manufacturing process 1 by dv shirbhate

8/14/2019 MANUFACTURING PROCESS 1 BY DV SHIRBHATE

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MANUFACTURING PROCESS I

UNIT NO. 1

Q.1 What are master patterns? How does their size differ

from other pattern`s ? Explain

Ans: Master patterns are used for preparing the moulds

for metal castings which are later used as patterns for

further moulding work, called metal patterns. The master

patterns are accurately finished wooden patterns, which

carry double shrinkage allowance and the required machining

allowance. For example, an alluminium pattern is to be

made which is to be used further for making moulds for

brass castings. The alluminium pattern should, obviously,

be larger than the desired brass casting by an amount equal

to shrinkage that will take place during solidification of

this casting. For making this alluminium pattern a wooden

pattern is to be used which should be larger than the

alluminium pattern by an amount equal to the alluminium

shrinkage, added with proper machining allowance for

finishing the alluminium casting. Mathematically, it can

be represented thus :

Let Sb represent the size of the desired casting in

brass.

And Let Sa represent the size of alluminium pattern.

And Let Cb represent the contraction allowance for brass.

Then Sa=Sb+Cb

Again, let S represent the size of the master pattern.

And let Ca represent the contraction allowance for

alluminium.

Also let Am represent the machining allowance required

to finish the alluminium casting to the required size of

pattern and to give smooth surface finish.


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Then S = Sa+Ca+Am

= Sb+Cb+Ca+Am or

Size of master pattern = Size of the final casting to be

Made + shrinkage allowance for

the material of final

casting + shrinkage allowance

of the metal of which the

pattern is to be made

+ Finishing allowance for the

metal pattern.

Q.2. What considerations are necessary while designing a

pattern?

Ans. The following points should be considered, while

designing a pattern :

1. Proper allowances should be provided, wherever

necessary.

2. The parting line should be carefully selected so as to

allow as small portion of the pattern in the cope as

possible.

3. A proper material should always be selected for the

pattern after carefully considering the factors mentioned

in Art.9.4.

4. An endeavor should always be made to employ full cores

instead of jointed half cores as far as possible. This

will reduce cost and ensure greater dimensional accuracy.

5. The wall thickness and sections should be kept as

uniform as possible. Abrupt changes should invariably be

avoided.

6. The use of offset parting, instead of cores, should be

encouraged to as great an extent as it is possible.

7. For large-scale production of small castings, the use

of gated or match-plate patterns should be encouraged

wherever the existing facilities permit.


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8. All sharp corners and edges should be invariably

provided with suitable fillets or otherwise rounded to

enable an easy withdrawal of pattern, smooth flow of

molten metal and ensure a sound casting.

9. All those surfaces of the casting which are

specifically required to be perfectly sound and clean

should be so designed that they will be molded in the

drag.

10. The pattern should be given a high-class surface

finish as it directly effects the corresponding finish of

the casting.

11. If gates, runners and risers are attached to the

pattern, they should be properly located and their sudden

contraction or enlargement should be avoided.

12. Shape and size of the casting and that of the core

should be carefully considered to decide the size and

location of the core prints.

Q.3. Shortly explain the following :

(1)Segmental patterns(2) Core prints

Ans: Segmental patterns:-These patterns are used for

preparing moulds of large circular castings, avoiding the

use of a solid pattern of the exact size. In principle

they work like a sweep, but the difference is that a sweep

is given a continuous revolving motion to generate the

desired shape, whereas 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


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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. A typical example is shown in

Fig.

(2) Core prints :

When a casting is required to have a hole, through or

blind, a core is used in the mould to produce the same.

This core has to be properly seated in the mould on formed

impressions in the sand. To form these impressions, extra

projections are added on the pattern surface at proper

places. These projections are known as core prints.

Q.4.(a) What do you understand by acid and basic cupolas?

Where each type is preferred and why?

Ans : Cupolas are termed as basic or acid according to the

type of refractory lining used inside them. The refractory

lining is provided by setting bricks, made of refractory

material all along the inside surface of the cupola shell.

Basic cupolas find their specific use in the production of

ductile cast iron. They help in the reduction of sulphur

content in iron to the tune of 0.005 percent. They also

enable a higher carbon pick-up than acid cupola, with the

use of same raw material. This enables use of a higher

proportion of steel scrap in the metal charge. However, if

higher percentage, say upto 0.1 percent of sulphur is

admissible in the cast metal the acid cupola proves to be

relatively cheaper.

(b) Write a short notes on following casting defects.

(1)Metal penetration(2)WarpageAns.: (1) Metal penetration :

This defect occurs as a rough and uneven external

surface on the casting. It takes place when the molten


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metal enters into the spaces between the sand grains and

holds some of the sand tightly with it even after fettling.

The principal causes for the promotion of this defect are

the use of coarse sand, having high permeability and low

strength, and soft ramming. Use of fine sand with low

permeability and hard ramming will minimise this defect.

(2) Warpage :

It is an undesirable deformation in the casting, which

may occur during or after solidification. The deformation

takes place due to the internal stresses developed in the

casting due to differential solidification in different

sections. Such stresses are also developed and

differential solidification occurs in case of castings

having very large and wide flat surfaces. Both the causes

can be attributed to faulty design of the casting, which

needs modification to ensure proper directional

solidification.

(c) Explain, how the grain size and shape effect the

performances of a foundry sand.

Ans. Sand Grain :

The shape and size of the sand grains has a remarkable

effect on the physical properties of the foundry sand. The

sand grains may have smooth, conchoidal or rough surfaces.

Out of these the first type i.e., smooth, is preferred for

moulding for the reason that such a surface renders higher

permeability, sinter point and plasticity to the sand

mass, but the percentage of binder required is also equally

high.

Similarly the sand grains may have different shapes.

The commonly formed shapes are rounded, sub-angular,

angular and compound. The rounded grains do not bind

together two well when rammed and, hence, render the sand


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mould highly permeable but the strength of the mould is

also reduced.

Sub-angular grains give a relatively stronger bond

than above but the permeability is reduced. Angular or

sharp grains produce a much stronger bond and a low

permeability when rammed. Thus they enable a mould of

greater strength. Sand grains which are cemented together

such that they do not separate when screened are called

compound. They may consist of one, two or a combination of

all the above three shapes. They are not

much preferred.

Like the shape the size of sand grains also effects

the mould structure and its characteristics. Large, regular

and uniform grains increase permeability. Smaller grains

increase smoothness on mould surfaces.

Q.5. What are the common materials used for pattern making?

Discuss their relative merits and demerits.

Ans.: Pattern materials

The common materials of which the patterns are made

are the following:

1. Wood. It is the most common material used for pattern

making because of the following advantages :

(i) It is cheap and available in abundance.

(ii)It can be easily shaped into different forms and

intricate designs.

(iii) Its manipulation is easy because of lightness in

weight.


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(iv)Good surface finish can be easily obtained by only

planning and sanding.

(v) It can be preserved for a fairly long time by applying

proper preservatives like shellac varnish.

On the other hand, it has certain disadvantages also as

follows:

(i) It wears out quickly due to its low resistance to sand

abrasion. As such, a wooden pattern cannot stand a long

constant use.

(ii)It is very susceptible to moisture, which may lead to

its warping or splitting. This needs its careful

storing in a dry place and the application of

preservatives.

(iii)Its life, owing to the above reasons, is short as

compared to other pattern materials. This confines its

use to such cases only when a small number of castings

are required.

2) Metals :- Metals are used with advantage, as patternmaterial, only when the number of castings to be made is

very high and a closer dimensional accuracy is desired.

They have a much longer life than wooden patterns and

eliminate the inherent disadvantages of wood to a great

extent. But they also carry the following

Disadvantages :

(i) They are costlier than wood and, therefore, cannot be

used with advantage, where a smaller number of

castings is to be made.

(ii) For giving different shapes and fine surface finish

they need machining. This again adds to their cost.

(iii) Most of them are very heavy and in case of large

castings the weight of the pattern always poses a

problem in its manipulation.

(iv) A large number of them have a tendency to get

rusted.


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3) Plaster :- Plaster of Paris or gypsum cement is

advantageously used as a pattern material since it can be

easily casted into intricate shapes and can be easily

worked also. Its expansion can be easily controlled and it

carries a very high compression strength. Its specific use

is in making small patterns and core boxes involving

intricate shapes and closer dimensional control. A marked

feature of this cement is that contrary to the action of

metals, it expands on being solidified. Thus, if a cement

of proper coefficient of expansion is selected, the effect

of shrinkage of casting can be automatically neutralised.

4)Plastics :- Plastics are gradually gaining favour aspattern materials due to their following specific

characteristics :

1. Lightness in weight.

2. High strength.

3. High resistance to wear.

4. High resistance to corrosion due to moisture.

5. Fine surface finish.

6. Low solid shrinkage.

7. Very reasonable cost.

The plastics used as pattern materials are thermo-

setting resins. Phenolic resin plastic and foam plastic

suit best for this purpose. For making the pattern, first

the moulds are made, usually from plaster of Paris. The

resin is then poured into these moulds and the two heated.

At a specific temperature, the resin solidifies to give the

plastic pattern.

5) Wax :- Wax patterns are exclusively used in investmentcasting. For this a die or metal mould is made in two

halves into which the heated wax is poured. The die is

kept cool by circulating water around it. As the wax

sets on cooling, the die parts are separated and the wax

pattern taken out.


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Q.6. Explain the process of Sand preparation and

conditioning.

Ans.: None of the natural sand possesses the required

qualities to the required extent. They may lack in one or

more of these properties which we have to make up by

artificial means to make the sand suitable for use. Sand

mixing is the process through which we add those materials

to the sand which are rich in such characteristics, which

the sand lacks.

Sand to be used in moulding should be properly

conditioned before use in order to obtain good castings,

since most of the defects, which occur in castings, are due

to improper conditioning of the sand. It holds good

equally for the new as well as old or used sand. Proper

conditioning means the uniform distribution of the clay

bond over the sand grains, even distribution and proper

control of the moisture content in the sand and sorting out

the foreign materials like nails, gaggers and other metal

pieces from the sand by ridding and a thorough mixing of

the entire sand mass.

Even today the above operation is carried out by hand

in most of the small foundries. Since no testing equipment

is normally available in such foundries, the sand condition

is judged by the moulders themselves by virtue of their

practical experience only and the quality of the castings

produced in such foundries entirely depends upon this

factor. A common physical test, which is generally

followed by most of the moulders, for judging the sand

condition is to grip a handful of the prepared foundry sand

and then relieve the pressure of the fingers. The sand

mass thus produced is broken into two pieces by hand and

the edges formed at the broken section are carefully

observed. If there is no deformation in the edges the sand


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is supposed to be properly conditioned. If the upper

surface of the broken pieces appears to be setting down

gradually, as if it is being compressed, it indicates a

high moisture content. Gradual separation of sand grains,

as if they are being sprinkled from the parted surfaces,

indicate a weak bond and low moisture content. Mixing of

sand by hand is performed by first collecting the sand,

together with the other constituents to be mixed in it, in

the form of a help and then pouring adequate amount of

water on to it. After keeping it as such for some time it

is turned upside down by means of a shovel and the

operation repeated several times to ensure through mixing

of different constituents. It is then riddled to remove

the forming material from it and thus it is ready for use.

Q.7. Write short notes on :-

(1)Mould hardness test(2) Core hardness test

Ans.: (1) Mould hardness test.

The hardness of a sand mould can easily be tested by

means of a hardness tester. It is a very handy instrument

working on the principle of dryness hardness testing

machine. It carries a hemispherical ball or tip at its

bottom, which is penetrated into the mould surface. A

spring-loaded shaft inside the hollow body of the

instrument actuates the needle of the dial gauge fitted at

the top. The dial of this gauge provides direct reading of

the mould hardness.

(2) Core hardness test.

It is also a very simple and handy instrument used for

testing the hardness of dry sand cores, especially of the

dried oil-sand cores. It carries a cutter at its bottom,

which is provided with a pre-determined pressure, by means

of a spring inside the instrument, when it is pressed


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against the surface of the core. Corresponding to the

penetration the hardness is directly given by the scale

provided on the tester.

Q.8. What are crucible furnaces? Where are they preferred

and why?

Ans.: These are the simplest of all the furnaces used in

foundries. They are sparingly used in most of the small

foundries where melting is not continuous and a large

variety of metals is to be melted in small quantities. In

these furnaces the entire melting of metal takes place

inside a melting pot, called crucible, which is made of

clay and graphite. The sizes of these crucibles vary from

No.1 to No.400 each number representing a definite quantity

of metal that can be held conveniently by the crucible.

Q.9. How is the thermal efficiency of cupola is determined?

Ans. The thermal efficiency of the cupola is given by the

ratio of heat actually utilised in melting and superheating

the metal to the heat evolved in it through various means.

This ratio can be expressed mathematically as follows:

derecent cupola

= Heat utilised in melting and superheating the metal x 100

Cal. Value of coke + heat evolved due to oxidation of

iron,Si & Mn

In case of a hot blast cupola the above expression will

change as follows:

percent cupola

= Heat utilised in melting and superheating the metal x 100

Cal. Value of coke + Heat evolved due to oxidation of

iron,

Si and Mn + Heat supplied by the air

Blast


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Experiments reveal that the thermal efficiencies of

different cupolas normally range between 30 to 50 per cent.

Q.10. Explain in brief causes and remedies of the following

casting defects

Ans. (1) Blow holes:- They appear as cavities in a casting.

When they are visible on the upper surface of the casting,

they are called open blows. These blows are normally

rounded and have smooth walls. When they are concealed in

the casting and are not visible from outside, they are

known as blowholes. They are due to the entrapped bubbles

of gases in the metal and are exposed only after machining.

Possible causes :

1.Excess moisture content in moulding sand-leading to the

production of too much of steam and thereby

rendering the permeability of the mould as inadequate.

2.Cores not sufficiently baked.

3.Use of rusted or highly moistened chills, chaplets or

other metal inserts-giving rise to the production of a

high amount of steam and gases.

4.Excessive use of organic binders-resulting in the

production of high amount of gases.

5.Cores not adequately vented-resulting in their low

permeability.

6.Moulds inadequately vented resulting in their low

permeability.

7.Moulds rammed very hard-more addition to low

permeability.

Remedies :

1.Moisture content in the moulding sand should be properly

controlled.

2.Cores should be adequately backed.

3.Chills, chaplets and metal inserts used should be clean

and free from rust or any other gas producing substance.


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4.Organic binders should be used with restraint.

5.Cores and moulds should be adequately vented.

Moulds should not be rammed excessively hard.

(2) Shrinkage :- During solidification of metal, there is a

volumetric shrinkage. This should be adequately compensated

by feeding failing which voids will be produced in the

casting. These voids may exist on the surface as

depression, called surface shrinkage, or within the casting

called internal shrinkage. Too much shrinkage may lead to

crack, known as hot tears. This defect occurs on account

of inadequate and improper gating, risering and chilling so

that proper directional solidification does not take place.

As such it can be remedied by adopting corrective measures

in respect of the above factors.

(2)Hot tears :- They are also known as pulls or hot

cracks. The main reasons of their occurrence is the low

strength of metal after solidification, causing the

metal to fail in coping up with the excessively high

stresses set up by the solid shrinkage of the metal.

These cracks may be external or internal. They are

supposed to be more harmful when they are present

internally, because in that case their occurrence is not

revealed without machining or radiographic testing.

Their presence is identified by an oxidised surface

showing an irregular and ragged appearance on fracture.

The main reasons of their occurrence are lack of

collapsibility in the core and mould, faulty design

leading to exceptionally high residual stresses at

certain portions in the casting and very hard ramming of

sand resulting in restricted contraction of casting. An

improvement over these shortcomings will help

elimination of hot tears.


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Q.11. Describe the following types of sands in respect of

their composition, particular properties and uses

Ans: Loam Sand :- It is a mixture of clay and sand with

water to a thin plastic form and from which moulds are

built. It contains moisture 18-20% and the loam is dried

very slowly. It is used for producing larger castings.

A typical mixture of loam sand consists of

Floor coke 10 Vol

Loam sand 10 Vol

New sand 6 Vol 80% mixture + 20% moisture

Silica sand 22 Vol

Clay 5 Vol

Other gradients 5 Vol

1.Parting Sand :- Conventionally, mould is prepared into

two or more boxes. These boxes are to be separated

without adhering to each others sand. For this purpose

parting sand is used. One of the parting compounds is

lycopedium, which is used when oil is mixed with moulding

sand. Very fine brick powder can also be used.

2.Green Sand :- It is the sand in green condition and after

preparing the mould, casting (pouring of molten metal) is

done in moist state. While preparing the mould, the

rammed sand is dense but porous and further the structure

is made porous by venting. Green sand is generally used

for small or medium sized casting.

(a) Mixture of green sand for light work purpose contains

Floor sand ...... 80%

New sand........ 13.5% 95% mixture + 5% moisture

Super fine coal dust ... 6.5%

(b) Mixture of grey n sand for general purpose contains

Floor sand.. 60%

New sand .. 30% 95% mixture + 5% moisture

Coal dust .. 10%


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(c) Mixture of green sand for high finish castings

contains

Floor sand 51%

New sand 23%

Coal dust 8.5% 95% mixture + 5% moisture

Carbon blacking 8.5%

Talc 2.8%

3.Backing Sand :- Backing sand or floor sand is used to

back up the facing sand and to fill the whole volume of the

flask. Old, repeatedly used moulding sand is mainly

employed for this purpose.

The backing sand is sometimes called black sand

because of the fact that old, repeatedly used moulding sand

is black in colour due to the addition of coal dust and

burning on coming in contact with molten metal.

Q.12. Describe the utility of following moulding tools and

give neat sketches of each.

Ans. Bellows :- A hand operated bellow is shown in Fig.

It is used to blow but the loose or unwanted sand from

the surface and cavity of the mould.

Hand Riddle :- It consists of a wooden frame fitted with a

screen of standard wire mesh at its bottom. It is used for

hand ridding of sand to remove foreign material from it.


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Rammer:- A hand rammer is a wooden tool used for

packing or ramming the sand into the mould. One end,

called the peen, is

wedge shaped, and the opposite end, called the butt, has a

flat surface. Floor rammers are similar in construction but

have long handles. Pneumatic rammers are used in largemoulds saving considerable labor and time.

Q.13. What do you understand by casting? List the merits

and demerits of casting process.

Ans. Casting is one of the most versatile form of

mechanical process for producing components; casting is a

replica of pattern, in metal which is obtained by pouring

molten metal into the mould.

Principal of casting consists of introducing the

molten metal into a cavity or mould of the desired shape

and allowing it to solidify. When it is removed from mould,

the casting is of same shape but slightly smaller due to

contraction of metal. The molten metal passes through the

four stages i.e. liquid stage, musy stage, plastic stage,

and solid stage till the solidification takes place.

Today we have a variety of moulding processes and melting

equipments, thus we are capable to produce castings of

different, materials and their alloys. Though, there is a

tremendous improvement in the production methods, but the

basic principles are still the same. One can realise the

importance of castings and their role in modern


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development. It is difficult to visualise any product

which do not have one or more casted parts in different

sizes. Though there are other metal shaping process such

as metal-machining, metal-forging, stamping which can

fulfil the needs of the community. But casting caries

inherent advantages, which have made it as the backbone of

industrial production.

Design Advantages

(a) Size Casting can be prepared upto 200 weight and the

least size that can be made is weighing few grams. Its

advantage lies with the production of massive objects in

one piece.

(b) Complexity Most simple/complex shaped products can

be prepared by casting easily. Such production depends

on the preparation of pattern and mould. Complicated

shapes cannot be easily produced by other shaping

methods.

(c) Weight Saving Since the metal can be placed at the

exact location where it is needed, thus lot of metal can

be saved by adopting this process.

(d) Production of Prototype It is capable to produce

prototype models/exact product as desired.

(e) Wide Range of Properties This process offers a large

range of mechanical and physical properties in the

castings as per requirement. Usually the use the metal-

alloys is one variable.

Advantages of Casting Process

(a) Low cost It is usually found to be the cheapest

method of metal shaping.

(b) Dimensional Accuracy Tolerances as close as 0.1 mm

can be achieved depending on metal to be casted, casting

process, shape and size of casting. Surface finish can

also be controlled from 5 microns to 50 microns.


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(c) Versatility in Production This process is adaptable

to all types of production.

Metallurgical advantages

(i) Fibrous structure.

(ii) Controlled grain size,

(iii)Uniform density.

Merits of casting:-

1.Versatile form of mechanical process for producing

components.

2.There is no limit to the size and shape of the articles

that can be produced by casting.

3.Casting offers one of the cheapest methods and gives high

strength and rigidity even to intricate parts, which are

difficult to produce by other methods of manufacturing.

Demerits :-

1.Casting is not always the best method of the various

production techniques.

2.Metals having good fluidity and small shrinkage can only

be casted in a best way.

Q.14. Which factors need to be considered in selecting a

particular type of pattern? Explain split pattern, sweep

pattern and match-plate pattern with the help of neat

sketches

Ans. Factors affecting selection of a particular tape of

pattern.

The type of pattern to be used for particular casting

depends upon many factors like

1.The bulk of casting i.e. whether a small or large number

of casting is wanted.

2.Ease or difficulty of moulding operation.

3.Type of moulding process.

Split pattern :- Many times the design of casting offers

difficulty in mould making and withdrawal of pattern, if a


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solid pattern is used. For such castings, split or two

piece patterns are employed. They are made in two parts,

which are joined at the parting line by means of dowels.

While moulding one part of the pattern is contained by the

drag and the other by the cope.

Match plate patterns :- These patterns are used where a

rapid production of small and accurate castings is desired

on a large scale. Their construction cost is quite high,

but the same is easily compensated by a high rate of

production, greater dimensional accuracy and minimum

requirement for machining in the casting. These patterns

are made in two pieces; one piece mounted on one side and

the other on the other side of a plate, called match-plate.

The plate may carry only one pattern, or a group of

patterns mounted in the same way on its two sides. The

plate may be of wood, steel magnesium or alluminium. Gates

and runners are also attached to the plate alongwith the

pattern.

Sweep pattern :- Sweeps can be advantageously used for

preparing moulds of large symmetrical castings,

particularly of circular cross-section. This effects a

large saving in time, labour and material. The full


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equipment consists of a base, suitably placed in the sand

mass, a vertical spindle and a wooden template, called

sweep. The outer end of the sweep carries the contour

corresponding to the shape of the desired casting. The

sweep is rotated about the spindle to form the cavity.

Then the sweep and spindle are removed, leaving the base in

the sand. The sweep and spindle are removed, leaving the

base in the sand. The hole made by the removal of spindle

is patched up by filling the sand.

Q.15. What is pattern? How does it differ from the actual

product to be made from it?

Ans. A pattern may be defined as a replica or facsimile

model of the desired casting which, when packed or embedded

in a suitable moulding material, produces a cavity called

mould. This cavity, when filled with molten metal,

produces the desired casting after solidification of the

poured metal. Since it is a direct duplication, the

pattern very closely conforms to the shape and size of the

desired casting, except for a few variations due to the

necessary allowances. The ways in which a pattern differs

from an actual component are :

1.It carries an additional allowance to compensate for

metal shrinkage.

2.It carries additional allowances over those portions,

which are to be machined or finished otherwise.


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3.It caries the necessary draft to enable its easy removal

from the sand mass.

4.It carries additional projections, called coreprints, to

produce seats for cores.

Q.16. What is permeability? What is the effect of Moisture

and clay content permeability?

Ans. It is also termed as porosity. It is that property of

the sand, which allows the gases and steam to escape

through the sand mould. When the hot molten metal is poured

in the mould a very large volume of gases and steam is

formed due to heating to moisture, coal dust, oil and

similar other materials present in the sand. If these

gases are not allowed to go out they will either make the

casting unsound or blast the mould. Therefore, this is

very important property required in the moulding sand. It

largely depends upon the same grain size and shape and the

proportion of moisture and clay present in the sand.

Rounded grains of uniform size lead to a high permeability.

This property is also effected by ramming of sand. A soft

ramming will increase the permeability and hard ramming

will reduce it. In practice it is further increased by

applying vent wires in the prepared mould.

Q.17. Write note on Skeleton pattern

Ans. Skeleton pattern :- When the size of the casting is

very large, but easy to shape, and only a few numbers are

to be made, it is uneconomical to make a large solid

pattern of that size. In such cases, a pattern consisting

of a wooden frame and strips is made, called skeleton

pattern, it is filled with loam sand and rammed. The

surplus sand is removed by means of a strickle. The core

can be prepared separately, either with the help of


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a core box or another skeleton made for that, and assembled

in position in the mould.

Q.18. Give reason for Rat-tails or buckles in casting.

Ans. 1) Continuous large flat surface on casting.

2) Excessive mould hardness.

3) Lack of combustible additives in moulding sand.

Q.19. What care is to be taken in operating cupola?

Ans. The following considerations should be made for

operating the cupola successfully:

1.A superior refractory lining should be used to withstand

high temperature produced inside the furnace.

2.The man who fires the coke and charge should place the

metal charge in the centre.

3.The molten metal should be tapped out well before its

level rises too high in the well.

4.The tap hole should be property closed by means of a

well-prepared clay bolt or plug.

5.In closing the tap hole care should be taken to press the

plug downward in the hole so that the splash of the

molten metal does not fall on the hands.

6.The amount of air supply should be property controlled.

An excess amount of air will result in lowering to

temperature inside.

Q.20. Define

(a)(i) Pattern(ii) Mould


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iii) Core Baking : Generally baking is carried out in

ovens equipped with drawers, shelves or other holding

devices. The operation is generally continuous and cores

are put either in batches or continuously over or moving

shelves. The heat in oven is produced by burning oil or

coke or by electric resistance. Core-baking time depends

upon the types and quantity of binder used, the amount of

moisture used in sand, and size of core. The temperature

for baking depends on the core material used. When cores

are baked, they are more easily supported on a flat

surface, which should be incorporated in the design.

iv) Finishing : After receiving them from ovens, the

cores are properly finished by rubbing or filing, etc. to

bring them to correct dimensions, remove extra sand

projections from their surfaces and provide a good

surface finish. Then only they become suitable for being

placed in the moulds.

(c) (i)Sound test : It consists of suitably suspending the

casting, free of floor and all other abstractions, and then

gently striking it with hammer. The sound produced is

carefully noted. Tapping by the hammer is done at

different points and a change in the pitch and quality of

sound indicates a discontinuity within the mass of the

castings. However, it is difficult to locate the

discontinuity and the extent to which it is present.

Types of cores


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(i) Magnetic particle testing : This method can be used

only for those metals and alloys, which can develop

magnetic properties, e.g. iron and steel. The principle

involved in this test is that in a magnetised metal if

its magnetic field is interrupted by a crack its

continuity is broken. Due to low magnetic permeability

of air some magnetic flux lines leak out of the metal,

and if a magnetic material is spread over that portion

some of it is held there by the flux lines to show the

presence of a crack or void there. So, for this test,

the casting is first magnetised and then fine particles

of iron or steel are spread over its surface. The

presence of cracks is revealed by the help up particles

on the surface.

(ii) Penetrant testing : This method is used to detect

small surface cracks and can be conveniently used for all

metals and alloys. It consists of applying a thin

penetrating liquid over the surface of the casting,

allowing it to penetrate into the cracks by capillary

action and then cleaning the whole surface which draws

back some of the liquid on to the surface. The surface

is then exposed to an ultraviolet light where the

presence of liquid is clearly seen, indicating a crack

there.

Q.21. What do you understand by gravity die-casting?

State its advantages.

Ans.Advantages:

1)It is a very speedy process and each cast takes between

2 to 4 minutes time only.

2)Permanent moulds have a very long life in as much as

one mould can be conveniently used for producing

between 3,000 to 10,000 castings in cast iron and

between 10,000 to 25,000 castings in alluminium.


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3)Surface finish through this method is better than sand

castings but inferior than pressure die-castings.

4)Dimensional tolerances of the order of 0.508 per 100

mm. can be conveniently obtained.

5)For the same amount of production it requires less

floor area than sand casting.

6)Less skill is required of the operator than in sand

casting.

7)Many of the defects found in sand castings are

eliminated totally.

8)Castings produced through this method are found to have

a finer grain structure.

9)Castings in large quantities can be produced more

economically.

10) A minimum thickness of 2.4 mm. Can be easily cast.

Q.22. Explain with the help of neat cross sectional sketch

of cupola, indicating its various zones.

Ans. Various zones of cupola are shown in Figs.

A number of chemical reaction take place in these zones

which are explained below :

1.Well :- It is the space between the bottom of the tuyeres

and the sand bed. The metal, after melting,, trickles

down and collects in this space before it is tapped out.

2.Combustion zone :- It is also known as oxidising zone.

It is located between the top of the tuyeres and a

theoretical level above it. The total height of this

zone is normally from 15 cm. To 30 cm. The actual

combustion takes place in this zone, consuming all free

oxygen from the air blast and producing a lot of heat,

which is sufficient enough to meet the requirements of

other zones of cupola. More heat is evolved due to

oxidation of silicon and manganese. A temperature of

about 15400C to 18700C is produced in this zone. The


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exothermic reactions taking place in this zone can be

represented thus.

3.Reducing zone :- It is also known as the protective

zone. It is located between the top of the combustion

zone and the top level of the coke bed. CO2 is reduced

to CO in this zone through an endothermic reaction, as a

result of which the temperature falls from combustion

zone temperature to about 12000C at the top of this zone.

The reaction is as follows :

Nitrogen, the other main constituent of the

upward moving hot gases does not participate in the

reaction. This zone, on account of the reducing

atmosphere in it, protects the charge against oxidation.

4.Melting zone :- The first layer of metal charge above the

coke bed constitutes this zone. The solid metal charge

changes to molten state in this zone and trickles down

through the coke to the well. The molten metal picks up

sufficient carbon content in this zone as represented by

the following reaction :

5.Preheating zone :- It extends from above the melting zone

to the bottom level of the charging door and contains a

number of alternate layers of coke and metal charges.

The function of this zone is to preheat the charges from

atmospheric temperature to about 10930C before they

settle downwards to enter the melting zone. This

preheating takes place due to the upward advancing hot

gases, from which the solid metal also picks up some

sulphur content.

6.Stack :- The empty portion of cupola above the preheating

zone, which provides the passage to hot gases to go to

atmosphere, is known as stack.

Q.23. Explain the common allowances provided on patterns.

Ans. Pattern Allowances


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(i) Shrinkage:- When any metal cools, it naturally

shrinks in size. Hence, if the actual object itself is

used for the pattern, the resulting casting would be

slightly smaller than desired. To compensate for this

possibility, a shrink rule is used in laying out of

measurements for the pattern. A shrink rule for cast

iron is 10 mm per meter (the average shrinkage for cast

iron) longer than the standard rule.

When metal patterns are to be cast from the original

patterns, double shrinkage must be allowed.

(ii) Draft :- When a pattern is drawn out from a mould, the

tendency to tear away the edges of the mould in contact

with the pattern is greatly decreased if the surfaces of

the pattern are given a slight taper in a direction

parallel to which it is being withdrawn. This tapering

of the sides of the pattern, known as draft, is done to

provide a slight clearance for the pattern as it is

lifted up. The amount of draft on exterior surfaces is

about 10 to 20 mm per meter. On interior holes, which are

fairly small, the draft should be around 30 mm per meter.

(iii)Finish :- When a draftsman draws up the details of a

part to be made each surface to be machined is indicated

by a finish mark. The mark indicates that additional

metal must be provided at this point so that there will

be some metal to machine. The amount that is to be added

depends upon the size, shape of casting, but in general,

the allowance for small castings and average sized

castings is 3 mm.

(iv) Distortion :- This allowance applies only to those

castings of irregular shapes, which are distorted in the

process of cooling as a result of metal shrinkage. Such

an allowance depends on the judgment and experience of

the pattern maker, who understands the shrinkage

characteristics of the metal.


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(v) Shake :- When a pattern is rapped in the mould before

it is withdrawn, the cavity in the mould is slightly

increased. In an average sized casting this increase in

size can be ignored. In large castings or in one that

must fit without machining, however, shake allowance must

be considered by making the pattern slightly smaller.

Q.24. Discuss the advantages split and multi-piece patterns.

Ans. Following are the advantages of split and multi-

piece patterns.

1.Complicated designs can be constructed in these types of

patterns.

2.They facilitate easy withdrawal from cope and drag box.

3.These pattern are easy to contact as compared to solid or

single piece pattern.

Q.25. Why testing of foundry sand is necessary? What are the

common tests performed on foundry sands?

Ans. In progressive foundries it is recognized that the

foundry sand deserves as much attention as the casting

metal. The foundry sand may account for one-third of the

cost of the finished casting. In modern mass production of

sand castings, the moulding sand, which constitutes the

chief moulding material, is therefore, required to be

tested periodically in order that control of its

composition and properties may be maintained. Test may be

either chemical or mechanical. Chemical tests are used

only to determine the undesirable elements in the sand, and

in most cases mechanical tests are employed.

The essential mechanical tests include fineness,

moisture content, clay content, permeability, strength in

compression, and mould hardness.


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Q.26. What are the factors, which should be considered

before designing a casting?

Ans. The important factors to keep in mind when designing a

casting to obtain maximum strength and minimum casting

include :

1.Design for directional solidification.

2.Design for minimum stresses.

3.Design for metal flow.

4.Cast-well design.

5.Design for minimum casting.

6.Design for expected tolerances.

7.Functional design.

Q.27. Write short notes on the following casting defects.

1)Cuts and washes2)DropsAns. 1) Cuts and washes :- These defects occur due to the

erosion of sand from the mould or core surfaces by the

molten metal. The cavities formed on the mould and core

surfaces due to this erosion are filled by the molten metal

and the same appear on the casting surface as an surface as

an excess metal in the form of ragged spots. These spots

are called scabs. The eroded sand appears as a sand

inclusion some-where else in the casting. These cuts and

washes take place due to insufficient strength of mould and

core, lack of binding material in the facing and core sand

and faulty gating. Obviously, the remedy of the defect

lies in adequate ramming, additional of sufficient binders

in facing and core sands and improved gating system.

2) Drops :- This defect appears as an irregular

deformation of the casting. It occurs on account of a

portion of the sand breaking away from the mould and

dropping into the molten metal. The above breaking takes

place due to low green strength in the sand, too soft


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MP -I Unit - 1 31

ramming, insufficient reinforcement of the cope or other

sand projections. Increase in green strength of the sand

by suitable modification in its composition, hard ramming

and adequate reinforcing of cope and other sand projections

by means of bars, nails and gaggers etc. are the principal

remedies of this defect.

Q.28. What are the factors which govern the selection of a

proper material for pattern making.

Ans. Factors effecting the selection of pattern material:-

The selection of a particular material for making the

pattern is influenced by the following factors : -

1.Number of castings to be made.

2.Method of moulding to be used, i.e., hand or machine.

3.Type of casting method to be used.

4.Degree of accuracy in dimensions and the quality of

surface finish required on the castings.

5.Design of casting.

Q.29. How are the patterns classified ? Explain the use of

solid pattern.

Ans: On the basis of material used in construction of

patterns, they are classified as :

(1) Wooden patterns

(2) Metal patterns

(3) Plaster patterns

(4) Plastic patterns

(5) Wax patterns

(1) on the basis of number of pieces used in construction,

patterns are classified as

(1) Solid or single piece pattern

(2) Two piece or split pattern

(3) Multi-piece pattern

(4) plate pattern


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Solid or Single piece pattern :-

A single piece pattern is the simplest of all the

patterns, is made in one-piece and carries no joint,

partition or loose pieces. Depending upon the shape, it

can be moulded in one or two boxes. This pattern is the

cheapest but its use can be done to a limited extent of

production only since its moulding involves a large number

of manual operations like gate cutting, providing runners

and risers and the like.

Q.30. Write short notes on

1) Functions of a pattern

2) Core boxes

Ans. 1) Functions of a pattern :-

The main functions of a pattern are :

(i) To produce the mould cavity of appropriate shape and

size in which the molten metal can be poured to obtain

desired casting.

(ii) To produce seats for cores in the mould in which cores

can be placed to produce cavity in the casting. These

seats in the mould are called coreprints and the

corresponding projections on the pattern, which

produce these seats, are also known as coreprints.

(iii)To establish the parting surfaces and lines in the

mould.

(iv) To establish distinct locating points in the moulds of

which the corresponding points on the casting are used

as reference points, for checking the casting


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dimensions and relative location of machined and other

surfaces.

(v) To minimise defects in castings.

(vi) To enable production of green sand or rammed-up cores

within the mould itself.

(vii)To minimise the cost of casting.

2) Core boxes :-

Core boxes are used for making cores. They are either

made single or in two parts. Their classification is,

generally, according to the shape of the core or the method

of making the core. The common types of core boxes are the

following :-

(i) Half core box :- To prepare the core in two halves

which are later on cemented together to form the

complete core.(See fig. )

(ii) Dump core-box :- Used to prepare complete core in it.

Generally, rectangular cores are prepared in these

boxes.(See fig.)


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(iii)Split core-box :- It is made in two parts, which can

be joined together by means of dowels to form the

complete cavity for making the core.(See fig.)

(iv) Strickle type core-box :- It is used to form cores of

irregular or unsymmetrical shapes, as shown in fig.

(v) Loose piece core-box:- It is used to prepare, in the

same core box, the two halves of a core of which the

halves are not identical in shape and size. (See fig.)

Q.31. What are the factors which govern the choice of a

particular type of furnace for melting a particular metal?

Loose piece core box


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Ans. The choice of a particular type of furnace is largely

based on the following factors :-

1.Rate of melting desired, depending upon the quantity of

metal required to be melted per hour.

2.Type of metal to be melted.

3.Temperature required.

4.Capability of melting medium for absorbing impurities.

5.Method of pouring the molten metal.

6.Economic considerations, i.e. initial investment to be

made as cost of equipment and its installation,

maintenance cost and cost of fuel to be consumed, etc.

Q.32. What is a hot blast cupola? What are its advantages?

Q.33. Explain the causes and remedies of following casting

defects.

Ans.

Sr.

No

Defects Possible courses Remedies

1 Fusion a)Low refractoriness in

moulding sand.

b) Faulty gating.

c) Too high pouring

temperature of metal.

d) Poor facing sand.

(a)Improve refractoriness.

(b)Modify gating system.

(c)Use lower pouring

temperature.

(d) Improve quality of

facing sand.

2 Short

metal

(a)Too low pouring

temperature.

a)Excess sulphur

Content in metal.

(a)Faulty gating.

(b)High moisture

content in moulding

sand.

(a)Use higher pouring

temperature.

(b)Reduce sulphur content.

(c)Modify gating system.

(d)Reduce moisture

content.


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3 Shift (a)Worn-out or bent

Clamping pins.

(b)Misalignment of

Two halves of pattern.

(c)Improper support

of core.

(d)Improper location of

core.

(e)Faulty core boxes.

(f)Insufficient

strength of moulding

sand and core.

(a)Repair or replace the

pins.

(b) Repair of replace

dowels causing

misalignment.

(c) Provide adequate

support to core.

(d) Locate the core

properly.

(e) Repair or replace

the core boxes.

(f) Increase strength

of moulding sand and core.

Note :- There is always a possibility of asking question on

Casting defects, their causes and remedies. Students are

advised to Refer table 11.1 containing total 18 defects in

Workshop technology byB.S.Raghuwanshi.

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MP -I Unit - 2 37

UNIT NO.2

Q.1. (a) Define the following terms.

1)Drag

2)Sprue

3)Parting line

4)Gate

(b) Discuss the following casting defects.

i) Sand Spots

ii) Run Out

Ans. 1) Drag:- Sand moulds are prepared in specially

constructed boxes called flasks. The purpose of the flask

is to import the necessary rigidity and strength to the

sand in moulding. They are usually made in two parts, held

in alignment by dowel pins. The top part is called the

cope and the lower part.

2) Sprue :- A vertical passage through the cope and

connecting the pouring basin to the runner and gate is

called Sprue. Conventionally a sprue should be tapered

with larger end to receive the molten metal and to act as

reservoir. Round sprue is preferred upto 20 mm dia, but

longer sprues may be rectangular (less turbulence in

rectangular sprue). The circular sprue has minimum surface

exposed to cooling and offers the least resistance to

flow).

3) Parting line :- It is the line along which the sand

surfaces of the drag and cope join each other.

4) Gate :- It is an opening through which the molten-

mental flows from runner to mould cavity. The size, and

location of gates are so arranged that the mould cavity can

be filled as quickly as possible without cutting the mould

surfaces, further crack in metal after solidification

should not develop.


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Vaccum die casting machine :- Complete evacuation of air

from the die prior to metal injection is a vital necessity

for preventing the air entrapment in the casting.

This difficulty is overcome in the vaccum die casting

machine. Some modifications are made in hot chamber die

casting machine to get vaccum die casting machine. The

additional equipment required in this case consist of an

encasement ground the die blocks, compressible seals at

the top and bottom of this encasement and a pipe connecting

the encasement to the vaccum pump through a valve and

vaccum tank. An additional plunger, adjacent to the main

plunger, for elasing the part, when vaccum is applied tothe die to prevent the molten metal

From being drawn into the die.

Q.3. What is the utility of various types of furnaces used

in foundry shops? Describe one widely used furnace with

special reference to its parts, working and other features.

Draw a neat sketch of the furnace also.

Ans. The main utility of various furnaces used in foundries

is for melting of various varieties of ferrous and non-

ferrous metals and alloys.

Describe Cupola furnace for 2nd

part of question.

Q.4. Write short notes on the following

(Covering main aspects only).


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(i) Inspection of castings(ii) Modernisation and mechanisation of foundries.

Ans: Inspection of castings :-

It serves two purposes (1) The rejection of the castings

which do not meet the specifications and (2) maintaining

the quality of workmanship.

There are many methods used to check the quality of

castings. Most of the methods mentioned below.

(i) Visual inspection.

(ii) Dimensional inspection.

(iii)Mechanical testing and chemical testing.

(iv) Defects in internal flows by non-destructive tests.

(v) Metallurgical testing.

(ii) Modernisation and mechanisation of foundries.

The term mechanization means substitution of

machinery to perform the operations which were otherwise

performed by hands. Such a substitution can be made of

sand preparation, moulding and core-making, pouring,

material handling and many other similar operations. Such

foundries, where machines have been employed to replace

hand operations, are called mechanized foundries. However,

the decision to switch over to mechanization in any foundry

is largely governed by economic considerations. The extent

to which it can be adopted in any foundry depends

considerably on the quantity and type of production.

Larger the production, more is the scope for mechanization.

This scope is further enhanced if the sad production is of

identical components on mass scale.

Advantages of mechanization

1.From the same floor area much higher production can be

achieved.

2.A large saving in labour and time is effected by doing

away with a number of laborious hand operations, like

sand preparation, mould making and material handling etc.


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3.It creates more hygienic and healthy working conditions

for the shop workers and effects an increase in their

earnings through higher production.

4.It improves the quality of the castings produced by

facilitating closer dimensional tolerances, minimising

defects and providing better surface finish.

5.Overall production cost is reduced due to faster rate of

production and elimination of a large number of laborious

hand operations.

Q.6. What is permanent mould? Specify its advantages and

disadvantages?

Ans. When the mould made from metals like C.I. or steel

then, while casting such mould is not destroyed or rebuilt

after every casting. Since can be use for long period such

mould is known as permanent mould.

Advantages of permanent mould

1)Increases the speed of casting process.

2)Have very long life.

3)Results in better surface finish than sand casting.

4)Castings in large quantities can be produced

economically.

5)Casting method requires less skill and at same time

number of rejections are less.

Disadvantages of permanent mould

1)These moulds are much costlier than sand mould.

2)It can be successfully used for casting very high

temperature alloys

3)Gates, runners and risers can not be shifted and

positioned any where at will.

4)May produced several defects in casting like stress and

surface hardness due to surface chilling effect.


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Q.7. Sketch cross section through permanent mould showing

its principal parts. Describe its construction in detail.

Ans. Construction of permanent mould:

Generally these moulds are made in two halves, parting

surface of which is in a vertical plane. Cores may be

designed as part of the mould itself known as fixed cores

or fitted separately known as movable cores. For easy and

quick removal of movable cores separate mechanism is

incorporated. Clamping arrangement is used to avoid mould

to open under the hydrostatic pressure of molten metal.

Mould generally comprises of several blocks joined together

as shown in figure.

The form block and base block together form the actual

mould cavity whereas the runner block incorporates the

runner and riser. Runner and riser are generally kept on

parting line.

Q.8.What is gravity die casting? Explain with neat sketch.

Q.9. Explain the difference between gravity die casting and

pressure die casting.

Ans.

- In gravity die casting pouring is done simply due to

gravity and no external pressure is applied. But for

pressure die casting external pressure is applied to free

the molten or semi molten metal in to the die cavity.

- In P.D.C. the pressure is applied to the force the fluid

in die cavity. The fluid alloy fills the entire die


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including all minute cavities. Hence intricate can be

produced successfully but as compared to G.D.C.

- We get better dimensional tolerance and better surface

finish in P.D.C. compared to G.D.C.

- P.D.C. can be made fully or semi automatic.

- P.D.C. metal in semi molten state can be cast which not

possible in G.D.C.

- As the arrangement to develop the pressure is needed in

case of P.D.C. it needs some costly equipments which

increases the build up cost for P.D.C. Hence the

equipments use for P.D.C. are costlier than G.D.C.


(i) Repair of castings(ii) Pressed casting Ans: (i)Repair of castings:- When casting gets damaged or

found defective particularly in case of large castings, it

is not economical to reject it and recast it. In such case

we can repair it by proper method mostly by welding which

save time as well as money. Depending upon the casting

material, type of defect, we can use proper welding method

to repair defect. In most cases, bend or warped castings,

if slightly out of shape also possible to bring back to

original shape by hammering with soft hammers by hand,

jacks or by presses.

(ii)Pressed casting :- In this type fixed amount of molten

metal poured into the permanent mould and then close

fitting cores are pushed in the cavity, by this molten

metal force into the mould cavity. Cores are removed after

metal sets into cavity. We get thin walled hallow casting.

This method is limited for ornamental articles.

Q.10. Explain Jamming of cupola.

Ans. Jamming of cupola may be permanent or temporary.


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If the molten metal is not taped out before its level

rises to high in the well, the slag which floats on the

surface of molten metal, will start flowing into wind belt

through the tuyeres and air passage will be choked and the

cupola jammed. Thus, the furnace is put to an unusable

condition then it is known as permanent jamming. Iron and

slag around the tuyeres openings get solidify. Due to the

low temperature at the tuyeres openings which results in

the closing of air passage and supply of air is temporarily

stopped termed as temporary jamming. This can be prevented

by frequent poking of this solidified material by poking

bar, through tuyeres.

Q.11. Explain the process of sand preparation and

conditioning.

Ans. Following steps are carried out for sand preparation

and conditioning :-

Sand found in nature doesnt content all required

qualities in required extent necessary for proper moulding.

Hence some additives are mixed with sand. Generally clay,

lime, magnesia, potash, soda, horse manure, saw dust, cow

dug, coal dust etc. used in small quantities.

This additives are mixed by hand or by mixing machine

which ensures uniform distribution of clay, moisture and

other constituent between sand grains.

Then adequate amount of water is poured over sand,

then the sand turned upside and downside by means of

shovel. This moistens the clay making it adhesive.

This mixture is riddled to remove the foreign

material.

Q.12. State the advantages and disadvantages of die

casting?

Ans.Advantages of die casting are


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alloys having high melting

point usually above 5000C.

are cast.

melting point usually below

5000C. are cast.

Requires operating pressure

of 300 to 1600 kgf/cm2.

Requires comparatively low

operating pressure below 150

kgf/cm2.

Usually 75 to 150 castings

per hour can be produced.

Usually 300 to 350 castings

per hour can be produced.

Semi solid metals and alloys

can be cast.

Semi solid metals and alloys

can not be cast.

Usually nickel-chrome steel

is used for die.

Usually hardened and tempered

chrome-vanadium or chrome-

tungsten steel is used for

die.

Q.13. How permanent mould casting differ from sand casting?

Ans.

Permanent Mould Casting Sand Casting

1. Mould is a permanent one

and is neither destroyed

nor remade after each cast.

1. Mould is not permanent.

2. Requires less floor

space area.

2. Requires more floor space

area.

3. Moulds are costly. 3. Cost of mould is less.

4. Rate of production is

high.

4. Rate of production is

slow.

5. Economical for large

quantity production.

5. Used for small quantity of

production.

6. In order to enable and

easy and unrestricted

removal of casting, the

runner and riser are

normally kept on the

parting line.

6. The runner and riser can

be suitably positioned at

will.


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7. Better surface finish is

obtained.

7. Surface finish inferior

than permanent mould casting.

8. Less skill operator is

required.

8. Comparatively more skill

operator is required.

9. Eliminates many defects

found in sand casting but

need greater precaution

against chilling effect on

the casting surface.

9. No need of such precaution

against chilling effect on

casting surface.

Q.14. What is gravity die casting? Explain with neat

sketch?

Q.15. Sketch and explain the construction and operation of

hot chamber die casting machine.

Ans. In the hot chamber die casting machine the metal

melting unit forms an integral part of machine. It mainly

consist of hot chamber and a goose neck type metal

container made of cast iron.

Construction

This type of machine having goose neck type container

which always remains immersed in the metal pot.

Cylindrical shape is formed at the end of goose neck, a

plunger acts inside the cylinder. A port is provided near

the top of the cylinder. Goose neck injector is connected

to stationary die by nozzle and movable die can move to

from die casting and injecting the casting. Die is

provided with proper injecting mechanism.

Operation

As the plunger move upward the port get open and

molten metal enters into cylinder. Downward movement of

plunger closes the port and forces the molten metal inside

die cavity through nozzle. After solidification plunger

moves upward at the same time movable die move away from

stationary die to inject the casting.


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Injecting mechanism cause to inject the casting. Plunger

starts downward movement and movable die moves towards the

stationary die to form required casting. The cycle

is further repeated.


1) Slush casting2) Pressed castingAns: Slush casting :- Slush casting is a method of

producing hallow casting by using permanent moulds without

the use of cores. In this method the molten metal poured

in to the mould and allowed to solidify upto the required

thickness, then remaining molten metal made to fall out.

Because of this we cannot precisely control the thickness

of casting, hence this method is adopted for ornaments,

statues, toys and other novelties were controlled thickness

is not too important.

Pressed casting :- In this type fixed amount of molten

metal poured into the permanent mould and then close

fitting cores are pushed in the cavities, by this molten

metal force into the mould cavity. Cores are removed after

metal sets into the cavity and we get thin walled hallow

casting. This method is limited for ornamental articles.

Q.17. Explain with neat sketch, the construction and

operation of a die casting die?

Ans. Construction :-


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Die castings are made into halves, one is stationary

and other is movable. These two halves when closed have

vertical parting surface. Dowel pins are provided for

perfect alignment. Provision of vent for escape of air

into parting surface is usually provided. A proper

ejecting mechanism is provided. The die is so designed

that after solidification the casting will always cling to

the ejector die.

Operation :- Movable die moves and comes in contact with

stationary die. Movable die perfectly aligned with

stationary die, which forms the required die cavity. This

cavity is usually the required casting. Then the molten

metal is injected into the die cavity. After

solidification, inserted cores are firstly withdrawn. Then

the die opens, casting cling to the movable die.

Then either the movable half is slighted backward

over the ejector pins or the ejector plate attached to this

is advanced to project the ejector pins beyond the movable

die to eject the casting from the die.

Q.18. What are the common forms in which the die casting

dies are designed? Describe.

Ans. Die casting dies are generally designed in three

forms.

1)Single impression dies :- In this form die have single

cavity by this die only one casting at a time can

produce.


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2)Multi impression dies :- In this form die have more than

one die cavities. These die cavities are alike. By this

die, castings equal to the number of cavities in them can

be produced at a time.

3) Combination dies :- In this form die have more than onedie cavities, but these die cavities are not alike. By

this die, casting equal to the number of cavities in

them can be produced.

Q.18. Explain the working of a cold chamber machine with

the help of a diagram.

Ans. The working principle of a cold chamber machine is

illustrated in Fig.15.3. The word cold chamber is used

to denote horizontal cylinder into which the injection

plunger work. For these machines the metal is melted

separately in a furnace and transferred to these by means

of a small hand ladle. After closing the die the molten

metal is poured into the horizontal chamber through the

metal inlet. The plunger is pushed forward hydraulically

to force the metal into the die. After solidification, the

die is opened and the casting is ejected. The plunger is

again drawn back and the cycle repeated as usual for next

casting. These machines are widely used for casting a good

number of alluminium alloys and brasses which cannot be

cast in hot chamber machines as they require higher melting

points. Moreover, the chances of iron pick up by

alluminium are almost finished in these machines as it

takes place only at elevated temperatures, and also because

the molten alloy remains in contact with the steel cold

chamber and plunger for a very small period.

Q.19. State the advantages and disadvantages of die

casting?

Ans. Advantages of die castingare


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1. It requires less floor space as compare to other casting

processes.

2. Rate of production is high. 75 to 150 casts per hour in

cold chamber. 300 to 350 casts per hour in hot chamber

process.

3. Die casting dies retain their and more accuracy for a

very long time.

4. Very thin sections can be cast and Holes upto minimum of

1.6 mm.

5. High surface finish is obtained and often no further

finishing is required.

6. Cost per unit is minimum hence economical.

Disadvantages of die casting are

1.All metals and alloys cannot be cast.

2.The cost of machine dies and other equipment used is

high.

3.Not economical for small quantity production.

4.Heavy castings cannot be cast.

5.Special precautions are necessary for evacuation of air

from die cavity, otherwise cause porosity.

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MP -I Unit - 3 52

UNIT NO.03

Q.1. (a) Explain the method of carbon dioxide moulding with

its neat sketch.

Ans. In this method silica sand and 3.5 to 5 % Sodium

silicate liquid base binder, mixed for 3 to 4 minutes in a

muller. Silica sand should be clean, dry and free from

clay. To obtain collapsibility property some additives

like, wood, coal dust, flour, graphite etc are added.

Moisture content should not exceed 3%. This prepared

sand is put in to the mould (around the pattern) by any

convenient method.

After packing CO2 gas is forced into mould at a

pressure of about 1.41 kg/cm2, called gassing. CO2 reacts

with sodium silicate, following reaction takes place and

silica jel is formed.

Na2SiO3 +H2O+CO2 Na2CO3 +(SiO2+H2O)

(Sodium silicate) (Sodium (Silica jel)

carbonate)

Silica jel binds the sand grains together to provide

the strength and hardness to the Mould. Steps of carbon

dioxide moulding shown below.

Q.2.Describe the process of true centrifugal casting with

the help of neat diagram.


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MP -I Unit - 3 53

Ans. In this process, the castings are made in a

hollow, cylindrical mould rotated about an axis, common to

both casting and mould, the axis may be horizontal,

vertical or inclined. The mould used may be either of

permanent type or a sand lined mould usually end cores are

used to prevent the molten metal, thrown out from end.

Fig shows horizontal true centrifugal casting machine.

Having a large cylindrical mould for casting cast iron

pipes. The mould consists of an outer metallic flask

provided with rammed sand lining inside. The mould is

rotated between two sets of rollers, mounted on a shaft

driven by a variable speed motor. Pouring basin is formed

on the body of a trolley.

Molten metal is poured while Mould is rotating,

due to the centrifugal force metal is directed towards the

periphery. While pouring Mould is rotated at slower speed,

after pouring, speed is increased to effect even

distribution of the metal all along the inside surface of

the mould and proper directional solidification. After

solidification flask is replaced by new one and the process

is repeated.

The use of this process is limited only for

symmetrical shaped objects, such as pipes, rolls, cylinder

sleeves and liners, piston-ring stock, bearings bushing

etc.

Q.3. Explain the core with the use. What are the

characteristics of a good core?


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MP -I Unit - 3 54

Ans. A core can be defined as a body of sand,

generally prepared separately in a core box, which is used

to form a cavity of desired shape and size in a casting.

However, there are some exceptions to this definition. For

example in a pattern can be used to form a core as a part

of the mould, this being known as a green sand core.

Similarly, in permanent moulds or dies, the cores are

formed by the metallic moulds themselves as an integral

part of them. Cores which are prepared separately in core

boxes are called dry sand cores, and held and located in

the moulds in the seats formed by the core prints provided

on the patterns. The main characteristics required in a

good core are the

following :

1. It must be sufficiently permeable to allow an easy

escape to the gases formed.

2. It should be highly refractory to withstand the intense

heat of molten metal.

3. It should be enough hard and strong to bear its own

weight and the force of molten metal.

4. It should have high collapsibility i.e.; it should be

able to disintegrate quickly after the solidification of

the metal is complete.

5. It should not carry such constituents, which will give

rise to excessive gases on coming in contact with the

molten metal.

(The main ingredients of core sand mixtures

and their essential characteristics have already been

discussed in the last chapter.)

Q.4. What is investment costing? What are its main

advantages and disadvantages?

Ans. Following are the steps of investment casting:

1.First of all master pattern is made from wood or metal.


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2 By using gelatin or an alloy of low melting point and

master pattern, master mould is formed.

3.The master Mould is filled with liquid wax or by thermo

plastic polystyrene resin which when solidified forms a wax

pattern.

4.The wax pattern is coated with slurry consisting of

silica flour and small amounts of kaolin and graphite

mixed with water. This process referred to as the

investment of the pattern.

5.The pattern is then used to make up moulds similar to

those used in the conventional moulding process, but the

pattern within the mould is not taken out of the mould,

which is not opened after this moulding process.

6.Finished mould is dried in air for 2 to 3 hrs. and then

baked in an oven about 2 hrs. to melt out the wax or

remove the wax with the help of a solvent degreaser.

7.After this the mould is sintered at about 10000C to

improve its resistivity. Finally it is cooled down to a

temp. between 800 and 7000C for casting. The castings

are obtained by gravity, pressure vacuum or centrifugal

operations. After the metal is cooled the plaster is

broken away and gets feeders are cut out. The castings

so obtained are finally cleaned by sand blasting,

grinding or other finishing processes.

Advantages

1. Better dimensional accuracy, the normal tolerance being

0.005 mm.

2. Better surface finish

3. Thin sections of the order of 0.75 mm can be cast

4. Intricate machining of the casting is avoided

5. Castings are sound and have large grains as the rate of

cooling is slow.


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Disadvantages

1. Moulds used are single purpose, i.e.they can be used only

once.

2. It is expensive process and hence is adopted only where

small number of intricate and highly accurate parts are

to be manufactured.

3. This process is suitable for small size parts.

4. They present the same difficulty where cores are to be

used.

5. Process is slow.

Q.5. Which are the moulding machines used in practice?

Describe any one with the help of sketch if required?

Ans. Types of moulding machines:

The large variety of moulding machines that are

available in different designs can be classified as:

1. Jar or Jolt machine. This machine consists of an air-

operated piston and cylinder. The air enters from the

bottom side of the cylinder and acts on the bottom face

of the piston to raise it up. At the top of the piston

is attached the platen or table of the machine which

carries the pattern and moulding flask with sand over it.

When the piston, and hence the table, has been raised to

a certain height the air below the piston is suddenly

released, resulting in an even packing of sand around the

pattern in the flask. The operation is repeated several

times and quite rapidly. It is known as jolting.

2. Squeezer machine: These machines may be hand operated

or power operated. The pattern is placed over the machine

table, followed by the moulding flask. In hand-operated

mechanism whereas in power machines it is lifted by the

action of air pressure on a piston in the cylinder in the

same way as in jolt machine. The difference is that the

table is not dropped from height but is raised gradually.


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On the top of the machine column is provided an overhead

plate and the sand in the flask is squeezed between this

plate and the upward rising table. This enables a uniform

pressing of sand in the flask. A specific advantage of

power operated machines over hand-operated ones is that

more pressure can be applied in the former which

facilitates handling of a wider range of jobs.

Pin lift or push-off type machines. In these machines

the mechanism is almost similar to the above except that

the supporting ram is stationary and four pins are suitably

incorporated passing through the machine platen and the

pattern plate. No stripping plate is, however, required.

After the ramming is over the moulding flask is lifted off

the pattern by the four pins which are mechanically

operated by a mechanism suitably incorporated in the

machine.

Q.6. Describe centrifugal casting and state its advantages

and limitations. Draw a sketch of the same.

Ans. Centrifugal casting is casting method in which, molten

metal is poured while mould is rotating, due to the

centrifugal force metal is directed towards the periphery.

The cold metal is forced towards the outer side of the

casting by the centrifugal force, where as the hotter metal

remains on the inner side of the casting to provide the

required feeding of metal during solidification.

Although many different shapes can be cast through

this process, but those with symmetrical shapes are best

suited for it.

Centrifugal casting methods can be classified as

follows: -

a)True centrifugal casting

b)Semi centrifugal casting

c)Centrifu

manufacturing process 1 by dv shirbhate

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