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Manufacturing Process Amey D. Patwardhan

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Chapter 1

1. MANUFACTURING PROCESSES AND CLASSIFICATION

Manufacturing processes are the steps through which raw materials are

transformed into a product. The manufacturing processes can be broadly classified into

three categories viz. shaping,joining and finishing processes as shown schematically in

Fig.1.1.

Fig.1.1. Classification of manufacturing processes

2. METAL CASTING

2.1. Introduction

Metal Casting is one of the oldest materials shaping methods known. Casting isthe process in which liquid molten metal is poured into the casting cavity whose shape is

same as that of shape of object to be produced. When solidified, the desired metal object

is taken out from the mould either by breaking the mould or taking the mould apart. The

solidified object is called the casting. There are castings in locomotives, cars trucks,

aircraft, office buildings, factories, schools, and homes.


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Fig.1.2 Casting Process

2.1.1. Advantages

1. Molten material can flow into very small sections so that intricate shapes can

be made by this process.

2. It is possible to cast practically any material that is ferrous or non-ferrous.

3. As the metal can be placed exactly where it is required, large saving in weight

can be achieved.

4. The necessary tools required for casting Moulds are very simple and

inexpensive. As a result, for production of a small lot, it is the ideal process.

5. There are certain parts made from metals and alloys that can only be

processed this way.

6. Size and weight of the product is not a limitation for the casting process.


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2.1.2. Limitations

1. Lesser dimensional accuracy.

2. The metal casting process is a labour intensive process.

History

3200 B.C.A copper frog, the oldest known casting in existence, is cast in Mesopotamia.

233 B.C.Cast iron plowshares are poured in China.

500 A.D.Cast crucible steel is first produced in India, but the process is lost until 1750,

when Benjamin Huntsman reinvents it in England.

1455Dillenburg Castle in Germany is the first to use cast iron pipe to transport water.

1480Birth of Vannoccio Biringuccio (1480-1539), the "father of the foundry industry," in

Italy. He is the first man to document the foundry process in writing.

1809Centrifugal casting is developed by A. G. Eckhardt of Soho, England.

Metal Casting History (India)

3000 BC Earliest castings include the 11 cm high bronze dancing girl found at Mohen-jo-

daro.

2000 BCIron pillars, arrows, hooks, nails, bowls and daggers or earlier have been found

in Delhi, Roopar, Nashik and other places.

500 BC Large scale state-owned mints and jewellery units, and processes of metal

extraction and alloying have been mentioned in Kautilya'sArthashastra

500 A.D.Cast crucible steel is first produced in India, but the process is lost until 1750,

when Benjamin Huntsman reinvents it in England.

2.2.GATING SYSTEM/CASTING TERMS

Gating system is the path provided to the liquid molten metal in order to fill the mould

cavity. Elements of gating system are shown in fig.1.5.

Flask: A metal or wood frame, without fixed top or bottom, in which the mould is formed.

Drag lower moulding flask, Cope upper moulding flask, Cheek intermediate

moulding flask used in three piece moulding.


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Pattern: It is the replica of the final object to be made. The mould cavity is made with the

help of pattern.

Parting line: This is the dividing line between the two moulding flasks that makes up the

mould.

Moulding sand: Sand, which binds strongly without losing its permeability to air or gases.

It is a mixture of silica sand, clay, and moisture in appropriate proportions.

Facing sand: The small amount of carbonaceous material sprinkled on the inner surface

of the mould cavity to give a better surface finish to the castings.

Core:A separate part of the mould, made of sand and generally baked, which is used tocreate openings and various shaped cavities in the castings.

Pouring basin: A small funnel shaped cavity at the top of the mould into which the molten

metal is poured. It helps in maintaining the required rate of liquid metal flow. It also helps

in separating dross, slag and foreign element etc. from molten metal before it enters the

sprue.

Sprue: The passage through which the molten metal, from the pouring basin, reachesthe mould cavity. In many cases it controls the flow of metal into the mould.

Runner: The channel through which the molten metal is carried from the sprue to the

gate.

Gate: A channel through which the molten metal enters the mould cavity.

Chaplets: Chaplets are used to support the cores inside the mould cavity. Chaplets are

made up of same material which molten metal have. During solidification, chaplets

become part of casting.

Riser: A column of molten metal placed in the mould to feed the castings as it shrinks

and solidifies. Also known as feed head.


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Vent: Small opening in the mould to facilitate escape of air and gases.

Fig.1.3. Terms in Casting Process

Fig.1.4. Gating System (Elements of gating system)


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Fig.1.5. Alternative diagram for gating system

Fig.1.6. Molten metal flow though gating system

2.2.1. Goals of Gating System

1. To minimize turbulence to avoid trapping gasses into the mould

2. To get enough metal into the mould cavity before the metal starts to solidify

3. To avoid shrinkage

4. Establish the best possible temperature gradient in the solidifying casting so

that the shrinkage if occurs must be in the gating system not in the required

cast part.

5. Incorporates a system for trapping the non-metallic inclusions


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2.2.2. Types of gating system

1. Pressurized Gating System

1. The total cross sectional area decreases towards the mould cavity

2. Back pressure is maintained by the restrictions in the metal flow

3. Flow of liquid (volume) is almost equal from all gates

4. Back pressure helps in reducing the aspiration as the sprue always runs full

5. Because of the restrictions the metal flows at high velocity leading to more

turbulence and chances of mould erosion

2. Un-Pressurized Gating System

1. The total cross sectional area increases towards the mold cavity

2. Restriction only at the bottom of sprue

3. Flow of liquid (volume) is different from all gates aspiration in the gating

system as the system never runs full

4. Less turbulence

Fig.1.7. Pressurised(left) and non-pressurised(right) gating systems


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2.2.3. Role of riser in sand casting

1. Metals and their alloys shrink as they cool or solidify and hence may create a partial

vacuum within the casting which leads to casting defect known as shrinkage or

void. The primary function of riser as attached with the mould is to feed molten

metal to accommodate shrinkage occurring during solidification of the casting.

2. Riser permits the escape of evolved air and Mould gases as the Mould cavity is

being filled with the molten metal.

3. It also indicates to the foundry man whether Mould cavity has been filled

completely or not. The suitable design of riser also helps to promote the directional

solidification and hence helps in production of desired sound casting.

2.3. STEPS IN MAKING SAND CASTINGS

There are six basic steps in making sand castings:

1. Patternmaking

2. Core making

3. Moulding

4. Melting and pouring

5. Cleaning

1. Pattern making

Pattern can be said as a model or the replica of the object to be cast except for the

various al1owances a pattern exactly resembles the casting to be made. It may be

defined as a model or form around which sand is packed to give rise to a cavity known

as Mould cavity in which when molten metal is poured, the result is the cast object. When

this mould/cavity is filled with molten metal, molten metal solidifies and produces a

casting (product). So the pattern is the replica of the casting. If the casting is to be hollow,

as in the case of pipe fittings, additional patterns, referred as cores are used to form

these cavities.


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2. Core making

Core is used to produce hollowness in castings in form of internal cavities. Cores are

forms, usually made of sand, which are placed into a mould cavity to form the interior

surfaces of castings.

3. Moulding

Moulding consists of all operations necessary to prepare a mould for receiving molten

metal. Moulding usually involves placing a moulding aggregate/sand around a pattern

held with a supporting frame, withdrawing the pattern to leave the mould cavity, setting

the cores in the mould cavity and finishing and closing the mould.

4. Melting and Pouring

The preparation of molten metal for casting is referred to simply as melting. Melting is

usually done in a specifically designated area of the foundry, and the molten metal is

transferred to the pouring area where (mould cavity) the moulds are filled.

5. Cleaning

Cleaning refers to all operations necessary to the removal of sand, scale, and excess

metal from the casting. Burned-on sand and scale are removed to improve the surface

appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and

gates, is removed. Inspection of the casting for defects and general quality is performed.

2.4. PATTERN

It is the replica of the object to be made by the casting process, with some

modifications. The main modifications are the addition of pattern allowances, and the

provision of core prints. The quality of the casting produced depends upon the material

of the pattern, its design, and construction. The costs of the pattern and the related

equipment are reflected in the cost of the casting.


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2.4.1. Functions of the Pattern

1. A pattern prepares a mould cavity for the purpose of making a casting.

2. A pattern may contain projections known as core prints if the casting requires a core

and need to be made hollow.

3. Runner, gates, and risers used for feeding molten metal in the mould cavity may

form a part of the pattern.

4. Patterns properly made and having finished and smooth surfaces reduce casting

defects.

5. A properly constructed pattern minimizes the overall cost of the castings.

2.4.2. Pattern Material

Patterns may be constructed from the following materials. Each material has its own

advantages, limitations, and field of application. Some materials used for making

patterns are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax,

and resins. To be suitable for use, the pattern material should be:

1. Easily worked, shaped and joined

2. Light in weight

3. Strong, hard and durable

4. Resistant to wear and abrasion

5. Resistant to corrosion, and to chemical reactions

6. Dimensionally stable and unaffected by variations in temperature and humidity

7. Available at low cost

1. Wood

Wood is the most popular and commonly used material for pattern making. It is very

light and can produce highly smooth surface. Wooden patterns are preferred only when

the numbers of castings to be produced are less. The main varieties of woods used in

pattern-making are Shisham, Kail, Deodar, Teak and Mahogany.


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Advantages of wooden patterns

1. Wood can be easily worked.

2. It is light in weight.

3. It is easily available.

4. It is very cheap.

5. It is easy to join.

6. It is easy to obtain good surface finish.

7. Wooden laminated patterns are strong.

8. It can be easily repaired.

2. Metal

Metallic patterns are preferred when the number of castings required is large enoughto justify their use. These patterns are not much affected by moisture as wooden pattern.

The wear and tear of this pattern is very less and hence posses longer life. Moreover,

metal is easier to shape the pattern with good precision, surface finish and intricacy in

shapes. It can withstand against corrosion and handling for longer period. It possesses

excellent strength to weight ratio.

The main disadvantages of metallic patterns are higher cost, higher weight and

tendency of rusting. It is preferred for production of castings in large quantities with same

pattern. The metals commonly used for pattern making are cast iron, brass and bronzes

and aluminium alloys.

i) Cast Iron

Advantages

1. It is cheap

2. It is easy to file and fit

3. It is strong

4. It has good resistance against sand abrasion

Disadvantages

1. It is heavy

2. It is brittle and hence it can be easily

broken3. It may rust

Disadvantages

1. It is susceptible to moisture.

2. It tends to warp.

3. It wears out quickly due to sand

abrasion.

4. It is weaker than metallic patterns.


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ii) Brasses and Bronzes

Advantages

1. Better surface finish than cast iron.

2. Very thin sections can be easily casted.

iii) Aluminium Alloys

Advantages

1. Aluminium alloys pattern does not rust.

2. They are easy to cast.

3. They are light in weight.

4. They can be easily machined.

iv) White Metal (Alloy of Antimony, Copper and Lead)

Advantages

1. It is best material for lining and stripping

plates.

2. It has low melting point around 260C

3. It can be cast into narrow cavities.

3. Plastic

Plastics are getting more popularity now a days because the patterns made of

these materials are lighter, stronger, moisture and wear resistant, non sticky to Moulding

sand, durable and they are not affected by the moisture of the Moulding sand. Moreover

they impart very smooth surface finish on the pattern surface. These materials are

somewhat fragile, less resistant to sudden loading and their section may need metal

reinforcement. The plastics used for this purpose are thermosetting resins. Phenolic resin

plastics are commonly used.

4. Plaster

This material belongs to gypsum family which can be easily cast and worked with

wooden tools and preferable for producing highly intricate casting. The main advantages

of plaster are that it has high compressive strength and is of high expansion setting type

Disadvantages

1. It is costly

2. It is heavier than cast iron.

Disadvantages

1. They can be damaged by sharp edges.

2. They are softer than brass and cast iron.

3. Their storing and transportation needs prope

care.

Disadvantages

1. It is too soft.

2. Its storing and transportation needs proper

care

3. It wears away by sand or sharp edges.


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which compensate for the shrinkage allowance of the casting metal. It is also preferred

for production of small size intricate castings and making core boxes.

5. Wax

Patterns made from wax are excellent for investment casting process. The

materials used are blends of several types of waxes, and other additives which act as

polymerizing agents, stabilizers, etc. The commonly used waxes are paraffin wax, shellac

wax, bees-wax, cerasin wax, and micro-crystalline wax. The properties desired in a good

wax pattern include low ash content up to 0.05 per cent, resistant to the primary coat

material used for investment, high tensile strength and hardness, and substantial weld

strength. Maximum size of casting produced by wax pattern is 5kg only. Wax pattern

should not be used for green sand mould. No machining required after casting since it

wax pattern produces good surface finish.

2.4.3. Factors effecting selection of pattern material

The following factors must be taken into consideration while selecting pattern

materials.

1. Number of castings to be produced. Metal pattern are preferred when castings are

required large in number.

2. Type of mould material used.

3. Kind of Moulding process.

4. Method of Moulding (hand or machine).

5. Degree of dimensional accuracy and surface finish required.

6. Minimum thickness required.

7. Shape, complexity and size of casting.

8. Cost of pattern and chances of repeat orders of the pattern

2.4.4. Pattern Allowances

Pattern is always larger in size as compared to final casting, because it carries certain

allowances due to metallurgical and mechanical reasons.


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1. Shrinkage Allowance

In practice it is found that all common cast metals shrink a significant amount when they

are cooled from the molten state. The total contraction in volume is divided into the

following parts:

Liquid shrinkage:

1. The contraction during the period in which the temperature of the liquid metal

or alloy falls from the pouring temperature to the liquidus temperature.

2. Contraction on cooling from the liquidus to the solidus temperature, i.e.

solidifying contraction.

Solid shrinkage

The first two of the above are taken care of by proper gating and risers. Only thelast one, i.e. the solid contraction is taken care by the pattern makers by giving a positive

shrinkage allowance.

Exercise 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


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Solution 1

The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet (as

per Table 1)

For dimension 18 inch, allowance = 18 X0.125 / 12 = 0.1875 inch 0.2 inch

For dimension 14 inch, allowance = 14 X0.125 / 12 = 0.146 inch 0.15 inch

For dimension 8 inch, allowance = 8 X 0.125 / 12 = 0.0833 inch 0. 09 inch

For dimension 6 inch, allowance = 6 X 0.125 / 12 = 0.0625 inch 0. 07 inch

The pattern drawing with required dimension is shown below:

2. Draft or Taper Allowance

Making vertical surfaces of the pattern into inclined surfaces is called draft

allowance and it is mainly given for easy removal of the pattern from the mould. Without

provision of draft until the last point of the pattern comes out from the mould, pattern will

have contact with mould, any vibration taking place to the human hand during removal of

pattern it will cause damages taking place to the mould. With provision of draft allowance,

as soon as small amount of pattern is lifted to the mould, clearance is formed between

pattern and mould even hand is vibrating.

If pattern is made by wax, Hg or Polystyrine as a pattern material, there is no draft

allowance is provided on the pattern. (Wax and Hg removed in the form of liquid and


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Polystyrine removed in the form of vapors hence draft allowance is not required) For

manufacturing of large size castings i.e. machine tool beds, Polystyrine is used as a

pattern material. For manufacturing of complex shape of casting, wax and Hg is used.

Fig 1.8. Taper allowance

3. Distortion or Camber Allowance

Fig. 1.9. Distortions in Castings

Sometimes castings get distorted, during solidification, due to their typical shape. For

example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract at

the closed end causing the vertical legs to look slightly inclined. This can be prevented

by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that

the casting after distortion will have its sides vertical.The distortion in casting may occur

due to internal stresses. These internal stresses are caused on account of unequal

cooling of different section of the casting and hindered contraction. Measure taken to

prevent the distortion in casting includes:


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i. Modification of casting design

ii. Providing sufficient machining allowance to cover the distortion affect

iii. Providing suitable allowance on the pattern, called camber or distortion

allowance (inverse reflection)

4. Machining or Finish Allowance

It is a positive allowance given to compensate for the amount of material that is lost in

machining or finishing the casting. If this allowance is not given, the casting will become

undersize after machining. The amount of this allowance depends on the size of casting,

methods of machining and the degree of finish. In general, however, the value varies from

3 to 18 mm.

5. Shake allowance

Because of adhesiveness property of moulding sand, during removal of pattern, the

adhered moulding sand to the pattern is trying to come out allowing to the pattern and

damages the moulding wall. To avoid this before removal of pattern from mould, thepattern should be shake so that the adhered moulding sand is separating and avoids

damages to the moulding wall. But due to shaking of pattern the size of cavity becomes

greater than size of pattern which increases casting size. To maintain casting size as

required the original pattern dimensions as to be reduced by as amount equal to shake

allowance. Shake allowance depends on mould making person. Earlier allowances are


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increasing dimensions of pattern but shake allowance reduces dimension of pattern.

Hence it is called negative allowance. (Fig- In class notes)

2.4.5.Types of pattern

The types of the pattern and the description of each are given as under.

1. Single piece or solid pattern

2. Two piece or split pattern

3. Cope and drag patter

4. Loose piece pattern

5. Gated pattern

6. Sweep pattern

7. Skeleton pattern

8. Match plate pattern

9. Follow board pattern

10. Segmental or part pattern

1. Single Piece Pattern

A single piece pattern is the simplest of all forms. As the name indicates they are made

of a single piece as shown in fig. 1.10. This type of pattern is used only in cases where

the product is very simple and can be easily withdrawn from the mould. This pattern is

contained entirely in the drag. One of the surfaces is usually flat which is used as the

parting plane.

Fig. 1.10. Typical Single piece pattern

2. Split or Two Piece Pattern


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This is the most common type of pattern for intricate castings. When the contour of

the casting makes its withdrawal from the mould difficult or when the depth of the casting

is too high, then pattern is split into two parts. One part is contained in the drag and the

other in the cope. The split surface of the pattern is same as the parting plane of the

mould. The two halves of the pattern should be aligned properly by making use of dowel

pins which are fitted to the top half.

Fig. 1.11. Typical Two piece/split pattern

3. Cope and drag pattern

When very large castings are to be made the complete pattern becomes too heavy to

be handled by a single operator. Such a pattern is made in two parts which are separately

moulded in different moulding boxes. After completion of the moulds, the two boxes are

assembled to form the complete cavity. One part is contained by the drag and the other

by the cope. Thus it is different from split pattern in which both pieces are moulded

separately instead of being moulded in the assembled position.

Fig. 1.12. Cope and drag pattern


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4. Loose-piece Pattern

Certain single piece patterns are made to have loose pieces in order to enable their

easy withdrawal from the mould. These pieces from an integral part of the pattern during

moulding. After the mould is complete the pattern is withdrawn leaving the pieces in the

sand. These pieces are later withdrawn separately through the cavity formed by the

pattern as shown in fig. 1.13. Moulding with loose piece is a highly skilled job and is

generally expensive.

Fig. 1.13. Loose piece pattern

5. Gated pattern

It is used when more no of small castings to be produced. Such moulds are formed

by joining a number of patterns and gates and providing a common runner for the molten

metal, as shown in Fig.1.14. These patterns are made of metals, and metallic pieces toform gates and runners are attached to the pattern. Moulding time and efforts becomes

lesser and it reduces cost of process.

Fig. 1.14. Gated pattern


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6. Sweep pattern

Actually a sweep is a template of wood or metal and is attached to the spindle. As

sweep rotates around pole it goes down in mould and at the same time it removes sand

from moulding box. After it completely went inside the mould, sweep is removed from the

mould and required shape of cavity will be created. It is used for large castings of circular

sections and symmetrical shapes. This is a two dimensional pattern. It saves time to

making pattern and in making complex, symmetrical and circular castings, it is very useful.

Fig. 1.15. Sweep pattern

7. Skeleton pattern

When only a small number of large and heavy castings are to be made, it is not

economical to make a solid pattern. In such cases, however, a skeleton pattern may be

used. It is hollow pattern used for huge castings. This is a ribbed construction of wood

which forms an outline of the pattern to be made. This frame work is filled with loam sand

and rammed. The surplus sand is removed by strickle board. For round shapes, the

pattern is made in two halves which are joined with glue or by means of screws etc. It is

used for turbine, impellers etc. It saves the pattern material.


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Fig. 1.16. Skeleton pattern

8. Match plate pattern

This pattern is made in two halves and is on mounted on the opposite sides of a

wooden or metallic plate, known as match plate. The gates and runners are also attached

to the plate. This pattern is used in machine Moulding.

Fig. 1.17. Match plate pattern

9. Follow board pattern

When the use of solid or split patterns becomes difficult, a contour corresponding to

the exact shape of one half of the pattern is made in a wooden board, which is called a

follow board and it acts as a moulding board for the first moulding operation as shown.


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Fig. 1.18. Follow board pattern

10. Segmental pattern

These pattern are used for preparing moulds of large circular castings, avoid the

use of a solid pattern of exact size. In principle they are similar to sweep patterns. But thedifference is that while a sweep pattern is given a continuous revolving motion to generate

the desired shape, a segmental pattern is a portion of the solid pattern itself and the mould

is prepared in parts by it. It is mounted on a central pivot and after preparing the part

mould in one position, the segment is moved to the next position. The operation is

repeated till the complete mould is ready.

Fig. 1.19. Segmental pattern

2.4.6. Pattern color code:

Standard colours have been recommended for the finishing of wood patterns. The

colour scheme adopted by the American Foundry mensSociety is outlined below:

1. Cast surface to be left unmachined-Black


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2. Cast surface to be machined- Red

3. Loose pieces and seatings-Red strips on yellow base.

4. Core prints seats-Yellow

5. Stop offs or supports- black strips on a yellow background

6. Parting surfaces-clear or no color

7. Core prints for machined openings-yellow strips on black background

* Stop-offs are portions of a pattern that form a Mould cavity which is filled with

sand before pouring. Stop-offs may, for example, be reinforcing members to

prevent breakage of a frail pattern.

2.5.CORE AND CORE PRINTS

Castings are often required to have holes, recesses, etc. of various sizes and

shapes. These impressions can be obtained by using cores. Cores are mainly used to

create hollowness in casting. So where coring is required, provision should be made to

support the core inside the mould cavity. Core prints are used to serve this purpose. The

core print must be of adequate size and shape so that it can support the weight of the

core during the casting operation. Depending upon the requirement a core can be placed

horizontal, vertical and can be hanged inside the Mould cavity. A typical job, its pattern

and the mould cavity with core and core print is shown in fig.1.20.

Fig. 1.20. Typical Job, its Pattern, Mould cavity and Core.


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Fig. 1.21. Half section of mould shows function of Core, Core print and Chaplet

There are various functions of cores which are given below

1. Core is used to produce hollowness in castings in form of internal cavities.

2. It may form a part of green sand Mould

3. It may be deployed to improve Mould surface.

4. It may provide external undercut features in casting.

5. It may be used to strengthen the Mould.

6. It may be used to form gating system of large size Mould

7. It may be inserted to achieve deep recesses in the casting

2.5.1. Types of Cores

There are various types of cores such as horizontal core (Fig. 1.22), vertical core (Fig.

1.23), balanced core (Fig. 1.24), drop core (Fig. 1.25) and hanging core (Fig. 1.26).


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Fig. 1.22. Horizontal Core Fig. 1.23. Vertical Core

Fig. 1.24. Balanced Core Fig. 1.25. Drop Core Fig. 1.26. Hanging

Core

2.5.2. Core making

Following steps are involved in core making:

1. Core sand preparation:

Core sand consist of Granular refractories, core binders, water and special additives.

2. Making the core

Core is made by two ways i.e.

A) Hand making-for simple cores.

B) Using machines- i) Jolt machine ii) Core roll over machine iii) Sand slinger iv) Core

extrusion machine v) Core blower f) Shell core machine


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Hand making of cores

For hand making of cores, core boxes are used. Cores are made by means of core

boxes comprising of either single or in two parts. Core boxes are generally made of wood

or metal and are of several types. Core boxes are having cavity of required shape of cores

in which sand is filled and sand core is obtained.

Types of Core Box

The main types of core box are half core box, dump core box, split core box, strickle

core box, right and left hand core box and loose piece core box.

1. Half core box

This is the most common type of core box. The two identical halves of a symmetrical

core prepared in the half core box are shown in Fig. 1.27. Two halves of cores are pasted

or cemented together after baking to form a complete core.

Fig.1.27. Half core box

2. Dump core box

It is similar in construction to half core box. These cores do not require pasting, rather

they are complete by themselves. A dump core-box is used to prepare complete core in

it. Generally cylindrical and rectangular cores are prepared in these boxes.


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Fig.1.28. Dump core box

3. Split core box

Split core boxes are made in two parts as shown in Fig. 1.29. They form the complete

core by only one ramming. The two parts of core boxes are held in position by means of

clamps and their alignment is maintained by means of dowel pins and thus core is

produced.

Fig.1.29. Split core box

4. Right and left hand core box

Some time the cores are not symmetrical about the centre line. In such cases, right

and left hand core boxes are used. The two halves of a core made in the same core box

are not identical and they cannot be pasted together.


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5. Strickle core box

This type of core box is used when a core with an irregular shape is desired. The

required shape is achieved by striking of the core sand from the top of the core box with

a wooden piece, called as strickle board. The strickle board has the same contour as that

of the required core.

6. Loose piece core box

Loose piece core boxes are highly suitable for making cores where provision for

bosses, hubs etc. is required. In such cases, the loose pieces may be located by dowels,

nails and dovetails etc. In certain cases, with the help of loose pieces, a single core box

can be made to generate both halves of the right-left core.

7. Gang core box

It contains a number of core cavities so that more than one cores can be rammed at

a time.

3. Core baking

Cores kept on core driver or core plate in its green state and then they sent to oven

for baking. At 2120F, the moisture from the core is driven off. Cores are baked up to 6500F.

4. Finishing of cores

Finishing of core involves following operations

A) Cleaning

1. Trimming: Removing fins and other sand projections2. Brushing: It removes loose sand from the cores.

3. Coating: Fine refractory coating(Graphite & silica) is applied by dipping or spraying

4. Mudding: It is localised coating which is applied to make core completely smooth

B) Sizing

Sizing is making core dimensionally accurate by grinding, filing etc.


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C) Core assembly

Core assembly is the process of joining subparts by pasting or bolting.

D) Setting of cores

Core setting means placing cores in the mould in proper position.

2.6.MOULDING SAND

A large variety of moulding materials is used in foundries for manufacturing moulds

and cores. They include moulding sand, system sand or backing sand, facing sand,

parting sand, and core sand.

2.6.1. Moulding Sand Composition

The main ingredients of any Moulding sand are:

1) Base sand

2) Binder

3) Moisture

4) Additives

1. Base Sand

Silica sand is most commonly used base sand (75-85%). Other base sands that are

also used for making Mould are zircon sand, Chromite sand, and olivine sand. Silica sand

is cheapest among all types of base sand and it is easily available. Silica sand particles

provides strength to the moulding sand.

2. Binder

Binders are of many types such as Clay binders, Organic binders and Inorganic

binders. Clay binders are most commonly used binding agents mixed with the moulding

sands to provide the strength (15-20%). The most popular clay types are: Kaolinite or fire

clay (Al2O32 SiO22 H2O) and Bentonite (Al2O34 SiO2nH2O). Of the two the Bentonite

can absorb more water which increases its bonding power.

3. Moisture


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Clay acquires its bonding action only in the presence of the required amount of

moisture (6-8%). 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.

4. Additives

Additives are added up to 2% into moulding sand for special purposes

Wood powder or Saw dust: For improving porosity property and collapsibility

Coal powder: For improving refractoriness property of moulding sand

Starch or Dextrin: For improving resistance to deformation and strength.

2.6.1. Types of moulding sand

1) Green sand

Green sand is also known as tempered or natural sand which is a just prepared

mixture of silica sand with 18 to 30 percent clay, having moisture content from 6 to

8%. The clay and water furnish the bond for green sand. It is fine, soft, light, and

porous. Green sand is damp, when squeezed in the hand and it retains the shape

and the impression to give to it under pressure. Moulds prepared by this sand are not

requiring backing and it contains moisture hence are known as green sand moulds.

This sand is easily available and it possesses low cost. It is commonly employed for

production of ferrous and non-ferrous castings.

Green sand = Silica sand +Clay+Water

2) Dry sand

Green sand that has been dried or baked in suitable oven after the making Mould and

cores, is called dry sand.

It possesses more strength, rigidity and thermal stability. It is mainly suitable for larger

castings. Mould prepared in this sand are known as dry sand moulds.

Dry sand = Silica sand +Clay+ Sodium silicate


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3) Loam sand

Loam is mixture of sand and clay with water to a thin plastic paste. Loam sand

possesses high clay as much as 30-50% and 18% water. Patterns are not used for

loam Moulding and shape

is given to Mould by sweeps. This is particularly employed for loam Moulding used

for large grey iron castings.

Loam sand= Green/dry sand + 50% Clay

4) Facing sand

Facing sand is just prepared and forms the face of the mould. It is directly next to the

surface of the pattern and it comes into contact molten metal when the mould is

poured. Initial coating around the pattern and hence for mould surface is given by this

sand. This sand is subjected severest conditions and must possess, therefore, high

strength refractoriness. It is made of silica sand and clay, without the use of used

sand. Different forms of carbon are used to prevent the metal burning into the sand.

A facing sand mixture for green sand of cast iron may consist of 25% fresh and

specially prepared and 5% sea coal. They are sometimes mixed with 6-15 times as

much fine moulding sand to make facings.

5) Backing sand

Backing sand or floor sand is used to back up the facing sand and is used to fill the

whole volume of the moulding flask. Used moulding sand is mainly employed for this

purpose. The backing sand is sometimes called black sand because that old,

repeatedly used moulding sand is black in colour due to addition of coal dust and

burning on coming in contact with the molten metal.

2.6.2. Moulding sand properties

The properties that are generally required in Moulding materials are:


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1) Porosity

It is the ability to escape air or gases through the moulding sand. As molten metal

filed into the cavity, the air already present in the cavity starts to compress and the

pressure of the air becomes greater than the atmospheric pressure. Hence air starts

to flow inside to outside using porosity property of the moulding sand otherwise

blowhole defect will form in casting. It is desirable property of moulding sand.

Methods to improve porosity:

i) By selecting large grain size of silica sand particles

ii) By reducing percentage of clay

iii) By reducing ramming force

iv) By adding wood powder or saw dust

v) By providing vent holes

2) Strength

Strength is resistance to deformation. Strength is divided into Tensile, Compressive

and Shear strength.

3) Cohesiveness Property

It is the ability of formation of bond between same material particles and it is

desirable property of moulding sand so that sand particles should be stick with one

another.

4) Adhesiveness Property

It is the ability of formation of bond between different material particles and it is non-

desirable property of moulding sand so that sand particles should not stick on

surface of the pattern.

5) Refractoriness

It is the ability of the moulding material to resist the temperature of the liquid metal

to be poured so that it does not get fused with the metal. The refractoriness of the

silica sand is highest.

6) Permeability

During pouring and subsequent solidification of a casting, a large amount of gases

and steam is generated. These gases are those that have been absorbed by the


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metal during melting, air absorbed from the atmosphere and the steam generated

by the moulding and core sand. If these gases are not allowed to escape from the

mould, they would be entrapped inside the casting and cause casting defects. To

overcome this problem the moulding material must be porous. Proper venting of the

mould also helps in escaping the gases that are generated inside the mould cavity.

7) Green Strength

The moulding sand that contains moisture is termed as green sand. The green sand

particles must have the ability to cling to each other to impart sufficient strength to

the mould. The green sand must have enough strength so that the constructed

mould retains its shape.

8) Dry Strength

When the molten metal is poured in the mould, the sand around the mould cavity is

quickly converted into dry sand as the moisture in the sand evaporates due to the

heat of the molten metal. At this stage the moulding sand must possess the sufficient

strength to retain the exact shape of the mould cavity and at the same time it must

be able to withstand the metallostatic pressure of the liquid material.

9) Hot Strength

As soon as the moisture is eliminated, the sand would reach at a high temperature

when the metal in the mould is still in liquid state. The strength of the sand that is

required to hold the shape of the cavity is called hot strength.

10) Collapsibility

The moulding sand should also have collapsibility so that during the contraction of

the solidified casting it does not provide any resistance, which may result in cracks

in the castings. Besides these specific properties the moulding material should be

cheap, reusable and should have good thermal conductivity.

11) Flow ability

It is the ability of flowing of moulding sand into each and every corner of mould.

Fluidity and flow ability are indicating flow behaviour of substances. Fluidity indicates

flow behaviour of fluid and flow ability indicates flow behaviour of solid


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2.6.3. Methods of mould making (Refer class notes)

1) Hand moulding

2) Machine moulding

i) Jolting

ii) Squeezing

iii) Jolting and squeezing

iv) Sand slinging

2.7. CLASSIFICATION OF CASTING PROCESSES

Casting processes can be classified into following FOUR categories:

1. Conventional Moulding Processes

i) Green Sand Moulding

ii) Dry Sand Moulding

iii) Flask less Moulding

2. Chemical Sand Moulding Processes

i) Shell Moulding

ii) Sodium Silicate Moulding

ii) No-Bake Moulding3. Permanent Mould Processes

i) Gravity Die casting

ii) Low and High Pressure Die Casting

4. Special Casting Processes

i) Centrifugal Casting

ii) Investment Casting

iii) Continuous Casting

iv) Ceramics Shell Moulding

v) Evaporative Pattern Casting

vi) Vacuum Sealed Moulding

i) Green Sand Moulding


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Green sand is the most diversified moulding method used in metal casting

operations. The process utilizes a mould made of compressed or compacted moist

sand. The term "green" denotes the presence of moisture in the moulding sand.

The mould material consists of silica sand mixed with a suitable bonding agent

(usually clay) and moisture.

Advantages

i) Most metals can be cast by this method.

ii) Pattern costs and material costs are relatively low.

iii) No Limitation with respect to size of casting and type of metal or alloy used

Disadvantages

Surface Finish of the castings obtained by this process is not good and

machining is often required to achieve the finished product.

ii) Dry Sand Moulding

Dry sand casting is a sophisticated form of green sand process, in which the

sand mould is baked at a given temperature to make it stronger. This process

in mostly used in large foundries to produce big ferrous and non-ferrous castings

like engine blocks, construction parts, etc. Dry sand casting ensures precise

size and perfect dimensions. Two types of drying of Moulds are often required.

i) Skin drying and

ii) Complete mould drying.

In skin drying a firm mould face is produced. Shakeout of the mould is almost

as good as that obtained with green sand moulding. The most common method

of drying the refractory mould coating uses hot air, gas or oil flame. Skin drying

of the mould can be accomplished with the aid of torches, directed at the mould

surface.

Permanent Mould Process

In all the above processes, a mould need to be prepared for each of the casting

produced. For large-scale production, making a mould, for every casting to be produced,


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may be difficult and expensive. Therefore, a permanent mould, called the die may be

made from which a large number of castings can be produced. , the moulds are usually

made of cast iron or steel, although graphite, copper and aluminum have been used as

mould materials. The process in which we use a die to make the castings is called

permanent mould casting or gravity die casting, since the metal enters the mould under

gravity. Some time in die-casting we inject the molten metal with a high pressure. When

we apply pressure in injecting the metal it is called pressure die casting process.

1. Gravity die casting

Fig.1.30. Typical gravity die casting setup

In gravity die casting, molten metal is poured into the mould under gravity only and

no external pressure is applied to force the liquid metal into the mould cavity. However,

the liquid metal solidifies under pressure of metal in the risers, etc. The metallic mould

can be reused many times before it is discarded or rebuilt. These moulds are made of

dense, fine grained, heat resistant cast iron, steel, bronze, anodized aluminium, graphite

or other suitable refractoriness. The mould is made in two halves in order to facilitate the

removal of casting from the mould.


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2. Pressure die casting

When we apply pressure in injecting the metal it is called pressure die casting process.

There are two general types of molten metal ejection mechanisms adopted in die casting

set ups are:

(i) Hot chamber type

(a) Gooseneck or air injection management-Air pressure is used to inject molten

metal

(b) Submerged plunger management-Plunger is used to inject molten metal

(ii) Cold chamber type

i) Hot chamber die-casting

In the hot-chamber die casting process, the furnace to melt material is part of the die

itself and hence, this process is suitable primarily for low-melting point temperature

materials such as aluminum, magnesium etc. This process may be of gooseneck or air-

injection type or submerged plunger type-air blown or goose neck type machine is shown

as in Fig. 1.31. It includes following steps:

(i) Holding two die halves finally together.

(ii) Closing the die.

(iii) Injecting molten metal into die.

(iv) Opening the die.

(v) Ejecting the casting out of the die.

(1) (2)


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Fig.1.31. Hot chamber pressure die casting (1) Before applying plunger pressure (2)

After applying plunger pressure

(1)

(2)


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Fig.1.32. Hot chamber pressure die casting (1) Before applying plunger pressure and

dies are open (2) After applying plunger pressure and dies are closed

ii) Cold chamber die casting

Cold chamber die casting process differs from hot chamber die casting in following

respects.

1. Melting unit is generally not an integral part of the cold chamber die casting

machine. Molten metal is brought and poured into die casting machine with help

of ladles.

2. Molten metal poured into the cold chamber casting machine is generally at lower

temperature as compared to that poured in hot chamber die casting machine.

3. For this reasoning, a cold chamber die casting process has to be made use ofpressure much higher (of the order of 200 to 2000 kgf/cm 2) than those applied in

hot chamber process.

4. High pressure tends to increase the fluidity of molten metal possessing relatively

lower temperature.

5. Lower temperature of molten metal accompanied with higher injection pressure

with produce castings of dense structure sustained dimensional accuracy and free

from blow-holes.

6. Die components experience less thermal stresses due to lower temperature of

molten metal. However, the dies are often required to be made stronger in order

to bear higher pressures.


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Fig.1.33. Cold chamber pressure die casting (1) Before applying plunger pressure (2)

After applying plunger pressure

(1)


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(2)

Fig.1.34. Cold chamber pressure die casting (1) Before applying plunger pressure and

dies are open (2) After applying plunger pressure and dies are closed

2.1. Advantages of die casting over sand casting

1. Die casting requires less floor space in comparison to sand casting.2. It helps in providing precision dimensional control with a subsequent reduction

in machining cost.

3. It provides greater improved surface finish.

4. Thin section of complex shape can be produced in die casting.

5. More true shape can be produced with close tolerance in die casting.

6. Castings produced by die casting are usually less defective.

7. It produces more sound casting than sand casting.

8. It is very quick process.

9. Its rate of production is high as much as 800 casting/hour. (One set of die can

produce about 10000 castings)

10. Semiskilled worker can do the job.

11. Casting surface is free from sand


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2.2. Limitations of die castings

1. Cost of die is high

2. Not suitable for heavy castings

3. Suitable for only non-ferrous castings

4. Not suitable for small scale production

2.3. Applications of die casting

1. Carburettor bodies

2. Hydraulic brake cylinders

3. Refrigeration castings

4. Washing machine

5. Connecting rods and automotive pistons

6. Oil pump bodies

7. Gears and gear covers

8. Aircraft and missile castings, and

9. Typewriter segments

3. Investment Casting Process

The investment casting process also called lost wax process begins with the

production of wax replicas or patterns of the desired shape of the castings. A pattern is

needed for every casting to be produced. The patterns are prepared by injecting wax or

polystyrene in a metal dies. A number of patterns are attached to a central wax sprue to

form an assembly. The mould is prepared by surrounding the pattern with refractory slurry

that can set at room temperature. The mould is then heated so that pattern melts and

flows out, leaving a clean cavity behind. The mould is further hardened by heating and

the molten metal is poured while it is still hot. When the casting is solidified, the mould is

broken and the casting taken out.

Advantages

1. Formation of hollow interiors in cylinders without cores


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2. Less material required for gate

3. Fine grained structure at the outer surface of the casting free of gas and shrinkage

cavities and porosity

Disadvantages

1. More segregation of alloy component during pouring under the forces of rotation

2. Contamination of internal surface of castings with non-metallic inclusions

Inaccurate internal diameter


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Fig.1.35. Steps in investment casting process


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4. Centrifugal Casting

In this process, the mould is rotated rapidly about its central axis as the metal is

poured into it. Because of the centrifugal force, a continuous pressure will be acting on

the metal as it solidifies. The slag, oxides and other inclusions being lighter, get separated

from the metal and segregate towards the centre. This process is normally used for the

making of hollow pipes, tubes, hollow bushes, etc., which are axisymmetric with a

concentric hole. Since the metal is always pushed outward because of the centrifugal

force, no core needs to be used for making the concentric hole. The mould can be rotated

about a vertical, horizontal or an inclined axis or about its horizontal and vertical axes

simultaneously. The length and outside diameter are fixed by the mould cavity dimensions

while the inside diameter is determined by the amount of molten metal poured into themould.

(1) True centrifugal casting

(2) Semi-centrifugal casting and

(3) Centrifuged casting

4.1. True Centrifugal Casting

Fig.1.36. True centrifugal casting


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The two processes namely De Lavaud casting process and Moore casting process

are commonly used in true centrifugal casting. True centrifugal casting is carried out as

follows:

1. Applying ceramic slurry to the mould wall then drying it and baking

2. Rotating the mould at a predetermined speed (300-3000 rpm).

3. Pouring molten metal directly into the mould (no gating system is employed).

4. The mould is stopped after the casting has solidified.

5. Extraction of casting from the mould

4.2. Semi-Centrifugal Casting

Fig.1.37. Semi-centrifugal casting Fig.1.38. Centrifuging casting


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It is similar to true centrifugal casting but only with a difference that a central core

is used to form the inner surface. Semi- centrifugal casting setup is shown in Fig. 1.37.

This casting process is generally used for articles which are more complicated than those

possible in true centrifugal casting, but are axi-symmetric in nature. A particular shape of

the casting is produced by mold and core and not by centrifugal force. The centrifugal

force aids proper feeding and helps in producing the castings free from porosity.

Symmetrical objects namely wheel having arms like flywheel, gears and back wheels are

produced by this process.

4.3. Centrifuging Casting

Centrifuging casting setup is shown in Fig. 1.38. This casting process is generally

used for producing non-symmetrical small castings having intricate details. A number of

such small jobs are joined together by means of a common radial runner with a central

sprue on a table which is possible in a vertical direction of mould rotation.

Advantages of centrifugal casting

1. Formation of hollow cavities in cylinders without cores.

2. Non-metallic and slag inclusions and gas bubbles are forced to the inner surface

of the casting by centrifugal force.

3. No gating system and hence casting yield is high (100% in many cases)

4. Fettling costs are reduced. Cost of production is less.

5. Casting free of gas and shrinkage cavities and porosity

6. Find outside details (castings) can be successfully cast

7. Easy to inspect the castings (defects occur on the surface)

Disadvantages of centrifugal casting

1. More segregation of alloy components during pouring under the forces of

rotation

2. Suitable for only axial symmetrical components

3. Skilled workers are required for operation

4. Inaccurate internal diameter


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5. Continuous casting

Continuous casting process is widely used in the steel industry. In principle,

continuous casting is different from the other casting processes in the fact that there is no

enclosed mould cavity. The continuous casting process is used for casting metal directly into

billets or other similar shapes that can be used for rolling. The process involves continuously

pouring molten metal into an externally chilled copper mould or die walls and hence, can be

easily automated for large size production. Since the molten metal solidifies from the die wall

and in a soft state as it comes out of the die wall such that the same can be directly guided

into the rolling mill or can be sheared into a selected size of billets.

5.1. Types of continuous casting

i) Vertical continuous casting

ii) Horizontal continuous castings

iii) Continuous casting in travelling mould

i) Vertical continuous casting

. Fig.1.39

Vertical continuous casting I (detailed) Fig.1.40. Vertical continuous casting II (Complete)


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Steps involved in vertical continuous casting

1. Molten metal is transferred from the holding furnace into a special ladle called a

tundishfrom where the same is poured into the top of the bottomless water cooled

copper mould of the desired shape.

2. The process is started by placing dummy bar in the mould upon which the first

liquid metal falls. The liquid metal gets cooled and is pulled by pinch rollsalong

with the dummy bar.

3. Heat from the molten metal being poured dissipates fast through the mould walls

and a skin of solid metal forms quickly at the mould-metal interface and shrinks

from the mould walls. The shrinking effect provides a very small gap between the

metal and the mould thereby reducing friction between the two and permitting castshape to move continuously through the mould.

4. Pinch and guide rolls regulate the rate of settling of cast shape and keep proper

alignment.

5. As casting passes out of the pinch rolls it is cut to desired length by a saw or

oxyacetylene torch moving briefly along with the casting.

6. The cut length may be straightened, rolled and inspected before shipping as shown

in Fig.1.39,

a) Argon is provided to remove all reacting gases including Oxygen and to

provide inert atmosphere to avoid atmospheric contamination of molten

metal.

b) X-ray unit controls the pouring rate of the molten metal from the ladle and

also monitors the casting defects.

Advantages of continuous casting

1)100 percent casting yield (Casting yield = Weight of final casting100/Weight of

poured metal)

2) Cheaper to produce ingots

3) Better surface finish

4) Grain structure can be regulated

5) Process is automatic hence requires less labour


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Applications of continuous casting

1) Long billets of any cross section can be obtained (Round, square, hexagonal,

toothed gear)

2) Solid and hollow ingots can be made.

3) Bushings and pump gears

4) Production of copper bar (wire)

ii) Horizontal continuous casting

It uses graphite mould. It is commonly used to cast non-ferrous alloys. Molten

metal passes through the holding furnace into the water cooled mould as shown in

Fig. 1.41. As molten metal passes through the mould, a bar is generated. The bar is

fed continuously outwards by the rollers.

Fig.1.41. Horizontal continuous casting

iii) Vertical continuous casting

In the case of vertical and vertical continuous casting mould is stationary. But

in continuous casting in traveling mould, region between two belts acts as a mould which

rotates continuously and molten metal trapped between these two rollers. Molten metal

passes through the holding furnace, tundish to the nozzle. Two blocks consists of chilling

blocks. Cast strip comes out as shown in Fig. 1.42. and as it advances casting becomes

cool. Casting rate is 0.5-10 meter/min.


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Fig.1.42 Continuous casting in travelling mould

6.0 Shell moulding

Shell moulding, also known as shell-mould casting, is an expendable mould

casting process that uses a resin covered sand to form the mould. As compared to sand

casting, this process has better dimensional accuracy, a higher productivity rate, and

lower labour requirements. Shell mould casting is a metal casting process similar to sand

casting, in that molten metal is poured into an expendable mould. However, in shell mould

casting, the mould is a thin-walled shell created from applying a sand-resin mixture

around a pattern. The pattern, a metal piece in the shape of the desired part, is reused to

form multiple shell melds. A reusable pattern allows for higher production rates, while the

disposable moulds enable complex geometries to be cast. Shell mould casting requiresthe use of a metal pattern, oven, sand-resin mixture, dump box, and molten metal.Shell

mould casting allows the use of both ferrous and non-ferrous metals, most commonly

using cast iron, carbon steel, alloy steel, stainless steel, aluminium alloys, and copper

alloys. Typical parts are small-to-medium in size and require high accuracy, such as gear

housings, cylinder heads, connecting rods, and lever arms.


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6.1 The process of creating a shell mould consists of six steps:

Fine silica sand that is covered in a thin (36%) thermosetting phenolic resin and

liquid catalyst is dumped, blown, or shot onto a hot pattern. The pattern is usually

made from cast iron and is heated to 230 to 315 C (450 to 600 F). The sand is

allowed to sit on the pattern for a few minutes to allow the sand to partially cure.

The pattern and sand are then inverted so the excess sand drops free of the

pattern, leaving just the "shell". Depending on the time and temperature of the

pattern the thickness of the shell is 10 to 20 mm (0.4 to 0.8 in).

The pattern and shell together are placed in an oven to finish curing the sand. The

shell now has a tensile strength of 350 to 450 psi (2.4 to 3.1 MPa).

The hardened shell is then stripped from the pattern.

Two or more shells are then combined, via clamping or gluing using a thermoset

adhesive, to form a mould. This finished mould can then be used immediately or

stored almost indefinitely.

For casting the shell mould is placed inside a flask and surrounded with shot, sand,

or gravel to reinforce the shell. It is used for small to medium parts that require high

precision.


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6.2 Advantages:

Can form complex shapes and fine details

Very good surface finish, High production rate

Low labour cost.

Low tooling cost. Little scrap generated.

Can produce very large parts.

Can form complex shapes.

Many material options.

Low tooling and equipment cost.

Scrap can be recycled.

Short lead time possible.

6.3 Disadvantages:

High equipment cost.

Poor material strength.

High porosity possible.

Poor surface finish and tolerance.

Secondary machining often required.

Low production rate, High labour cost.

6.4 Applications:

Cylinder heads

Connecting rods

Engine blocks and manifolds

Machine bases, gears, pulleys.


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7.0 CO2 Moulding

Hardening the sand in mould by gassing it with the help of the CO2 gas is the CO2

moulding. The CO2 moulding idea conceived about 1898. However the process is

introduced in 1952 by Petrzela and then by Atterton in 1955. Since then CO2 moulding

has been adopted on a wide scale.

7.1 Principle

The highly flow able mixture of pure dry silica sand and sodium silicate binder is

rammed or blown into the mould. CO2 gas at pressure of about 1.5 kg/cm2 is diffused

through the mixture. To initiate the hardening reaction which takes from a few seconds to

a few minutes depending upon the size of core or mould.

Passage of CO2 through the sand containing sodium silicate produces carbonic acid

in the aqueous solution. This causes the formation of the colloidal silica gel. Which gets

hardened and forms a bond between the sand grains. This reaction is as below,

(Sodium silicate) Na2SiO3+ CO2Na2CO3+ SiO2 (silica gel)

This reaction proceeds rapidly in the early stages, and the compression strength of

the given sand mixture reaches to maximum level. But if the gassing is continued beyond

critical point then the strength of bond gets impaired. (for different types of arrangement

of gassing see class notes).

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