Metal casting process part 1

Unit –II
Casting
Part 1
Dr. L.K. Bhagi
Associate Professor
School of Mechanical Engineering
Lovely Professional University
MEC323: PRIMARY MANUFACTURING
(Dr. L K Bhagi)

cONTENT
Introduction to casting, Major classifications of casting, Casting terminology,
Characteristics of molding sand, Constituents of foundry sand,
Patterns and their types,
Cores and types of cores,
Gating system, Types of gates, Solidification, Riser system, Types of riser, Types of
allowances,
Directional Solidification,
Defects in casting,
Riser design(Chvorinov's rules),
Advanced casting techniques:Shell molding, Permanent mould casting, Vacuum die
casting, Low pressure die casting, Continuous casting, Squeeze casting, Slush casting,
Vacuum casting, Die Casting, Centrifugal casting, Investment castingMEC323: PRIMARY MANUFACTURING
(Dr. L K Bhagi)

Casting
 Oldest Manufacturing Process
According to historical records, casting dates back ~ 5000 years B.C (for Arrow
heads, weapons e.t.c)
(Dr. L K Bhagi)

 Oldest Manufacturing Process
3200 B.C, A copper frog (oldest known casting in existence) was cast in
Mesopotamia.
460 B.C, Bronze statue of Zeus was cast in Greece…
Casting
(Dr. L K Bhagi)

Iron Pillar ~ 400 A.D.
This iron pillar dating to 400 A.D.,
remains standing today in Delhi,
India. Corrosion to the pillar has
been minimal, and this skill is lost
to current ironworkers.
Casting
Iron Pillar, Quthub Minar, Delhi.
(Dr. L K Bhagi)

Casting
(Dr. L K Bhagi)

Capabilities and Advantages of Casting
• Can create complex part geometries that can not be
made by any other process
• Can create both external and internal shapes
• Some casting processes are net shape; others are near
net shape
• Can produce very large parts (with weight more than
100 tons), like m/c bed
• Casting can be applied to shape any metal that can melt
• Some casting methods are suited to mass production
• Can also be applied on polymers and ceramics
(Dr. L K Bhagi)

• Some casting processes can produced parts to net shape
(no other manufacturing operations are required)
100 tons), like m/c bed
(Dr. L K Bhagi)

• Some casting processes can produced parts to net shape
(no other manufacturing operations are required)
100 tons)
(Dr. L K Bhagi)

Disadvantages of Casting
Different disadvantages for different casting processes:
– Limitations on mechanical properties
– Poor dimensional accuracy and surface finish for
some processes; e.g., sand casting
– Safety hazards to workers due to hot molten metals
– Environmental problems
(Dr. L K Bhagi)

Casting
Process in which molten metal flows by gravity or other force into a mold
where it solidifies in the shape of the mold cavity.
• Steps in casting seem simple:
1. Melt the metal
2. Pour it into a mold
3. Let it freeze
(Dr. L K Bhagi)

Casting > Casting Processes
Process Advantages Limitations
Sand Casting Almost any metal can be cast; no limit to part size,
shape, or Weight; low tooling cost
Some finishing required;
relatively coarse surface finish;
Wide tolerances
Shell mold Good dimensional accuracy and surface finish; high
production rate
Part size limited; expensive
patterns and equipment
Evaporative
pattern
Most metals can be cast, with no limit to size;
complex part shapes
Patterns have low strength and
can be costly for low quantities
Investment Intricate part shapes; excellent surface finish and
accuracy; almost any metal can be cast
patterns, molds, and labor
Permanent
mold
Good surface finish and dimensional accuracy; low
porosity; high production rate
High mold cost; limited part
shape and complexity; not
suitable for high-melting point
metals
Centrifugal Large cylindrical or tubular parts with good quality;
high production rate
Expensive equipment; limited
part shape
(Dr. L K Bhagi)

Wide tolerances
production rate
Evaporative
pattern (Lost-
foam casting)
polystyrene
foam
complex part shapes
patterns, molds, and labor
Permanent
mold
metals
part shapeMEC323: PRIMARY MANUFACTURING
(Dr. L K Bhagi)

Wide tolerances
production rate
Evaporative
pattern
complex part shapes
patterns, molds, and labour
Permanent
mold
metals
part shape
(Dr. L K Bhagi)

Principle of the Process
 Mould with required cavity is created
 Metal is heated above its melting temperature
 Liquid metal is poured into mould
 Metal solidifies inside the cavity of the mould (casting)
 Casting is removed from the mould
Casting
(Dr. L K Bhagi)

Which typically are made of sand, plaster, ceramics, and similar
materials and generally are mixed with various binders (bonding
agents) for improved properties.
A typical sand mold consists of 90% sand, 7% clay, and 3% Water.
After the casting has solidified, the mold is broken up to remove the
casting.
The mold is produced from a pattern; in some processes, such as
sand and shell casting, the mold is expendable, but the pattern is
reused to produce several molds. Such processes are referred to as
expendable-mold, permanent-pattern casting processes. On the
other hand, investment casting consumes a pattern for each mold
produced; it is an example of an expendable-mold, expendable
pattern process.
Casting> Expendable molds
(Dr. L K Bhagi)

Made of metals that maintain their strength at high temperatures.
As the name implies, they are used repeatedly and are designed in
such a Way that the casting can be removed easily and the mold used
for the next casting.
Metal molds are better heat conductors than expendable nonmetallic
molds; hence, the solidifying casting is subjected to a higher rate of
cooling, which in turn affects the microstructure and grain size
Within the casting.
Casting> Permanent molds
(Dr. L K Bhagi)

Examples of Castings
Casting
The Polaroid PDC-2000 digital camera
Jaivana Canon, Jaigarh Fort, Jaipur
(Dr. L K Bhagi)

Casting Terminology
Schematic illustration of a sand mould,
showing various features
Flask: A metal or a wooden frame with out top or a bottom, in which mould is made.
Cope: Upper moulding flask
Cheek: Intermediate moulding flask
Drag: Lower moulding flask
Casting
(Dr. L K Bhagi)

Casting Terminology
Source: Kalpakjian & Schimd
Pattern: A physical model or the replica of the final objected to be casted. Mould cavity
is created with the help of the pattern.
Casting
(Dr. L K Bhagi)

Casting Terminology
Parting Line: A imaginary line that divides the drag & cope (two parts of the moulding
flask) .
Casting
(Dr. L K Bhagi)

Casting Terminology
Moulding Sand: Binding sand which is used to make the mould . It is the mixture of
silica sand, clay and moisture in appropriate proportions and it is not supposed to loose
permeability.
Casting
(Dr. L K Bhagi)

Casting Terminology
Core: A separate part of the mould made of sand (conventionally baked), which is used
to make various internal cavities inside the castings.
Casting
(Dr. L K Bhagi)

Casting Terminology
Pouring Basin: A small funnel shaped cavity at the top of the mould into which molten
metal is poured.
Casting
(Dr. L K Bhagi)

Casting Terminology
Sprue: The passage through which the molten metal flows from pouring basin to mould
cavity.
Casting
(Dr. L K Bhagi)

Casting Terminology
Gate: The channel through which the molten metal enters the mould cavity.
Runner: The channel through which the molten metal is carried from Sprue to Gate.
Casting
(Dr. L K Bhagi)

Casting Terminology
Chaplets: The metallic supports used to help Core inside the mould cavity, to with
stand its own weight and resist metallostatic forces.
Casting
(Dr. L K Bhagi)

Casting Terminology
Riser: The extra void created in the mould that will be filled by the molten material. It
simple functions as a reservoir of molten metal for the castings, to compensate material
shrinkage , which occurs during solidification.
Casting
(Dr. L K Bhagi)

Casting Terminology
Vent: Small opening provided in the mould to facilitate escape of air (from the mould)
and the gases (from the molten metal).
Casting
(Dr. L K Bhagi)

Casting Procedure
Casting
Outline of production steps in a typical sand-casting operation
(Dr. L K Bhagi)

Casting Procedure
Casting
Schematic Illustration of Sand
Casting Process...
(Dr. L K Bhagi)

Casting
(a) A mechanical drawing of the part is used to generate a design for the pattern. Considerations
such as part shrinkage and draft must be built into the drawing. (b-c) Patterns have been mounted
on plates equipped with pins for alignment. Note the presence of core prints designed to hold the
core in place. (d-e) Core boxes produce core halves, which are pasted together. The cores will be
used to produce the hollow area of the part shown in (a). (f) The cope half of the mold is
assembled by securing the cope pattern plate to the flask with aligning pins, and attaching inserts
to form the sprue and risers.
Casting Procedure
(Dr. L K Bhagi)

Casting
Casting Procedure
(g) The flask is rammed with sand and the plate and inserts are removed. (h) The drag half is
produced in a similar manner, with the pattern inserted. A bottom board is placed below the drag
and aligned with pins. (i) The pattern, flask, and bottom board are inverted, and the pattern is
withdrawn, leaving the appropriate imprint. (j) The core is set in place within the drag cavity. (k)
The mold is closed by placing the cope on top of the drag and buoyant forces in the liquid, which
might lift the cope. (l) After the metal solidifies, the casting is removed from the mold. (m) The
sprue and risers are cut off and recycled and the casting is cleaned, inspected, and heat treated
(when necessary).
(Dr. L K Bhagi)

 Very complicated shapes (Close tolerances),
 Internal cavities,
 Very large castings,
 Very small castings,
 Use is widespread; technology well developed
and more diverse range of products.
 Materials are inexpensive (sand)
 Process is suitable for both ferrous, non-ferrous
metal and alloys castings,
 Cast metal is isotropic, It has same physical and
mechanical properties on all directions.
Casting
Advantages
(Dr. L K Bhagi)

 Very complicated shapes (Close tolerances),
 Internal cavities,
 Very large castings,
 Very small castings,
 Use is widespread; technology well developed
and more diverse range of products.
 Materials are inexpensive (sand)
 Process is suitable for both ferrous, non-ferrous
metal and alloys castings. ,
 Cast metal is isotropic, It has same physical and
mechanical properties on all directions.
Casting
Advantages
~ 8 Tons
~ 5 gms
(Dr. L K Bhagi)

Casting
Disadvantages
 Material waste often ~20 – 50 %
 Slow production rate
 Labor intense job,
 Rough surface finish,
 Sand : metal ratio is relatively high.
 High level of waste is typically generated, particularly sand, refractory linings,
bag house dust, spent shot, timber (from pallets and pattern making), e.t.c.
(Dr. L K Bhagi)

Patterns and Pattern Making - Casting
Pattern
Pattern is a principle tool in casting process. It is replica/physical model of the
final object to be casted, with some small variations.
Major modifications include:
 Pattern Allowances,
 Provision of core.
Functions of Pattern:
 It helps in preparation of the mould,
 It enables creation of core prints,
 It makes provision for runner, gates and riser,
 Patterns properly made and having smooth surfaces reduce further
finishing and reduces casting defects.,
 Properly constructed castings reduce the overall cost of the casting.
(Dr. L K Bhagi)

Some of the materials used for the patterns include:
Wood, Metals, Alloys, Wax, Rubber, Plaster of Paris, Resins, Plastic etc…
There is no perfect pattern material for all applications, every material has its
own advantages and limitations respective to their field of applications.
Properties expected in Pattern:
 It has to be easily workable, shapeable and joined,
 Light in weight, strong, hard and durable.
 Resistant to corrosion, wear, abrasion and even inert to chemical reactions.
 Stable dimensionally, and inert towards temperature and humidity
variations,
 Efficient in terms of cost and time.
Pattern Materials:
(Dr. L K Bhagi)

Single Piece Pattern:
 Inexpensive and Simple type of pattern,
 Used only in the case very jobs or simple (most cases for prototype),
 This pattern is expected to be totally inside the drag,
 One surface is expected to be flat and lying on the parting line.
(not mandatory)
Types of Patterns:
(Dr. L K Bhagi)

Types of Patterns:
Split Pattern:
 Most widely used for intricate castings,
 Used in the case withdrawal from the mould is difficult or depth of the
pattern is high,
 Pattern is split into two parts, where 1st part is in cope and 2nd in drag,
 Split surface matches the parting line
and aligned properly
(Dr. L K Bhagi)

Types of Patterns:
Cope & Drag Patterns
 They are similar to split patterns , however the cope & drag halves are fitted along
with sprue, gating system and riser, and fitted to the wooden or a metal
plate, where there is a further provisions for alignment pins.
 Cope and Drag moulds are produced separately by two different moulders and
and assembled together to form a total mould,
 Used for heavy castings which are
inconvenient for handling.
 Used for mass production.
(Dr. L K Bhagi)

Types of Patterns:
Match Plate Pattern
 One side of the pattern is for cavity in cope, whereas the other in drag,
 Total plate is put in the mould and the sand is compressed, later the cope & drag are
separated and pattern is removed.
 Completely pattern is usually made of aluminum (light weight and easy
machinability)
 Generally used for small casts with high dimensional accuracy
(Dr. L K Bhagi)

Types of Patterns:
Gated Pattern
 When several casting are needed, gated patterns are used.
Usually made of metal,
Gates/ runners for the molten metal are made by connecting parts between individual
parts (objects),
Cost effective, high productivity and time efficient.
(Dr. L K Bhagi)

Types of Patterns:
Loose Piece Pattern
This type of pattern is used when the shape of the final object is complicated and total
pattern is impossible to remove.
(Dr. L K Bhagi)

Types of Patterns:
Sweep Pattern
It is used to sweep the complete casting by means of a plane sweep. These are used for
generating shapes which are axi-symmetrical or prismatic in nature such as bell shaped
or cylindrical. total pattern is impossible to remove.
(Dr. L K Bhagi)

Types of Patterns:
Follow Board Pattern
Is used when portion of casting is structurally weak and couldn’t support force
of ramming sand, or its own weight. The bottom of the board is modified in such a way
that it gives support to the weaker section and to withstand any external forces,
protecting the pattern. (used only for drag, no need for cope).
Skeleton Pattern
Is used when castings are of enormous size and in small numbers. Simple
wooden frame / structure outlining the shape of the cast is used to guide the moulder
for hand shaping the mould. Used when complete wooden frame is not justified.
(Dr. L K Bhagi)

Pattern Allowance:
The Proper selection of allowances greatly helps in reduction of production costs and
Reduces rejections…
Allowances to be taken into consideration:
 Shrinkage or contraction allowances,
 Draft or taper allowance,
 Machining / surface finish allowance,
 Distortion or chamber allowance,
 Shake or Rapping allowance.
(Dr. L K Bhagi)

Pattern Allowance:
Shrinkage or contraction allowances :
When a heated metal is allowed to cool, it shrinks or contracts. These shrinkage
allowances are provided to compensate these contractions. It happens in three stages:
 The contraction of the liquid from the poring
temperature to the freezing temperature,
 The Contraction associated with the change
of phase from solid to liquid,
 The contraction of the solid casting from the
freezing temperature to room temperature.
Compensated by riser
Shrinkage allowance
(Dr. L K Bhagi)

Liquid Shrinkage> Solid Shrinkage>Solidification
(Dr. L K Bhagi)

(Dr. L K Bhagi)

Shrinkage Allowance:
Shrinkage or contraction allowances α the thermal expansion of the material
Normally denoted as unit length for a given material, and applied linearly.
(Dr. L K Bhagi)

Numerical -1
The casting shown is to be made in cast iron using a wooden pattern.
Assuming only shrinkage allowance, calculate the dimension of the
pattern. All Dimensions are in Inches
(Dr. L K Bhagi)

Numerical -1
(Dr. L K Bhagi)

The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet
For dimension 18 inch, allowance = 18 X 0.125 / 12 = 0.1875 inch » 0.2 inch
For dimension 8 inch, allowance = 8 X 0.125 / 12 = 0.0833 inch » 0. 09 inch
The pattern drawing with required dimension is shown:
Numerical -1
(Dr. L K Bhagi)

The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet
The pattern drawing with required dimension is shown above:
Final dimensions of the pattern
Numerical -1
(Dr. L K Bhagi)

Dimensions of the Aluminium casting as height (H), width (W), Depth
(D), and hole diameter (d) are shown in sketch.
Dimension of wooden pattern will be.
(Assuming linear allowances for Al =.013 mm/mm)
Numerical -2
H=202.6, W=303.9, D=151.95, d=29.61
(Dr. L K Bhagi)

While cooling, a cubical casting of side 40 mm undergoes 3%, 4% and
5% volume shrinkage during the liquid state, phase transition and solid
state, respectively. The volume of the metal compensated from the riser
is
(A) 2 %
(B) 7 %
(C) 8 %
(D) 9%
Numerical -3
(Dr. L K Bhagi)

Heat is removed from a molten metal of mass 2 kg at a constant rate of
10 kW till it is completely solidified. The cooling curve is shown in the
figure.
Asuming uniform temperature throughout the volume of the metal
during solidification, the latent heat of fusion of the metal (in kJ/kg) is
____
Rate of Heat removed from a molten (P) = 10 kW =  KJ/s
time
Heat
H= P×t
Latent heat for a given mass of a substance is
H = m×L
H is the amount of energy released or absorbed during the
change of phase of the substance (in kJ)
m×L = P×t
2×L = 10×103×10
L = 50×103 = 50 KJ/Kg
Numerical -4
(Dr. L K Bhagi)

A cubic casting of 50 mm side undergoes volumetric solidification
shrinkage and volumetric solid contraction of 4% and 6% respectively.
No riser is used. Assumed uniform cooling in all directions. The side of
the cube after solidification and contraction is………..
a = 50mm
Volume of cube after solidification
shrinkage =
solidification shrinkage (4%)
Side of cube after contraction   mm2349120000 3
1
.a 
    333
mm1200000.961250000405050
100
4
V-V  .
Solid Contraction (6%)
    33
11 mm112770.90306013249
100
6
V-V  .. mm2348.a 
Numerical -5
(Dr. L K Bhagi)

Gray cast iron blocks 200 ×100 × 10 mm are to be cast in sand mould
shrinkage allowance for pattern making is 1% the ratio of the volume
of pattern to that of casting will be
(A) 0.97
(B) 0.99
(C) 1.01
(D) 1.03
= 0.97
Numerical -5
(Dr. L K Bhagi)

Draft or Taper Allowance:
Tapering is provided to the sides of the pattern that will allow the smooth
withdrawal of the pattern form the mould cavity without causing any damage
to the edges of the mould is known as draft allowance.
Taper is put on the surface parallel to the direction of the withdrawal of the
pattern from the mould cavity.
(Dr. L K Bhagi)

Machining allowance:
 Usually the surface finish of a casting is too rough to be used in the same way as
the surface of the final product.
 Hence, further machining operations are required to produce the final product ,
 To compensate loss of such material for machining losses, machining allowances
are allowed.
(Dr. L K Bhagi)

Distortion/ Camber allowance:
Some times castings get distorted inside the camber while cooling, due to typical
shape.
 Internal stresses,
 Non-uniform cooling of casting.
To overcome distortion:
 Sufficient machining allowances are to be provided to cover the distortion
allowance.
 Providing suitable (camber)allowance on the pattern (Inverse Reflection)
Unit-1
(Dr. L K Bhagi)

core
(Dr. L K Bhagi)

Requirements for cores:
Green strength: In the green condition, there must be adequate
strength for handling
In the hardened state, it must be strong enough to handle the
forces of casting
Permeability must be very high to allow for the escape of gases.
Collapsibility: As the casting or molding cools, the core must be
weak enough to break down, they must be easy to remove during
shakeout.
Good refractoriness
A smooth surface finish.
Minimum generation of gases during metal pouring.MEC323: PRIMARY MANUFACTURING
(Dr. L K Bhagi)

Types of Cores
Generally, cores are of two types:
1. Green Sand Core
2. Dry Sand Core
(Dr. L K Bhagi)

Types of Cores
1. Green Sand Core
A core formed by the pattern itself,
in the same sand used for the
mould is known as green sand
core.
The pattern is so designed that it
provides the core of green sand.
The hallow part in the pattern
produces the green sand core.
(Dr. L K Bhagi)

Types of Cores
2. Dry Sand Core
A core is prepared separately in
core boxes and dried, is known as
dry sand core. The dry sand cores
are also known as process cores.
They are available in different
sizes, shapes and designs as per till
requirement.
(Dr. L K Bhagi)

Types of Cores
Dry Sand cores –Dry sand core contains dry sand, binders (clay) and linseed oil so they
develop strength on baking. The types of Dry sand cores are:
(i). Horizontal
(ii). Vertical
(iii). Balanced
(iv). Hanging.
(Dr. L K Bhagi)

• Metallic Cores- These cores are made of metals, normally of MS, CI and SS in
case of low melting temperature metals.
(Dr. L K Bhagi)

 70-85% Silica sand (SiO2)
 10-12% Bonding material e.g., clay etc.
 3 – 6 % Water
Properties of Sand - Casting
Composition :
Base Sand :
Silica sand is most commonly used base sand. Other base sands that are also used for
making mold are zircon sand, Chromite sand, and olivine sand. Silica sand is cheapest
among all types of base sand and it is easily available.
Unit-1
(Dr. L K Bhagi)

Binder :
Binders are of many types such as:
1. Clay binders,
2. Organic binders and
3. Inorganic binders
Clay binders are most commonly used binding agents mixed with the molding sands to
provide the strength. The most popular clay types are:
Kaolinite or fire clay (Al2O3.2SiO2.2H2O) and Bentonite (Al2O3.4SiO2 nH2O)
Of the two, the Bentonite can absorb more water which increases its bonding power.
Moisture :
Clay acquires its bonding action only in the presence of the required amount of
moisture.
When water is added to clay, it penetrates the mixture and forms a microfilm, which
coats the surface of each flake of the clay. The amount of water used should be properly
controlled. This is because a part of the water, which coats the surface of the clay
flakes, helps in bonding, while the remainder helps in improving the plasticity.
Unit-1
(Dr. L K Bhagi)

 70-85% Silica sand (SiO2)
 10-12% Bonding material e.g., clay etc.
 3 – 6 % Water
 Refractoriness – Ability to remain solid at high temp
 Cohesiveness – Bonding
 Permeability – Gas flow through mould
 Collapsibility – Ability to permit metal to shrink after solidification
Composition :
Requirements :
Criteria :
 Permeability
 Green strength
 Dry strength
Unit-1
(Dr. L K Bhagi)

Properties of Sand- Casting
Flowability:
It is the ability of the moulding sand to flow and get compacted all around the
pattern and take up the required shape.
Refractoriness:
It is the ability of the moulding sand to withstand the high temperature of the
molten metal which is to be poured. Silica sand has the high refractoriness…
Permeability:
It is the ability of the moulding sand to allow hot gasses to pass through it…
Unit-1
(Dr. L K Bhagi)

Properties of sand- Casting
Green Strength:
The moulding sand that contains the moisture is termed as green sand. Green
strength is the ability of the green sand to retain the shape of the construct the
model.
Dry Strength:
It is the ability of the moulding material to retain the exact shape of the mould
cavity in the dry condition (when the molten metal is poured in the mould) and to
withstand the metallostatic forces of the molten metal.
Hot Strength:
It is the ability of the moulding material to retain the exact shape of the mould
cavity at an elevated temperature.
Unit-1
(Dr. L K Bhagi)

Elements of Gating System
Main Elements :
Unit-1
(Dr. L K Bhagi)

Functions :
Unit-1
(Dr. L K Bhagi)

CHARACTERISTICS OF GATING SYSTEM
1. Permit complete filling of the mold cavity
2. Requires minimum time to fill the mold cavity
3. Minimum turbulence so as to minimize gas pickup
4. Regulate rate at which molten metal enters the mold cavity.
5. Prevent unwanted material from entering mould cavity
6. Establish suitable temperature gradients.
7. No mould erosion
8. Simple and economical design
9. Easy to implement and remove after solidification
10. Maximum casting yield
(Dr. L K Bhagi)

Pouring Basin :
(Dr. L K Bhagi)

Pouring Basin :
 Molten metal is not poured directly in a mould cavity / sprue to avoid possible sand erosion,
 Pouring basin acts as dam, controlling the flow of the molten metal , ensuring the smooth
flow into sprue,
 Skim core/bob is added to control dirt/slag from entering into mould cavity.
Unit-1
(Dr. L K Bhagi)

Sprue :
 Molten metal when moving from top of the cope to the parting line gains velocity, and
requires smaller cross sectional area at the end of the sprue,
 If the sprue is in straight cylindrical shape, then molten metal would not be full at the bottom,
but some low pressure area would be created around the metal in the sprue,
 Sand is permeable, hence air is breathed into the low pressure area , which would be then
carried into mould cavity.
Hence, Sprue is tapered.
Unit-1
(Dr. L K Bhagi)

Runner :
 Runners are always made on parting plane, conventionally in trapezoidal shape,
 Traditionally for ferrous metals, runners are cut in the cope and the ingates are cut in the drag,
this is done trap slag/dirt in the runner,
 Amount of the molten metal flowing from sprue should be > Amount of molten metal flowing
through ingates.
Unit-1
(Dr. L K Bhagi)

Types of Gating System
 Top Gate
 Bottom Gate
 Side/ Parting Gate
 Step Gate
Unit-1
(Dr. L K Bhagi)

 Top gates are usually limited to relatively small castings of simple design.
 The turbulence of metal as it enters the mould cavity causes erosion, which Is a
major problem in the manufacture of steel castings
 As such, top gates are used in steel foundries only for broad shapes of low heights.
Top Gate
Unit-1
(Dr. L K Bhagi)

Bottom Gate
 Bottom gating reduces the turbulence and erosion of the mould to a minimum, but
creates unfavorable thermal gradients ..
 Whereas local hot spots results at the gate entrance, cold metal appears in the riser.
Unit-1
(Dr. L K Bhagi)

Side/ Parting Gate
In this case, metal enters the mould cavity at
the same level as the mould joint or parting
line. Molten metal enters through the sprue
and reaches the parting surface where the
sprue is connected to the runner or gates in a
direction horizontal to the casting.
The arrangement of providing a gate at the
parting line allows the use of devices that
can effectively trap any slag, dirt, or sand,
which passes with the metal down the sprue.
Unit-1
(Dr. L K Bhagi)

Step Gate
 Used for heavy castings, where the molten metal enters the mould cavity through
number of ingates arranged in vertical steps,
 Size of ingate is varied from bottom to top, to avoid gradual filling and sand erosion.
Unit-1
(Dr. L K Bhagi)

Riser :
Unit-1
(Dr. L K Bhagi)

Chill :
 Chills are large heat sinks, used to provide progressive solidification or to avoid
shrinkage cavities,
 Chills are placed close to cavity, such that more heat is absorbed from the larger mass
of molten metal, and supporting cooling rate equal to that of thinner sections,
 Helps to avoid shrinkage cavities.
Unit-1
(Dr. L K Bhagi)

Casting Yeild :
 All the metal poured will not end up in casting. There will be huge wastage,
 Sprue, Runner, Riser etc. are direct loss.
 Machining, Surface finish etc. are indirect losses.
Casting yield = W/w x 100
W = Actual casting mass
w = mass of metal poured into mould
Unit-1
(Dr. L K Bhagi)

Gating System Design
Unit-1
(Dr. L K Bhagi)

Metal flow rate:
 Volume of the metal flowing at any
section in the mould is constant.
Mathematically :
Q = A1V1 = A2V2
Q = Rate of flow, m3 / second
A = Area of cross section, m2
V= Velocity of metal flow, m / second
Unit-1
(Dr. L K Bhagi)

Metal flow rate:
 The liquid metal that runs through the various channels in the mould obeys the
Bernoulli's theorem, which states that the total energy head remains constant at any
section. Mathematically:
 All though, Bernoulli’s theorem can be applied only theoretically, it helps understand
the metal flow in sand mould qualitatively.
 When metal enters the mould it only has potential energy, no kinetic and pressure
energies,
 There will be friction loss because of molten metal contact with mould walls,
 Heat is also continuously lost in the system, which helps the metal to solidify.
Unit-1
(Dr. L K Bhagi)

Pouring Time:
 One of the objective of the gating system is to fill the mould in the smallest possible
time. The time needed to completely fill the mould is called as Pouring Time.
 Too long pouring time require very high pouring temperature,
 Too short pouring time induces turbulent flow, making casting defect prone.
 Pouring time casting materials α Complexity of castings,
Section thickness &
Casting size
Casting material
 Cast iron tends to loose heat fast, so faster pouring time is opted, where as non-ferrous
materials tend to cool slowly, hence, less pouring time is preferred.
Unit-1
(Dr. L K Bhagi)

Pouring Time:
Formula for calculating pouring time for different materials:
Grey cast iron mass < 450 kg pouring time t = k [1.41 + (T/ 14.59)]√w sec
Grey cast iron mass > 450 kg pouring time t = k [1.236+(T/16.65)] sec
Where K = Fluidity of iron in inches / 40
T = average section thickness in mm,
W = Mass of the casting in Kg .
For Steel castings:
Pouring time t = ( 2.4335- 0.3953 log W) √w sec
Calculate the optimum pouring time for a casting whose mass is
20 Kg and having an average section thickness of 15mm.The
material of the casting are grey cast iron and steel. Take the
fluidity of iron as 28 inches?
Unit-1
(Dr. L K Bhagi)

Choke Area:
 Main control area which controls the metal flow into the mould cavity so that mould is
filled completely within the stipulated pouring time. Normally choke happens to be at the
bottom of the sprue, and the choke area is given as:
Unit-1
(Dr. L K Bhagi)

Sprue:
 Sprues should be tapered down to take into account the velocity gain of molten metal,
as it flows down reducing the air aspiration. The exact tapering can be obtained by the
equation of continuity.
Unit-1
(Dr. L K Bhagi)

Sprue Base Well:
 The provision of sprue base well at the bottom of the sprue helps in reducing the
velocity of the incoming metal and also mould erosion. Reasonable proportions are give
in Fig:
A general guide line says that
sprue base well area would be 5
times that of sprue choke area and
the well depth should be equal to
that of runner
Unit-1
(Dr. L K Bhagi)

Pouring Basin:
 The main function of a pouring basin is to reduce the momentum of the liquid flowing
into the mould by settling first into it.
 In order that the metal enters into the sprue without any turbulence it is necessary that
the pouring basin is deep enough (2.5 times the radius of the top of sprue),
 and also the entrance into the sprue
be a smooth radius of atleast 25mm.
Unit-1
(Dr. L K Bhagi)

Gating Ratios :
 Gating system refers to the proportion of the cross sectional areas between sprue,
runner and is generally denoted as:
Sprue area : Runner area : Ingate area
Depending on the choke area, there can be two types of gating systems
1) Non-pressurized Gating System
2) Pressurized gating system
Unit-1
https://nptel.ac.in/content/storage2/courses/1
12107144/metalcasting/fig17.htm
https://nptel.ac.in/content/storage2/courses/1
12107144/metalcasting/fig18.htm
(Dr. L K Bhagi)

Non-pressurized Gating System:
Gating Ratios :
 A non pressurized gating system has choke at the end of the sprue base. In this
system total runner area and ingate area will be higher than sprue area.
 In this system there will no pressure existing in the metal flow system, and it helps
in reduction of turbulence.
 It is used for casting drossy alloys such as aluminum and magnesium alloys.
Sprue area : Runner area : Ingate area : : 1 : 4 : 4
 Casting yield gets reduced because of large metal is used in runners and gates.
 The gating system has to be carefully designed to see all the parts of flow full. Failing
which, some elements of the gating system may flow partially allowing air aspiration.
Unit-1
(Dr. L K Bhagi)

Pressurized Gating System:
Gating Ratios :
 In this case normally the ingate area is the smallest, thus maintaining a back
pressure throughout the gating system.
 Because of this back pressure in the gating system, the metal is more turbulent and
generally flows full and thereby, reduces the possible air aspiration.
 When multiple gates are used this system alloys all the gates to flow full.
 This system generally provide a higher casting yield since the volume of the metal
is used up in the runners and gates is reduced.
Sprue area : Runner area : Ingate area : : 1 : 2 : 1
 Because of the turbulence induced, high amount of dross is formed, hence this system can
not be used for light alloys, but can be very advantageous for ferrous castings.
Unit-1
(Dr. L K Bhagi)

Ingate Design:
 The ingate can be considered as a weir with no reduction in cross section of the stream
at the gate. Then the rate of flow of molten metal through the gates depends on the free
height of the metal in the runner and the gate area and the velocity with which metal is
flowing in the runner. The free height h, can be calculated as:
Unit-1
(Dr. L K Bhagi)

Riser Design
(Dr. L K Bhagi)

Riser Design
Design of a Riser:
1. Riser Shape
2. Riser Size
3. Location of Riser
4. Grouping of castings
5. Riser connection to castings
6. Use of Chills
7. Insulations & exothermic
compounds,
Riser Shape
 Casting loses its thermal energy by transferring it to its surroundings by radiation,
conduction & convection.
 Solidification time is expressed as: ts = V2
C / A2
C
 According to the solidification time, riser shape is designed, the minimum riser
shape is a sphere…However voids are formed in spheres.
 Practice says that cylindrical shaped risers are ideal…
(Dr. L K Bhagi)

Riser Size
 Riser is similar to the casting in its solidification behavior, hence the riser
characteristics are also specified by the ratio of its surface area to volume.
 The shape of the riser should be designed such that it has minimum heat loss
and is able to maintain the molten metal in liquid state as long as possible.
i.e., volume of the riser should be minimum and cooling rate of molten metal is
slower
 Optimum riser size for casting is obtained by following methods:
Riser Design
Chvorinov’s Rule
Freezing Ratio ‘X’
(Dr. L K Bhagi)

Riser Design
Caine’s Method :
Riser Size :
(Dr. L K Bhagi)

Riser Location
 The fall in the temperature at the ends is more rapid as compared to rest of
casting.
 If the casting have variety of section thickness, the riser must be placed at the
thickest portion.
Riser Design
Grouping of Castings
 Grouping of several castings around a single riser helps in increasing the
casting yield, since the same riser will be able to feed to more than one casting.
 Also by a small variation in the moulding practice, it is possible to reduce
risering requirements.
(Dr. L K Bhagi)

Riser Connection to the Casting
 The way that a riser is attached to the casting is important because it
determines:
o How well the riser can feed the casting,
o How readily the riser can be removed from the casting.
 It may also control to some extent the depth of shrinkage cavity by solidifying
just before the riser freezes, thereby preventing the cavity from extending into
casting.
Riser Design
(Dr. L K Bhagi)

Use of Insulators and Exothermic Compounds:
 A riser can be made more efficient by employing some artificial means to keep
the top of the riser from freezing over so that the molten metal beneath can be
exposed to exothermic pressure.
 This can be done by use of certain additions made to the surface of the molten
metal in the riser, preferably as soon as possible after the metal enters the riser.
 Ex: Graphite, Charcoal , rice or oat hulls, and refractory powders (insulators)
Riser Design
(Dr. L K Bhagi)

In the casting of steel under certain mold conditions, the mold
constant in Chvorinov's rule is known to be 4.0 min/cm2,
based on previous experience. The casting is a flat plate whose
length = 30 cm, width = 10 cm, and thickness = 20 mm.
Determine how long it will take for the casting to solidify.
Surface area of flat plate (A) = 2(30 x 10 + 30 x 2 + 10 x 2) = 760 cm2
According to Chorinov’s relation
30 cm
10 cm
20 mm
Volume of the steel casting (V) = 30 x 10 x 2 = 600 cm3
ts = V2
C / A2
C
= 4(600/760)2 = 2.49 minMEC323: PRIMARY MANUFACTURING
(Dr. L K Bhagi)

Volume of a cube of side ‘l’ and volume of a sphere of
radius ‘r’ are equal. Both the cube and the sphere are solid
and of same material. They are being cast. The ratio of
the solidification time of the cube to the same of sphere is
Surface area of cube = 6l2 Surface area of sphere = 4 π r2
(Dr. L K Bhagi)

Surface area of cube = 6l2 Surface area of sphere = 4 π r2
According to Chorinov’s relation ts = V2
C / A2
C
(Dr. L K Bhagi)

A spherical drop of molten metal of radius 2 mm solidifies
in 10 seconds. Then the solidification time of a similar
spherical drop of molten of radius 4 mm is
Surface area of sphere = 4 π r2
683
4
23
4
V
3
1
3
1
3
1
1
ddd







2
1
2
1
2
1
1
4
4
2
4A d
dd







(Dr. L K Bhagi)

Surface area of sphere = 4 π r2
6
V
3
2
2
d
 2
22A d
(Dr. L K Bhagi)

Metal casting process part 1

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Metal casting process part 1