A Basic Course on Casting Process

DME-II: A Basic Course on Casting process

Page 1 of 16

Casting

I. Patterns:

Definition: A pattern is a replica or facsimile model of the desired casting. Using it, we can

produce a dimensionally similar cavity called mould into which molten metal can be poured

and original casting can be produced. Pattern is used in Casting Process for direct duplication

of the desired component.

a. Materials used:

i. Wood1: is the most common material for patterns because

1. It is cheap and easily available.

2. It can be easily shaped into different forms and intricate designs

3. It is light weight. So it can be easily handled.

4. By sanding and planing, good surface finish can be obtained.

5. It can be preserved for long time by applying shellac. Hence it can be

repeatedly used.

Pine wood, Deodar, Teak, Shisham and Mahagony are generally used.

ii. Metals: are used only when large number of castings is to be manufactured and

close dimensional accuracy is desired. Cast Iron2 patterns are cheap, have good

machinability, good surface finish and high resistance to abrasion. Brass3 is used

for small patterns in bench and machine moulding. Aluminum4 is cheap and

light. Hence aluminum and its alloys like Duralumin are used for large castings.

iii. Plaster: Plaster of Paris5 (POP) or Gypsum cement has very high compression

strength and is used in small patterns and core boxes having intricate design and

close dimensional control.

iv. Plastic: is light weight, high wear resistant, high compressive strength, doesnt

absorb moisture or corrode and cheap. Thermo-setting plastic (phenolic resins &

foam plastics) is used. A pattern is made from POP and mould cavity is prepared.

Resin is poured into the cavity. When it solidifies, plastic pattern is prepared.

v. Wax: is used in investment casting.

b. Types: Type of pattern to be used depends on number of castings required, type of

moulding process, size of casting and the degree of intricacy in design.

1 Disadvantages of Wood: Due to sand abrasion, it wears out quickly. By absorbing moisture, it changes shape by

warping and splitting. Preservation of wood patterns is therefore difficult. 2 Cast Iron is very heavy. Its weight becomes a disadvantage is pattern is huge.

3 Brass is heavier than Cast Iron. It is also costly.

4 Aluminum has to be handled very carefully since it is light weight.

5 POP expands on solidification. Hence allowance has to be provided suitably.


Page 2 of 16

i. Split Pattern: is used because it is easy to withdraw the pattern from the mould

box. The two pieces are joined at the parting line by dowel pins. One piece goes

to the drag and the other into the cope.

ii. Multi-piece pattern: If the casting has a complicated design, then, we may have

3, 4 or more pieces of pattern. As many pieces of pattern, so many moulding

boxes are required. The bottom box is called Drag, top box is called Cope and

other middle boxes are called Cheeks.

iii. Match-plate pattern: is used when large number of castings is to be prepared. It

is used in machine moulding. The high construction cost is compensated by high

rate of production, greater dimensional accuracy and minimum machining. This

has two pieces. Runners, risers and gating system are already incorporated in

these patterns. The plate is made of wood, steel or aluminum.

iv. Gated pattern: is used for mass production of an item. Patterns are connected to

each other by means of gates. These gates help in easy flow of molten metal into

the cavities. This enables considerable saving in moulding time and a uniform

feeding of molten metal.

v. Skeleton pattern: When size of pattern is very large, but shape is simple, it is

not economical to make a large solid pattern of that size. In such cases, a pattern

consisting of a wooden frame and strips of wood is made. This is called skeleton

pattern. It is filled with loam sand and rammed. Surplus sand is removed by

means of strickle.

vi. Sweep pattern: is used for large symmetrical castings of circular cross-section.

This saves time, labor and material. The equipment consists of a base, suitably

placed in the sand mass, a vertical spindle and a wooden template called Sweep.

The outer edge of the sweep has 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 the spindle are removed, leaving the base in the sand. The hole

made by the spindle is filled up by ramming more sand. Separately prepared

cores are placed using chaplets.

vii. Segmental pattern: are used for preparing moulds of large circular castings,

avoiding the use of a solid pattern of the exact size. It is almost similar to sweep

pattern, but here, a sweep is not given a continuous revolving motion to generate

the mould cavity. Here, the pattern is a portion of the solid pattern itself. Mould

is prepared by it in parts. The segment is mounted on a central pivot and after

preparing the part mould in one position, the segment is moved to the next

position, and so on, till the complete mould is ready.


Page 3 of 16

c. Allowances: A pattern is always made larger than the required size of the casting in order

to allow for various factors such as shrinkage, machining, distortion, rapping, etc. This

extra dimension added on the pattern is called Allowance.

i. Shrinkage allowance: Metals contract when they cool from melting temperature

to room temperature. This contraction is in three forms: Liquid contraction,

solidifying contraction and solid contraction. Gates and risers compensate for the

first two contractions. Shrinkage allowance compensates for solid contraction.

Pouring temperature of molten metal, design and dimensions of casting, type of

mould material, moulding method & casting metal are the factors that affect this

allowance.

ii. Machining allowance: Casting may require machining on all surfaces or specific

surfaces. Corresponding portions on the pattern are given adequate allowance.

Amount of allowance depends on the casting metal, method of machining

employed, method of casting used, size & shape of casting and degree of finish

required. Ferrous metals need more allowance than non-ferrous metals. Large

and slender castings need more allowance.

iii. Draft allowance: All patterns are given a slight taper on all vertical surfaces to

help in easy withdrawal of pattern from mould cavity. It is provided on both

internal and external surfaces. Internal surface draft allowance is more than

external surface draft allowance. This allowance depends on design of pattern,

vertical height and moulding method.

iv. Rapping allowance: When a pattern is withdrawn from the mould, it is first

rapped or shaken, by striking over it from side to side. This rapping makes the

sides free from the walls of the mould. As a result, size of the mould increases.

So a negative allowance is provided to compensate for this.

v. Distortion allowance: In some shapes such as large flanges or U-shapes or I-

sections, thermal stresses set in while cooling. So, shape gets distorted. To


Page 4 of 16

eliminate this, an opposite distortion is provided in the pattern. This allowance

neutralizes the distortion.

d. Cores, Core allowance, Core prints: When a casting has a hole, a core is used in the

mould to produce the same. This core is made from special sand. It must be seated

properly in the mould. This is done by adding extra projections in the pattern at proper

places. These projections are called Core Prints. Cores are suspended inside the mould.

They should not shift when molten metal is poured. So they are supported with Chaplets.

Chaplet is a metal rod with a flat or curved metal surface riveted on the rod.

Core is prepared using sand and binder. Silica, zircon sand, olivine sand & chamotte sand

are used. Rosin (a resin from pinewood), bentonite, core oil, dextrin, etc are used as

binders.

II. Moulds:

a. Materials: Moulds can be prepared from sand, various metals like grey cast iron, steel,

anodized aluminum, POP, ceramic and wax.

b. Types of sands:

Natural sand: contains sufficient amount of binding clay and hence no binder is added.

Silica sand6: does not contain clay. Hence binders and additives are used.

Green sand7: is the most common sand used in Indian foundries. It contains just

enough moisture to give it sufficient bonding. Moulds prepared from green sand

do not require baking before pouring molten metal into them.

Dry sand: is obtained by drying green sand in an oven. It has no moisture.

Facing sand8: This sand forms the face of the mould. It is rammed around the

surface of the pattern. It is actually fresh prepared, well tempered green sand. The

6 Silica sand is also called Sharp Sand.

7 Green sand is also called Tempered Sand.

8 Facing sand is also known as Fat Sand.


Page 5 of 16

initial coating around pattern is given with this sand and rest of the mould box is

filled with floor sand for economy.

Floor sand9: After a casting is done, most of the sand is reclaimed. Then it is

riddled and re-used. This re-used sand is called Floor Sand.

Fine grained sand is used for small castings which carry intricate details. Medium

grained sand is used for bench moulding and other medium castings. Heavy grained sand is

used for large iron and steel castings.

Main constituents of moulding sand are silica sand, binder, additives and water.

Organic (dextrin, linseed oil) and inorganic (bentonite, kaolonite, limonite) binders are used.

Coal dust, wood flour, fuel oil are used as additives to improve certain characteristics of

moulding sand. The sand should have 2% - 8% water10

content.

Properties of Moulding Sand:

Refractoriness: enables sand to withstand high temperature of molten metal

without fusing.

Permeability11: allows gases and steam to escape through the mould. If they

are trapped inside, casting will get damaged or mould could blast.

Flowability12: allows sand to flow during ramming to all portions of the

mould and pack properly around the pattern.

Cohesiveness13: allows the rammed sand grains to bind together firmly so

that mould retains its shape even after pattern is removed. Also, the sand

grains retain shape even when molten metal is poured over it.

Collapsibility: allows automatic collapse during solidification of casting to

enable free contraction of metal. If collapsibility is less, casting will develop

tears and cracks.

c. Moulding processes:

i. Sand moulding14: Two boxes are generally used for preparing the mould. The

lower box is called Drag and upper box is called Cope. Pattern is placed inside

the Drag and is filled up with green sand. This sand is rammed well and pattern is

removed. The mould is ready for molten metal to be poured into it. No further

baking of sand is required.

9 Floor sand is also called Black Sand or Baking Sand.

10 Water is present as film between sand grains. This water is called Pore Water. Upto 8% water content is held in

this form within the sand. If water content increases beyond 8%, it remains as Free Water. This is harmful for moulding. 11

Permeability is also called Porosity. 12

Flowability is also called Plasticity. 13

Cohesiveness is also called Green Strenght. 14

Sand Mould: If green sand is not used and dry sand is used, then some binders are mixed with dry sand. In that case, the mould has to be heated for sand & binder to bond together. Such a sand mould is called Dry Sand Mould.


Page 6 of 16

ii. Pit moulding: When the casting is so large that it cannot be accommodated

inside a mould box15

, it is moulded in pits in the ground. Moulding floor is dug in

square or rectangular shape and lined with brick work or concrete, particularly at

the bottom, to withstand heavy fluid pressure during pouring. The pit acts as the

drag box. Separate drag box is not used. However, a cope is used for the second

half of the pattern. This method is used for extremely large castings only.

iii. Machine moulding: is used for mass production of castings, especially when

identical castings are required in large numbers. Various machines are used

which ram the sand, roll over the mould16

, form gates, rap the pattern and

withdraw the pattern.

1. Jolt Machine: has an air operated piston and a cylinder. Air enters from

the bottom side of piston and raises it. When piston rises, the table

attached to it also rises. Mould box is kept securely on the table. Air is

then suddenly released. The table falls with a jolt. This jolt properly

packs the sand around the pattern in the box.

2. Squeezer Machine: Box is placed between two plates. The lower plate

is lifted slowly and the top plate presses down on the sand in the box and

squeezes it. After squeezing, the lower plate is lowered slowly and box is

removed.

3. Jolt-squeezer Machine: This combines both machines in one. First a

drag is kept and by jolting the drag mould is prepared. Then the box is

turned upside down and cope is placed over it. Cope is prepared by

squeezing method. It is a complicated machine as the cylinders in the top

and bottom must be synchronized properly.

15

Mould Box is also called Flask. 16

When mould is prepared, drag is prepared first. When drag is prepared, pattern is placed on the bench or floor, sand is rammed into it, and then the drag box is turned over so that pattern comes up. This process is called Roll over.


Page 7 of 16

4. Sand slinger: It has a hopper to carry sand, a bucket elevator with a

number of buckets, and a sand impeller head on a swinging arm.

Prepared sand is poured into the hopper. The conveyor system brings the

sand to the impeller. The swinging arm is used to aim the impeller

properly on the mould box. The impeller shoots a steady stream of sand

at high velocity and sand is filled.

5. Roll-over Machine: It has a roll-over frame attached on trunnions.

Pattern is mounted on a pattern plate and is secured to the roll-ver frame.

Box is placed on the pattern plate and clamped. Sand is filled by a slinger

and rammed. Then the entire table is rotated by 180O

which takes the

drag box down below. This suspended box is lowered onto a plate and

unclamped. Again the process continues for a cope box.

6. Continuous Casting Combined Machine: combines the characteristics

of all the machines mentioned above.

iv. Shell moulding17: A metallic pattern is prepared and kept on a metal plate.

Runners, gates and risers are added. This unit is heated to 232OC and sprayed

with the silicon release agent. This agent prevents the shell from sticking to the

pattern and enables easy removal. Sand-Resin mixture is blown over the hot

pattern. A shell of uniform thickness is formed. The pattern with the shell is

removed and heated again to temperatures between 315OC & 427

OC to cure the

resin and harden the shell. The curing time is 1 minute. Shell is then stripped off

from the pattern. Cores are inserted in position and the two halves of the shells

are clamped together. Molten metal is then poured into it. Cores are prepared

separately using a sand-resin mixture in a pre-heated core box.

Advantages of this process are that it can be easily mechanized and very high

surface finish is obtained. Complex shapes and designs can be moulded and cast,

which is not possible in other methods.

Disadvantages are the high cost of pattern and the sand-resin mixture.

III. Melting practice:

a. Types of furnace with specific applications: A Foundry uses different types of furnaces

in order to re-melt metals and pour the molten metal into previously prepared mould

cavities to obtain castings. The type of furnace used depends on rate of melting desired,

type of metal to be melted, temperature required, method of pouring and cost of melting.

i. Crucible Furnace: It uses a large container called crucible, made of refractory

material. It can be coke or oil or gas fired crucible furnace depending on fuel

17

Shell moulding is also called Croning Process or C-Process.


Page 8 of 16

used. Coke fired crucible furnace is used for melting small quantity of pig iron

and other alloys.

ii. Oil-fired tilting furnace

iii. Cupola furnace

iv. Electric direct arc furnace

v. Electric induction furnace

b. Cupola furnace: is generally used in foundries for melting cast iron. Its initial cost amd

maintenance cost is low. It requires small floor space compared to other furnaces of

similar capacity.

i. Parts of Cupola: It has a hollow vertical cylinder made of MS plates18 which are

riveted or welded. This is lined with refractory bricks inside. Bottom door is

supported on a Prop. There is a door and platform for charging near the top called

Charging Door & Platform. The top has a mesh screen and spark arrestor. Mesh

screen prevents ash from flying out. Spark arrestor deflects sparks back to inside.

A wind belt encircles the cupola shell above the bottom19

. This has a set of pipes

called Tuyures to carry air to the burning area. It is connected to the furnace

18

The upper portion is made of 6mm MS plates and lower portion is from 10mm MS plates. 19

Distance between Bottom Door and Tuyures is 5 times the dia of Cupola.


Page 9 of 16

blower by means of a blast pipe. There are two spouts on opposite sides for

tapping molten metal and slag. The slag spout20

is placed above the metal spout.

The area between bottom door and tuyures is called Well21.

ii. Preparation of Cupola: Waste material & slag from previous melting is

cleaned. Fire bricks are checked and replaced if required. After cleaning, bottom

door is closed. A sand bed is prepared by ramming sand from the charging door.

Average thickness of this bed is 10cm. the bed slopes towards the tapping spout

for metal to flow out.

iii. Charging the Cupola: Soft wood mixed with broken pieces of coke is placed on

the bed. This is known as Bed Charge. Coke is introduced from charging door

after kindling the fire, up to a height of about 30cm and then a layer of metal

charge, alternating with coke till the height of charging door. The metal charge

consists of pig iron, scrap and flux. About 50kgs of Limestone is used as flux. In

each layer, metal to coke ratio will be 8:1. O2

iv. Zones in a Cupola:

1. Well: This is the area where molten metal will get collected.

2. Combustion Zone22: is located up to 30cm from tuyures. Combustion

takes place here consuming all free oxygen from air blast and producing

a lot of heat which meets all the requirements of other zones of cupola.

Temperatures of 1540OC to 1870

OC are achieved here.

3. Reducing Zone23: is located between top of combustion zone and top of

coke bed. CO2 is reduced to CO here through an endothermic reaction.

As a result, temperature falls to 1200OC.

4. Melting Zone: is the first layer of metal charge. Solid metal melts and

trickles down through the coke to the well of cupola. Molten metal picks

up sufficient carbon content in this zone.

5. Preheating Zone: extends from melting zone up to charging door. It

contains a number of alternate layers of coke & metal charges. The

20

Slag hole is kept about 30cm from bottom door. 21

Depth of Well is about 60cm. 22

This is also known as Oxidizing Zone. 23

This is also known as Protective Zone because this zone protects the metal charge from getting oxidized.


Page 10 of 16

advancing hot gases from combustion zone heats up metal in this zone up

to 1093OC.

6. Stack: is the empty portion of cupola above preheating zone.

c. Electric arc furnace: consists of a steel shell having a spherical bottom. The furnace is

mounted on rollers so that it can be tilted for pouring the melt into ladles. The hearth

inside the furnace is bowl-shaped and is lined with magnesite or dolomite. Two spouts

are provided on either side, one for slag and other for molten metal. Roof is detachable

and charge is fed in through roof. Three vertical electrodes are suspended through the top.

A 3-phase current is passed. These electrodes can be raised and lowered as desired.

After charging, the roof is closed and electrodes are lowered. Current is switched on till

arc is generated. This creates a high temperature of about 2000OC and above, which

melts the metal. As the level of molten metal rises, electrodes are also raised

automatically. Charge is mainly steel scrap with flux. Alloy additions are made later on

for getting final composition.

IV. Casting principle & operation: Casting is an important manufacturing process. Innumerable

components are manufactured by this process. It is also the oldest known manufacturing

process dating back to many centuries before Christ. The whole process has three distinct

phases. First of all, a replica of the desired component is prepared, called Pattern. Using this

replica, a cavity is prepared called Mould. Metal is melted and poured into this cavity. When

the metal cools and solidifies, it takes the desired shape. Each of these phases has developed

to great extent today.


Page 11 of 16

V. Special casting processes:

a. Die casting24: In this process, molten metal is forced into a permanent mould (called

Die) under pressure. The advantage is that this method helps for speeding,

mechanization & automation of casting process. There are two types:

i. Hot-chamber die casting: It has a plunger which acts inside a cylinder formed

at the end of a goose-neck injector. This goose-neck injector is submerged inside

molten metal. This goose-neck portion is immersed in a hot bath of molten metal.

There is a port in the cylinder which allows entry of molten metal into the

cylinder. The down stroke of plunger closes this port and applies pressure on

molten metal present in the injector. Molten metal rushes into the die cavity

through injector nozzle. The die then opens and the casting is ejected. Process is

repeated. This is generally used for low melting point alloys.

ii. Cold-chamber die casting: Here, the metal is melted separately in a furnace and

poured into the cold chamber by means of a ladle. Molten metal is poured in the

cold chamber after the dies are closed. Plunger is pushed hydraulically to force

metal into the die cavity. After solidification, die opens and casting is ejected.

This process is used for aluminum and brass casting which require high

temperatures.

24

This is also called Pressure Die Casting. If pressure is not applied, then it becomes Permanent Mould Casting.


Page 12 of 16

b. Centrifugal casting: consists of rotating the mould at high speed as molten metal is

poured into it. Due to centrifugal force, metal is forced away from center. This results in

uniform thickness of casting. Impurities in molten metal, being lighter, remain near the

center and do not enter the casting. Thus castings of great accuracy and better physical

properties are manufactured by this process.

i. True centrifugal casting: has a large cylindrical mould with rammed sand

lining. Mould is rotated between two sets of rollers as shown. Pouring is done in

the mould through a pouring basin formed on the body of a trolley. Initially

during pouring, mould is rotated at slow speed. After pouring is over, mould is

rotated at very fast speed to have even distribution of metal all along the inside

surface. The end of the mould is covered with an end core. Wall thickness of

casting is controlled by volume of metal poured. Pouring temperature for cast

iron is around 1500OC. This is mainly used for casting huge pipes.

ii. Semi-centrifugal casting25: is used for symmetrical castings only, such as

pulleys, wheels, gear-blanks, etc. Mould is rotated about a vertical axis and metal

is poured through a central sprue. Several moulds can be stacked together, one

over the other and fed simultaneously through a common central sprue. This

increases rate of production. The speed of rotation26

is much lower than in true

centrifugal casting.

25

This is also called Profile Centrifugal Casting. 26

Speed of rotation in semi-centrifugal casting is generally such that a linear speed of 180m/min is obtained at the

outer edge of the casting.


Page 13 of 16

c. Investment casting27: consists of preparing an expandable wax pattern by pouring wax28

into a metal die. This wax pattern is used for making a mould of investment material,

which consists of a refractory material and a binder. This investment mould is used for

casting with metal. The metal die is generally made of steel. Cores, if required, are made

of metal and then removed from wax pattern. The two halves of die are closed and

molten wax is injected. Solidification takes about a minute. A number of these wax

patterns are welded using wax to a gating system so that they are held in one mould. This

is wax pattern is then dipped in a slurry mixture of refractory grains & binder, repeatedly

for multiple coat of the investment material. This invested mould is then heated up to

1000OC till all the wax is melted and drained out. This wax can be re-used. The

investment mould is preheated to a suitable temperature and molten metal is poured into

it. After cooling, the mould is fettled and individual cast parts are separated from the

gating system.

27

It is also called Lost Wax Process. 28

Mercury also is used instead of wax. Then the process is called Mercast Process. Here, the die filled with liquid mercury is dipped in refrigerated acetone bath. Mercury freezes. Die is removed and mercury pattern is ready. It is

dipped in refractory slurry at the refrigerated temperature till investment coating is obtained. Then it is brought to

room temperature and all mercury is drained. The advantage of using mercury is no extra heat is needed.


Page 14 of 16

Advantages of Investment Casting are high dimensional accuracy can be obtained; very

thin sections can also be cast; complex designs can be cast; high quality castings are

obtained and surface finish is high. Disadvantages are size of casting is limited; large

castings cannot be produced; cost of production is high and rate of production is low.

VI. Casting defects:

Factors of defects: Casting defects arise from design of casting or pattern, defects in

moulding and core-making equipments, defects in mould and core materials, defects in gates

and risers, defects in melting and pouring and defects in metal composition.

a. Blowholes29: are cavities in the casting. They occur due to entrapped gas bubbles in the

casting. Excess moisture content, insufficiently baked Core, use of rusted chills, chaplets

and metal inserts, and inadequately rammed mould are the causes.

b. Porosity30: During solidification, gases are released. When these gases escape out of the

metal, they leave a trail of very small holes throughout the casting. These holes are called

Pinholes. These holes are so small that only X-Rays can detect them. This defect is due

to higher temperature of molten metal and slower solidification of metal. Remedy is to

maintain adequate temperature of metal, add adequate amount of flux, proper gating and

risering to be provided for quicker solidification, increase the permeability of mould.

c. Shrinkage31: There is volumetric shrinkage when metal solidifies. This should be

adequately compensated by proper feeding. If that is not done, voids are created in the

casting. This occurs due to inappropriate gating, risering and chilling.

d. Misrun & cold shut: Misrun is an incomplete casting because molten metal could not

reach every part of the mould. When molten metal flows into the mould from two

different directions, the liquid must meet. If it doesnt meet, there is discontinuity in the

casting. This is called Cold Shut. This defect is due to improper design (such as very thin

sections, where misrun happens) and improper temperature of metal during pouring.

e. Inclusions: are foreign materials present in the casting. They may be oxides, slag, dirt,

sand or gas. This happens due to improper fluxing and inadequate skimming of molten

metal surface in the ladle before pouring. Oxide formation is minimized when proper

gating and proper pouring is done. Sand inclusions are due to improper ramming and bad

choice of moulding sand.

f. Hot tears32: occurs due to excessive shrinkage. When the metal solidifies, if its strength

is low, then it wont be able to withstand the high thermal stresses induced by solid

shrinkage. If they are present on the surface, they can be detected. Internal hot tears are

very dangerous and can be detected by X-Rays and NDT33

. Hot tears can be avoided by

proper designing.

g. Cuts & Washes: occur due to erosion of sand from the mould or core surface by molten

metal. Molten metal fills up these areas and appear as Scabs on the surface. The eroded

29

Blow holes on the surface are called Open Blows. Internal holes are called Blow Holes. 30

Porosity defects are also called Pinhole Porosity or Gas Porosity. 31

Shrinkage is of two types: Surface Shrinkage & Internal Shrinkage. 32

Hot tears are also called Pulls or Hot Creaks. 33

NDT: Non-Destructive Testing.


Page 15 of 16

sand could get included somewhere else in the casting. This occurs due to inadequate

ramming, improper use of binders and additives and improper gating.

h. Metal penetration: occurs as a rough and uneven external surface on the casting. It

happens when molten metal enters into the spaces between the sand grains and holds

some of the sand tightly with it even after fettling34

. This occurs due to use of coarse sand

grains and soft ramming.

i. Drop: occurs when a portion of sand breaks away from the mould and falls into the

molten metal. This happens due to low sand strength and soft ramming.

j. Fusion: appears as a rough glassy surface over the casting. When molten metal enters the

mould cavity, it comes in contact with the sand and due to excessive heat of metal; sand

gets fused into the casting surface. It occurs due to low refractoriness of sand. It is

remedied by proper additives in sand.

k. Shot metal: Sometimes when metal is poured into the cavity, it may splash. The small

particles of molten metal separated from the main stream during the spray are thrown

ahead and solidify quickly to form small metal shots. If these shots fail to fuse with the

flowing metal, they remain embedded in the casting. Proper pouring temperature is to be

maintained to avoid this defect. Also, sulphur content must be regulated to avoid

splashing.

l. Shift: is a misalignment between two mating surfaces, leaving a small gap between them

and changing their relative location. This may occur at the parting line (Mould shift) or at

the core-print (Core shift). This occurs due to improper matching of the drag and cope,

and due to defective mould boxes. Also, when the molten metal is poured in large

quantities for large castings, there is tremendous pressure created inside the mould. If

proper weights are not placed on the mould box, shifts happen.

m. Swells: are unintentional bulging on casting surface. This is caused by liquid metal

pressure which pushes back the mould sand at certain places. It occurs due to soft

ramming.

n. Warpage: is an undesirable deformation in the casting which may occur during

solidification. It occurs due to internal stresses developed in the casting due to differential

solidification in different sections. The main cause is faulty design.

*******************

1. What is pattern making? What are the advantages of wood as pattern material? 2. Define:

a. Pattern allowance b. Shrinkage allowance c. Machine allowance d. Draft allowance e. Sand casting f. Die casting g. Malleable casting h. Investment casting i. Core j. Chaplets

34

Fettling: After casting is removed from mould, the sand is removed by vibrating on a vibratory screen. This

process is called fettling.


Page 16 of 16

3. Write short notes on: a. Split pattern b. Cope and drag pattern c. Sweep pattern d. Loose piece pattern e. Solid pattern

4. What are master patterns? 5. What are the factors which govern the selection of a proper material for pattern making? 6. How are patterns classified? Explain the specific advantage of Match Plate patterns? Describe how they are

used for making moulds?

7. What are the common allowances provided on patterns and why? 8. Classify moulding process. 9. What is the best shape of a riser and why? 10. What are the desired properties of moulding sands? 11. How are the different mould materials classified? What are the factors which influence their selection for a

particular use?

12. Describe different steps of moulding sand preparation 13. What is core? Describe different steps of core-making. 14. What is the purpose of using a core? 15. What are the different types of machines used in core-making? 16. What are the characteristics of a good core? 17. What is meant by green strength and dry strength as applied to moulding sand? 18. What do you understand by the term Gating System? 19. Why are risers used in moulds? 20. How does a cold chamber die casting machine differ from a hot chamber machine? 21. Describe with neat sketch hot chamber die casting method and cold chamber die casting method. 22. What are the main methods of casting applied to investment casting? 23. What are the advantages & disadvantages of true centrifugal casting? 24. Name furnaces used for melting metals. 25. What are the defects of casting system? 26. What are the causes and remedies of following casting defects?

a. Fusion b. Hot tears c. blowholes

27. Explain the term Directional Solidification as applied to castings.

****************

Documents

A Basic Course on Casting Process