24-12-2022

Casting
By Rashin Khera

Syllabus for Casting

• Introduction to sand and die casting
• Advantages/disadvantages of Casting
• Applications of sand casting
• Pattern, pattern materials, types of patterns (Single piece, double piece, loose
piece, gated, match plate, sweep and segmental)
• Pattern allowances
• Moulding sand composition
• Desirable properties and study of various types of moulding sand
• Green sand mould making and moulding tools

1
 24-12-2022

Manufacturing Process
• It is a process in which number of activities are performed to convert
raw material/ semi finished product into a finished product.
• Casting
• Forming
• Fabrication Process (Welding, joining Process)
• Metal Cutting operation/Metal Removal process

What is Casting? Its Advantages

• One of the oldest processes is casting, which basically involves pouring
molten metal into a mold cavity. Upon solidification, the metal takes the
shape of the cavity.
• Casting processes are most often selected over other manufacturing
methods for the following reasons:
1. Casting can produce complex shapes and can incorporate internal cavities
or hollow sections.
2. Very large parts can be produced in one piece.
3. Casting can utilize materials that are difficult or uneconomical to process
by other means.
4. The casting process can be economically competitive with other
manufacturing processes.

2
 24-12-2022

Disadvantages of Casting
• It is a laborious and time consuming process.
• Surface finish of casting components are very poor.
• It is very difficult to eliminate casting defects completely (gas defects).
• Casting components are not having uniform mechanical property
throughout its cross section.

Application of Casting: machine bases, large turbine impellers,

propellers, plumbing fixtures, and a wide variety of other products
and components

Sand Casting

3
 24-12-2022

Sand Casting Processes

• (a) Placing a pattern in sand to make an imprint,

• (b) Incorporating a gating system,
• (c) Removing the pattern and filling the mould cavity with molten
metal,
• (d) Allowing the metal to cool until it solidifies,
• (e) Breaking away the sand mould and removing the casting

Pattern: Pattern is the replica of the object to be made or replica of the

casting to be made with some modifications(core and allowances).

Pattern materials
• Wood: Master Pattern for making pattern and casting material
• Wooden pattern have double shrinkage allowances
• Metal and alloy: Al, cast Iron, Steels
• Plastics: Polystyrene, foam, thermocol, rubber
• Wax and Hg

4
 24-12-2022

1. Solid or Single Piece Pattern

• Object to be produced is very simple
in shape and size without any
complex surfaces.
• Kept inside the drag box, one of its
surface will match with parting line
• They generally are made up of wood
and are inexpensive

5
 24-12-2022

2. Split Piece Pattern

• If the object is having complex
surfaces, it is difficult to remove
the pattern from the mould.
• Pattern is splitted into number of
pieces along the symmetry.
• The individual split piece can be
removed from cope and drag boxes
separately.

3.Loose Piece Pattern

• If the pattern is having internal projections and by splitting the

pattern if it is difficult to remove from the mould then we can select
loose piece pattern.
• The individual loose piece pattern is removed from the mould after
removal of the main parts of the pattern.
• Loose piece is recovered from the gap generated by main pattern.

6
 24-12-2022

4. Match Plate pattern

• If the complexity of the pattern is more
and number of small size product has to
be produced in large production then
match plate pattern is selected.
• The individual pattern can be split into two
pieces along the symmetry and will be
added on both sides of match plate along
with gatting element.

5. Gated pattern
• Many times it is desired to produce the
multiple small parts in one go.
• In such case, small part cavities are created in
one mould and these cavities are joined by the
system of runners and gates.
• This arrange helps to achieve higher
productivity.

7
 24-12-2022

6. Sweep Pattern
• For axi-symmetrical castings such as bells,
the full cavity of the casting can be
created by sweep in the pattern around
the spindle.
• The pattern is a part of the full shape of
the end product.

7. Skeleton Pattern
• As the name of the pattern suggest, this type of
pattern are made of strips of wood or plastic.
• These strips are used for building the Skeleton
of the pattern giving the correct shape and size
of the pattern.
• Around and inside Skeleton of pattern molding
sand is packed.
• This type of pattern is used for very large
casting, large size shell and containers.

8
 24-12-2022

5 Types of Pattern Allowances

• The difference in the dimension of pattern and final product of
casting is known as the pattern allowance.
Shrinkage or Contraction allowance
Draft or taper allowance
Machining or finish allowance
Rapping or shake allowance
Distortion or Camber allowances

Pattern Allowances

9
 24-12-2022

1. Shrinkage or Contraction allowance

• During the solidification of metal in the cavity the volume
of the casting gets reduced due to shrinkage of material
• To overcome this, the size of pattern is increased by
adding shrinkage allowances.
Liquid shrinkage : shrinkage between pouring temperature
and freezing temperature
Solidification Shrinkage: shrinkage during phase
transformation process
Solid shrinkage: shrinkage between freezing temperature
and room temperature
• Both liquid and solidification shrinkages can be
compensated by providing the riser.
• Solid shrinkage can be compensated by increasing the size
of pattern in the form of shrinkage allowances.

2.Draft or taper allowance

• Draft allowances is provided on the vertical
surface of pattern:
(a)For easy removal of pattern and
(b)To minimize the continuous contact of vertical
surfaces of pattern with the mould surface
• Draft allowance depends upon the height of
the pattern.

10
 24-12-2022

3.Machining or finish allowance

• Negative machining allowances: For internal dimensions the machining allowances is

reduced from its size of casting

4. Rapping or shake allowance

• During ramming operation the moulding sand sticks to
the surface of pattern material due to adhesive property
of moulding sand.
• For easy removal of pattern some clearance is required
between pattern and mould wall.
• This can be achieved by shaking the pattern. By this
action, size of cavity increases and hence, size of casting
will increase.
• To overcome this, shake allowances is reduced from the
size of pattern when compared to size of casting.
• Shake allowance is a negative allowances.

11
 24-12-2022

5. Distortion or Camber allowances

• Depending on the shape and size of the casting, due
to differences in shrinkage rate, shrinkage stresses will
be developed in the material which causes distortion.
• To arrest this, distortion allowances is provided in the
pattern against the direction of distortion.
• This value depends upon the linear dimension and
property of material.

Composition of Molding Sand

The main constituents of molding sand involve
• Silica sand: 70 to 85%
• Binder -10 to 20%
• Moisture content : 2 to 6%
• Additives: 1-3%

12
 24-12-2022

• Silica Sand
Silica sand in form of granular quarts is the main constituent of molding sand
having enough refractoriness which can impart strength, stability and
permeability to molding and core sand.
But along with silica small amounts of iron oxide, alumina, lime stone (CaCO3),
magnesia, soda and potash are present as impurities.
The silica sand can be specified according to the sand grain size and the shape
(angular, sub-angular and rounded) of the sand.
• Binder
Inorganic group : In foundry shop, the clay acts as binder which may be Kaolinite,
Limonite, Bentonite.
• Organic group: Dextrin, Molasses, cereal binders, linseed oil and resins like
phenol formaldehyde, urea formaldehyde etc. Binders of organic group are
mostly used for core making.

• Moisture content: 2 to 6%.

This amount of water added to the mixture of clay and silica sand for developing
bonds.
This amount of water required to activate clay and to fill the pores between the
particles of clay without separating them.
This amount of water is held rigidly by the clay and is mainly responsible for
developing the strength in the sand.
• Additives
Additives are the materials generally added to the molding and core sand mixture
to develop some special property in the sand.
Examples: coal dust, corn flour, dextrin, sea coal, pitch, wood flour, silica flour

13
 24-12-2022

Moulding sand Properties

• Refractoriness i.e., it should be able to with stand high temperatures.
• Permeability i.e., ability to allow gases, water vapour and air to pass through it.
• Green sand strength i.e., when a mould is made with moist sand, it should have
sufficient strength, otherwise mould will break.
• Good flowability i.e., when it is packed around a pattern in a moulding box, it
should be able to fill all nooks and corners, otherwise the impression of pattern in
mould would not be sharp and clear.
• Good collapsibility i.e., it should collapse easily after the casting has cooled
down and has been extracted after breaking the mould. It is particularly important
in case of core making.
• Cohesiveness i.e., ability of sand grains to stick together. Without cohesiveness,
the moulds will lack strength.
• Adhesiveness i.e., ability of sand to stick to other bodies. If the moulding sand
does not stick to the walls of moulding box, the whole mould will slip through the
box.

14
 24-12-2022

1.Green Sand:
• Silica sand + Clay (18 to 30 %) + Water (6 to 8 %)
• It is porous, light, and soft, and the binding between the green sand grains is
provided by clay and water.
• Under pressure, it can retain its shape and impression.
• The green sand is inexpensive and easily accessible.
• Used to create a mould that is known as a green sand mould.
2. Dry Sand:
• The Green sand that has been dried or backed after the mould is made is called
dry sand.
• This Moulding sand is stronger, more thermally stable, and more rigid.

3. Loam Sand:
• mixture of sand and clay (50%) and water
• forms a thin plastic paste.
• suitable for large castings
4. Parting Sand:
• Parting sand is used to avoids sticking of green sand to the pattern and also to
prevent the sticking of cope and drag part.
• Pure silica sand without any binder and clay
5. Facing Sand:
• It forms the face of the mould.
• The facing sand is used directly next to the patterned surface and comes into
direct contact with the molten metal.
• high strength and refractivity.
• It is made of clay and silica sand in addition to unused sand.

15
 24-12-2022

6. Backing Sand:
• The backing sand is also called floor sand used to back up the facing sand.
• It is sometimes called black sand because of the addition of coal dust and
burning due to in contact with the molten metal.
7. Core Sand:
• The core sand is the sand for making cores.
• It is also called oil sand because it is a mixture of silica sand and core oil.
• Core oil is a mixture of linseed oil, resin, light mineral oil and other binding
materials.
CORE: A core is a preformed, bonded, sand insert placed into the mold to
shape the interior of a casting or a part of the casting that cannot be shaped by
the pattern. Cores are frequently used to create hollow sections or cavities in a
casting.

CASTING DEFECTS
Blow-holes:
• They appear as small holes in the casting.
• They may be open to surface or they may be below the surface of the casting.
• They are caused due to entrapped bubbles of gases.
• They may be caused by excessively hard ramming, improper venting, excessive
moisture or lack of permeability in the sand.
Shrinkage cavity:
• Sometimes due to faulty design of casting consisting of very thick and thin
sections, a shrinkage cavity may be caused at the junction of such sections.
• Shrinkage cavity is totally internal.
• It is caused due to shrinkage of molten metal. Remedy is to use either a chill or
relocation of risers.

16
 24-12-2022

Misrun:
• This denotes incomplete filling of mould cavity.
• It may be caused by bleeding of molten metal at the parting of
cope and drag, inadequate metal supply or improper design of
gating.
Cold shut:
• A cold shut is formed within a casting, when molten metal from
two different streams meets without complete fusion.
• Low pouring temperature may be the primary cause of this
defect.
Mismatch:
• This defect takes place when the mould impression in the cope
and drag do not sit exactly on one another but are shifted a little
bit.
• This happens due to mismatch of the split patter (dowel pin may
have become loose) or due to defective clamping of cope and
drag boxes.

Drop:
• This happens when a portion of the mould sand falls into the molten
metal.
• Loose sand inadequately rammed or lack of binder may cause this
defect.
Scab:
• This defect occurs when a portion of the face of a mould lifts or breaks
down and the recess is filled up by molten metal.
• Penetration of liquid metal on top surfaces due to improper ramming.
Hot tear:
• These cracks are caused in thin long sections of the casting, if the part
of the casting cannot shrink freely on cooling due to intervening sand
being too tightly packed, offers resistance to such shrinking.
• The tear or crack usually takes place when the part is red hot and has
not developed full strength, hence the defect is called “hot tear”.
• Reason may be excessively tight ramming of sand.

17
 24-12-2022

DIE CASTING
• A sand mould is usable for production of only one casting. It cannot be used twice.
Die is essentially a metal mould and can be used again and again.
• A die is usually made in two portions. One portion is fixed and the other is
movable. Together, they contain the mould cavity in all its details.
• After clamping or locking the two halves of the dies together molten metal is
introduced into the dies. If the molten metal is fed by gravity into the dies, the
process is known as gravity die casting process.
• if the metal is forced into the dies under pressure (e.g., a piston in a cylinder
pushes the material through cylinder nozzle), the process is called “pressure die
casting”.

DIE CASTING

• The material of which the dies are made, should have a melting point much higher
than the melting point of casting material. A great number of die castings are made
of alloys of zinc, tin and lead, and of alloys of aluminium, magnesium and copper.
• Hence dies are made out of medium carbon low alloy steels. The dies are usually
water or air blast cooled.
• Since most materials contract on cooling, extraction of castings from dies becomes
important otherwise they will get entangled in the die as they cool.
• Therefore, in the design of dies, some arrangement for extraction of casting is
incorporated.

18
 24-12-2022

STEPS IN DIE CASTING

1. Close and lock the two halves of a die after coating the mould cavity
surfaces with a mould wash, if specified:
2. Inject the molten metal under pressure into the die.
3. Maintain the pressure until metal solidifies.
4. Open die halves.
5. Eject the casting along with runner, riser etc.
6. The above cycle is repeated.

19
 24-12-2022

• Two pressure die casting methods are used:

1. Hot chamber process:
• This uses pressures up to 35 MPa and is used for zinc, tin, lead, and their alloys
(lower melting point materials)
• In this process the chamber, in which molten metal is stored before being pressure
injected into the die, is kept heated.
2. Cold chamber process:
• In this process, pressures as high as 150 MPa are used.
• The storing chamber is not heated.
• This process is used mainly for metals and alloys having relatively higher melting
point e.g., aluminium, magnesium and their alloys.

Advantages of die casting:

It is used for mass production of castings of small and medium size. e.g., pistons
of motorcycle and scooter engines, valve bodies, carburettor housings etc.
This process produces high quality, defect free castings.
The castings produced by this process are of good surface finish and have good
dimensional control and may not require much machining. All castings produced
are identical.
In case of mass production, castings can be produced cheaply.
The process does not require use of sand and requires much less space as
compared to a conventional foundry using sand moulds.
Disadvantages of die casting:
The initial cost of manufacturing a die is very high.
Large size castings cannot be produced by this process.
Castings with very complex shapes or with many cores are difficult to produce by
die casting.

20