UNIT -1 METAL
CASTING PROCESS
CASTING
• Casting is one of the process used for making
components of complicated shapes in larger
quantity.
• It is the process of producing metal parts by
pouring molten metal into the mould cavity of the
required
shape
and
allowing
the
metal
to
solidify.The solidified metal piece is called as
“casting”.
Block diagram of casting process
Casting Methods
• Sand Casting
High Temperature Alloy,
Complex Geometry,
Rough Surface Finish
• Investment Casting
High Temperature Alloy,
Complex Geometry,
Moderately Smooth Surface
Finish
• Die Casting
High Temperature Alloy,
Moderate Geometry,
Smooth Surface
Types of pattern
• Solid or single piece pattern
• Split pattern
• Loose piece pattern
• Match plate pattern
• Sweep pattern
• Skeleton pattern
• Segmental pattern
• Shell pattern
Solid pattern
• These patterns are made of single
solid piece without joints, partings
or loose piece.
• This pattern is used for making a
few large size simple castings.
Split pattern
• One pattern which is having complex geometry cannot
be removed from mould if they
are made by single piece .
• Generally split pattern is made
into two parts.
i.lower half
ii.upper half
• These two parts are arranged
together in correct position
by pins “dowel pins”
Loose piece pattern
If a pattern is made from a single piece having projections above
the parting plane ,it is impossible to withdraw it from the mould.
In such case the pattern is made into solid piece and loose
piece.
Match plate pattern
• These patterns are made in two pieces. One piece is
mounted on one side and the other on the other side of a
plate called match plate. Gates and runners are also
attached to the plate along with the pattern. After
moulding when the match plate is removed a complete
mould with gating is obtained by joining the cope and drag
together. The complete pattern with match plate is entirely
made of metal, usually aluminium for its light weight and
machinability.
Sweep pattern
• Sweep patterns are mainly used to generate surfaces of
revolution like cylinder, cone, sphere in large castings.
• A sweep is a section or board made of wood or metal to
the required cross section that is rotated about one edge
to shape mould cavities having shapes of rotational
symmetry.
Skeleton pattern
• For larger casting of simple shape, if a solid pattern is made of
wood,it will require large amount of wood.Hence, it is very
expensive.In such cases, a skeleton pattern is used instead of
a full pattern.This shape is made of wooden strips with a lot of
openings and fitted to end supports.Moulds for water
pipes,turbine castings,pipe bends are made by skeleton
pattern.
Segmental pattern
• This pattern is used for forming circular moulds. When
making the mould using this pattern ,a vertical spindleis
fixed of the center of the drag box . The pattern is
attached to the spindle.after ramming ,the pattern is
moved to the next segment. Thus , the full mould is
completed.
Pattern materials
• Wood – teak wood, mahogany, white pine,etc..
• Metal – cast iron,brass, aluminium, white metal etc..
• Plaster
• Plastics
• Wax
Wood
• Teak wood, mahogany, white pine, rose wood…
• Laminated wooden sheets are used for accuracy,surface finish
and long life.
• Metal spray coating
upto 0.25mm thick may be given on
wooden pattern.(zinc,aluminium) to avoid moisture absorption.
• Advantages :
1.light in weight, cheap.
2.easy to work, repair.
3.easily smoothened by varnish and paints.
• Limitations :
1.It absorb water from sand.
2.It cannot be used for mass production
Metal
• Cast iron, Brass, Aluminium…
• Advantages :
1. long life, smooth surface.
2. mass production is possible.
3. does’nt absorb any water & free from corrosion.
• Disadvantages :
1. costlier and heavier
2. difficult to make desired shape.
Plaster
• Plaster of paris or gypsum cement…
• Plaster can be easily made into difficult shape.
• Compressive strength is high.
• It is not affected by moisture.
Plastics
• A plastic has many advatages over other materials.
• It is light in wight but strong.
• Not affected by moisture and smooth glassy surface.
• The following plastics are widely used for pattern making with
the composition based on epoxy,phenol formaldehyde..
i. Poly acrylates
ii.Poly ethylene
iii.poly vinyle chloride….
wax
• Wax pattern primarily used in investment casting.
• the most commonly used waxes are
i.paraffin wax
ii.shelloc wax
iii.microcrystalline wax
• It will not absorb moistures.
• Cost is very less ,but it is used only for small patterns.
Pattern allowance
• Patterns are not made into exact size of the castings to be
produced.Patterns are made slightly larger than required
castings.This extra size given on pattern is called pattern
allowance.If the allowances are not given on the pattern,
the casting will become smaller than the required size
Types of allowance
• Shrinkage allowance
• Machining or finish allowance
• Draft or taper allowance
• Distortion or camber allowance
• Rapping or shake allowance
Shrinkage allowance
• The metal shrinks on solidification and contracts further
on cooling to room temperature.To compensate this , the
pattern is made larger than the required shape.
C.I
10.4mm
ALUMINIUM
17mm
BRASS
15.3mm
STEEL
20.8mm
Machining allowance
• The surface finish obtained in sand castings is generally
poor (dimensionally inaccurate), and hence in many
cases, the cast product is subjected to machining
processes like turning or grinding in order to improve the
surface finish. During machining processes, some metal
is removed from the piece. To compensate for this, a
machining allowance should be given in the casting.
Metal
Dimension (inch)
Allowance (inch)
Cast iron
Up to 12
12 to 20
20 to 40
0.12
0.20
0.25
Cast steel
Up to 6
6 to 20
20 to 40
0.12
0.25
0.30
Non ferrous
Up to 8
8 to 12
12 to 40
0.09
0.12
0.16
Draft allowance
• When the pattern is to be removed from the sand mold,
there is a possibility that any leading edges may break off,
or get damaged in the process. To avoid this, a taper is
provided on the pattern, so as to facilitate easy removal of
the pattern from the mold, and hence reduce damage to
edges.
• The taper angle provided is called the Draft angle. The
value of the draft angle depends upon the complexity of
the pattern, the type of molding (hand molding or machine
molding), height of the surface, etc.
Distortion allowance
• During cooling of the mold, stresses developed in the
solid metal may induce distortions in the cast. This is
more evident when the mold is thinner in width as
compared to its length. This can be eliminated by initially
distorting the pattern in the opposite direction.
Types of moulding sand
• Green-sand molds - mixture of sand, clay, and water;
• “Green" means mold contains moisture at time of
pouring
• Dry-sand mold - organic binders rather than clay
• And mold is baked to improve strength
• Skin-dried mold - drying mold cavity surface of a
green-sand mold to a depth of 10 to 25 mm, using torches
or heating lamps
Properties of moulding sand
• Porosity or permeability
• Plasticity or flowability
• Adhesiveness
• Strength or cohesiveness
• Refractoriness
• Collapsibility
Core making
• A core is a body made of sand which is used to make a
cavity or hole in a casting. The shape of the core is similar
to the required cavity in the casting to be made.
Core making materials:
• Core sand
• Binders
• Additives
Types of cores
• According to the state of core
i. Green sand core
ii. Dry sand core
• According to the position of the core in the mould
i. Horizontal core
ii. Vertical core
iii. Balanced core
iv. Hanging core
v. Drop core
Furnaces for Casting Processes
• Furnaces most commonly used in foundries:
• Cupolas
• Crucible furnaces
• Electric-arc furnaces
• Induction furnaces
Cupolas
Vertical cylindrical furnace equipped with tapping spout
near base
• Used only for cast irons
• Although other furnaces are also used, the largest
tonnage of cast iron is melted in cupolas
• The "charge," consisting of iron, coke, flux, and possible
alloying elements, is loaded through a charging door
located less than halfway up height of cupola
Crucible Furnaces
Metal is melted without direct contact with burning fuel
mixture
• Sometimes called indirect fuel-fired furnaces
• Container (crucible) is made of refractory material or
high-temperature steel alloy
• Used for nonferrous metals such as bronze, brass, and
alloys of zinc and aluminum
• Three types used in foundries: (a) lift-out type, (b)
stationary, (c) tilting
Crucible Furnaces
Figure 11.19 Three types of crucible furnaces: (a) lift-out crucible, (b)
stationary pot, from which molten metal must be ladled, and (c) tiltingpot furnace.
Electric-Arc Furnaces
Charge is melted by heat generated from an electric arc
• High power consumption, but electric-arc furnaces can be
designed for high melting capacity
• Used primarily for melting steel
Induction Furnaces
Uses alternating current passing through a coil to develop magnetic
field in metal
• Induced current causes rapid heating and melting
• Electromagnetic force field also causes mixing action in liquid metal
• Since metal does not contact heating elements, environment can be
closely controlled to produce molten metals of high quality and purity
• Melting steel, cast iron, and aluminum alloys are common
applications in foundry work
Shell casting
Steps in shell-molding
• (1) a match-plate or cope-and-drag metal pattern is heated and
placed over a box containing sand mixed with thermosetting resin.
• (2) box is inverted so that sand and resin fall onto the hot pattern,
causing a layer of the mixture to partially cure on the surface to form a
hard shell;
• (3) box is repositioned so that loose uncured particles drop away
• (4) sand shell is heated in oven for several minutes to complete
curing;
• (5) shell mold is stripped from the pattern;
Advantages of Shell Mould Casting
•
•
•
•
•
Good surface finish
High dimensional tolerance
towards automation
Castings weighing upto 450 kgs can be cast by this process
Thin sections (upto 0.25 mm) can be cast by this process
Limitations
• Patterns are expensive
• Castings weighing more than 450 kgs cannot be made
• Highly complicated shapes cannot be made
Applications
• Cast iron, Aluminium and copper alloys are cast by this process
Investment casting (lost wax casting)
(a) Wax pattern
(injection molding)
(d) dry ceramic
melt out the wax
fire ceramic (burn
wax)
(e) Pour molten metal (gravity)
 cool, solidify
[Hollow casting:
pouring excess metal before
solidification
(b) Multiple patterns
assembled to wax
sprue
(c) Shell built 
immerse into ceramic slurry
 immerse into fine sand
(few layers)
(f) Break ceramic shell
(vibration or water
blasting)
(g) Cut off parts
(high-speed friction
saw)
 finishing (polish)
Centrifugal casting
• Schematic illustration of the centrifugal casting process.
Pipes, cylinder liners, and similarly shaped parts can be
cast by this process.
• The Centrifugal accelerating force is high almost 70 to 80 times
that of gravity which helps in pushing the molten metal to the
walls of the mold
• Outer shape is controlled by the mold contour while the inner
shape is controlled by the amount of the molten metal poured in.
• There are horizontal as well as vertical axis machines. Only
short tubes aremanufacture in the vertical machines
Die casting
•
•
Designed to hold and accurately close two mold halves
and keep them closed while liquid metal is forced into
cavity
Two main types:
1. Hot-chamber machine
2. Cold-chamber machine
Hot-Chamber Die Casting
Metal is melted in a container, and a piston injects
liquid metal under high pressure into the die
• High production rates - 500 parts per hour not
uncommon
• Applications limited to low melting-point metals that
do not chemically attack plunger and other
mechanical components
• Casting metals: zinc, tin, lead, and magnesium
Figure : Cycle in hot-chamber casting: (1) with die closed and
plunger withdrawn, molten metal flows into the chamber (2)
plunger forces metal in chamber to flow into die, maintaining
pressure during cooling and solidification.
Cold-Chamber Die Casting
Machine
Molten metal is poured into unheated chamber
from external melting container, and a piston
injects metal under high pressure into die cavity
• High production but not usually as fast as
hot-chamber machines because of pouring step
• Casting metals: aluminum, brass, and magnesium
alloys
• Advantages of hot-chamber process favor its use
on low melting-point alloys (zinc, tin, lead)
Figure : Cycle in cold-chamber casting: (1) with die closed
and ram withdrawn, molten metal is poured into the
chamber
Figure : Cycle in cold-chamber casting: (2) ram forces metal to
flow into die, maintaining pressure during cooling and
solidification.
• Advantages of die casting:
• Economical for large production quantities
• Good accuracy and surface finish
• Thin sections are possible
• Rapid cooling provides small grain size and good
strength to casting
• Disadvantages:
• Generally limited to metals with low metal points
• Part geometry must allow removal from die
General Defects: Misrun
A casting that has solidified before completely
filling mold cavity
Figure : Some common defects in castings: (a) misrun
General Defects: Cold Shut
Two portions of metal flow together but there is
a lack of fusion due to premature freezing
Figure : Some common defects in castings: (b) cold shut
General Defects: Cold Shot
Metal splatters during pouring and solid globules
form and become entrapped in casting
Figure : Some common defects in castings: (c) cold shot
General Defects: Shrinkage Cavity
Depression in surface or internal void caused by
solidification shrinkage that restricts amount of
molten metal available in last region to freeze
Figure : Some common defects in castings: (d) shrinkage cavity
Sand Casting Defects: Sand Blow
Balloon-shaped gas cavity caused by release of
mold gases during pouring
Figure : Common defects in sand castings: (a) sand blow
Sand Casting Defects: Pin Holes
Formation of many small gas cavities at or slightly
below surface of casting
Figure : Common defects in sand castings: (b) pin holes
Sand Casting Defects: Penetration
When fluidity of liquid metal is high, it may penetrate
into sand mold or core, causing casting surface to
consist of a mixture of sand grains and metal
Figure : Common defects in sand castings: (e) penetration
Sand Casting Defects: Mold Shift
A step in cast product at parting line caused by
sidewise relative displacement of cope and drag
Figure : Common defects in sand castings: (f) mold shift