2. Title of slide
Lesson Objectives
In this chapter we shall discuss the following:
Learning Activities
1. Look up
Keywords
2. View Slides;
3. Read Notes,
4. Listen to
lecture
Keywords:
3. Casting
• Most widely used casting process, accounting for a
significant majority of total tonnage cast
• Nearly all alloys can be sand casted, including metals with
high melting temperatures, such as steel, nickel, and
titanium
• Castings range in size from small to very large
• Production quantities from one to millions
Fig: A large sand casting weighing over 680 kg
for an air compressor frame
6. Steps in Sand Casting
1. Pour the molten metal into sand mold
2. Allow time for metal to solidify
3. Break up the mold to remove casting
4. Clean and inspect casting
5. Heat treatment
7. Pattern Making
• In pattern making, a physical model of casting, i.e. a pattern
is used to make the mold.
• The mold is made by packing some readily formed
aggregated materials, like molding sand, around the pattern.
• After the pattern is withdrawn, its imprint leaves the mold
cavity that is ultimately filled with metal to become casting.
• In case, the castings is required to be hollow, such as in the
case of pipe fittings, additional patterns, known as cores,
are used to develop these cavities.
8. Making the Sand Mold
• The cavity in the sand mold is formed by packing sand
around a pattern, then separating the mold into two
halves and removing the pattern.
• The mold must also contain gating and riser system
• If casting is to have internal surfaces, a core must be
included in mold.
•A new sand mold must be
made for each part produced
9. Melting and Pouring
• Melting is a process of
preparing the molten
material for casting.
• It is generally done in a
specifically designated
part of foundry,
• The molten metal is
transported to the pouring
area wherein the molds are
filled.
10. Two Categories of Casting
Processes
1. Expendable mold processes – sand mold is
sacrificed to remove part
– Advantage: more complex shapes possible
– Disadvantage: production rates often limited by
time to make mold rather than casting itself
2. Permanent mold processes - mold is made of
metal and can be used to make many castings
– Advantage: higher production rates
– Disadvantage: geometries limited by need to open
mold
11. The Pattern
• A full-sized model of the part, slightly enlarged to
account for shrinkage and machining allowances in the
casting.
• Pattern materials:
– Wood - common material because it is easy to work, but it warps
– Metal - more expensive to make, but lasts much longer
– Plastic - compromise between wood and metal
Top center is the clay original, then the two part
plaster mold used for casting the lead at above, and
wax cast from mold, sprued for better brass casting,
not yet cast.
homepages.waymark.net/mikefirth/tapper6881b.jpg
12. Types of Patterns
Types of patterns used in sand casting are shown below:
– Solid pattern
– Split pattern
– Match-plate pattern
– Cope and drag pattern
13. Pattern Allowances
• Pattern is having different size as compared to casting
because it carries certain allowances due to
metallurgical and mechanical reasons.
• The various allowances are as follows:
1. Shrinkage or contraction allowance.v
2. Machining or finish allowance.
3. Draft or taper allowance.
4. Distortion or camber allowance.
5. Shake or rapping allowance (-).
6. Mould wall movement allowance (-).
14. Moulds
• Mould or Mould cavity contains molten metal and is
essentially a negative of the final product.
• Mould is obtained by pattern in moulding material (sand).
• Mould material should posses refractory characteristics
and withstand the pouring temperature.
15. Types of Moulds
Basically moulds are two types:
1. Expendable moulds-
– are made of sand and is used for single casting which
break upon solidification.
2. Permanent moulds-
– are made of metal or graphite (costly) and used
repeatedly for large number of castings which do not
break upon solidification.
Fig. Expendable moulds
16. Types of Sand Mold
• 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 - mold cavity surface of a green-sand
mold is dried to a depth of 10 to 25 mm.
Open mould
closed mould
17. Moulding
• Moulding is the process of making sound mould of sand
by means of pattern.
Types of moulding:
1. Hand moulding-
are used for odd castings generally less than 50 no.
and ramming is done by hands which takes more
time.
2. Machine moulding-
are used for simple castings to be produced in large
numbers. Ramming is done by machine so require
less time.
18. Desirable Mold Properties
• Strength - to maintain shape and resist erosion
• Permeability - to allow hot air and gases to pass
through voids in sand
• Thermal stability - to resist cracking on contact with
molten metal
• Collapsibility - ability to give way and allow casting to
shrink without cracking the casting
• Reusability - can sand from broken mold be reused to
make other molds?
19. Finer the grain
low is the permeability
Addition of water
increases permeability upto a limit
Fig (a) Effect of grain size permeability
Fig (b) Water content on permeability
20. Material used for making green sand moulds consists following:
1. Sand (70-85%): to provide refractoriness
2. Clay (10-20%): to act as binder, along with water, impart
tensile and shear strength to the molding sand
3. Water (3-6%): to activate the clay and provide plasticity
4. Organic additives (1-6%): to enhance desired sand properties
• Moulding sand composition must be carefully controlled to assure
Satisfactory and consistent results.
• Good molding sand always represents a compromise between
conflicting factors such as:
– Size of sand particles, Amount of bonding agent (such as clay),
Moisture content, Organic matter
Composition Of Moulding Sand
21. Constituent Of Moulding Sand
CLAY
Clay is generally used as binding agent in the molding sand to provide
the strength, because of its low cost and wider utility.
The most popular types of clay used are:
1. Kaolinite or fire clay (melting point: range of 1750 to 1787°c )
2. Bentonite (melting point: range of 1250 to 1300 0c), two types
3. Sodium bentonite or western bentonite
4. Calcium bentonite or southern bentonite
Bentonite can absorb more water which increases its bonding
power.
Sodium bentonites produce better swelling properties (volume
increases some 10 to 20 times), high dry strength which lowers the
risk of erosion, better tolerance of variations in water content, low
green strength and high resistance to burnout which reduces clay
consumption.
22. • Water activates clay so that it develops
the necessary plasticity and strength.
• Amount of water used should be
properly controlled.
• Water in molding sand is often referred
as “tempering” water.
• Water in excess -------- free water
• A part of the water absorbed by clay
helps in bonding while the remainder up
to a limit helps in improving the
plasticity.
• Excessive water decreases the
strength, permeability and formability.
• Normal percentages of water used are
from 2 to 8%.
Constituent of Moulding Sand
Water
Fig. Effect of Water content on (a) sand properties (b) green strength
23. • Additives are added to sand to enhance the specific properties.
• Since molding material is often reclaimed and recycled, the
temperature of the mold during pouring and solidification is also
important.
• If organic materials are added to provide collapsibility, a portion will
bum during the pour. Some of the mold material may have to be
discarded and replaced with new one.
Constituent 0f Moulding Sand
Additives
24. Variables Affecting Molding
Sand Properties
Following are the main variable:
1. Sand grain shape and size
2. Clay and water
3. Method of preparing sand mold
25. • Coarse grains: good permeability and better refractoriness.
• Finer grains: lower permeability but better surface finish.
• Purity of sand grains improves the refractoriness.
• Uniform-size grains: good permeability, while a wide distribution
of sizes enhances surface finish.
• Round grains: good permeability and require less amount of clay.
• Angular grains: better green strength.
Sand Grain Shape And Size
26. Clay and Water
• An optimum amount of water is to be
used for a given clay content to obtain
maximum green compression
strength.
• During the sand preparation clay is
uniformly coated around the sand
grains.
• Water then reacts with the clay and
forms a linkage of silica-water-clay-
water-silica throughout moulding sand.
• Any additional amount of water
increases the plasticity and dry
strength but reduces the green
compression strength.
27. Degree of ramming increases the bulk density.
Increased ramming increases the strength.
Permeability of green sand decreases with degree of
ramming.
Machine moulding provide better and uniform density.
Sling moulding is better than jolt moulding
Method of Preparing Sand Mold
29. CORES
• Full-scale model of interior surfaces of part.
• It is inserted into the mold cavity prior to pouring
• The molten metal flows and solidifies between the mold cavity.
• Core forms the casting's external and internal surfaces.
• Cores may require supports to hold it in position in the mold
cavity during pouring.
Fig: (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c)
casting with internal cavity.
30. Core Parts
• A core consists of two portions:
– The body of the core and
– one or more extensions called prints
• The body of the core is surrounded by molten metal
during casting process.
• Body of core has all the features which are required in
final internal surface (e.g. hole) of the castings.
• The prints are necessary to support the core in the
mould.
• They also conduct the heat (and gases produced by a sand
core) to the mould.
31. Core, Core Print & Core Box
CORE: sand body that is
inserted into the mold to
produce the internal features
of a casting, e.g. holes.
CORE PRINT: region added to
the pattern, core, or mold which
is used to locate and support
the core within mold
CORE BOX: the mold or die
used to produce casting cores
32. Essential Characteristics
of Core (Sand)
A good core must possess followings:
High permeability to allow an easy escape to gases formed.
High refractoriness to withstand high temperature of
molten metal
Smooth surface.
High collapsibility i.e. it should be able to disintegrate
quickly after the solidification of the metal is complete.
Sufficient strength to support itself.
33. Functions (Purposes) of Cores
Cores are required for following :
• The cores are used to form the internal cavities.
• Cores are used to form a part of a green sand mould.
• Cores are used to strengthen the moulds.
• Cores are used as a part of the gating system.
34. Post Solidification Operations
In general following operation are performed on castings:
1. Trimming: Removal of sprues, runners, risers, parting-line
flash, fins, chaplets, and any other excess metal.
2. Removing core: Cores are fall out own, as the binder
deteriorates or are removed by shaking casting or
dissolved chemically.
3. Surface cleaning: Casting surface are cleaned by tumbling,
wire brushing, buffing, and chemical pickling etc. to enhance
surface appearance and detect defects.
4. Inspection: To detect defects & assure quality of castings
5. Repair : to fix general and casting related defects.
6. Heat treatment : Castings are often heat treated to
enhance properties and relieving stresses.
35. • Discontinuities in castings
that exhibit a size, shape,
orientation, or location that
makes them detrimental to
the useful service life of
the casting
• Some casting defects are
remedied by minor repair
or refurbishing techniques,
such as welding
• Other casting defects are
cause for rejection of the
casting.
Casting Defects
36. • Fins are excessive amounts of metal created by
solidification into the parting line of the mold.
Fins are removed by grinding or sandblasting.
• Swells are excessive amounts of metal in the vicinity of
gates or beneath the sprue.
• Scabs are surface slivers caused by splashing and rapid
solidification of the metal when it is first poured and
strikes the mold wall.
Casting Defects:
Metallic Projections
37. – Blowholes and pinholes are holes formed by gas entrapped
during solidification
– Shrinkage cavities are caused by lack of proper feeding or
non-progressive solidification and have a rougher shape.
Porosity is pockets of gas inside the metal caused by micro-
shrinkage, e.g. dendritic shrinkage during solidification.
Casting Defects:
Cavities
38. • Cracks in casting and are caused by hot tearing, hot cracking,
and lack of fusion (cold shut)
– A hot tear is a fracture formed during solidification
because of hindered contraction
– A hot crack is a crack formed during cooling after
solidification because of internal stresses developed in the
casting
– Lack of fusion is a discontinuity caused when two streams
of liquid in the solidifying casting meet but fail to unite
Rounded edges indicate poor contact between various metal
streams during filling of the mold
Casting Defects:
Discontinuities
39. • Casting surface irregularities that are caused
by incipient freezing from too low a casting
temperature
• Wrinkles, depressions and adhering sand
particles
Casting Defects:
Defective Surfaces
40. • Particles of foreign material in the metal matrix
• The particles are usually nonmetallic compounds but
may be any substance that is not soluble in the matrix
– Slag, dross, and flux inclusions arise from melting
slags, products of metal treatment, or fluxes
They are often deep within the casting
– Mold or core inclusions come from sand or mold
dressings and are usually found close to the surface
Casting Defects:
Inclusions
41. Gating System
• Gating systems refer to all those elements which are
connected with the flow of molten metal from the
ladle to the mould cavity.
• Following are the elements of gating systems:
1. Pouring Basin
2. Sprue
3. Sprue Base Well
4. Runner
5. Runner Extension
6. Gate or Ingate
7. Riser
42. Objective of The Gating System
Four main points, which enables a proper gating system, are:
1. Clean molten metal.
2. Smooth filling of the casting cavity.
3. Uniform filling of the casting cavity.
4. Complete filling of the casting cavity.
• The mold cavity must be filled with a clean metal so that
it prevents the entry of slag and inclusions into the mold
cavity, which in turn minimizes the surface instability.
• If the mold has smooth filling then it helps to reduce the
bulk turbulence. If it has a uniform filling it means that
the casting fill is in a controlled manner.
• Complete filling of the cavity makes the metal thin with
minimum resistance at the end sections.
43. Factor Affecting The
Performance Of Gating System
• To achieve sound casting and other objectives following
factors should be controlled properly:
1. The type of ladle and ladle equipment.
2. The size, type, and location of sprue and runner.
3. The size, number & location of gates entering mold cavity.
4. The rate of pouring.
5. The position of the mold during casting.
6. The temperature and fluidity of the metal.
44. Elements of Gating System
1. Pouring basin : collects the molten metal, which is poured,
from the ladle.
2. Sprue : leads the molten metal from the pouring basin to
the sprue well.
3. Sprue Well : It changes the direction of flow of the
molten metal to right angle and passes it to the runner.
4. Runner : takes the molten metal from sprue to the casting.
5. Ingate: moves molten metal from runner to the mold cavity.
6. Slag trap : It filters the slag when the molten metal moves
from the runner and ingate.
45. ELEMENTS OF GATING SYSTEM
Pouring basin
• A reservoir for the molten metal poured from the
ladle.
• This is otherwise called as bush or cup.
• It is circular or rectangular in shape.
• It collects the molten metal, which is poured, from the
ladle.
• It prevent the mould erosion.
• Prevent slag and other impurities from entering the
mould cavity.
46. ELEMENTS OF GATING SYSTEM
• Sprue : It is circular in cross section. It leads the
molten metal from the pouring basin to the sprue well.
• Sprue Well : It changes the direction of flow of the
molten metal to right angle and passes it to the
runner.
47. ELEMENTS OF GATING SYSTEM
• Slag trap : It filters the slag when the molten metal
moves from the runner and ingate.
• It is also placed in the runner.
48. ELEMENTS OF GATING SYSTEM
• Runner : The runner takes the molten metal from
sprue to the Ingates of casting.
• This is the final stage where the molten metal moves
from the runner to the mold cavity.
50. TYPES OF GATES
1. Horizontal Gating System : This is used most widely. This type is
normally applied in ferrous metal's sand casting and gravity die-
casting of non-ferrous metals. They are used for flat casting,
which are filled under gravity.
2. Vertical Gating System : This is applied in tall castings were
high-pressure sand mold, shell mold and die-casting processes are
done.
3. Top Gating System : this is applied in places where the hot metal
is poured form the top of the casting. It helps directional
solidification of the casting from top to bottom. It suits only flat
castings to limit the damage of the metal during the initial filling.
4. Bottom Gating System : it is used in tall castings where the
molten metal enters the casting through the bottom.
5. Middle Gating System : It has the characteristics of both the
top and bottom
54. Solidification of Casting
•During solidification metal experience shrinkage which
results in void formation.
•This can be avoided by feeding hot spot during
solidification.
•Riser are used to feed casting during solidification.
60. Modulus Method
• Modulus is the inverse of the cooling characteristic ( surface
area/ Volume) and is defined as
Modulus = Volume / Surface area
• In steel casting riser with height to diameter ration of 1 is
generally used.
• Volume of cylindrical riser =
• Surface area =
• For sound casting modulus of riser shoud be greater than the
modulus of casting by a factor of 1.2. Therefore
Mr = 1.2 Mc
• On simplification D =6 Mc
• Considering contraction of metal
66. CHILLS
• Chills are pieces of material placed in the mold to
speed up heat transfer in thicker areas of the part
to prevent shrinkage porosity
• Internal chills are left within the cast part; external
chills are removed
67. Chills
FIGURE 5.35 Various types of (a) internal and (b) external chills (dark areas at corners), used in castings to
eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal,
as shown in (c).
