UNIT IV : METAL CASTING PROCESSES 1. State uses of metal ...

Unit 4 : Metal Casting Processes Dr. Abdul Khader A A P a g e | 41

UNIT IV : METAL CASTING PROCESSES

1. State uses of metal casting (Nov-Dec-2017)

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

2. Explain metal casting process (April-May 2016)

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.


3. Define pattern and list the materials used for pattern (April-May 2019, Nov-Dec-2018)

4. List different pattern making materials (Nov-Dec 2016)

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.

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

5. Explain briefly cope, drag and core used in casting process (Nov-Dec-2018, Nov-Dec

2016)

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 as shown in fig.

The core is normally a disposable item that is

destroyed to get it out of the piece.

6. List any 10 patterns used in casting (April – May 2018, April May 2017, April-May

2016)

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


6. List pattern making allowances (Nov-Dec-2017, Nov-Dec 2015)

7. Explain casting allowance (April – May 2018, Nov-Dec 2017, April-May 2016)

Pattern allowances

Shrinkage allowance.

1. Machining allowance.

2. Pattern draft or taper allowance.

3. Corners and fillets allowance.

4. Rapping or shake allowance.

5. 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.


8. Explain with a sketch, a split pattern (April-May 2019, Nov-Dec-2017, Nov-Dec 2015,

April-May 2017

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.


9. Explain sweep pattern with a sketch (Nov-Dec-2018)

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:

10. Explain loose piece pattern with a sketch (April – May 2018)

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


11. State properties of moulding sand (Nov-Dec-2017, April May 2017, April-May 2016,

Nov-Dec 2015)

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 replica 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 foundries, 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


12. State ingredients of foundry sand (April-May 2019, Nov-Dec 2016)

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

13. List types of foundry sand (Nov-Dec 2016, Nov-Dec 2015)

Types of moulding sandGreen sand

1. Dry sand

2. Loam sand

3. Facing sand

4. Backing sand

5. System sand

6. Parting sand

7. Core sand

8. C02 sand

9. Shell sand or synthetic sand

10. Oil and molasses sand

11. Mould washes.

14. With a neat sketch, explain centrifugal casting (April-May 2019, Nov-Dec-2017)

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


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.


15. Explain with sketches slush casting (Nov-Dec-2017, Nov-Dec 2015)

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.


16. Explain briefly die casting with its applications (Nov-Dec-2017, April-May 2017)

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.

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.


17. Explain with a sketch, 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.

18. Explain briefly “Runner” and “Raiser” used in casting process (Nov-Dec 2016)

Runner and riser

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

around the cavity through a passageway called the runner.

• Runner is generally preferred in the drag as shown in fig. Sometimes 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


19. Differentiate between sand casting and die casting (April-May 2016, Nov-Dec 2017)

20. List defects in casting (Nov-Dec 2015)

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.

Sand casting Die casting

1 From pattern mould is prepared

in sand

Pattern itself is mould

2 Pattern is made of wood, metal

or plastic

Made up of hardened steel

2 Suitable for one time

production

Suitable for mass production

3 Not accurate dimensional

tolerance

Close dimensional tolerance

4 Not good surface finish Good surface finish

5 Very thin sections can not be

cast

Thin sections can be cast

6 Mould is prepared for each

piece

Long die life

7 Molten metal will not come in

contact with pattern

Comes in direct contact


S No

Defect Cause Remedies

1. Shift Mismatching of top and bottom

parts of the casting

Maintaining proper alignment of

pattern or die

2. Warpage

It is undesirable deformation in casting due to different rates of solidification.

Proper casting design.

3. Fin

It is a thin projection of metal at

the 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

Adjusting the moisture content

6.

Drop

Surface of mould cracks and pieces fall into the molten metal leading to low strength

Providing sufficient strength and proper ramming of sand.

7. Honeycombing

or sponginess

These are small cavities very close

to each other caused by dirt in

molten metal

Removing the slag particles in

molten metal

8 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.

21. Explain the following casting defects (i) Blow hole (ii) Shrinkage cavity (iii) Misrun (iv)

Cold shut (v) Mismatch) (Nov-Dec-2018)

Blow hole

When gases entrapped on the surface of the casting due to solidifying metal, a rounded or oval

cavity is formed called as blowholes. These defects are always present in the cope part of the

mold.

Causes

(i) Excessive moisture in the sand.

(ii) Low Permeability of the sand.

(iii) Sand grains are too fine.

(iv) Too hard rammed sand.

(v) Insufficient venting is provided


Shrinkage Cavity

The formation of cavity in the casting due to volumetric contraction is called as shrinkage

cavity.

Causes

(i) Uneven or uncontrolled solidification of molten metal.

(ii) Pouring temperature is too high.

Cold Shut

It is a type of surface defects and a line on the surface can be seen. When the molten metal

enters into the mold from two gates and when these two streams of molten metal meet at a

junction with low temperatures than they do not fuse with each other and solidifies creating a

cold shut (appear as line on the casting). It looks like a crack with round edge.

Causes

(i) Poor gating system

(ii) Low melting temperature

(iii) Lack of fluidity

Misrun

When the molten metal solidifies before completely filling the mold cavity and leaves a space

in the mold called as misrun.

Causes

(i) Low fluidity of the molten metal.

(ii) Low temperature of the molten metal which decreases its fluidity.

(iii) Too thin section and improper gating system


Shift or Mismatch The defect caused due to misalignment of upper and lower part of the casting and misplacement of the core at parting line.

Cause:

(i) Improper alignment of upper and lower part during mould preparation.

(ii) Misalignment of flask (a flask is type of tool which is used to contain a mould in metal

casting. it may be square, round, rectangular or of any convenient shape.)

Classification 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.

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

UNIT IV : METAL CASTING PROCESSES 1. State uses of metal ...

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