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Pattern Making and

Foundry

UNIT 2 PATTERN MAKING AND FOUNDRY

Structure

2.1 Introduction

Objectives

2.2 Pattern Making2.2.1 Pattern Materials

2.2.2 Types of Pattern

2.2.3 Pattern Making Allowances

2.2.4 Colour Coding for Patterns

2.2.5 Core Prints

2.2.6 Core Boxes

2.2.7 Master Pattern

2.3 Foundry

2.3.1 Composition of Moulding Sand2.3.2 Types and Properties of Moulding Sand

2.3.3 Sand Testing

2.3.4 Methods and Types of Moulding Processes

2.3.5 Definition of Gating System

2.3.6 Function of an Ideal Gating System

2.3.7 Types of Gate

2.3.8 Factors of Directional Solidification

2.3.9 Types of Core

2.3.10 Metal Melting Furnaces

2.4 Summary2.5 Answers to SAQs

2.1 INTRODUCTION

The process of making a wooden or metallic pattern is known as pattern making.

Pattern acts as a principle tool during the casting process and can be defined as a

model of desired casting, so constructed that may be used for forming an

impression or cavity called mould in moulding sand or other suitable material.

This mould or cavity when filled up with molten metal, forms the desired shape

of casting after solidification of the poured metal.Flow ability of metals is a very important and useful characteristics of metals.

Foundry engineering deals with the process of producing castings in moulds

prepared by patterns. The whole process of producing castings may be classified

into different stages. Except pattern making stage, all other stages are done in

foundry shop.

Objectives

After studying this unit, you should be able to

classify the pattern materials,

describe the types of patterns, distinguish between core and core print,

explain the pattern making allowances,

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Workshop Technology classify the types and properties of moulding sand,

describe the different methods of moulding process,

explain parts and functions of gated system,

elaborate classification of cores, and

conceptualize about crucible and cupola furnace.

2.2 PATTERN MAKING

For producing a mould or impression of desired shape in moulding sand or other

materials, one needs to have a wooden or metallic pattern similar to the shape of

the mould. The art and science of preparing the pattern is calledpattern making.

2.2.1 Pattern Materials

Some of the common materials used for pattern making are wood, metal, plaster,

wax and plastic.

WoodWood is the most common material used for pattern making as it satisfies

most of the essential requirements which are considered for a good pattern.

It is light in weight and easily available at low cost, may be easily shaped

into different forms as obtained good surface finish easily. The most

common woods used for pattern are Deodar, Teak, Shishum and Mohogany.

Metal

It is used for pattern when a large number of casting with a closer

dimensional accuracy is desired. The pattern of metal has a much longer life

than wooden pattern as it does not change its shape when subjected to moist

conditions. A metal pattern is itself cast from a wooden pattern calledMaster Pattern. Cast-iron, aluminium and its alloys, brass and white metal

are commonly used as a pattern metals.

Plaster

Plaster of paris (gypsum cement) is also used for making patterns and core-

boxes. It can be easily worked and casted into desired shape. It has a high

compressive strength (up to 300 kg/ cm2). Its specific use is in making small

patterns and core-boxes involving intricate shapes and closer dimensional

control.

Wax

Patterns which are generally used in investment casting process are madeby wax. The wax patterns are made by pouring the heated wax into a split

die or metal mould. The die is kept cool by circulating the water around it.

After complete cooling, the die parts are separated and wax in shape of

pattern is taken out.

Plastic

At present, plastics are finding their place as a pattern materials due to their

specific characteristics such as high strength and resistance to wear,

lightness in weight, fine surface finish and low solid shrinkage etc.

2.2.2 Types of PatternThe type of patterns selected for a particular casting depends upon many factors

such as type of moulding process, number and size of casting and anticipated

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Pattern Making and

Foundrydifficulty of moulding on account of design or typical shape of casting. The most

common types of pattern are listed and described below :

(a) Solid or Single Piece Pattern

(b) Split Pattern

(c) Gated Pattern

(d) Loose Piece Pattern

(e) Sweep Pattern

(f) Match Plate Pattern

(g) Multipiece Pattern

Solid or Single Piece Pattern

This type of pattern is the simplest of all the patterns. It is made without

joints, partings or loose pieces (Figure 2.1). For moulding with two

patterns, one or two moulding boxes may be used. Moulding operation withthis pattern takes more times as the moulder has to cut his own runners,

risers and feeding gates. This type of patterns are usually used for simple

and large sizes of casting.

Figure 2.1 : Single Piece Pattern

Split Pattern

Whenever the design of casting offers difficulty in making of mould and

withdrawal of pattern with a single piece pattern, split or two-piece pattern

is most suitable. This type of pattern eliminates this difficulty and can be

used to form the mould of intricate design or unusual shape of casting. Split

patterns are made in two parts so that one is placed in cope and other in

drag with the dowel pins holding the two together (Figure 2.2). The surface

formed at the line of separation of the two parts, usually at the centre line of

the pattern, is called parting line.

Parting Li ne

Core Prints

Figure 2.2 : Split Pattern

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Gated PatternWorkshop Technology

In mass production, a number of castings are prepared in a single

multicavity mould by joining a group of patterns. In such type of

multicavity mould, gates or runners for the molten metal are formed by

connecting parts between the individual patterns as shown in Figure 2.3.

Figure 2.3 : Gated Pattern

These are made of wood or metal and specially used for mass productions

of small castings.

Loose Piece Pattern

As per requirement, some solid or single piece type of patterns are made as

assemblies of loose component pieces. Loose pieces are arranged in such a

way that it can be removed from the mould easily as shown in Figure 2.4.

Figure 2.4 : Loose Piece Pattern

Usually, this type of pattern requires much maintenance and are slower to

mould.

Sweep Pattern

Large sizes of symmetrical moulds are generally prepared by means of

sweep patterns. It consists of a base, a wooden sweep board and a vertical

spindle. The outer end of sweep board carries a shape corresponding to the

shape of desired casting. Usually, sweep patterns are employed formoulding part carrying circular sections. The sweep board is attached with

the vertical spindle.

Section on AB

A B

RammedPattern

LoosePieces

CorePrints

Sand

MouldCavity

Loose Piecesbeing Withdrawn

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Pattern Making and

FoundryAfter holding the spindle in vertical position, the moulding sand is rammed

in place. As the sweep board is rotated about the spindle it will form a

desired cavity in the moulding sand as depicted in Figure 2.5.

Figure 2.5 : Sweep Pattern

Spindle

Sweep

Match Plate Pattern

Such type of patterns are widely used for producing small sizes of castings

in mass scale and are made of metal. Match plate patterns are made in two

pieces like split patterns. It consists of a wooden or metallic plate, called

match plate. Both the parts of split pattern are mounted on both sides of this

match plate (Figure 2.6). Groups of patterns on both sides of match plates

are used to prepare the moulds at a time separately, i.e. for group of patterns

on one side is prepared in drag while the other side group of patterns in

cope.

Figure 2.6 : Match Plate Pattern

Multipiece Pattern

Sometimes, it is necessary to prepare a pattern in more than two parts in

order to facilitate an easy moulding and withdrawal of pattern (Figure 2.7).

PartingLine

Parting Line

Core Print

Core Print

Figure 2.7 : Multipiece Pattern

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Workshop Technology This type of pattern is known as multipiece pattern. This type of pattern is

used for casting having a more complicated design. For the preparation of

mould this type of pattern requires generally three moulding boxes.

2.2.3 Pattern Making Allowances

Usually, the pattern is always made larger than the desired size of the casting on

account of allowance which should be allowed for machining, shrinkage,distortion and rapping etc. For a pattern, the following allowances are provided :

Machining Allowance

The extra amount of metal provided on the surfaces of casting to be

machined is called as a machining allowance. The amount of this allowance

depends upon the method of casting used, metal of casting, method of

machining. Size and shape of casting etc. Ferrous types of metals require

more allowance comparative to non-ferrous metals.

Shrinkage Allowance

Metals used for casting usually shrink and contract due to solidification andcooling. It is compensated by providing adequate amount of allowance in

the pattern which is called as shrinkage allowance.

Distortion Allowance

Casting of irregular shape and design tend to distort during cooling period.

Distortion of casting will take place due to uneven metal thickness,

shrinkage and rate of cooling. To eliminate this defect, distortion in

opposite direction is provided in the pattern so that this effect of distortion

may be neutralized.

Rapping AllowanceWhen a pattern is withdrawn from a mould, rapping is used in the pattern.

As a result of this rapping, the cavity in the mould is slightly increased.

Therefore, a negative allowance is to be provided in the pattern to

compensate the same.

Draft Allowance

To facilitate easy and early withdrawl of pattern from the mould without

injuring the vertical surfaces and edges of mould, patterns are given a slight

taper on all vertical surfaces. This slight taper inward on the vertical

surfaces of a pattern is known as the draft or draft allowance. Draft

allowance may be expressed either in degrees or in terms of millimeter permetre on a side. Its amount varies from 10 mm to 25 mm per metre on

external surfaces and from 40 mm to 70 mm per metre on internal surfaces.

2.2.4 Colour Coding for Patterns

Representation of different types of surfaces by means of different colours is

known as colour coding. By accepted colour code on pattern, we can judge the

casting surfaces either to be machined or not. Parts of pattern as a core print or

seat for loose piece are also justified by it. A widely accepted colour code for

common practice is given below:

Black colour Surfaces to be left unmachined

Red colour Surfaces to be machined

Yellow colour Core prints

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Pattern Making and

FoundryRed strips on Yellow base Seats for loose pieces

Black strips on Yellow base Stop offs

No colour or Clear Parting surface

2.2.5 Core Print

A core print is an added projection on a pattern. It forms a seat in the mould

which is used to hold and locate the core. The cores are used to produce throughor blind holes and recesses in the casting. The shapes and sizes of these

impression depend upon the shapes and sizes of cores.

Core prints may be of the following types :

Horizontal Core Print

This forms seat for a horizontal core. Horizontal core print is often found on

the split or two-piece pattern.

Vertical Core Print

It forms seat to support a vertical core in the mould.

Balancing Core Print

It forms seat on one side of the mould and the core is supported at one end

only, i.e. the core remains partly in this formed seat and partly in the mould

cavity. The print of core in the mould cavity should balance the part which

rests in the core seat.

Cover or Handing Core Print

It is used when the whole surface of pattern is rammed in the drag and the

core is suspended from top of the mould.

Wing Core PrintAt that place, where the cavity to be cored is above or below the parting

line in the mould, wing core print is referred.

2.2.6 Core Boxes

These are used for making cores. A core box is a wooden or metallic type of

pattern and are made either single or in two parts. They may be classified

according to the method of making the core or shape of core. The common types

of core boxes are described below:

Half Core Box

Half core box is used when a symmetrical core is prepared in two identical

halves which are later on pasted or cemented together to form a complete

core (Figure 2.8).

Core Box ProducedHalf-core

Figure 2.8 : Half Core Box

Split Core Box

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It is made in two parts like a split pattern. Both the parts are joined together

by means of dowel pins to form the complete hollow cavity for making the

core as shown in Figure 2.9.

Workshop Technology

Core Box Produced Half-core

Figure 2.9 : Split Core Box

Dump Core BoxFor making the slab or rectangular shape of core, dump core box is used. In

construction, it is similar to half core box. The box is made with side

opening.

Figure 2.10 : Dump Core Box

Loose Piece Core Box

It is used for the preparation of core with the provisions of boxes or hubs

and also when the two halves of a core of which the halves are not identical

in shape and size is to be prepared in the same core-box as shown in

(Figure 2.11).

Figure 2.11 : Loose Piece Core Box

Strickle Type Core Box

For the preparation of unsymmetrical or irregular shapes of cores, strickle

type of core-boxes are often used as shown in (Figure 2.12).

Core Box

ProducedCore

Loose Piece forLeft Hand Core

Loose Piece forRight Hand Core

Core Box

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Pattern Making and

Foundry

Figure 2.12 : Strickle Type Core Box

Colour coding for core boxes is followed in the same manner as in case of

pattern.

2.2.7 Master Pattern

A wooden pattern is usually used for preparing the mould of metal casting. When

this casting is used as a pattern for moulding work of further desired casting the

wooden pattern is called as a master pattern of this later casting.

SAQ 1

(a) What is pattern? Name some of the materials of which pattern aremade.

(b) What are the common allowances provided on patterns and why?

(c) How are the pattern classified? Explain the use of a gated pattern?

(d) Write short notes on the following :(i) Core prints

(ii) Sweep pattern

(iii) Pattern colours

(iv) Core boxes

2.3 FOUNDRY

Foundry or casting may be defined as a process of forming desired metallic

products by melting the metal, pouring this molten metal into a mould and then

allowing it to solidify. When this solidified shape of metal is separated from the

mould it will be of same shape as the mould. The mould is a cavity or impression

in the moulding sand which is produced by means of pattern. The process of

producing this cavity is known as moulding. A core is pre-determined shaped

mass of dry sand which is made separately or within the mould to obtain the

desired recesses and cavities in the mould. The process of producing the cores is

called Core Making.

2.3.1 Composition of Moulding Sand

The principal constituent of moulding sand are silica sand, binder, additives andwater. These are described below :

Silica Sand

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Workshop Technology As per composition, silica sand is the main constituent of moulding sand. It

is a product of the breaking up of quarry stone or decomposition of granite.

Silica sand imparts permeability, chemical resistivity and refractoriness to

the moulding sand. Silica sand is specified according to the average shape

and size of its grains.

Binder

The main function of binder is to impart the sufficient strength and

cohesiveness of the moulding sand, so that it may retain its shape after

ramming. The common binders may be divided as

(i) organic binders, and

(ii) inorganic binders.

The organic binders such as molasses, dextrin, linseed oil and resins are

usually used in core making while in the inorganic group the common

binders are portland cement, clay and sodium silicate. Amongst all, the clay

binders are widely used.

Additives

Materials which are added to the moulding sand to improve its existing

properties or to include certain new properties, are known as additives. As

per demand coal dust, wood flour, mollases, cornflour and pitch may be

used as an additive.

Water

When water is added to clay it furnishes the bounding action of clay. It

penetrates the mass of clay and forms a microfilm. The bonding quality ofclay totally depends on the maximum thickness of microfilm it can hold. In

general, water quantity varies from 2 to 8 percent.

2.3.2 Types and Properties of Moulding Sand

Types of Moulding Sand

Moulding sands are classified according to their use. These are classified

and described below :

Green Sand

It is a mixture of silica sand with 18 to 30 percent clay, having

quantity of water 6 to 8 percent. Green sand in its natural state

contains enough moisture to give it sufficient bonding property. It is

soft, light, porous and retains the shape easily when squeezed in the

hand. Moulds prepared by this sand are known as green sand moulds

which are used for small and medium castings only.

Dry Sand

When moisture from green sand mould is removed, it is known as dry

sand mould and is used for large size of casting. By drying the mould

in moulding box it becomes stronger and compact.

Facing Sand

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Pattern Making and

FoundryIt is used directly next to the surface of pattern. When the mould is

poured with the molten metal it comes directly in contact with the

molten metal. As it is subjected to most severe conditions, it must

possess high strength and refractoriness. It is made of silica sand and

clay in fine powder form.

Loam Sand

It is a mixture of clay (about 50%), sand and water (about 18-20%) toobtain a thin plastic paste which is used to plaster on moulds with soft

bricks and hardens on drying. This is particularly employed for loam

moulding usually for rough and large castings.

Backing Sand

It is the sand obtained from mould and is used again and again. Due to

its black colour which is due to burning and addition of coal dust, it is

also known as black sand.

Parting Sand

It is fine sharp dry sand used to keep the green sand from sticking tothe pattern and also to keep the moulding boxes (drag and cope)

separated.

Core Sand

This is silica sand mixed with core oil which is composed of linseed

oil, light mineral oil, resin and other binding materials. For the sake of

economy, pitch or flours and water may also be used in case of large

cores.

Properties of Moulding Sand

A good moulding sand must possess the undermentioned properties of

porosity, plasticity, adhesiveness, cohesiveness and refractoriness etc. All

the properties are supposed not only by the chemical composition, but by its

moisture content, by the amount of clay and lastly by the size and shape of

the silica sand grains.

Porosity or Permeability

The passage of gaseous materials, water and steam vapour through the

moulding sand is related to porosity. Molten metal always contains a

certain amount of dissolved gases, which are evolved during the

solidification of metal. A very large volume of gas and steam is alsogenerated when the molten metal is poured into the mould due to the

heating of moistures, coal dust and similar other materials present in

the sand. If these gases are not allowed to escape completely through

the mould, they will form pores and gas holes in the casting. So, for a

good sand it must be sufficiently porous to allow the gases or

moisture present or generated into the atmosphere freely. This

property of sand is called porosity.

Plasticity or Flowability

It refers to the condition of acquiring predetermined shape under

pressure and to retain it when the pressure is removed. This propertyof moulding sand increases as clay and water content increase.

Adhesiveness

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The sand particles must be capable of sticking to the other bodies

particularly to the moulding box of flask and it is due to only the

property of adhesiveness that moulding sand mass is held in the

moulding box properly. Due to this property, moulding sand can be

manipulated as desired without any chance of its falling out.

Workshop Technology

Cohesiveness

The ability of sand particles to stick together is denoted as

cohesiveness or the strength of moulding sand. Due to this property,

mould retains its shape even after the molten metal is poured in the

mould. In green state this property is termed as green strength or

green bond while for dry state as a dry strength or dry bond.

Cohesiveness property is largely effected by the clay and moisture

content, and size of grains.

Refractoriness

It is ability of the silica sand to withstand high temperature withoutfusing or breaking down as due to poor refractoriness sand may burn

at high temperature. This property of sand is measured by the sinter

point rather than melting point of sand.

2.3.3 Sand Testing

We have already discussed the main properties of a good moulding sand. Foundry

sand plays role as a chief constituents of moulding sand. Therefore, the properties

of moulding sand depend upon the properties of foundry sand. To control its

composition and properties, it should be tested periodically. The common types of

tests are as follows :

(a) Grain fineness test

(b) Moisture content test

(c) Clay content test

(d) Permeability test

(e) Strength test

(f) Mould and core hardness test

Grain Fineness Test

The size of grain of a sand is designated by a number called Grain fineness

number which points out the average size as well as proportion of smaller

and larger grains. Size of sand grain provides a significant effect on its

porosity or permeability. Grains of similar sizes increase porosity whereas

those of different sizes reduce the porosity but increase the compactness.

The foundry sand for its grain size is tested by sieve shaker unit shown in

Figure 2.13. It consists of a set of standard sieves which are graded and

numbered according to the fineness of their meshes or apertures. The

coarsest sieve is placed at the top and the finest one at the bottom, rest of

being placed below one another in the same order.

Sieve Cover

Ti

ghtening Rubber Strap

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Pattern Making and

Foundry

Figure 2.13

The sand sample to be tested is washed to remove the clay and then dried. A

weighed quantity of this sand is placed in the coarsest sieve, mounted on the top

of sieve shaker and the unit shaken for about 15 minutes. The test of fineness isconducted by screening sand grains by means of standard sieves. After due time,

the percentage amount of sand remaining on each sieve is calculated and

multiplied by a constant. The products of this multiplication are added to find out

total product. By applying the given formula, fineness number may be calculated

screenonretainedsandofPercentageTotal

ProductTotalNumberFineness =

Moisture Content Test

For determining moisture content, a moisture teller instrument is widely

used. It consists of a cast iron stand, an infra-red-heater bulb fitted in theshade and a drying pan with a handle.

For test, take about 25 gms sand as a sample in pan and then arrange this

pan under the shade. The bulb is switched on for about 2-3 minutes and

then switched off. Remove pan and weigh the sand again. The difference in

the weight of sample before and after drying indicates the amount of

moisture content. It is expressed as a percentage of the total weight of sand

sample.

Clay Content Test

The difference between original weight of a dried sand sample and the final

weight of a dried sand sample after the mud has been washed away givesthe mud content of the sand. It may be easily expressed as a percentage of

the original weight of the sand sample.

The method for determining the clay content of sand consists of stirring the

sand sample in distilled water at room temperature so as to separate the clay

particles from the sand which remains suspended in water. The material

which fails to settle within a period of 5 minute is designated as a clay

content.

Permeability Test

The volume of air in cubic centimeter that will pass per minute under apressure of 1 gm per cm2 through a standard specimen of sand having 1 cm2

in cross-sectional area and 1 cm deep is defined as a permeability number.

The permeability number can be calculated by using the following formula.

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Permeability Number =tap

hV

Workshop Technology

where, V= volume of air in cubic cm,

h = height of specimen in cm,

p = air pressure in gm/cm2,

a = cross sectional area of the specimen in cm2, and

t= time in minutes taken by the air to pass.

Strength Test

The foundry sand should be capable to develop a maximum compressive

strength in moist condition. For strength test, a well rammed sand specimen

of 5.08 cm high and 5.08 diameter is pushed out of the specimen tube and

then placed on the upper plate of universal testing machine with its end. A

continuously increasing load at a rate of about 200 kgf/cm2 is applied on

this specimen until rupture of the specimen takes place. The compression

value may be read directly on the green compression scale of the testingmachine.

Mould and Core Hardness Test

The hardness of a mould and core can be tested easily by means of a

hardness tester. The tester is about the size of a pocket watch and the

hardness test can be performed within few seconds. It carries a tip at its

bottom which is penetrated into the testing surface. A spring loaded shaft

inside the hollow body of the instrument actuates the needle of dial gauge

fitted at the top. The dial of this gauge indicates direct reading of the

hardness of testing surface.

2.3.4 Methods and Types of Moulding Processes

The different moulding processes may be classified as follows :

According to the method used

(a) Floor Moulding

(b) Bench Moulding

(c) Pit Moulding

(d) Machine Moulding

According to the mould materials

(a) Green Sand Moulding

(b) Dry Sand Moulding

(c) Loam Sand Moulding

(d) Core Sand Moulding

Floor Moulding

This method of moulding is commonly used for preparing the mould of

heavy and large size of jobs which cannot be conveniently moulded through

bench moulding method. In floor moulding, the floor itself acts as a drag. It

is preferred for such rough type of castings where the upper surface finishhas no importance.

Bench Moulding

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Pattern Making and

FoundryBench moulding is done on a work bench of a height convenient to the

moulder. It is best suited to prepare the mould of small and light items

which are to be casted by non-ferrous metals.

Pit Moulding

Large size of jobs which cannot be accommodated in moulding boxes are

frequently moulded in pits. Here, the pit acts as a drag. Generally, one box,

i.e. cope is sufficient to complete the mould. Runner and riser, gates and

pouring basin are cut in it.

Machine Moulding

Machine moulding method is preferred for mass production of identical

casting as most of the moulding operations such as ramming of sand, rolling

over the mould, and gate cutting etc. are performed by the moulding

machine. Therefore, this method of moulding is more efficient and

economical in comparison to hand moulding.

Green Sand MouldingGreen sand consists of silica sand, 10 to 15 percent clay and 4 to 6 percent

moisture content. All these materials are thoroughly mixed and riddled. It

should also be given the required condition by proper tempering.

The main methods of green sand moulding are as follows :

(a) Open sand method

(b) Bedded in method

(c) Turn over method

Open Sand Method

The complete mould is prepared in the floor as already discussed in

floor moulding. It is the simplest form of green sand moulding

method and suitable for solid types of patterns. The moulding sand is

rammed highly, just to support the weight of metal during its pouring.

After proper levelling of moulding sand, the pattern is pressed down

in the sand bed for making mould.

This method is employed for simple types of casting, grills, railings,

floor plates and gates etc.

Bedded in Method

This type of moulding technique is usually employed when the upper

surface of casting is not flat or should be smoother than produced by

open-sand method or the parting line of solid pattern is not marked

clearly. In this method, the drag is filled partially and the pattern is

pressed down to bed it into the moulding sand to form the mould

cavity.

After the finishing of mould cavity, the pattern is again pressed

downward until properly rammed mould cavity is obtained. A cope is

then placed over the pattern and rammed properly. The rest of theprocedure followed in the usual way.

Turn Over Method

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This type of moulding method is most suitable for split patterns as

well as for solid type of patterns. Most of the small and medium sized

castings of non-ferrous metals are made by this method.

Workshop Technology

In this method, one half of the pattern is placed on the moulding plate

or board with its flat side. The drag part of the moulding box is then

kept over it and rammed. Excess moulding sand from the top of dragpart is parted off by means of strike off bar and the box turned over.

The other half part of pattern is assembled in position with 1st half by

means of dowel pins which control the proper alignment of both the

parts. After that, the parting sand is sprinkled over the top surface of

drag and pattern. The parting sand is used to prevent the joints

between the halves of a mould from adhering to one another when the

two parts of the moulding box are separated.

Figure 2.14

The upper half pattern is followed by cope and two tapered wooden

pegs are placed in proper position on the pattern which serve as

Pattern

MouldingBoard

(First Step)

Parting Line

Drag

Cope

Drag

Riser Sprue

(Second Step)

Pin

Runner

Riser

MouldingBoard

Core

VentsWeight

Pouring Basin

Gate

(Third Step)

(Completed-Mould)

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Pattern Making and

Foundryrunner and riser. Next, the cope is filled with moulding sand and

rammed. Excess sand is then cut off. Wooden pegs are taken out and

the pouring basin is made as shown in Figure 2.14. Venting process is

also applied to provide the vents. The cope is then rolled over, the

pattern pieces are taken out from both the moulding boxes and gates

are cut in the drag from the pattern to runner side. The dry sand core,

if any, is also located in position and the mould is closed for pouring

of molten metal. The metal is poured into the mould through the

runner.

Dry Sand Moulding

This process of moulding is just similar to green sand moulding except the

composition of constituents in mixture. Here, in the preparation of mixture

for dry sand moulding, special binding materials such as resin, molasses,

flour, or clay are mixed to give strong bond to the sand. All parts of mould

are completely dried before casting. Dry sand moulding is widely used for

large size of work such as parts of engine, large size of fly wheel and rolls

for rolling mill. This process is costlier than green sand moulding but muchsuperior in quality.

Loam Sand Moulding

This process is used for extremely large size of casting which are to be

made in very small numbers. Loam sand moulds are prepared with coarse

grained silica sand, clay, coke, horse manure and water. This process of

moulding is performed in different way. First, a rough structure of desired

shape is made by hand by using bricks and loam sand. This structure is then

finished by means of strickle and sweep. The surfaces of structure are

blackened and dried before being casted.

Core Sand Moulding

For core sand moulding, mixture is prepared with silica sand, olivine,

carbon and chamotte sands. Sand that contains more than 5% clay may not

be used as a core sand. For core making by hand, the core sand is filled and

rammed in the core box properly. The whole operation takes a short time

after that the core box is withdrawn and the core removed.

2.3.5 Definition of Gating System

All the channels or passages through which the molten metal is delivered to

mould cavity is termed as gating system. Gating system includes the runner, riser,pouring basin and gate etc.

2.3.6 Function of an Ideal Gating System

The main functions of an ideal gating system are to

(a) distribute the molten metal with the least disturbance in order toreduce erosion of the mould material.

(b) facilitate complete filling of the mould cavity.

(c) fill the mould cavity with molten metal at the earliest possible time toavoid temperature gradient.

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(d) develop such temperature gradient in molten metal and the mouldwhich will lead to the directional solidification of the casting towards

riser.

Workshop Technology

(e) prevent the formation of oxide and dross in the molten metal whileflowing through it.

(f) prevent the entry of slag, sand and the other particles from the mould.2.3.7 Types of Gate

A gate is a passage or channel through which the molten metal flows from the

runner to the mould cavity. However, it should be located where it can be easily

removed without any loss to the casting. As per their position in the mould cavity,

gates may be classified as follows :

Parting Line Gate

It is the simplest type of gate and the molten metal enters the mould cavity

at the parting line. Such type of gate is cut by hand when the cope and drag

are separated or it can be formed by an attached gate to the pattern.

Top Gate

In this type of gate, the molten metal from the top flows down directly into

the mould. As all the molten metal enters the casting at the top therefore,

the hottest metal comes to rest at the top of casting. With the result, proper

temperature gradient is formed to enable directional solidification of casting

from the bottom side towards the riser. The gates themselves may be cut to

serve as the risers. Main drawback of this type of gating is the erosion of

the mould, which takes place by the falling metal. The cavity of mould,

therefore, should be much hard and strong to resist this impact.

Strainer Core

Figure 2.15

Bottom Gate

In this type of gate, the molten metal from the pouring basin flows down and

enters to the mould cavity at or near its bottom. Bottom type of gate

facilitates the mould to be prepared in two moulding boxes. During pouring

of molten metal, bottom types of gate enable to reduce the erosion of mould

and core and minimize the turbulence of metal (Figure 2.16).

Figure 2.16

Pouring Basin

Casting

Mould Cavity

Horn Sprue Choke

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Pattern Making and

Foundry2.3.8 Factors of Directional Solidification

As the molten metal cools and solidifies in the mould, shrinkage of metal will

take place which creates voids inside. Solidification of metal free of internal

voids and shrinkage is called as a directional solidification. The factors which are

used to control this directional solidification are

(a) proper design and positioning of risers,

(b) proper design and positioning of gating systems,

(c) use of padding,

(d) use of metal chills, and

(e) use of exothermic materials.

Riser

It is a passage of sand made in the cope part of moulding box through

which the molten metal rises after the mould is filled up completely. The

main functions of a riser are given below :

(a) Riser acts as a reservoir and feeds the molten metal to the casting tocompensate the shrinkage during solidification.

(b) It permits the escape of gas, air and steam as the mould cavity is beingfilled up with the molten metal.

(c) It controls the solidification time, which should be greater in it thanthat in the mould cavity.

(d) It helps to ensure that the mould cavity has been completely filled upwith molten metal.

Use of Padding

Padding means adding of some extra metal to the original section of casting

in varying thickness to attain the required directional solidification.

Use of Chills

Chills in shape of extra metal are also used in achieving directional

solidification. If a casting consists of sections of uneven thickness, rate of

cooling will be different as per thickness of section. The thin sections tend

to solidify earlier than the thick ones, resulting in uneven shrinkage and

severe distortion. To accelerate the cooling rates of thick sections, chills areinserted in these sections and thus, obtain the desired directional

solidification.

Use of Exothermic Materials

The proper directional solidification of casting can be further controlled by

the use of exothermic materials. These materials produce large amount of

heat when come in contact with the molten metal. These are added to the

surface of molten metal through riser side. The materials used as

exothermic materials are the oxides of iron, copper and nickel etc. mixed

with suitable amount of aluminium.

2.3.9 Types of Core

A core is made by core sand and prepared separately in a core box. It is used to

form a desired recess and cavity in casting. Different types of cores are used in

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foundry work and are employed according to their shape and their position in the

mould. The main types of cores are described below :

Workshop Technology

Horizontal Core

It is the simplest type of core which is placed horizontally at the parting line

of the mould. As per cross section, it may be of any shape but cylindrical

shaped core is mostly used as shown in Figure 2.17.

Sand

Mould

Core

Figure 2.17

Vertical Core

It is similar to horizontal core, only differs in its position. Vertical core is

placed in the mould with its axis vertical. Normally, top and bottom ends of

the core are provided with a toper as shown in (Figure 2.18).

Figure 2.18

Balanced Core

It is suitable to produce a blind hole along a horizontal axis in casting. The

overhanging length of the core is supported by means of chaplets as shown

in Figure 2.19.

Figure 2.19

Hanging or Cover Core

The core which has no support at the bottom and hangs vertically from the

cope (Figure 2.20) is known as hanging core. In this case, the entire mouldcavity is prepared in the drag only.

Cope

Drag

CoreMould

Cavity

Parting Line

Cope

Drag

Core

Mould

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Pattern Making and

Foundry

Cope

Drag

Core

Mould

Figure 2.20

2.3.10 Metal Melting Furnaces

A large number of melting furnaces have been developed and the choice of a

particular type of furnace depends upon the quantity of metal to be melted at a

time and the nature of metal to be melted.

The main types of melting furnaces used in foundry work are described below :

Crucible Furnace

It is the simplest type of furnace and widely used at that place where the

melting is not continuous and different types of metal are to be melted insmall quantities. A crucible furnace comprises a crucible which is made of

clay and graphite. The whole process of melting of metal takes place inside

the crucible. Generally, two types of crucible furnaces are used in practice

such as Pit-type and Tilting type.

Pit Type Crucible Furnace

As the name signifies, this type of furnace is prepared in the form of a

pit. These are used for melting small quantities of ferrous and

non-ferrous metals. Usually, pit furnace is fired with coke. They are

provided with the fire bricks lining inside and a chimney for natural

draught as shown in Figure 2.21.

ConcreteLining

Pit

Steel Shell

Fire Bricks Lining

Cover

Chimney

SlidingDoor

Gate

NaturalDraght

Crucible

Figure 2.21 : Pit Type Crucible Furnace

Tilting Type Crucible Furnace

This type of furnace is mounted on two pedestals, raised above

ground level and rotated with the help of geared hand wheel. Such

types of furnaces are employed with the provision of forced draught

and usually fired by coke, oil or gas (Figure 2.22). For oil and gasfired furnaces, the crucible is supported with a block of refractory

material while for coal or coke type, rests on the fuel bed.

To Atmosphere

Cover

RefractoryLinin

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Workshop Technology

Figure 2.22 : Tilting Type Crucible Furnace

Cupola

Cupola is used for melting and refining of pig iron along with scrap. It is

basically a hollow vertical shell or cylinder made of mild steel and linkedwith fire bricks. The cylinder or shell is mounted either on steel column or

on a brick work foundation. The bottom of the shell is provided with drop

ChargingPlatform

Tapperd SandBottom

Charging Door

Refractory Lining

Spark Arrester

Spout

Wind Belt

Blast Pipe

Coke Bed

Coke Charges

Metal Charges

TapHole

Melting ZoneReducing Zone

Shell

Stack

PreheatingZone

Combustion Zone

WellTuyeres

Slag Spout

Prop Leg

Figure 2.23

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Pattern Making and

Foundrybottom door. When the cupola is in operation, the bottom door is supported

by a prop. At the end of operation, the charge feeding is stopped, air supply

cut off and the prop removed. As soon as the prop is removed, the bottom

door drops down providing a passage for the residue of molten metal with

slag to fall down. The amount of air required is forced into the wind belt by

blower which enters the furnace, through tuyeres. Charging door is

provided above the charging platform. Through charging door, the charge is

fed into the furnace. The shell is continued above the charging door to form

a chimney. At the top of furnace a conical construction called the spark

arrester is attached to prevent the spark from emerging to the outside as

shown in Figure 2.23.

Zones in a Cupola

A number of combustion reactions take place in the cupola. Therefore, the

entire shell of cupola may be divided in zones which are as under :

Well Zone

The metal after melting is collected here and then tapped out. Well zone is

the space between top of sand bed and the bottom of tuyeres.

Combustion Zone

It is located about 15 cms to 30 cms above the top of tuyeres and also

may be called as oxidizing zone. As the actual combustion takes place

in this zone, a lot of heat is produced which is supplied from here to

other zones. A temperature of about 1550oC to 1850oC is produced in

this zone.

Reducing Zone

Reducing zone is located from the top of combustion zone to the top

of the coke bed. In this zone, the temperature falls to about 1200 oC at

the coke bed on account of reducing atmospheres. This zone protects

the charge against oxidation.

Melting Zone

The 1st layer of metal charge above the coke bed and extend upto a

height of about 90 cm. Being temperature around 1600oC, the

complete combustion of coke and iron takes place in this zone.

Preheating Zone

It extends from above the melting zone to the bottom of charging door

and contains the cupola charge (alternate layers of coke, flux and

metal). In this zone, the charge is preheated at a temperature of about

1100oC before coming to the melting zone.

Stack Zone

It is the empty portion of cupola above the preheating zone to the top

of the cupola which carries the gases generated within the furnace to

the atmosphere.

Preparation and Charging of Cupola

First of all the waste material and slag etc. are removed from the cupola

which are dumped under the furnace after the previous melting. The bottom

door is brought and secured in position by means of prop, and then a sand

bed is laid at the bottom. The surface of the sand bed is sloped towards the

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tapping hole so that the molten metal may be drained from the cupola at any

time.

Workshop Technology

As a bed charge, soft and dry wood is placed over the sand bed followed by

a bed of coke. The wood is ignited through the tap hole. As soon as the

coke bed is built up to the correct height and ignited uniformly throughout,

an alternate layers of pig iron, coke and limestone are charged from thecharging door until the cupola is full to the charging door. Usually, in

practice, the charge ratio between metal and coke is kept 8 : 1 to 10 : 1.

Amount of limestone in charge depends upon the amount of metal which is

about 40 to 50 kg per metric ton of metal charge.

SAQ 2

(a) What is moulding? What are the main characteristics, which a goodmoulding sand should possess?

(b) Why testing of foundry sand is necessary? What are the common testsperformed on foundry sands?

(c) How do you classify the various moulding sand processes?

(d) Distinguish between green sand moulding and dry sand moulding.

(e) What do you understand from the term Gating system? What arethe main requirements expected of an ideal gating system?

(f) What is a core? How many types of core are there?

(g) Write short notes on the following :

(i) Pit moulding

(ii) Exothermic materials

(iii) Crucible

(iv) Cupola.

2.4 SUMMARY

Pattern is the model of the desired casting, which is used for forming an

impression, called mould in the damp sand or other suitable moulding materials.

They are made of timber, metal, plaster of paris or plastic. As per nature of work,

different types of pattern are used in practice. Number of casting, appearance and

surface finish of casting are the important consideration which a pattern maker is

to make in order to plan the pattern successfully.

A mould is the cavity or impression which is prepared in moulding sand by

means of pattern. It will produce a casting when filled with molten metal. The

process of producing this cavity is known as moulding. According to use,

moulding sands are classified into a number of varieties. A good moulding sand

must possess the properties of porosity, adhesiveness, cohesiveness and

refractoriness etc.

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FoundryCores are separate shapes of sand that are generally required to form the hollow

interior of the casting or a hole through the casting. A sand casting is produced by

pouring the molten metal into the mould through a passage called gate. As per

situation, gates may be classified as top gates, parting line gates and bottom gates.

Crucible and cupola furnaces are commonly used in foundries for melting of

various varieties of ferrous and non-ferrous metals and alloys.

2.5 ANSWERS TO SAQs

Refer the relevant preceding text in the unit or other useful books on the topic

listed in section Further Reading to get the answer of the SAQs.