Pattern and Mould
– A pattern is made of wood or metal, is a replica of the
final product and is used for preparing mould cavity
– Mould cavity which contains molten metal is
essentially a negative of the final product
– Mould material should posses refractory
characteristics and with stand the pouring
– When the mold is used for single casting, it made of
sand and known as expendable mold
– When the mold is used repeatedly for number of
castings and is made of metal or graphite are called
– For making holes or hollow cavities inside a casting,
cores made of either sand or metal are used.
– Several types of furnaces are available for
melting metals and their selection depends
on the type of metal, the maximum
temperature required and the rate and the
mode of molten metal delivery.
– Before pouring provisions are made for the
escape of dissolved gases. The gating
system should be designed to minimize the
turbulent flow and erosion of mould
cavity.The other important factors are the
pouring temperature and the pouring rate.
Solidification and Cooling
– The properties of the casting significantly depends
on the solidification time cooing rate.
– Shrinkage of casting, during cooling of solidified
metal should not be restrained by the mould
material, otherwise internal stresses may develop
and form cracks in casting.
– Proper care should be taken at the design stage of
casting so that shrinkage can occur without casting
Removal, Cleaning, Finishing and Inspection
– After the casting is removed from the mould it is
thoroughly cleaned and the excess material usually
along the parting line and the place where the
molten metal was poured, is removed using a
– White light inspection, pressure test, magnetic
particle inspection, radiographic test, ultrasonic
inspection etc. are used
Production steps in sand casting including
pattern making and mold making
Variety of patters are used in casting and the
choice depends on the configuration of casting
and number of casting required
Follow board pattern
Cope and drag pattern
Match plate pattern
Major part of Moulding material in sand casting are
70-85% silica sand (SiO2)
10-12% bonding material e.g., clay cereal etc.
Requirements of molding sand are:
The performance of mould depends on following
Effect of moisture, grain size and shape
on mould quality
Melting and Pouring
The quality of casting depends on the method of melting. The
melting technique should provide molten metal at required
temperature, but should also provide the material of good quality
and in the required quantity.
Molten metal is prevented from oxidation by covering the molten metal with
fluxes or by carrying out melting and pouring in vacuum
Ladles which pour the molten metal from beneath the surface are used
The two main consideration during pouring are the temperature and pouring
Fluidity of molten metal is more at higher temperature but it results into more
amount of dissolved gases and high temperature also damage the mould
walls and results into poor surface quality of the casting
To control the amount of dissolved gases low, the temperature should not
be in superheated range
In ferrous metals, the dissolved hydrogen and nitrogen are removed by
passing CO. In non-ferrous metals, Cl, He, or Ar gases are used.
Therefore, fluidity and gas solubility are two conflicting requirements. The
optimum pouring temp. is therefore decided on the basis of fluidity
requirements.The temp. should be able to fill the whole cavity at the same
time it should enter inside the voids between the sand particles.
Cooling rate depends on casting material and configuration. It
also depends on volume and surface area of the casting also.
The pouring rate should be such that solidification does not start
and the cavity is completely filled without eroding mould surface
and undue turbulence.
On the basis of experience following empirical relations are
developed for pouring time
K: Fluidity factor
W: Weight In kg
Tp: Poring time in sec
The Gating System
Minimize turbulent flow so that absorption of
gases, oxidation of metal and erosion of mould
surfaces are less
Regulate the entry of molten metal into the
Ensure complete filling of mould cavity, and
Promote a temperature gradient within the
casting so that all sections irrespective of size
and shape could solidify properly
Mechanism of Solidification
Pure metals solidifies at a constant temp. equal to its
freezing point, which same as its melting point.
The change form liquid to solid does not occur all at
once. The process of solidification starts with nucleation,
the formation of stable solid particles within the liquid
metal. Nuclei of solid phase, generally a few hundred
atom in size, start appearing at a temperature below the
freezing temperature. The temp. around this goes down
and is called supercooling or undercooling. In pure
metals supercooling is around 20% of the freezing temp.
A nuclease, more than a certain critical size grows, and
By adding, certain foreign materials (nucleating agents) the
undercooling temp. is reduced which causes enhanced
In case of pure metals fine equi-axed grains are formed near
the wall of the mold and columnar grain growth takes place
upto the centre of the ingot.
In typical solid-solution alloy, the columnar grains do not extend
upto the center of casting but are interrupted by an inner zone
of equiaxed graines.
My adding typical nucleating agents like sodium, magnesium or
bismuth the inner zone of equiaxed grained can be extended in
In alloys, such as Fe-C, freezing and solidificaion occurs overa
wide range of temp. There is no fine line of demarcation exists
between the solid and liquid metal.
Here, ‘start of freezing’ implies that grain formation while
progressing towards the center does not solidify the metal
completely but leaves behind the islands of liquid metals in
between grains which freeze later and there is multidirectional
tree like growth.
Once the material cools down to freezing
temperature, the solidification process for the
pure metals does not require a decrease in
temperature and a plateau is obtained in the
cooling curves, called thermal arrest. The
solidification time is total time required for the
liquid metal to solidify.
Solidification time has been found to be directly
proportional to volume and inversely proportional
to surface area.
Location of Risers and Open and
•Top riser has the
head and smaller
feeding distance over
the side riser.
•Blind risers are
generally bigger in
size because of
additional area of
shrinkage occurs in three stages,
1. When temperature of liquid metal drops from
pouring to zero temperature
2. When the metal changes from liquid to solid state,
3. When the temperature of solid phase drops from
freezing to room temperature
The shrinkage for stage 3 is compensated by
providing shrinkage allowance on pattern, while the
shrinkage during stages 1 and 2 are compensated by
The riser should solidify in the last otherwise liquid
metal will start flowing from casting to riser. It should
promote directional solidification. The shape, size and
location of the risers are important considerations in
Cleaning and Finishing
Casting is taken out of the mould by shaking and
the Moulding sand is recycled often with suitable
The remaining sand, some of which may be
embedded in the casting, is removed by means of
The excess material in the form of sprue, runners,
gates etc., along with the flashes formed due to
flow of molten metal into the gaps is broken
manuaaly in case of brittle casting or removed by
sawing and grinding in case of ductile grinding.
The entire casting is then cleaned by either shot
blasting or chemical pickling.
Sometimes castings are heat treated to achieve
better mechanical properties.
Defects may occur due to one or more of
the following reasons:
– Fault in design of casting pattern
– Fault in design on mold and core
– Fault in design of gating system and riser
– Improper choice of moulding sand
– Improper metal composition
– Inadequate melting temperature and rate of
Classification of casting defects
Internal Defect Visible defects
These are due to poor design and quality of sand
molds and general cause is poor ramming
Blow is relatively large cavity produced by gases
which displace molten metal from convex surface.
Scar is shallow blow generally occurring on a flat
surface. A scar covered with a thin layer of metal is
called blister . These are due to improper permeability
or venting . Sometimes excessive gas forming
constituents in moulding sand
Drop is an irregularly-shaped projection on the cope surface
caused by dropping of sand.
A scab when an up heaved sand gets separated from the
mould surface and the molten metal flows between the
displaced sand and the mold.
Penetration occurs when the molten metal flows between the
sand particles in the mould. These defects are due to
inadequate strength of the mold and high temperature of the
molten metal adds on it.
Buckle is a vee-shaped depression on the surface of a flat
casting caused by expansion of a thin layer of sand at the
mould face. A proper amount of volatile additives in moulding
material could eliminate this defect by providing room for
The internal defects found in the castings are mainly due to
trapped gases and dirty metal. Gases get trapped due to hard
ramming or improper venting. These defects also occur when
excessive moisture or excessive gas forming materials are
used for mould making.
Blow holes are large spherical shaped gas bubbles, while
porosity indicates a large number of uniformly distributed tiny
holes. Pin holes are tiny blow holes appearing just below the
Inclusions are the non-metallic particles in the metal matrix,
Lighter impurities appearing the casting surface are dross.
Insufficient mould strength, insufficient metal, low pouring
temperature, and bad design of casting are some of the
Wash is a low projection near the gate caused by erosion of
sand by the flowing metal. Rat tail is a long, shallow, angular
depression caused by expansion of the sand. Swell is the
deformation of vertical mould surface due to hydrostatic
pressure caused by moisture in the sand.
Misrun and cold shut are caused by insufficient superheat
provided to the liquid metal.
Hot tear is the crack in the casting caused by high residual
Shrinkage is essentially solidification contraction and occurs
due to improper use of Riser.
Shift is due to misalignment of two parts of the mould or
incorrect core location.
Casting with expendable mould:
Advantages and Limitations
Parts of greater complexity and intricacy can be
Close dimensional control ±0.075mm
Good surface finish
The lost wax can be reused
Additional machining is not required in normal
Preferred for casting weight less than 5 kg,
maximum dimension less than 300 mm,
Thickness is usually restricted to 15mm
Al, Cu, Ni, Carbon and alloy steels, tool steels
etc. are the common materials
Permanent mould casting: Die casting
General Configuration of a Die
In Die casting the molten metal is forced to
flow into a permanent metallic mold under
moderate to high pressures, and held under
pressure during solidification
This high pressure forces the metal into
intricate details, produces smooth surface and
excellent dimensional accuracy
High pressure causes turbulence and air
entrapment. In order to minimize this larger
ingates are used and in the beginning
pressure is kept low and is increased
•A permanent mold made of metal or ceramic is rotated at high
speed (300 to 3000 rpm). The molten metal is then poured into the
mold cavity and due to centrifugal action the molten metal conform
to the cavity provided in the mould.
•Castings are known for their higher densities in the outer most
•The process gives good surface finish
•Applications: pipes, bushings, gears, flywheels etc.