3. What is casting
• Casting means pouring molten metal into a refractory mould with a
cavity of the shape to be made and allowing it to solidify.
• Similar to ice making.
• When solidified, the desired metal object is taken out from the
refractory mould either by breaking the mould or taking the mould
apart.
• The solidified object is called casting.
• This process is also called founding.
4. Mold flask – Upper part – COPE
Lower part – DRAG
Mold cavity – formed by PATTERN
6. Molding Flask – for holding sand mold
Cope – upper molding flask
Drag – lower molding flask
Cheek – intermediate molding flask (in case of three-piece molding)
Parting line – dividing line between the two molding flasks
Molding Sand – mixture of silica, clay and water for making mold cavity
Pouring Basin – small funnel shaped cavity at top of mold to molten metal
Sprue – passage through which molten metal reaches mold cavity from
pouring basin
Runner – passage in parting line through which molten metal flow
reaches mold cavity
Gate – entry point to mold cavity
Riser – reservoir of molten metal
Core - used for making hollow cavities in casting
Chaplet - to support core in mold cavity
8. STEPS IN SAND CASTING PROCESS
Pattern making:
Pattern is the replica or model of the shape of product to
be made by the casting process.
Wood, metal, wax or plastic is used to make the pattern.
Always the pattern is larger in size compared to the
actual casting to be made.
Core making:
If the casting is hollow, additional patterns called cores
are used to produce the hollow surfaces.
The core is usually made by a special sand mixture called
core sand.
The core is baked to impart strength before placing it
inside the mould cavity.
Moulding:
It is the process of creating a void or cavity in compacted
sand mass with the help of a pattern.
9. STEPS IN SAND CASTING PROCESS
Melting and pouring:
metal is melted in a furnace and then it is poured into the mould cavity
through the gating system.
Solidification:
Along with pouring, cooling of the liquid metal starts and the metal
solidifies.
During cooling and solidification, the metal shrinks in size, and acquires
a solid structure.
Shakeout and cleaning:
When the solidification is completed and mould has cooled down to
room temperature the casting is removed from the mould. This is called
shakeout.
The surface of casting may have sand particles and scales attached to
it, which are removed by cleaning process. The gating and risering
system are also cut and removed now.
10. Types of Sand Molds
1. GREEN SAND MOLD – mixture of silica sand, clay (binder) and
water (to activate clay).
2. DRY SAND MOLD – Green sand with cereal flour (1-2%) and
pitch (1– 2%) is baked in oven for several hours to expel the
moisture.
3. SKIN-DRY SAND MOLD – Instead of entirely drying, mold is
partially dries around the cavity (depth of around 25 mm).
4. LOAM SAND MOLD – Percentage of clay is high with
refractories, graphite and fibrous reinforcements.
5. CEMENTED SAND MOLD – 10 -15% cement as binder.
6. RESIN BONDED SAND MOLDS - Green sand with thermosetting
resins (polymers) or an oil (linseed or soyabean). During baking
the resin oxidises and polymerises around the sand.
11. Moulding sand-components
Silica Sand:
Major portion of the moulding sand (nearly 96%) is silica grains.
The melting point of the silica sand is in the range 1500-1600O deg. Cel and is suitable for
ferrous castings.
Olivine, chromite and zircon are other types of sand used as moulding sand.
Clay:
Clay is used as the binding agent to provide cohesiveness and strength to the moulding
sand. The two types of clay used for this purpose are bentonite and fire clay.
Water:
Normal requirement of water is in the range of 2 to 8% for necessary plasticity and
strength to the moulding sand. The binding effect of clay is activated by the presence of
water
Additives:
To enhance the properties of moulding sand, certain additives are added
Cornflour or cereals: Added in the range 0.25 to 2% to improve strength, toughness and
collapsibility.
Pitch: It is a by-product of coke making and is used upto 3% to improve hot strength.
Saw dust: To improve collapsibility and to increase permeability. Added upto 2%.
Silica flour: Added upto 35% to increase hot strength and to decrease metal penetration
into the mould.
12. Types of Moulding Sands
Natural sand
it refers to the sand collected from river beds.
It contains sufficient amount of binding clay so that it can be used as
moulding sand without adding other binders.
suitable for cast iron and non-ferrous castings.
Synthetic sand
This type of sand is artificially compounded by mixing washed silica
sand with selected type of clay binders in the required proportions.
It is used for steel and non-ferrous alloys.
Synthetic sand possesses high refractoriness and permeability.
Facing sand
The sand used next to the pattern is called facing sand.
All the faces of mould will be covered with facing sand to obtain a
smoother surface for the casting.
The facing sand is coming into direct contact with molten metal;
hence it needs high hot strength and refractoriness.
13. Types of Moulding Sands
Backing sand
This sand forms bulk of the mould. After covering the pattern with a thin layer of facing sand,
the remaining space in the moulding box is filled with backing sand.
It contains burnt facing sand along with reconditioned foundry sand with proper clay and
water content.
Parting sand
This is washed silica sand free of clay which is a non-sticky material. Parting sand is sprinkled
over the pattern and parting (open) surface of the mould. This allows easier separation of
cope and drag without sticking to each other. The removal of pattern from mould also is
made easier by the presence of parting sand.
Core sand
This is used to make the cores. It contains special binders and additives to provide green and
cured (baked) strength to the cores. Core sand usually has very low clay content and larger
grain size.
Loam sand
This type of sand is used for making moulds of heavy and large castings. It contains more clay
content, even upto 50%. Upon drying it attains higher hardness.
14. Properties of Mold sand
Green strength: It is the strength of the moulding sand in the green or moist condition.
Dry strength: It is the strength of moulding sand in the dry or nearly dry state. The mould
should have sufficient dry strength to withstand erosion of mould walls and mould
enlargement.
Hot strength: When the molten metal is poured into the mould the mould also becomes hot.
The strength of moulding sand in the hot condition is referred to as hot strength.
Refractoriness: It is the property of moulding sand, by which it can withstand high
temperature of liquid metal without any physical change.
Permeability and porosity: The ability of the moulding sand to allow the gases and steam to
escape through the mould walls.
Adhesiveness: It is the property of moulding sand due to which it adheres to the surface of
other materials. Due to this property, sand adheres to the walls of moulding box.
Cohesiveness or strength: It is the property due to which sand particles stick together. The
moulding sand should have sufficient strength so that it does not collapse or get damaged
during its preparation or use.
Collapsibility: Due to this property, the sand mould collapses automatically after solidification
of the casting. If this property is absent, it will lead to defects like hot tears and cracks in
15. Sand testing
1. Moisture content test – by loss of weight after evaporation
- by moisture teller
2. Clay content test
3. Permeability test
4. Strength test – Compressive strength, Tensile strength, Shear strength, Bending strength
5. Hardness test – to check the ramming density; by dial indicating hardness number
6. Fineness test – by GFN (Grain Fineness Number)
GFN =
𝑊𝑖 𝐹𝑖
𝐹𝑖
where, 𝑊𝑖 - weightage factor
𝐹𝑖 - amount of sand retained in sieve
16. Types of Patterns
1. Single piece pattern
2. Split or two-piece pattern
3. Multi piece pattern
4. Match plate pattern
5. Cope and drag pattern
6. Gated pattern
7. Sweep pattern
17. 1. Single piece pattern
This is the simplest type of pattern made as a single piece.
It is also called as solid pattern.
This type of patterns are made from wood or metal and is used
either in the cope or drag part of moulding box.
18. 2. Split or two-piece pattern
This type of pattern will have two pieces known as upper (cope part)
and lower (drag part) parts.
These parts are kept is position by the dowel pins.
The line of separation of the two pieces of pattern is called parting
line.
Two-piece pattern
19. 3. Multi piece pattern
Larger and complicated castings require more than two pieces of pattern.
The pieces of pattern are fastened with the help of dowel pins.
When there are three pieces of pattern, each part will be moulded in separate boxes.
Top part of moulding box is called cope, the middle one is cheek and the bottom part is
drag.
20. 4. Match plate pattern
When the two pieces of a split pattern are attached to
opposite sides of a metal or wooden plate, it is called match
plate pattern.
Match plate pattern
21. 5. Cope and drag pattern
This is similar to match plate pattern, but the two halves of
split pattern are attached to two separate plates.
The gating and risering system also may be included in this
type of patterns.
22. 6. Gated pattern
For producing large number of small castings, gated pattern is used.
Here, a number of patterns are connected together with runner and
gates. The gated pattern is usually made of metals.
It helps in reducing time and labour required for the moulding work.
23. 7. Sweep pattern
This type of pattern is used for generating axisymmetric or prismatic shapes of larger
dimensions.
The sweep pattern is a wooden board having semi-sectional shape of the casting to be
made.
This reduces the cost of a large 3-dimensional pattern. It is used with the help of a base
and vertical spindle.
The sweep is rotated around the spindle to obtain a cavity.
24. Pattern Materials
While selecting a material for pattern making, the following factors
needs to be considered.
Number of castings to be made.
Quality of finish required.
Dimensional accuracy required.
Moulding method used.
Shape, size and complexity of casting
Type of casting method used.
The commonly used materials are wood, metal, plastic, plaster of
paris and wax.
25. Pattern Allowances
While designing a pattern, certain allowances are given on the
dimensions of castings.
Therefore, pattern will be slightly enlarged than the casting of the
exact dimensions is obtained.
The important allowances applied to patterns are:
1. Shrinkage Allowance
2. Taper or Pattern Draft Allowance
3. Machining Allowance
4. Rapping or Shaking Allowance
5. Distortion Allowance
26. 1. Shrinkage Allowance
The shrinkage allowances are applied to patterns
compensate the shrinkage of metals during the
solidification.
Shrinkage allowance make the patterns bigger than the
dimensions of casting.
Positive allowance
27. 2.Taper or Pattern Draft Allowance
This is the allowance given on vertical faces of pattern for easier
removal from the mould without damaging the mould.
In general, a taper of around 1.5 degree or 10 to 20mm per meter
is given for the outside surfaces.
For interior surfaces and holes, 40 to 60 mm per meter is usually
given.
Positive allowance
28. 3. Machining Allowance
If the surface of the casting to be machined after casting, extra metal
needs to be provided.
After machining, the required dimensions is obtained.
The standard allowance for ferrous materials is 3mm and for
nonferrous materials, it is 1.5mm.
Positive allowance
29. 4. Rapping or Shaking Allowance
To remove the pattern from the mould, it is slightly
shaken inside the mould.
This expands the mould slightly and causes the size of
casting to increase.
To compensate this the pattern has to be made slightly
smaller along the horizontal dimensions.
This is the only negative allowance given to a pattern.
30. 5. Distortion Allowance
In addition to contraction, distortions are also observed in
casting due to non uniforms stresses developed during
cooling.
This depends on the shape and size of the casting.
In order to compensate this, the pattern may be made
distorted in the opposite way.
For casting very thin surfaces like U & v Shapes.
31. Example-problem 1
A casting of dimensions 20×50×80mm is to be made from cast iron. The mould for this
casting is to be made using a wooden pattern. Determine dimensions of pattern, if
machining allowance is 2mm on each side, shrinkage allowance is 2% and draft
allowance is 1O.
1. Machining allowance = 2 mm on each side
Adding the machining allowance, dimensions of casting become 24×54×84mm
32. Example-problem 1- continued….
2. Draft allowance of 1O
When the job is cast with the 54×84 mm horizontal surface, the vertical sides of
24mm need draft allowance of 1O.
From figure, x = 24tan1= 0.42mm
Adding draft allowance, dimensions of top surface become 84 + 2×0.42mm =
84.84mm and 54 +2×0.42 = 54.84mm
33. Example-problem 1- continued….
3. Shrinkage allowance of 2% on all dimensions
The side of 84.84 will become 84.84 + 2% = 86.54mm
The side of 84 will become 84 + 2% = 85.68mm
The side of 54.84 will become 54.84 + 2% = 55.94mm
The side of 54 will become 54 + 2% = 55.08mm
The side of 24 will become 24 + 2% = 24.48mm
34. CORES
Core is a sand shape used to make an internal cavity or hollow
structures in a casting.
Core is placed in a mould to prevent the molten metal from filling the
space occupied by the core.
The core sand is a special sand with good permeability, high
refractoriness, good strength and hot strength, high collapsibility and
good surface finish.
The main application of cores is to provide accurate internal shapes in
the casting.
Another application is to help in simplifying moulding of difficult
outer shapes.
35. Core Making
Core is made using a core box.
Core sand is rammed manually in the case of small cores.
To enhance the strength, reinforcement using metal wires
also are provided in specific cases.
Core making machines are used for making cores in large
quantity.
In order to attain sufficient strength (to support itself
inside the mould) the core is cured by baking.
36. Chaplets
Chaplets are used to keep the core in position and prevent it from
being shifted.
Chaplets are metallic inserts made of the same material of casting,
so that it melts and forms part of casting.
Core prints
Core prints are provided on the mould for positioning the cores
securely and correctly in the mould cavity.
Another function of the core print is to carry the core weight and
ensure that the core is not shifted during the pouring of the melt to
the mould cavity.
37. Types of Cores
1. Horizontal core: When the core is positioned horizontally in the mould. When it is having an axial
symmetry, it is placed at parting line.
2. Balanced core (Fig. a): If one end of core is supported and balanced, it is called as balanced core. The
longer core print helps to keep the core in position. Usually placed horizontally with the support of
chaplets.
3. Vertical core (Fig. b): When the axis of the core is vertical it is called vertical core. Major portion of
vertical core may be in the drag.
38. Types of Cores
4. Hanging core (Fig. a): This is supported along the parting line and hangs vertically in the mould. The
bottom portion is unsupported. This is also known as cover core.
5. Wing Core (Fig. b): This core is used when a hole is needed in casting either above or below the parting
line. Here, the side of core print is given sufficient taper for placing the core easily. This type of core is
also called as drop core, tail core or saddle core.
39. Types of Cores
6. Kiss core (Fig. a): The kiss core is kept in the drag part of the mould to make a hole in the cast. The core
is held in position due to the weight of cope. Multiple kiss cores are used for a number of holes.
7. External Core (Fig. b): External core is used to shape the external surface of the casting, if the shape of
the casting is not easy for moulding. Here, a plain mould is made and the cores are placed inside it to
obtain the complicated shapes.
40. SOLIDIFICATION OF METALS
Progressive or Parallel Solidification
As the liquid metal enters the mould, sudden cooling occurs near the mould
walls and progresses perpendicularly towards the interior.
When this happens, the last liquid to solidify will be surrounded by a shell of
solid metal and will lead to shrinkage cavities at this location.
Directional Solidification
If the solidification starts at the thinnest section and proceeds towards the thick
sections in the casting it is called directional solidification.
This will lead to defect free castings
41. SOLIDIFICATION OF METALS
Chvorinov’s Rule
Solidification time for a casting is a function of the ratio of square of volume to
square of surface area of casting.
Solidification time =
square of volume
square of surface area
=
𝑉2
𝐴2
Solidification time, 𝑡 = 𝐶
𝑉
𝐴
2
, where C is a constant related to mould material
and metal properties.
This ratio will be high for spheres and progressively lesser for cylinders, bars
and plates.
42. Example – problem 1
What will be the solidification time for a 1200mm diameter and 35mm
thick casting of aluminium having a mould constant, C =2.2 sec/mm2
1200 mm
35 mm
43. Example – problem 2
Which of the following casting shapes would have the least solidification time. (a) sphere of
diameter, D=35mm; (b) cylinder of diameter d = height, h = 35mm; (c) cube of side a=35mm
35 mm
35 mm
35 mm
35 mm
45. Pouring Basin: The molten metal from the ladle is poured into the pouring basin.
Sprue: The sprue is the vertical passage through which molten metal flows down from pouring basin to runner.
Sprue base: This is a reservoir for liquid metal at the bottom of sprue.
Runner: Runner connects the sprue base to the gates or ingates. Gates or Ingates: Molten metal enters the
mould cavity through the openings called gates or ingates.
Skim bob: This is an enlargement given along the runner for trapping both heavier and lighter impurities like
slag or sand particles moving along with liquid metal.
Choke: The region having smallest cross-sectional area in the gating system is called choke , used to control
flow.
Gating system
46. Gating system-Purpose
Metal should flow smoothly into the mould cavity without any
turbulance, so that mould erosion is prevented.
Unwanted inclusions like dross or slag should not enter into mould
cavity.
Metal entering into the mould should flow at a controlled speed.
The complete cavity should be filled in minimum time before
solidification occurs.
Ultimately, defect free casts.
47. Top Gate
If the molten metal enters the mould cavity
from the top, top gates are used.
Bottom Gate
In a bottom gate, the molten metal enters the
mould cavity through the bottom side (drag
part) and raises gradually upwards.
Types of Gates
48. Parting Gate
Parting gate is the commonly used gate in sand
casting.
Here the molten metal enters the mould along
the parting plane.
Step gate
To allow the molten metal to heavy and large
castings step gates are used.
The molten metal enters the cavity through a
number of ingates arranged vertically.
Types of Gates
49. Gating Ratio
Gating ratio is defined as the ratio of Sprue area: runner area: gate area
Pressurized gating system: In this system, the ingates function as choke
and the entire gating system become pressurised.
Unpressurised gating system: In this system, sprue base is narrow and
serves the function of choke.
50. Pouring Time
The time required for complete filling of a mould is called
pouring time.
d = Predominant wall thickness of the casting in mm
M = Mass of the casting in kg
A = Constant depending on the metal being cast.
𝑡 = 𝐴
3
𝑑 × 𝑚
51. Choke Area
Choke is the narrowest section in the gating system controls the
metal flow rate into the mould cavity.
𝑎𝑛 =
𝑀
𝜌𝑡𝜇 2𝑔𝐻
M = Mass of casting including risers, kg;
ρ = Density of molten metal, kg/m3;
t = Pouring time or Time for filling the mould, sec;
µ = Coefficient of discharge (0 <µ<1);
g = Acceleration due to gravity, 9.81 m/sec2;
H = Effective static head, m.
𝐻 = 𝐻𝑜 −
𝑃2
2𝐶
H0 = Vertical distance from runner to the feeding point of
mould, or Head;
p = Vertical distance from ingate to the top surface of mould
cavity;
c = Height of the mould cavity
52. Example-problem 1
Design a gating system for a sand mould to cast a steel flange. Weight of
casting is 30 N, pouring time 6 sec, coefficient of discharge is 0.4 and
effective static head is 22cm. Gating ratio is selected as 1:1.4:1.2. Using
the given data and principles of hydraulics determine the important
dimensions of sprue, runner and ingates.
53. This is an unpressurised gating system, where the choke or narrowest
passage is the sprue base.
54. Example-problem 2
A cast iron base plate weighing 230 kg is being cast using sand moulding
process by floor moulding method. Design a gating system to feed the
casting using a gating ratio of 1: 2.5: 3 with the following data. The
effective static head is 28 cm, predominant thickness of casting is 14mm,
coefficient of discharge, 0.45 and the constant A=2 for cast iron.
Determine the important dimensions of the gating system.
55.
56. RISERING OR RISERS
The volume shrinkage of metals occurs during solidification in any casting
process.
During solidification, the last region to solidify may require some additional
liquid metal to compensate for shrinkage already occurred.
This additional liquid metal is provided by the riser which acts as a reservoir of
liquid metal.
The cooling of different sections in a casting can be adjusted for directional
solidification, to obtain defect free castings.
The risers are to be included in a mould in such a way that the last metal to
solidify should be that in the riser.
Also the volume of metal in riser should be sufficient to compensate the
shrinkage demand.
57. Types of risers
Top riser: This is located on top of the casting. It is also called dead or cold riser. As it is on
top of the casting, it provides liquid metal faster to the casting.
Side riser: It is located on the side of casting. The side riser is filled last and contains hot
metal and hence called as live or hot riser. Since the metal in the riser is hot this is more
effective than the top riser.
Open riser: As the name indicates this is open to the atmosphere at the top surface of
mould. It is easy to mould open risers. Open riser also serves to collect floating nonmetallic
inclusions.
Blind riser: The riser completely surrounded by moulding sand is called blind riser.
58. CHILLS
Chill is a piece of metal placed near the casting surface or kept
inside the mould cavity so that more heat is absorbed from the
molten metal at a faster rate.
Use of chills eliminate hot pockets inside the mould and helps
to avoid formation of shrinkage cavities.
59. Insulators
As like chills to accelerate freezing, freezing can be delayed at
selected locations in the casting using local insulations and
exothermic padding.
Asbestos, plaster of paris, ceramics, sawdust etc. are used as
insulating materials for padding
Exothermic Padding
A riser can be made more efficient by employing some artificial means to
prevent the top surface of the riser from solidifying.
burning materials like powdered charcoal, rice hulls or refractory
powders
60. Riser Design - Caine’s method
This method is used to check the adequacy of the riser size for a casting.
Caine’s method is based on an equation for finding the riser volume 𝑋 =
𝑎
𝑌−𝑏
+ 𝑐. a, b and c are
constants
Caine defined X as the relative freezing time or freezing ratio of the riser to the casting.
The freezing time is proportional to surface area by volume. The freezing ratio is the relative
freezing time of the riser to that of the casting.
The freezing ratio, 𝑋 =
𝑆𝑐
𝑉𝑐
𝑆𝑟
𝑉𝑟
and the volume ratio, 𝑌 =
𝑉𝑟
𝑉𝑐
. Where Sr and Vr are the surface area
and volume of riser, and Sc and Vc are the surface area and volume of the casting.
Caine introduced a curve which represents this equation, where a, b and c are constants having
different values for different materials.
61. Riser Design - Caine’s method
When X>1, 𝑆𝑐
𝑉𝑐
> 𝑆𝑟
𝑉𝑟
Freezing time of
casting is more than
that of riser.
Riser will solidify after the solidification of
casting or the riser will be able to feed the
casting when X>1.
When X=1 The riser and casting
will solidify
simultaneously.
Riser will not do its function.
When X<1 The riser will solidify
prior to casting.
62. Riser Design - Caine’s method
The Caine’s equation can be diagrammatically
represented as shown in figure.
The, x-axis is the freezing ratio, X, and the y-
axis is the volume ratio, Y. This curve can be
used to predict whether the casting is defective
or not.
Diagrammatic representation of Caine's
equation
63. Example 1– Riser Design
Determine the size of a cylindrical riser to feed a steel casting of dimension 25 x 25 x 5 cm, given
height of riser is equal to diameter of cylindrical riser using Caine’s method.
64. Example 2– Riser Design
Assuming uniform cooling in all directions, determine the dimensions of a 80mm cube casting
after it cools down to room temperature. The solidification shrinkage for the cast metal is 4%
and the solid contraction is 7%.
65. Example 3– Riser Design
Calculate the ratio of solidification times of two steel cylindrical risers of sizes 25cm diameter by
50cm height and 50cm diameter by 25cm height, subjected to identical conditions of cooling.
66. SPECIAL CASTING PROCESSES
Sand casting process is suitable for a large range of applications and gives satisfactory
results at low cost.
Sand moulds are single use moulds which are to be destroyed for taking out the
castings.
But a permanent mould may save time and labour in mould making.
The accuracy and precision of sand casting are not good and consistent results are
rarely obtained.
Limitations of sand casting
67. Special casting processes
Greater dimensional accuracy
Higher production rate
Better surface finish
Lower labour costs
Lesser machining needs
Suitable for mass production
68. 1. Shell Moulding
Step 1: A metal pattern is used in this process. The pattern attached to a
match plate is heated to 200-250 OC temperature. Then it is clamped upside
down to a dump box containing the sand/resin mixture.
Step 2: The box is then inverted and the sand-resin mixture is allowed to fall
over the hot pattern. The heat softens and fuses the resin to form a soft shell
of sand grains along the surface of pattern.
Step 3: The dump box is inverted once again to stop the shell forming process
and to remove the remaining sand mixture. The pattern along with shell is
heated in an oven at 300 C for a few minutes to complete the curing.
Step 4: The shell is then removed from the pattern
Step 5: The shell is assembled with a similar shell to form a complete mould.
The assembly is placed inside a moulding box and supported by sand or metal
shots, thus completing the mould.
70. 2.Investment Casting
In this casting process an expendable pattern (usually wax) is used
which is coated with a refractory material to form the mould.
The pattern is then melted and removed prior to pouring of liquid
metal.
The term investment refers to the special covering made of
refractory material surrounding the wax pattern.
This is also a precision casting process, since it can make castings of
high accuracy and surface finish. The process is also known as lost
wax process.
72. 3. Permanent Mould Casting
For large scale production of simple casts reusable moulds are preferred.
The basic permanent mould process uses metallic moulds made in two
sections suitable for easy opening and closing.
Steel is usually used to construct the moulds.
The mould cavity, gating and risering systems are accurately machined to
good surface finish.
Metals like aluminium, magnesium, copper base alloys and cast iron are cast
using steel moulds made of refractory metals.
The permanent mould is preheated and lubricant coatings are sprayed prior to
pouring of liquid metal.
The mould halves are closed under external pressure and are kept like that
until the solidification is completed. After this, the mould is opened and
casting is removed.
Usually, metallic cores are used in permanent moulds.
Vacuum casting, slush casting and pressure casting also uses permanent
moulds.
73. 4.Die Casting
This is also a type of permanent mould casting, where the mould
cavity is filled by injecting the molten metal by the application of
external pressure.
The steps involved in die casting are:
1. Close and lock the two halves of dies
2. Force the molten metal into the die cavity under pressure
3. Maintain the pressure until the metal solidifies
4. Open the die
5. Eject the casting along with sprue and runners.
83. Shaping Faults Arising in pouring
The primary requirement of the liquid metal entering into the mould is that it should fill
the mould satisfactorily, else serious defects like misrun or short run occurs. Other
defects under this category are cold shuts and cold laps.
Cold laps
Cold laps occur when the liquid metal fails to flow freely over the mould surface. Due to
the broken flow, the liquid streams solidify without a continuity.
Misrun
When the castings solidify before completely filling the mould cavity, this defect is
observed.
insufficient fluidity of molten metal
lower pouring temperature
very slow pouring
improper gate design.
Cold shut
This defect occurs when two streams of liquid metal meet inside the mould, but fail to
fuse together, due to premature freezing. This defect is also due to the reason behind
the misrun.
84. Inclusions
1. Indigenous or endogenous inclusions - due to the product
of reactions within the liquid metal.
2. Exogenous inclusions - resulting from non-metallic
particles entrapped during melting and pouring. These
include dross, slag, flux residues, refractory particles and
eroded particles of moulding material.
Slag inclusions are usually of smooth rounded form, while dross and
refractory fragments are of irregular shape. These can be prevented
by careful skimming at the pouring stage.
85. Sand defects
Expansion
scab
It occurs due to the partial or complete
breakage of a section of mould face and
penetration of liquid metal to the mould
walls.
Sand
wash/Erosion
scab
This is caused by erosion of mould walls
during pouring of molten metal. The shape of
the eroded region is formed on the surface
of casting. This defect is also known as
erosion scab.
Metal
penetration
A rough casting surface may be resulted when
molten metal enters the gaps between sand
grains. This occurs when the grain size of
sand is coarse and when the ramming is too
light.
Mould crack
or rat tail
When the strength of mould is low, crack may
develop and metal can seep into it to form a
fin shaped projection on the casting surface.
This defect is also known as rat tail.
Swell This is also due to lower strength of mould
surface. The mould wall may move back due
to the metallostatic forces and this will lead
to a bulging or swell on the casting surface.
86. Gas Defects
Defects like blowholes, surface blows, airlocks, pinholes or intergranular
cavities are caused by the entrapped gases.
Defects poor permeability of the mould to pass on the air displaced by the
liquid metal.
The dissolved gases in metals like hydrogen and nitrogen can result in
cavities.
Blow holes:
• These are bubble shaped gas holes appearing in the
casting.
• The gases released from mould during pouring and
entrapped in liquid metal forms blow holes.
• When they are located at the top surface of casting it
is called sand blow or open blow.
87. Shrinkage Defects
Shrinkage occurs during cooling of liquid metal and during
solidification.
Proper gating and risering can compensate it.
When the compensation is inadequate shrinkage defects occur in
castings.
Primary shrinkage or Pipe cavity results from inadequate supply of feed metal from riser. The
pipe may extend into the casting due to the lack of sufficient feed metal.
Secondary shrinkage is internal and occurs in positions away from the riser. This may lead to
a sink in the casting surface (a small depression). In some cases, a local puncture occurs and
produces an inkwell cavity or draw.
88. Contraction Defects
The proper contraction of the casts is hindered by the mould, or by pressure
of residual liquid metal or by other parts of the casting.
If the resistance to contraction occurs at the highest temperature, when the
alloy is in a relatively brittle condition and hot tears occur.
Hot tears or pulls are characterised by irregular, partial or complete
intergranular fracture.
These are usually located at sharp changes in section where stress
concentration is associated with locally delayed cooling.
89. Dimensional Errors
The dimensions of a casting are subjected to variation from minor
changes in production conditions within the limits of normal working
practice. Such errors can occur in pattern making, moulding and
casting.
Alignment faults and mould distortion are two major types of casting
defects under this category.
Mould shift: This defect is due to the shifting of cope relative to drag.
The change in alignment results in a step in the casting along the
parting line.
Core shift: When the core is displaced from its intended position
inside the mould, core shift results. When the core is not properly
placed with the support of core prints and chaplets, the buoyancy of
liquid metal shifts it upwards and a defective casting is obtained.
Run out: When the molten metal leaks out of the moulding box, run
out occurs. This is due to faulty mould making or due to a faulty
moulding box.
90. Compositional Errors and Segregation
Compositional differences between different regions within every
solid grain is called as segregation.
The compositional differences extending over dimensions of the order
of a single grain or less is called micro segregation.
On the other hand, there may be zonal differences between one part
of the casting and another. This is referred to as macro segregation.
The mechanical properties like strength, ductility, impact and fatigue
resistance are affected by micro-segregation.
91. Significance of Casting Defects
The significance of a local defect depends not only upon the size and shape but upon position relative
to the stress pattern in the casting.
Among surface defects, a flaw on a flat surface is less serious than one situated at a change of cross-
section where there may be appreciable concentration of applied stress.
Important defects, like centre line porosity or segregation are less serious because the location is
close to a neutral axis of stress.
All defects produce an increase in the mean stress by reducing the cross-sectional area carrying the
load. The primary effect of this is in the redistribution of stresses.
If a cavity contains some foreign substance like a non-metallic inclusion, the stress concentration
would be less than that of an empty cavity.
Spherical defects like blowholes or slag inclusions also introduce stress concentration as in the case of
shrinkage cavities and sand inclusions.
A fine crack is a more dangerous defect, where the chance of failure is high.
Microporosity is having a drastic effect in lowering the mechanical properties of cast alloys.
Loss of mechanical properties due to segregation of alloying elements or impurities depends upon the
metallurgy of the individual alloy.
Segregated impurities also cause grain boundary embrittlement.
92. SUPERALLOYS
The basis of superalloys is iron, cobalt and nickel, the transition metals
located in a similar region of the periodic table.
The superalloys are created usually by adding significant levels of the alloy
elements like chromium, aluminum and titanium, plus appropriate refractory
metal elements such as tungsten and molybdenum to the base metal.
Demand for materials with high temperature stability, excellent creep
behaviour and high corrosion resistance, etc. led to the development of
superalloys.
The are the main techniques used to produce high quality superalloys are,
Vacuum induction melting (VIM)
Electroslag remelting (ESR)
Vacuum arc remelting (VAR).
93. Vacuum Induction Melting
A VIM furnace is simply a melting crucible inside a steel shell that is
connected to a high-speed vacuum system.
The heart of the furnace is the crucible with heating and cooling coils
and refractory lining. Heating is done by electric current that passes
through a set of induction coils. The coils are made from copper
tubing that is cooled by water flowing through the tubing.
The passage of current through the coils creates a magnetic field that
induces a current in the charge inside the refractory. When the
heating of the charge material is sufficient that the charge has
become all molten, these magnetic fields cause stirring of the liquid
charge.
In VIM process, an electrode or ingot created by melting prerefined
metal made using conventional air melting techniques is remelted
using induction to optimize the composition.
Vacuum induction melting is often done as the primary melting
operation.
94. Electro Slag Remelting
In the Electro Slag remelting (ESR) process, an electrode produced by VIM
is melted under air or gas using heat produced by electrical resistance in
a slag.
The liquid metal runs through a reactive slag of lower density into a
cooled copper crucible.
ESR is a secondary process that further refines alloys formed into
consumable electrodes by VIM.
The ESR controls both metal melting and solidification.
In ESR, alternating current is passed through the electrode formed by VIM
melting.
One end of the electrode is in contact with refractory slag, which acts
like a resistor in the electrical circuit.
Contact with the molten slag generates resistive heating which melts
the slag and bottom edge of the alloy electrode.
The mould is water cooled and efficiently extracts the heat created
during the melting process. The result is a homogeneous ingot with
minimal chemical segregation.
95. Vacuum Arc Remelting
The vacuum arc remelting process (VAR) is the first remelting process
used for the production of steels and superalloys.
A consumable electrode of the superalloy is remelted by a direct current
electric arc in a water-cooled copper crucible.
Like ESR, VAR depends on controlled cooling and solidification of the
remelted ingot to minimize chemical segregation.
The refining takes place in the arc zone between electrode tip and
molten metal pool, on top of the ingot which solidifies continuously
from the bottom upwards.
The controlled solidification of the ingot in the water cooled copper
crucible leads to a sound and homogenous material with excellent
metallurgical properties.
The main purpose of the VAR process is the production of a highly
homogeneous material by controlled melting and solidification.
