten metal
is poured in. When the hot metal enters the
mould cavity, it may generate various gases
due to chemical reactions. The mould design
should allow these gases to escape, so that the
molten metal can spread and fill the mould
cavity completely. It helps in producing defect
free, fully dense and quality casting
components.
Prior to the pouring, the mold produced must
be heated to increase the strength of mold by
the escape of gases, for no crack, to avoid
porosity and thermal instability. The mold can
burst due to the pressure of gases.
For the escape of gases, besides provisions,
some ingredients can also be applied. For
example wood charcoal.
Whenever the mold is used for pouring of
metal, pour gate is organized.

• 4- solidification process, better design and control at
this stage helps in getting quality output. Required
mechanical properties of the casting can be achieved
by controlling the rate of solidification of the molten
metal.
5- After proper solidification, the casting should be
removed from the mould i.e. casting removal.
Generally, expendable moulds are broken apart and
destroyed after each casting is produced without any
difficulty. But, using re-usable moulds may cause major
challenges from designers' point of view on the
removal of casting from permanent moulds.
6- Various cleaning, finishing and inspection
operations are performed after the casting removal
from the mould.

Pattern
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A pattern is simply the duplicate or model or replica of the component which has
to be manufactured by the casting process.
Important property of a Pattern
It is slightly larger in the size than the original size casting to be produced because
a pattern has:
Shrinkage allowance:
Almost all the metals shrink or contract volumetrically after solidification
and therefore the pattern to obtain a particular sized casting is made over sized by
an amount equal to shrinkage or contraction.
It depends upon mainly on the cast metal or alloy used for the casting and pouring
temperature of the metal or alloy. The shrinkage allowance given to pattern for
Aluminum casting is not suitable for the steel casting.
Machining allowance
It is given to the pattern for surface finishing of the casting produced, to
remove surface imperfections, to require exact casting dimensions (actual size
achievement by grinding).
How much machining allowance is applied to the pattern, depends upon:
Nature of metal: i.e. Ferrous and non-ferrous. Ferrous metals get scaled whereas
non-Ferrous ones don’t.
Size and shape of casting: Longer castings tend to warp and need more allowance
to be added to ensure that after machining the casting will be alright.

Selection of material for Pattern
• The following factors assist in selecting proper pattern
material:
a) The number of castings to be produced. Metal pattern
are preferred when the production quantity is large.
b) The desired dimensional accuracy and surface finish.
c) Nature of molding process. E.g. sand casting frequently
use wood pattern while investment casting require wax
pattern.
d) Shape, complexity and size of casting.
e) The chances of repeat order.

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Materials used for making Patterns
Wood
The most common material for making pattern for sand casting is the wood.
Advantages:
1.
in expensive and easyly available.
2.
for mass scale production.
3.
Easy machining and shaping.
4.
Easy to obtain good surface finish.
5.
Can be used for complex shapes and large castings.
6.
Light in weight.
7.
Can be repaired easily.
Limitations:
Absorbs moisture and hence swelling is there.
Short life patterns.
Poor wear and abrasion resistance.
Cannot withstand rough handling.
Weak as compared to metallic patterns.
Applications:
Wooden pattern are used where the number of castings to be produced is small
and the pattern size is large.
Recently, ply wood (synthetic material like wood) is used for the heavy castings
(40-50 tons). Natural wooden pattern used for the normal sized castings.

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Metals
Metal patterns are cast from wooden patterns.
Advantages:
They do not absorb moisture.
They possess life much longer than wooden patterns.
They do not warp.
Give good surface finish.
Excellent wear and abrasion resistant.
Good machine ability.
Limitations:
Expensive
Heavy in weight than wooden patterns.
A chance of rusting/oxidation.
Not easily repaired (Al patterns).
Machining is not easy as wooden patterns.
Applications:
Wooden and metallic patterns are used for mass scale production.
Patterns are also used, made by plastic, rubber, plaster and wax
(investment casting) and poly styrene.

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Wax
Wax patterns are used in investment casting.
Advantages:
Gives good surface finish
Has ceramic coating
High dimensional accuracy
Not need machining
After being molded, the wax pattern is not taken out of the mold
like other patterns as wooden, rubber and metal patterns; rather
the mold is inverted and heated; the molten wax comes out.
Wax can be recycled.
Important notes:
Only both the wax and poly styrene patterns are consumable
patterns. Both evaporate but wax has ability to only melt also. So
wax (molten obtained after molding) can be recycled but
polystyrene cannot be recycled (due to its fuming on heating).
Wood, rubber, metal patterns etc. are not consumable because
they are taken out of the mold after molding.

Four Important Qualities of Molding/Core Sands
• The sands used for making molds must have following four important
qualities:
• Refractoriness
• It is the ability to withstand high temperatures without melting, fracture
or deterioration. Refractoriness is provided by the basic nature of the
sand. This property is very important for very high M.P metals/alloys such
as steel whose high M.P depends upon the ingredients. Zircon sand has
highest refractoriness and can be used to make sand mold. For low
melting M.P metals, silica sand or chromite sand can be used.
• Cohesiveness
• Cohesiveness is the ability of molding sand to retain a given shape when
packed into a flask. It is obtained by using binders such as clay (bentonite,
kaolinite or illite) that becomes cohesive when moistened. Molasses (not
for cores, only for molding sand), resins, oils and sodium silicate are also
used. (Volcanic ash)
• Permeability
• It is the ability of mold or core to escape gases through the sand. It is a
function of the size and shape of sand particles, the amount and type of
clay used or any binder, the moisture content and the compacting
pressure. Gases are produced due to sand ingredients also. If permeability
is not there, there is a chance for cracking. Due to retention of air or gases
in the molten metal, casting defects may be produced.
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To increase permeability, saw dust or wood dust is added (in large
amount for fine particle sands). For the facilitation, vents are
produced through the mold.
• Collapsibility
• It is the ability to accommodate metal shrinkage after solidification
and provides easy removal of the casting through the mold on its
disintegration (shakeout).
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This property is sometimes enhanced by adding cereals or
other organic materials, such as cellulose that burn out when they
come into contact with the hot metal. This property can also be
enhanced by adding talcum powder, graphite powder. Graphite
powder plays role to avoid stickness of casting to the molding sand
cavity.
• Types of Molding Sands
• Natural Molding Sands
(2) Synthetic Molding Sands
• Natural Molding Sands:
• Natural sands are simply the base sands and natural sands have 510% water and 10-15% clay. They have some good properties
required for the molding but are not perfect.

• Types of Base Sands and their Properties:
• It is the type of sand which is used to make the mold with binder. They
have some applications without binders.
• Silica Sand
• On beach, coastal areas, in river beds mostly in India, Pakistan and Brazil
of good quality. Sub-continent is rich in good quality sand.
• Composition depends upon geological history, but main contents are not
changed. Main component is silica (SiO2). It has 94 to 98 % silica.
• Fusion point is 17600C (pure) but usually less melting point due to the
presence of impurities. For high melting temperature metals, it is not used
For Aluminum (low M.P), brass, bronze it is good to use but for steel (high
M.P) it is not good.
• It has poor refractoriness. It is mixed with other sands to get the good
refractoriness.
• Bad surface finish. Machining is required in the sand used casting process.
• It is chemically reactive with certain basic metals.
• It has high thermal expansion which can cause casting defects with high
melting point metals.
• It has low thermal conductivity which can lead to unsound casting.
• Because of common and abundance, it is of low cost.

• Olivine Sand((Mg,Fe)2SiO4.
• Olivine is a mixture of orthosilicates of iron and magnesium
from the mineral dunite.(Dunite granular, green igneous
rock composed of coarse grains of olivine, is the source of
the world's supply of chromium. It weathers to a dun
brown color)
• It is free from silica.
• It has good fusion point of 17600C.
• It has good thermal stability so has good refractoriness.
• Because it is free from silica, therefore it can be used with
basic metals, such as manganese steels.
• Olivine sand has high thermal conductivity.
• Olivine has low thermal expansion.
• This sand gives good surface finish.
• It is relatively high costly than the silica sand.

• Chromite Sand
• It is the solid solution of spinels.(MgAl2O4 )
• Chromite (Cr2O4) is the main content. Silica is in
very small amount.
• It has high fusion point of 18500C.
• It has good refractoriness.
• It has good chemical inertness.
• It has high thermal conductivity.
• It has low thermal expansion.
• It gives good surface finish.
• This sand is rare, so it is expensive. Therefore it’s
only used with expensive alloy steel casting and
to make cores.
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• Zircon Sand
• Zircon sand is a compound of approximately twothirds zirconium oxide (Zr2O) and one-third silica.
• Zircon persists in sedimentary deposits and is a
common constituent of most sands.
• It has the highest fusion point of all the base sands
at 2,600 °C.
• It has very low thermal expansion.
• It has a high thermal conductivity.
• Because of these good properties it is commonly used
when casting alloy steels and other expensive alloys.
• This sand gives good surface finish.
• It is expensive and not readily available.

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Chamotte
Chamotte is made by calcining fire clay (Al2O3-SiO2) above 1,100 °C.
Its fusion point is 1,750 °C i.e. gives relatively low refractoriness.
It has low thermal expansion.
This is mixed with other sands to improve refractoriness.
Its disadvantages are, coarse grains, which result in a poor surface finish.
It is the second cheapest sand but it is still twice as expensive as silica.
Silica and chamotte sands have bad surface finish and chromite, olivine
and zircon sands have good refractoriness but they all have not sufficient
cohesiveness. So these natural sands are not perfect in properties. So we
prepare synthetic molding sands using moisture, binders, additives etc.
Important Note:
Corrosion is the reverse of extraction.
Synthetic Molding Sand:
The main ingredients of any synthetic molding sand are:
Base sand,
Binder, and
Moisture
Molding sands are prepared synthetically to produce the good molding
properties.

Binders and their types
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Binders are used to adhere the particles of sand to each other for the
strength of molds. Or it is also defined as it is the glue that holds the mold
together. Binders improve cohesiveness.
Types of Binders:
Clay and water
Oil
Resin
Sodium silicate
Clay and water:
A mixture of clay and water is used as binder.
It is used where the clay content is less than 15% in the molding sand.
Oils:
Oil use as a binder has restricted conditions.
Long time use or overheating makes the mold hard and brittle.
However due to their increasing cost, they have been mostly phased out.
The oil also required careful baking at 100 to 200 °C to cure, otherwise
wasting of mold.

• Resins:
• Resin binders are natural or synthetic high melting
point gums.
• These are sticky and in liquid form.
• The two common types used are urea
formaldehyde (CH2O)(UF) and phenol
formaldehyde (PF) resins.
• PF resins have a higher heat resistance than UF
resins and cost less.
• There are also cold-set resins, which use
a catalyst instead of a heat to cure the binder.
• Resin binders are quite popular because different
properties can be achieved by mixing with various
additives.
• Other advantages include good collapsibility, low
gassing, and they leave a good surface finish on
casting.

• Sodium Silicate:
• Sodium silicate [Na2SiO3] is a
high strength binder used with
silica molding sand.
• To cure the binder, carbon
dioxide gas is used, after making
mold, which creates the
following reaction:
• The mold produced using this
binder is hard but has excellent
surface quality.
• Na2O(SiO2) + CO2---------Na2CO3 + SiO2 + Heat

Shell Molding
• Introduction:
• Shell molding is newest of casting processes. Shell molding
replaces conventional sand molds by shell molds made up of
relatively thin, rigid shells of approximately uniform wall thickness.
The molten metal is filled in the shell mold and then allowed to
solidify.
• Steps:
• Metal pattern production
• First of all, a metal or steel pattern having the profile of the
required casting. Let say a flask is to be produced. Cop and drag
part of the steel pattern are produced.
• Preparation of molding sand
• The mixture of silica sand and the thermosetting phenolic resin is
used to produce the molding sand to produce the mold shell. The
phenolic resin gives good strength and epoxide resin gives better
strength. There use is depending upon the metal used to be cast.
•

• Preparation of the shell
• There are two methods to produce the shell. The
patterns are sprayed with a solution of lubricating
agent or release agent containing silicone to prevent
the shell sticking to the metal pattern in both the
cases.
• (1) The steel pattern is preheated in oven 175-270oC.
The pattern is placed over the dump box and then
inverted. The molding sand is dumped over the steel
pattern. Heat from the pattern partially cures the
molding sand and the shell is produced of desired
thickness drying and makes the resin sticky present in
it. The pattern and sand mixture are then inverted,
allowing the excess (uncured) sand to drop free. The
pattern with adhering shell is then placed in an oven,
where additional heating completes the curing process
to give rigidity.

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(2) The sand mixture is spread over the steel pattern and then it is heated in the
oven up to 600-700oC. The resin hardens and hence the shell is produced.
The thickness of the shell depends on the pattern temperature and time of contact
but typically ranges between 10 and 20 mm. High temperatures produces high
thickness of the shell.
Stripping of the shell
The hardened shell, with tensile strength between 350 and 450 psi, is then
stripped from the steel pattern.
Clamping of two or more shells in a pouring jacket
Two or more cooled shells are then clamped or glued together with a
thermosetting adhesive to produce a mold. Before this, we use carbon powder or
talcum powder as parting agent for easy disintegration or to release easily. To
provide extra support during the pour of molten metal, shell molds are often
placed in a pouring jacket surrounded by the sand or steel shots.
Pouring of molten metal
The molten metal is poured into the shell. The metal should not come out of
parting line. Otherwise, we will require machining. The heat of molten metal starts
burning resin binder of the mold and the gases evolved escape through the
permeable shell walls.
To get the casting
By the time the casting has solidified, the binder has completely burn out and the
shell mold disintegrates easily. So, casting is extracted. Since shell is no re-useable
so it is an expendable mold casting process.

• Disadvantages:
• Not for heavy
castings for ferrous
or non-ferrous
(only up to 10 kg).
• Steel pattern cost
is high.
• Resins costs are
comparatively
high.
• Uneconomical on
small scale
production.

Advantages:
For complex shape castings.
For both ferrous and non-ferrous.
No separate runner, risers and sprue
are not required.
Excellent surface finish.
High dimensional accuracy.
Use of resin produces smooth surface
of casting.
Cleaning, machining and other
finishing cost can be significantly
reduced.
Low labor cost.
Thin shells provide easy permeability.
The burn out resin gives good
collapsibility and shakeout
characteristics.
Pouring jacket sand is re-useable.

INVESTMENT CASTING
• Introduction:
• It is a very old process-used in ancient china and Egypt and most recently
performed by dentists and jewelers for a number of years. Products such
rocket components and jet engine turbine blades required the fabrication
of high precision complex shapes from high melting point metals that are
not easily machined. It offers unlimited freedom to complexity of shapes
and types of materials to be cast.
• After being molded, the wax pattern is not taken out of the mold like
other patterns like wooden, rubber and metal patterns.
• This process is also known as lost wax casting process because wax pattern
during process melts on heating and comes out of the mold, in which the
molten metal is poured.
• Steps:
• Production of master pattern
• A modified replica of the desired product made from metal, steel, plastic
or wood is made for the production of master die.
• Production of master die from the master pattern
• A die is produced from the master pattern usually of metal or steel. It is
made of usually of Al because Al has tendency to extract heat very rapidly
and material cools down rapidly after melting.

• Production of wax patterns
• Wax patterns are made by pouring molten wax into the
master die and allowing it to harden. Release agents, such
as silicone sprays or talcum powder are used to assist in
pattern removal from the master die.
• Assembling of wax pattern on common wax sprue
• Using heated tools and melted wax, a number of patterns
can be attached to a central wax sprue and runner system
to create a pattern cluster or a tree. If the product is
sufficiently complex that is pattern could not be withdrawn
from a single master die, the pattern may be made in
pieces and assembled prior to attachment.
• Coating of the cluster or tree with a thin layer of
investment material
• The wax pattern and wax sprue assembly is dipped into
water slurry of finely ground refractory material. A thin but
very smooth layer of investment material is onto the wax
pattern, ensuring a smooth surface and good details in final
product.

• Investment of the wax pattern assembly for the production of mold
• After the initial layer of ceramic slurry, wax pattern assembly is then
invested to produce mold. The initially invested wax pattern assembly is
re-dipped in the ceramic slurry but this time, refractory sand is showered
on the wet ceramic and then dried for high temperature uses. Repeat the
procedure to get the required thickness of the shell. Allow the investment
to fully harden.
• Removal of wax pattern from the mold
• The wax pattern is removed from the investment mold by inverting the
mold, melting the wax pattern in the oven at 300-400oC. The melted wax
pattern comes out. Due to this step this process is called lost-wax process.
• Heating of mold prior to pouring of molten metal
• After removing the wax pattern, investment mold is heated at 800-900oC.
This baking ensures complete removal of wax from the mold, cures the
mold to give the aided strength, and allows the molten metal to retain its
heat and flow more readily into all of thin details and sections and good
dimensional accuracy.
• Filling of unbounded sand in the flask around the investment shell mold:
• That investment shell mold is kept in flask and filled with the unbounded
sand giving vibration to ensure the compaction, to provide extra support
during the pour of molten metal.

• Pouring of molten metal
• Molten metal is poured into the investment mold by
simply under gravity to ensure the complete filling of
mold. When complex thin sections are involved, the
molten metal is poured assisted by positive air
pressure.
• Removal of solidified casting from the mold
• After solidification, techniques such as mechanical
vibration or sand blasting are used to break the mold
and remove the mold material from the metal casting.
• Separation of casting from sprue
• For this purpose cutting or machining is applied, to get
the castings in their useable form.
• Inspection and testing
• Inspection and testing is done on the sample of casting
to investigate about its quality and standard
achievement.

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Advantages:
Complex shapes can be cast as single piece.
Mass scale and high rate of production.
Thin sections can be produced.
Excellent dimensional accuracy.
Excellent details and smoother surface.
Machining can be completely eliminated or greatly reduced.
Castings do not contain any disfiguring parting line.
Sounder and denser castings free from defects.
Wax melted is re-usable.
The economic value of this process lies in its ability to produce
intricate shapes in various alloys that could probably not be
produced at all by another casting process.
Disadvantages:
A complex process and expensive.
High cost of dies to make the wax pattern.
For small casting 2-2.5 kg.
Pattern is expendable i.e. one wax pattern is to make one
investment mold.
Slow process.

Casting

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