Chapter 4: Metal Casting Process

Karnataka (Govt.) Evening Polyetchnic Page 1 of 29 Unit 4: Metal Casting Process

Chapter 4: Metal Casting Process

Casting

• Casting is the most ancient process of manufacturing the metallic components.

• In this process the raw material is melted and heated to the required temperature and poured

into a mould cavity where it takes the desired shape of the component, allowing the molten

metal to cool and solidify in the mould.

• After the solidification of molten metal in a mould cavity the product is taken out from the

mould and clean it, which may be subjected to further treatment, if necessary.

• The solidified piece of metal, which is, taken out of the mould is called as 'Casting'. A plant

where the castings are made is called as 'Foundry'

Advantages of casting

1. Parts (both small and large) of intricate shapes can be produced.

2. A part can be made almost of the finished shape before any machining is done.

3. Almost all the metals and alloys and some plastics can be casted.

4. Good mechanical and service properties.

5. Mechanical and automated casting processes decrease the cost of casting.

6. Casting provides freedom in the design process.

7. Excellent vibration damping capacity.

8. Casting provides uniform directional properties.

Disadvantages of casting

1. High initial cost.

2. Casting of thin sections becomes difficult.

3. Great care is to be taken while handling molten metals and chemicals.

4. Casting is not economical for small number of production.

5. Casting is the tedious process.

6. Great care should be taken to control the cooling rate to get defect free castings.

7. Castings may have internal defects such as shrinkage, blowholes, etc


Uses of Casting

Casting process can be used for

• automobile engine blocks,

• cylinder blocks,

• pistons, piston rings,

• machine tool beds and frames,

• mill rolls, wheels and housings of steam and hydraulic turbines, turbine vanes and aircraft jet

engine blades,

• water supply and sewer pipes,

• sanitary fittings etc

Process of casting

1. Take the pattern (the material of the pattern may be wood, metal or plastic).

2. Prepare the moulding sand.

3. With the help of pattern, prepare the mould and necessary cores.

4. Melt -the metal or alloy to be cast.

5. Pour the molten metal/ alloy into mould cavity.

6. Allow the molten metal to cool and solidify.

7. Remove the casting from the mould. This operation is called 'shake out'.

8. Clean and finish casting. The operation is known as 'fettling'.

9. Test and inspect the casting.

10. Remove the defects if any and if possible (salvaging the casting).

11. Stress relieve the casting by heat treatment.

12. Again inspect the casting.

13. The casting is ready for use.

Pattern

• In casting, a pattern is a replica of the object to be cast, used to prepare the cavity into which

molten material will be poured during the casting process.

• Patterns used in sand casting may be made of wood, metal, plastics or other materials

• So, a pattern is an element used for making cavities in the mould, into which molten metal is

poured to produce a casting.

• A pattern is not an exact replica of the desired casting. There are certain differences. It is

slightly greater than the desired casting, due to various allowances (shrinkage, machining etc.)

and it may may also have extensions to get the runners and gates


Pattern making materials

Wood : White pine, Mahogany, Maple, Birch and Cherry, Teak, Shisham, Kail and Deodar.

Metal : Cast iron, Brass, Alluminium, White metal.

Plastic : Plastics of epoxy resins, acrylates, phenol formaldehyde and polyester resins.

Quick setting compounds : Gypsum, resin-impregnated materials, waxes

Wood

The wood used for pattern making should be properly dried and seasoned and should not contain

moisture more than 10%. It should be straight grained and free from knots.

Advantages

1. Light in weight.

2. Good workability.

3. Comparatively inexpensive.

4. Can be repaired easily.

5. Holds well varnishes and paints.

Limitations

1. Possess poor wear and abrasion resistance.

2. Cannot withstand rough handling.

3. Absorbs and gives off moisture.

Metal

A metal pattern may be either cast from a master wooden pattern or may be machined by the usual

methods of machining. These are generally used in machine moulding.

Advantages

1. More durable and accurate in size than wooden patterns.

2. Have a smooth surface.

3. Do not deform in storage.

4. Good resistant to wear, abrasion, corrosion and swelling.

5. Withstand rough handling.

Limitations

1. Expensive as compared to wood.

2. Not easily repaired.

3. Heavier than wooden pattern.

4. Ferrous patterns get rusted.


Plastic

The use of plastics for pattern material possess following advantages.

1. More economical in cost and labour.

2. High resistant to corrosion, lighter and stronger than wood.

3. No moisture absorption.

4. Smooth surface of patterns.

Types of pattern

1. Single piece or solid pattern.

2. Split pattern.

3. Match plate pattern.

4. Cope and drag pattern.

5. Loose piece pattern.

6. Gated pattern.

7. Skeleton pattern.

8. Sweep pattern.

9. Shell pattern.

10. Segmental pattern.

11. Boxed-up pattern.

12. Built up pattern.

13. Lagged-up pattern.

14. Left and right-hand pattern

Single piece pattern

• This pattern is made without joints, partings or any loose pieces and it is not attached to a

frame or plate, as shown in fig.

• This pattern is exactly like a desired casting. For making mould, the pattern is accommodated

either in cope or drag.

• This moulding process is quite inconvenient and time consuming. Therefore these are used for

large castings, for example, stuffing box of steam engine


Split pattern

These patterns are split along the parting plane

to facilitate the withdrawal of the pattern out of

the mould before the pouring operation.

• Fig. shows the split pattern for casting a bush.

• The two parts of the pattern are joined together

with the help of dowel pins.

• For a more complex casting, the pattern may be

split in more than two parts as shown in lower fig.

• These are used for casting of spindles, cylinders,

small pulleys, steam valve bodies etc.

Match plate pattern

A match plate pattern is a split pattern having the cope

and drag portions mounted on opposite sides of a plate

known as match plate (usually metallic) that conforms

to the contour of the parting surface.

The gates and runners are also mounted on the match

plate, so that very little hand work is required that

results in higher productivity

This type of patterns are used for large number of

castings. Several patterns can be mounted on one

match plate if the size of the casting is small.

When match plate patterns are used, the moulding is

generally done on a moulding machine. Piston rings of

I.C. engines are produced by this process.


Cope and drag pattern

• It is a split pattern having the cope (upper) and drag (lower) portions each mounted on

separate match plates.

• These patterns are used in the production of large castings, because complete moulds are too

heavy to be handled by a single worker.

• The patterns are accurately mounted on the plates and when two separate moulds are

assembled together, the mould cavity is properly formed.

• For a higher rate of production, each half of the pattern is mounted on a separate moulding

machine with one worker working on the cope part and the other worker on the drag part of

the mould.

Loose piece pattern

• Some patterns are produced as assemblies of loose component pieces. When a one piece solid

pattern has projections which lie above or below the parting plane, it is impossible to remove

the pattern from the mould. With such patterns, the projections are made with the help of

loose pieces. A loose piece is attached to the main body of the pattern by a pin or dovetail slide

• While moulding, sand is rammed securely around the loose piece. Then pins are removed. The

sand is then packed and rammed around the total pattern. When the main pattern is removed,

the loose pieces remain in the mould. These are then carefully rapped and drawn


Gated pattern

A gated pattern is simply a pattern where or

more loose patterns having attached with

gates and runners as shown in fig.

To produce good casting, it is necessary to

ensure full supply of molten metal flows into

every part of the mould, this is ensured by

providing easy passage gating system. Since

gates and runners are not to be cut by hand,

gated patterns reduce the moulding time.

• These patterns are used in mass production of small castings in a single multicavity mould by

joining a group of patterns, and gates. Gated patterns may be made of wood or metal.

Skeleton pattern

• Patterns of large castings usually require a huge amount of timber. In such cases, skeleton

patterns are used as shown in fig.

• These are simple wooden frames that outline the shape of the part to be cast and having a large

number of square or rectangular openings between the ribs.

• The framework is filled and rammed with clays, sand or loam, and a strike-off board known as

strickle board is used to scrap the excess sand out of the spaces between the ribs so that the

surface is even with the outside of the pattern.

• It is generally built with two parts, one for cope and other for the drag


Swept pattern

A sweep is a section or wooden board of proper contour that is

rotated about one edge to shape mould cavities having shapes of

rotational symmetry as shown in fig.

Sweep patterns are used when a large sized castings are to be

produced in a short time.

A complete pattern is not necessary and becomes very expensive for

very large castings.

The moulds are made manually, either in a pit or on the foundry

floor. Therefore, these are referred as pit moulding or floor moulding.

In this case loam sand is used and a brick or wooden frame work supports the loam sand.

• Once the mould is ready, the sweep pattern and the post about which it rotates, are removed

before pouring the molten metal into the mould cavity.

• Large kettles of C.I. are made by sweep patterns:

Shell pattern

The shell pattern is a hollow construction like a shell and the

outside shape is used as a pattern to make the mould, while the

inside is used as a core-box for making cores.

These patterns are usually made of metal, mounted on a plate and

parted along the centre line, the two sections being accurately

together as shown in fig.

The shell pattern is largely used for drainage fittings and pipe

work.


Segmental pattern

These are similar to sweep pattern, but it is in the form of

segment as shown in fig.

This is used for moulding the parts of circular shape. To

create the mould, it is rotated about the post in the same

way as in sweep pattern.

But it is not revolved continuously about the post to

prepare the mould, instead, it prepares the mould by

parts, when one portion is completed, the pattern is

lifted up and moved to the next portion to make the next

segment of the mould.

This process is continued until entire mould is completed. Wheel rims, big gears, etc., can be

produced by this pattern

Boxed up pattern

In this case planks or strips of wood are joined together by using nails

or glue or screws in the form of box to have a light weight pattern.

This type patterns are used for casting having a regular outline and

rectangular form.

Lagged up pattern

In this pattern the longitudinal strips of wood called "lags' or

"staves" which are bevelled on each side to make the joint tight

outside, then glued and screwed or nailed to the end pieces of

wood called "heads" as shown in fig.

The heads are half of a regular polygon or the shape of the

object to be casted.

These patterns are parted longitudinally through the centre.

These patterns are used for castings like cylinders, pipes or

columns.


Left and right hand pattern

• For some applications, parts of the pattern can not be

reversed

• For example, casting the legs of 'J' hangers, garden

bench or brackets have separate left and right pieces

• For such patterns are to separately made for Left and

right sides

Pattern allowances

1. Shrinkage allowance.

2. Machining allowance.

3. Pattern draft or taper allowance.

4. Corners and fillets allowance.

5. Rapping or shake allowance.

6. Distortion allowance.

Shrinkage allowance

• As the metal shrinks on solidification and contracts on cooling to room temperature.

• To compensate this, linear dismissions of patterns are increased in respect of those of the

finished casting to be obtained, which is known as shrinkage allowance.

• It is given as mm/ m

Steel = 20 mm/m

C.I. / Malleable Iron =10 mm/m

Brass, Cu, Al = 15 mm/m

Zinc, Lead = 25 mm/m


Machining allowance

Usually, rough surfaces of castings have to be machined to improve surface finish. So size of the

casting must be slightly more than the finished part represented in drawings. This extra amount of

metal provided on the surfaces to be machined is called machining

1. Kind of metal to be used.

2. Size and shape of the casting and

3. Methods of moulding.

Typical machining allowances

Draft allowance

A certain taper is given on the pattern surfaces that are

parallel to the direction in which the pattern is withdrawn

from the mould.

This is to avoid damage to the mould during withdrawal of

the pattern

The draft allowance depends on

1. Length of vertical side.

2. Intricacy of the pattern

3. The method of moulding.

Generally, draft is about 10 to 20 mm/ m on exterior surfaces and 40 to 60 mm/ m on interior surfaces.


Corner of fillet allowance

• The intersection of surfaces in casting should be smooth and form no sharp angles. For this, the

external and internal corners of patterns are suitably rounded.

• They are known as rounded corners or fillets.

• Fillets facilitates the removal of the pattern from the mould, prevents the formation of cracks

and shrink holes in the casting.

Rapping allowance

• To take pattern out of the mould cavity it is slightly rapped to detach it from the mould cavity.

• Due to this, the cavity in the mould increases slightly. So, the pattern is made slightly smaller.

Distortion allowance

• Some

castings, because of their size, shape and type of metal, tend to warp or distort during the

cooling period.

• This is a result of uneven shrinkage and is due to uneven metal thickness causing it to cool more

rapidly.

• The shape of the pattern is thus bent in the opposite direction to overcome this distortion as

shown in fig.

Moulding

• Moulding is the process of creating mould cavities of different shapes by using metal

or sand.

• The shape of mould cavity corresponds to the shape of the casting required except in

dimension.

• In general, a mould is referred as the exact riplica of the casting.

• A hot molten metal is poured into the mould cavity and allowing it to solidify for

getting the casting.


• In most of the foundaries, sand is used as moulding material. Sand moulds are prepared by

using materials such as base sand, binder, water and other ingredients.

• Moulding process may be carried out on the floor or bench known as floor moulding or bench

moulding. The moulding process may be conducted with hand tools by the moulder known as

hand moulding process or with the help of machine known as machine moulding process

Moulding sand

• Sand is the principal material used in the foundry shop for moulding process. Sand is obtained

from river bed, sea, lake and deserts.

• The sand should possess the properties which are vital for foundry purposes

Properties of moulding sand

1. The sand should have adequate strength in its green, dry and hot states.

2. He sand should have high permeability.

3. The sand should have high thermal stability.

4. It should have good refractoriness.

5. It should have good flowability.

6. It should have uniform sand texture.

7. It should be cheap and reusable.

8. It should have good thermal conductivity

9. It should have low collapsibility

10. It should be easy to prepare and control

11. It should have good adhesiveness

Principal ingredients of moulding sand

Silica sand grains - 80 to 90%.

Clay - 5 to 20%

Moisture - 2 to 8 %

Miscellaneous materials - below 2%

i) Oxides of iron

ii) Limestone

iii) Magnesia soda and

iv) Potash


Classfication of moulding sand

• Natural moulding sands

• Artificial or synthetic or high silica sands.

• Special sands.

Natural moulding sand

• Natural moulding sands are also called as green sands, and are taken from river beds or dug

from pits.

• They contain appreciable amount of clay about 5 to 20% which acts as a bond between the sand

grains and they are used as received with water added.

• The quantity and type of clay mineral present will affect the strength, toughness and

refractoriness of the sand.

Synthetic moulding sand

• These are basically high silica sands containing about 95 to 98% of silica and less than 2% of

clay or no clay (binder) in its natural form.

• They are made in foundry by crushing quartzite sandstones and then washing and grinding to

get desired grain size.

• The desired strength and bonding properties can be obtained by adding the binders such as

bentonite, water and other materials as required.

• The synthetic sands are more expensive than natural sand.

Special moulding sand

• Special sands are zircon, chromite, olivine, chamotte and chrome-magnesite.

• They possess the special characteristics which are not ordinarily obtained in other sands.

• Zircon sands are used for cores of brass and bronze castings, chamotte is used for heavy steel

castings.

• Chrome-magnesite sands are particularly useful for steel castings and olivine sand for non-

ferrous castings of intricate shape.

Types of moulding sand

1. Green sand

2. Dry sand

3. Loam sand

4. Facing sand

5. Backing sand

6. System sand

7. Parting sand

8. Core sand

9. C02 sand

10. Shell sand or synthetic sand

11. Oil and molasses sand

12. Mould washes.


Green moulding sand

• Green sand is a natural sand and it is a mixture of silica sand with 18 to 30% of clay and 6 to 8%

of water.

• Clay and water gives the bonding strength to the green sand. It is soft, light, porous and fine.

• It retains the shape, the impression given to it under pressure.

• Green sand is generally used for casting small or medium sized moulds.

• Coal dust is mixed in green sand to prevent defects in castings.

Dry moulding sand

• Green sand moulds are dried and baked to remove almost all moisture to get the dry sand

moulds.

• These are suitable for large castings.

• These moulds are strong and compact.

• These moulds offer greater rigidity for heavy castings

Lome sand

• It is a mixture of clay and sand mixed with water to a thin plastic paste, from which moulds are

prepared on a backing of soft bricks.

• It contains upto 50% clay and 18 to 20% water and dries hard. Chopped stray and manure are

used to give the binding strength.

• These are used for large castings.

Facing sand

• Facing sand is used directly next to the surface of the pattern and it comes into contact with the

molten metal when the mould is poured.

• As it is subjected to the severest conditions, it must possess high strength and refractoriness.

• It is prepared from silica sand and clay, without the addition of used sand.

• Facing sand layer in mould ranges from 20 to 30 mm

System sand

• System sand is used in machine moulding to fill the whole flask.

• In mechanical preparation and handling units, no facing sand is used.

• The used sand is cleaned and reactivated by the addition of water binders and special additives,

this is known as system sand.

• The system sand should have a higher strength, permeability and refractoriness than the

backing sand.


Parting sand

• Parting sand is a clean clay free silica sand.

• It is used to keep the green sand from sticking to the pattern and also separate the moulding

boxes from adhering to each other by spreading a fine sharp dry sand called parting sand.

Core sand

• Core sand is used for making cores.

• It is silica sand mixed with core oil i.e., linseed oil, rosin, light mineral oil and other binders.

• For the sake of economy, pitch or flour and water may be used as core sand for large cores.

CO2 sand

• In CO2 sand, the silica grains are coated with sodium silicate.

• This mixture is packed first around the pattern and then hardened by passing C02 through the

interstices for about a minute.

• The sand sets hard and produces a strong mould.

Shell or synthetic sand

• Shell sands are coated with phenol or urea-formaldehyde resins and cured by heated pattern to

produce very strong thin shell.

• No backup sand is required to provide support for the weight of the casting.

• As alloys solidify at higher temperatures, the resins are not disassociated.

• But moulds disintegrate when casting solidified, due to breaking up of chemical bond by heat

from solidifying casting

Oil or molasses sand

• It is the mixture of sand and oil binders or molasses binders.

• These binders imparts high dry strength and collapsibility to moulds and cores.

These are used for casting of small intricate sections.

Mould washes

• Mold washes are the slurries of fine ceramic grains, They can be applied over the mould

surfaces to minimise the fusing of facing sand grains.

• They help to produce smooth surface on casting by filling the interstices.


Cope and drag

In foundry work, the term cope and drag refer

respectively to the upper and lower parts of a two-part

casting flask used in sand casting as shown in fig.

In the production of large castings, the complete moulds

are too heavy to be handled by a single operator.

Therefore, cope and drag patterns are used to ease this

problem to efficient operation.

The patterns are made in halves, split on a convenient

joint line and separate cope and drag patterns are built

and mounted on individual plates or boards.

This arrangement permits one operator or group of

operators to prepare the cope half of the mould while another operator or group worked on the

drag half. This increases the production capacity

Core

A core is a device used in casting and

moulding processes to produce internal

cavities and reentrant angles as shown in

fig.

The core is normally a disposable item

that is destroyed to get it out of the piece.


Runner and riser

• Runner : In large castings, molten metal is usually carried from the sprue base to several gates

around the cavitv through a passageway called the runner.

• Runner is generally preferred in the drag as shown in fig. Simetimes it may be located in the

cope depending upon the shape of the casting.

• Riser : It is also known as feeder. Riser is a reservoir of molten metal built into a metal casting

mould to prevent cavities due to shrinkage, by supplying this material to sections of the mould

to compensate for any shrinkage during cooling

Special casting processes

• In sand castings the moulds are destroyed after solidification of castings. This will increase the

cost of production.

• In sand mould castings we cannot get closer dimensional tolerances, good surface finish and

greater mechanical strength.

• But, in special casting processes called as permanent casting processes, the moulds are reused

repeatedly and all cast metals can be cast by these processes.

• The material used for making moulds (dies) may be cast iron or alloy steels.

• In these processes, a closer dimensional tolerances, better surface finish, greater mechanical

strength with lower percentage of rejection can be obtained.

• These becomes economical production in larger quantities.


Classfication of special casting processes

1. Permanent mould casting

2. Slush casting.

3. Die casting .

I. Hot chamber die casting

II. Cold chamber die casting.

4. Centrifugal casting.

Permanent mould casting

• A casting made by pouring molten metal into a mould made up of some metallic alloy or other

material of permanance is known as permanent mould casting.

• This type of casting is also known as gravity die casting. Since permanent moulds are costly,

therefore it is used for mass production only.

• The permanent moulds are impractical for large castings and alloys of high melting

temperatures, but they are useful for small and medium sized non-ferrous castings.

• The mould is not destroyed after removing the casting and may be reused many times.

• The iron and steel moulds are suitable for non-ferrous castings.

• The steel moulds coated with refractory material, such as graphite are successfully used for

production of iron castings.

• The castings produced by these moulds require less skill, limited floor space, have improved

surface finish, high dimensional accuracy and less number of rejections than sand castings.

• The bronze moulds also used for low melting temperature metals.


Slush casting

• The slush casting is a variation of permanent mould casting that is used to produce hollow parts.

The process is based on the solidification of molten metal by the chilling effect.

• It is used for casting low melting temperature metals and alloys such as gold, silver, aluminium,

zinc, lead and their alloys etc. It is Also used for making hollow castings without the use of cores.

• In this process, the molten metal is poured into a metallic mould.

• The metal is retained in the mould long enough for the outer skin to solidify. Finally, the mould

is turned over to remove metal still in molten condition.

• Due to the chilling effect, only a thin layer of the metal sticks to the mould surface which is

taken out by opening the halves of the mould and which is the required product as shown in fig.

This method is only adopted for ornaments and toys of non-ferrous alloys.

Applications

1. Used for decorative and ornamental objects.

2. Used for bowls, candle sticks, lamps and statues.

3. Used for production of jewelleries, animal miniatures, handles for hollow wares etc.


Advantages of slush casting

1. Hollow parts can be produced without the use of cores.

2. Desired thickness can be achieved.

3. A variety of designed castings can be produced.

Die casting

• Die casting is the process of rapidly producing accurately dimensioned parts by forcing the

molten metal under pressure into a split metal dies.

• The molten metal fills the entire die within a fraction of second and solidifies quickly due to the

low temperature of the dies as they are water cooled.

• The casting is ejected by separating the die halves. If the parts are small, several parts may be

cast at one time known as multiple cavity die.

The machines used for producing the die casting are .

1. Hot chamber die casting

2. Cold chamber die casting.

Hot chamber die casting

• It is a submerged plunger type machine, in which plunger operates in one end of a gooseneck

casting which is submerged in the molten metal as shown in fig

• When the plunger moves in the upper position, the molten metal flows by gravity into this

casting through holes just below the plunger.

• When plunger moves down, the holes are closed and the entrapped liquid metal is forced into

the die through the gooseneck channel and in gate. As the plunger retracts, the channel is again

filled with the right amount of molten metal. The plunger made of refractory material may be

operated by manually or mechanically and hydraulically

• Heating is continued throughout the operation to keep the molten metal in liquid state.


Cold chamber die casting

• It is a horizontal plunger type machine as shown in fig.

• The plunger is driven by air or hydraulic pressure to force the molten metal into the die. As soon

as the ladle is emptied, the plunger moves towards left and forces the metal into the cavity.

• After the metal is solidified, the core is withdrawn; and then the die is opened.

• Ejectors are provided to remove the casting automatically from the die. This machine is suitable

for alluminium alloys which cannot be cast in hot chamber machines due to ready reactivity of

molten alluminium with steel.

• Plunger pressure usually ranges from 29 to 157 MN/m2

Advantages of die casting

1. More economical for mass production.

2. Close dimensional tolerances can be achieved.

3. Very high rate of production.

4. Unit cost is minimum.

5. Good surface finish can be achieved.

6. Very fine details can produced

7. Very thin sections of the order of 0.5mm ran be cast.

8. Longer die-life is obtained.

9. Less floor space is required.

10. High strength and excellent mechanical properties can be achieved.


Disadvantages of die casting

1. It is uneconomical for nonferrous alloys.

2. Not economical for small runs.

3. The heavy castings cannot be cast and maximum size of the casting is limited.

4. High cost of die and die casting equipment.

5. Complicated die design.

6. Usually the die castings contain some porosity due to the entrapped air.

Centrifugal casting

• In centrifugal casting, the molten metal is poured into moulds while they are rotating.

• The molten metal falling into the centre of the mould is thrown out by the centrifugal force

under high pressure towards the periphery and the impurities which are light in weight are

pushed towards centre.

• The solidification progresses from the outer surface to the inwards. The grains are refined and

castings are completely free from any porosity defect by the forced movement of the molten

metal, thus making the dense and sound castings.

• The use of gates, feeders and cores are eliminated, hence this method becomes less expensive

and complicated

• In this method, hollow cylindrical bodies like cast iron water supply pipes, sewerage pipes, steel

gun barrels and other symmetrical objects such as pulleys, gears, disk wheels can be produced

easily

Types of centrifugal casting

1. True centrifugal casting

2. Semi-centrifugal casting

3. Centrifuging.


True Centrifugal casting

• The method is used for the production of hollow cylindrical products which are symmetrical

about the axis such as bushings, gun barrels, C.I. pipes, etc.

• The metal mould of desired shape is prepared and walls are coated with refractory green or dry

sand coating as shown in fig.

• A cylindrical mould is made to rotate about its axis at high speed so that the measured quantity

of molten metal poured at the centre being thrown out towards the periphery by the centrifugal

force and holds the molten metal until it solidifies.

• The metal solidifies in the form of a hollow cylinder. The thickness of the cylinder can be

controlled by the amount of liquid metal poured.


Semi centrifugal casting

This process is generally used for making

large sized castings which are

symmetrical about their own axis such

as gears, disc wheels, propellers and

pulleys.

A metal mould consists of cope and drag

rotates about vertical axis. The molten

metal is poured through gate as shown

in fig.

The metal flows into hub and then

towards the rim by means of centrifugal

force. A dry sand core is used to get the

central hole.

The speed of rotation is slower than the true centrifugal casting.

Centrifuging

In this process several identical or nearly

similar moulds are located radially about a

vertically arranged central riser or sprue

which feeds the metal into the cavities

through a number of radially arranged gates.

The entire mould is rotated with the central

sprue which acts as the axis of rotation as

shown in fig.

Therefore, it is not a purely centrifugal

process. As the molten metal is poured

through central sprue, the metal enters into

the different gates and fills the radially

arranged cavities by means of centrifugal

force due to the high speed of rotation.

This method is suitable for small, intricate

parts where feeding problems are

encountered or it can be used for stack

moulding.


Advantages of centrifugal casting

1. Dense and sound castings can be produced.

2. Casting is clean and free from foreign inclusions.

3. Suitable for mass production.

4. Less rejection.

5. Use of runners, risers and cores is eliminated.

6. Improved mechanical and physical properties.

7. Closer dimensional tolerances can be achieved.

8. Saving in machining.

9. Thinner sections can be cast.

10. Any metal can be cast by this process

Disadvantages of centrifugal casting

1. This process is limited to only cylindrical and annular parts.

2. Limited range of sizes.

3. It involves high initial cost.

4. Requires skilled labour for its maintenance.

5. High speed may result in surface cracks.

Mould casting v/s die casting


Casting defects

1. Shift

2. Warpage

3. Fin

4. Swell

5. Blowholes

6. Drop

7. Dirt

8. Honeycombing or sponginess

9. Metal penetration and rough surface

10. Sand holes

11. Pin holes

12. Scabs

13. Shrinkage cavity

14. Hot tears (pulls)

15. Cold shut and missions

16. Poured short

17. Internal air pocket.

S No

Defect Cause Remedies

1. Shift Mismatching of top and bottom

parts of the casting or

misalignment of flask.

Maintaining proper alignment of

pattern or die part, moulding

boxes etc., before use.

2. Warpage

It is unintentional and undesirable deformation in casting during or after solidification due to different rates of solidification.

Proper casting design.

3. Fin

It is a thin projection of metal

which is not the part of casting

occurs at parting line due to

improper assembly or clamping.

Correct assembling and clamping.

4.

Swell It is an enlargement of mould

cavity by metal pressure, results

in overall. enlargement of casting.

The sand should be rammed

properly and evenly.

5. Blowholes

These are small holes below the surface of casting caused by excessive moisture in the sand, or low permeability of the sand, sand grains are too fine and sand is rammed too hard.

Adjusting the moisture content in sand, using sand of proper grain size and ramming should not be too hard.

6.

Drop

It occurs when upper surface of mould cracks 'and sand pieces fall into the molten metal. It occurs due to low strength and soft ramming of sand, insufficient flexing of molten metal or

Providing sufficient strength and proper ramming of sand. Sufficient flexing of molten metal and proper reinforcement of sand projections in cope.


insufficient reinforcement of sand projections in cope.

Particles of dirt and sand in the

casting surface due to crushing of

sand by rough handling, or

presence of slag particles in the

molten metal.

Use of dirt trap and proper flexing

Qf molten metal.

8. Honeycombing

or sponginess

These are small cavities very close

to each other caused by dirt in

molten metal or imperfect

skimming in the ladle.

Removing the slag particles in

molten metal by proper skimming

in the ladle.

9. Metal penetration and rough surface

Due to low strength and large

grain size of sand or high

permeability and soft ramming.

Removing all the causes.

10. Sand holes Due to loose sand washing into

the mould cavity and fusing into

the inferior of the casting.

Proper cleaning of mould cavity

and careful pouring of molten

metal.

11. Pin holes Due to sand with high moisture

content or absorption of hydrogen

or CO gas.

Using good melting and fluxing

practices and reducing moisture

content of sancl and increasing

permeability.

12. Scabs

These are rough & irregular

projections on the surface caused

due to too fine sand having low

permeability and moisture

content or by uneven sand

ramming.

Mixing additives like wood flour,

sea coal into the sand.

13.

Shrinkage

cavity

Uncontrolled and haphazard

solidification of metal.

By applying principle of directional

solidification in mould design.

14.

Hot tears They are internal or external

cracks occur during solidification

due to poor design and sudden

sectional changes, no proper

fillets and corner radii.

Improved design and proper

directional solidification.


15. Cold shut and

mission

It is an external defect caused

due to imperfect fusion of two

steams of metal in mould cavity

or unequal sections of pattern

assembled together.

Use hotter metal, frequent

inspection and proper design of

casting.

16. Poured short Cavity is not filling completely at

one pouring.

Sufficient pouring of metal in

mould cavity.

17. Internal air pocket

Pouring boiling metal or rapid

pouring of metal, faulty and poor

quality metal, excess moisture in

sand.

Correct pouring and using right

quality metal and dry sand.

Documents

Chapter 4: Metal Casting Process