1. MODULE 2
COURSE OUTCOMES
Fundamentals of Casting process
Solidification of metals
Fluid flow of molten metal
Various casting process
Casting defects & quality
2. : Casting / Foundry
Casting processes basically involve the introduction of a molten
metal into a mold cavity, where upon solidification, the metal
.takes on the shape of the mold cavity
:Applications
Cylinder blocks, liners, machine tool beds, pistons, piston rings,
mill rolls, wheels, housings, water supply pipes, bells
3. Examples of Cast Parts
Crank handle formed by casting; some areas
were machined and assembled after casting
4. Examples of Cast Parts
C-clamps formed by casting (left) and machining
)(right
6. Aluminum piston for an internal
: combustion engine
.a) as-cast (b) after machining (
7.
8. Flask : A molding flask is one which holds the sand mould intact
Drag : Lower molding flask
Cope : Upper molding flask
Pattern : It is a replica of final object to be made with some
modifications. The mold cavity is made with the help of pattern material
Molding sand : It is the freshly prepared refractory material used for
.making the mold cavity
.Core : It is used for making hollow cavities in the castings
Core Print : A region used to support the core
Pouring basin : A small funnel shaped cavity at the top of the mold
.into molten metal is poured
14. Parting Line / Parting Surface : Interface that separates the
cope and drag
Runner: The channel through which the molten metal is carried
.from the sprue to the gate
Gate: A channel through which the molten metal enters the mold
cavity
Chaplets: Chaplets are used to support the cores inside the mold
.cavity
. Riser: It is a reservoir of the molten metal provided in the casting
Vent: Small opening in the mold to facilitate escape of air and
.gases
Sprue : The passage through which the molten material from the
.pouring basin reaches the mold cavity
15. Pattern Material
Wood, metals & alloys, plastic, plaster of Paris, plastic
and rubbers, wax, and resins.
Material selection depends on size and shape of casting.
To be suitable for use, the pattern material should be:
1. Easily worked, shaped and joined
2. Light in weight
3. Strong, hard and durable
4. Resistant to wear and abrasion
5. Resistant to corrosion, and to chemical reactions
6. Dimensionally stable and unaffected by variations
in temperature and humidity
7. Available at low cost
17. Pattern Allowances
Pattern allowance is a vital feature as it affects the
dimensional characteristics of the casting
The selection of correct allowances greatly helps to
reduce machining costs and avoid rejections.
1. Shrinkage or contraction allowance
2. Draft or taper allowance
3. Machining or finish allowance
4. Distortion or camber allowance
5. Rapping allowance or shake allowances
18. Shrinkage or Contraction Allowance
All most all cast metals shrink or contract volumetrically on cooling.
The metal shrinkage is of two types:
(1) Liquid Shrinkage: It refers to the reduction in volume when the
metal changes from liquid state to solid state at the solidus
temperature.
To account for this shrinkage; riser, which feed the liquid metal to the
casting, are provided in the mold.
(2) Solid Shrinkage: It refers to the reduction in volume during the
cooling of the cast metal to room temperature.
To account for this, shrinkage allowance is provided on the patterns.
The rate of contraction with temperature is dependent on the
material.
For example steel contracts to a higher degree compared to
aluminum.
19. Shrinkage
Metal Percent Contraction (-)
Expansion(+)
Aluminum -7.1%
Zinc -6.5%
Gold -5.5%
Copper -4.9%
Brass -4.5%
Carbon Steel -2.5-4%
Lead -3.2%
Gray Cast Iron +2.5%
20.
21. Draft or Taper Allowance
Taper is provided on all vertical surfaces of the pattern so that it
can be removed from the sand without tearing away the sides of the
sand mold.
Draft allowance varies with the complexity of the sand job.
Inner details of the pattern require higher draft than outer surfaces.
The amount of draft depends upon the length of the vertical side of
the pattern to be extracted; the intricacy of the pattern; the method of
molding; and pattern material.
22. Pattern Height of the Draft angle Draft angle
material given surface (External (Internal
(inch) surface) surface)
1 3.00 3.00
1 to 2 1.50 2.50
Wood 2 to 4 1.00 1.50
4 to 8 0.75 1.00
8 to 32 0.50 1.00
1 1.50 3.00
1 to 2 1.00 2.00
Metal and
2 to 4 0.75 1.00
plastic
4 to 8 0.50 1.00
8 to 32 0.50 0.75
Draft Allowances of Various Metals
24. Machining or Finish Allowance
Machining or finish allowances are added in the pattern dimension
to have good surface finish or dimensionally accurate
The amount of machining allowance to be provided is affected by
the method of molding and casting used viz. hand molding or machine
molding, sand casting or metal mold casting.
The amount of machining allowance is also affected by the size and
shape of the casting; the casting orientation; the metal; and the degree
of accuracy and finish required.
25. Machining Allowances of Various Metals
Metal Dimension (inch) Allowance (inch)
Up to 12 0.12
Cast iron 12 to 20 0.20
20 to 40 0.25
Up to 6 0.12
Cast steel 6 to 20 0.25
20 to 40 0.30
Up to 8 0.09
Non ferrous 8 to 12 0.12
12 to 40 0.16
26. Distortion or Camber Allowance
.Castings get distorted, during solidification, due to their typical shape
For example, if the casting has the form of the letter U, V, T, or L etc. it
will tend to contract at the closed end causing the vertical legs to look
. slightly inclined
This can be prevented by making the legs of the U, V, T, or L shaped
pattern converge slightly (inward) so that the casting after distortion
will have its sides vertical
. The distortion in casting may occur due to internal stresses
These internal stresses are caused on account of unequal cooling of
. different section of the casting and hindered contraction
:To prevent the distortion in castings include
i. Modification of casting design
ii. Providing sufficient machining allowance to cover the distortion
affect
iii. Providing suitable allowance on the pattern, called camber or
)distortion allowance (inverse reflection
28. Rapping Allowance
Before the withdrawal from the sand mold, the pattern is rapped all
around the vertical faces to enlarge the mold cavity slightly, which
facilitate its removal.
Since it enlarges the final casting made, it is desirable that the
original pattern dimension should be reduced to account for this
increase.
There is no sure way of quantifying this allowance, since it is highly
dependent on the foundry personnel practice involved.
It is a negative allowance and is to be applied only to those
dimensions that are parallel to the parting plane.
29. Fluid flow
• 2 principles of fluid flow are relevant to
gating design: Bernoulli’s theorem and
the law of mass continuity.
30. Fluid flow
Bernoulli’s theorem
• Based on
- principle of conservation of energy
- frictional losses in a fluid system
2 h = elevation
p v p = pressure at elevation
h+ + = Constant
ρg 2 g v = velocity of the liquid
ρ = density of the fluid
• Conservation of energy requires that,
p1 v12 2
p 2 v2
h+ + = h2 + + +f
ρg 2 g ρg 2 g
31. Fluid flow
Mass continuity
• States that for an incompressible liquid
the rate of flow is constant.
Q = A1v1 = A2 v2
Q = volumetric rate of flow
A = cross-sectional area of the liquid stream
v = velocity of the liquid
• Subscripts 1 and 2 pertain to two different
locations in the system.
32. Fluid flow
Sprue profile
• Relationship between height and cross-
sectional area at any point in the sprue is
given by A1
=
h2
A2 h1
• Velocity of the molten metal leaving the
gate is v = c 2 gh
• When liquid level reached height x, gate
velocity is v = c 2g h − x
33. Fluid flow
Flow characteristics
• Reynolds number, Re, is used to characterize
aspect of fluid flow.
• It represents the ratio of the inertia to the viscous
forces in fluid flow and is defined as
vDρ v = velocity of the liquid
Re = D = diameter of the channel
η ρ = density
n = viscosity of the liquid.
34. Flow Characteristics
0 < Re < 2000 => laminar flow
2000 < Re < 20 000 =>mixture of laminar and turbulent
flow , generally regarded as harmless in gating
systems.
Re > 20 000 => severe turbulence
In gating systems, Re typically ranges from 2000 to
20,000
Techniques for minimizing turbulence
• Dross or slag can be eliminated by vacuum casting
• Use of filters eliminates turbulent flow in the runner
system
35. Turbulence can be reduced by the design of a gating system
that promotes a more laminar flow of the liquid metal.
Sharp corners and abrupt changes in sections within the
casting can be a leading cause of turbulence. Their affect
can be mitigated by the employment of radii.
36. Fluidity of molten metal
Fluidity of Molten Metal : The capability of molten metal to fill mold
cavities is called fluidity.
The following influence fluidity
Characteristics of molten metal
– Viscosity (How runny is it when hot)
– Surface tension (Development of film )
– Inclusions
– Solidification pattern of the alloy
Casting parameters
– Mold design (Risers, runners, gates, etc.)
– Mold material and its surface characteristics
– Degree of superheat
– Rate of pouring
– Heat transfer
37. Heat Transfer
Important consideration in casting
– Heat flow in the system
• Complex
• Depends of flow characteristics
Solidification Time
– A function of the volume of a casting and its surface
area
• Solidification time = C volume 2
surface area
– Effects on solidification time
• Mold Geometry
• Skin thickness
39. Solidification of Metals
Involves liquid metal turning back in to solid metal
The process is different for Pure metals and alloys
Can be divided into two steps:
Formation of stable nuclei
Growth of crystals
Pure Metals
• Have a clearly defined melting point
• Temperature remains constant during freezing
• Solidifies from the walls of the mold toward the
center of the part
40. Grain Structure for Pure Metals
• Two types of grains are formed for a pure metal
– Fine equiaxed grains
– Columnar
• Rapid cooling at the walls produces fine equiaxed grains
• Columnar grains grow opposite of the heat transfer
throughout the mold following the chill zone
Equiaxed Grains
• If crystals can grow approximately equally in all directions –
equiaxed grains will grow.
• Large amounts of under cooling is needed near the wall of
the mold.
42. Alloys
• Solidification in alloys begins when the temperature drops below
the liquidus TL and is complete when it reaches the solidus, TS.
43. Alloys
• Within the TL and TS Temperature range, the alloy is like a slushy with
columnar dendrites
44. Effects of Cooling Rates
• Slow cool rates results in course grain structures (102 K/s)
• Faster cooling rates produce finer grain structures (104 K/s)
• For even faster cooling rates, the structures are amorphous
(106 – 108 K/s)
• Grain size influences strength of a material
• Smaller grains have higher ductility and strength
• Smaller grains help prevent hot tearing and/or cracks in the
casting
48. Shell molding
– a mounted pattern, made of a ferrous metal or
aluminum, is heated to 175-370 0 C, coated with a
parting agent such as silicone, and clamped to a box
or chamber containing a fine sand coated with a 2.5 -
4.0% thermosetting resin binder
– the sand mixture is blown over the heated pattern,
coating it evenly
– the assembly is placed in an oven to complete the
curing of the resin
– the shell is formed by removing the pattern
– two half shells are made and are clamped together in
preparation for pouring
49. Shell molding
Advantages
• Better surface finish
• Better dimensional tolerances.
• Reduced Machining.
• Less foundry space required.
• Semi skilled operators can handle the process.
• The process can be mechanized.
51. Shell Molding
Applications
• -Crankshaft fabrication
• -Steel casting parts, fittings
• -Molded tubing fabrication
• -Hydraulic control housing fabrication
• -Automotive castings (cylinder head and
ribbed cylinder fabrication).
52. Expendable Mold
Uses a polystyrene foam pattern which evaporates
with molten metal to form a cavity for the casting.
Polystyrene foam pattern includes sprue, risers,
gating system and internal cores (if needed)
Polystyrene inexpensive and easily processed
into patterns
53. mo
me
support
sand pattern
polystyrene
pattern
54. Advantages of expanded polystyrene process:
1.Pattern need not be removed from the mold
2.Simplifies and speeds mold-making, because two
mold halves are not required as in a conventional
green-sand mold
Disadvantages:
1.A new pattern is needed for every casting
2.Economic justification of the process is highly
dependent on cost of producing patterns
55. Evaporative Pattern Casting of an Engine Block
(a ) (b )
a) Metal is poured into mold for lost-foam casting of a 60-hp. (
.3-cylinder marine engine; (b) finished engine block
58. Investment Casting – Characteristics
• Advantages:
Complex shapes possible
Thin wall sections possible
High production rates
High dimensional accuracy
• Disadvantages:
Limited weight range
Expensive Machinery & Dies
Expensive Unit Cost, Labor Intensive
Mold is not reusable
59. . Typical parts produced by investment casting
Products such as rocket components, and jet
engine turbine blades
60. Die casting
The molten metal is injected into die cavity under high
pressure
Pressure maintained during solidification
Die casting typically makes use of non-ferrous alloys.
The four most common alloys that are die cast are
Aluminum alloys, Copper alloys, Magnesium alloys,
Zinc alloys
63. Advantages of die casting
Excellent dimensional accuracy
Smooth cast surfaces
Thinner walls can be cast
Inserts can be cast-in (such as threaded inserts,
heating elements, and high strength bearing
surfaces).
Reduces or eliminates secondary m/c ing operations.
Rapid production rates.
64. Disadvantages of die casting
The main disadvantage - very high capital cost.
To make die casting an economic process a large
production volume is needed.
Die casting is limited to high fluidity metals (Zinc,
Aluminum, Magnesium, Copper, Lead and Tin) (Not
applicable for high melting point metals and alloys
(eg. steels)
Casting weights must be between 30 grams and 10 kg
Limited die life
65. Centrifugal Casting
In this process, the mold is rotated rapidly about its
central axis as the metal is poured into it.
Because of the centrifugal force, a continuous pressure
will be acting on the metal as it solidifies.
The slag, oxides and other inclusions being lighter, get
separated from the metal and segregate towards the
center.
This process is normally used for the making of hollow
pipes, tubes, hollow bushes, etc., which are ax- symmetric
with a concentric hole.
66. Centrifugal Casting
Since the metal is always pushed outward because of the
centrifugal force, no core needs to be used for making the
concentric hole.
The mold can be rotated about a vertical, horizontal or an
inclined axis or about its horizontal and vertical axes
simultaneously.
The length and outside diameter are fixed by the mold
cavity dimensions while the inside diameter is
determined by the amount of molten metal poured.
