5. Advantages:
Inexpensive
Easily available in large quantities
Easy to fabricate
Light in weight
Thay can be repaired easily
Easy to obtain good surface finish
Limitations:
Susceptible to shrinkage and swelling
Possess poor wear resistance
Abraded easily by sand action
Obsorb moisture, consequently get wraped
Cannot withstand rough handling
These are used where the no. of castings to
be produced is small and pattern size is large.
6. Advantages:
Do not absorb moisture
More stronger
Possess much longer life
Do not wrap, retain their shape
Greater resistance to abrasion
Accurate and smooth surface finish
Good machinability
Limitations:
Expensive
Not easily repaired
Ferrous patterns get rusted
Heavy weight
These are employed where large no. of
castings have to be produced from same patterns.
7. Advantages:
Durable
Provides a smooth surface
Moisture resistant
Does not involve any appreciable change in size or shape
Light weight
Good strength
Wear and corrosion resistance
Easy to make
Abrasion resistance
Good resistance to chemical attack
Limitations:
Plastic patterns are Fragile
These are may not work well when subject to conditions of
severe shock as in machine molding (jolting).
8. Advantages:
It can be easily worked by using wood working tools.
Intricate shapes can be cast without any difficulty.
It has high compressive strength.
Plastermaybe madeout of Plasterof parisor
Gypsumcement.
Plastermixtureis pouredintoa mould
madebya sweeppatternor a woodenmaster
pattern,in orderto obtaina Plasterpattern.
9. Advantages:
Provide very good surface finish.
Impatr high accuracy to castings.
After being molded, the wax pattern is not taken out of the mould
like other patterns; rather the mould is inverted and heated; the
molten wax comes out and/or is evaporated. Thus there is no
chance of the mould cavity getting damaged while removing the
pattern.
Wax patterns find
applications in Investment casting
process.
20. 11. Shell pattern:
• A pattern which has Hollow hole in it. only the outer
shape is used for making the mould
• The core is prepared inner surface of the pattern itself
Hole
21.
22. A pattern is larger in size as compared to the final
casting, because it carries certain allowances due to
metallurgical and mechanical reasons for example,
shrinkage allowance is the result of metallurgical
phenomenon where as machining, draft, distortion, shake
and other allowances are provided on the patterns
because of mechanical reasons.
23. The various pattern allowances are:
1. shrinkage or contraction
allowance.
2. Machining or finish allowance.
3. Draft of tapper allowances.
4. Distortion or chamber allowance.
5. Shake or rapping allowance.
24. All most all cast metals shrink or contract
volumetrically on cooling.
The metal shrinkage is of two types:
1. Liquid Shrinkage:
it refers to the reduction in volume when the metal
changes from liquid state to solid state at the solidus
temperature. To account for this shrinkage; riser, which
feed the liquid metal to the casting, are provided in the
mold.
2. Solid Shrinkage:
it refers to the reduction in volume caused when metal
loses temperature in solid state. To account for this,
shrinkage allowance is provided on the patterns.
28. Almost all cast metals shrink or contract volumetrically
after solidification and therefore the pattern to obtain a
particular sized casting is made oversize by an amount
equal to that of shrinkage or contraction.
Different metals shrink at different rates because
shrinkage is the property of the cast metal/alloy.
The metal shrinkage depends upon:
1. The cast metal or alloy.
2. Pouring temp. of the metal/alloy.
3. Casted dimensions(size).
4. Casting design aspects.
5. Molding conditions(i.e., mould
materials and molding methods
29. A Casting is given an allowance for machining, because:
i. Castings get oxidized in the mold and during heat treatment;
scales etc., thus formed need to be removed.
ii. It is the intended to remove surface roughness and other
imperfections from the castings.
iii. It is required to achieve exact casting dimensions.
iv. Surface finish is required on the casting.
How much extra metal or how much machining allowance
should be provided, depends on the factors listed below:
i. Nature of metals.
ii. Size and shape of casting.
iii. The type of machining operations to be employed for cleaning the
casting.
iv. Casting conditions.
v. Molding process employed
30. It is given to all surfaces perpendicular to parting line.
Draft allowance is given so that the pattern can be
easily removed from the molding material tightly
packed around it with out damaging the mould cavity.
The amount of taper depends upon:
i. Shape and size of pattern in the depth
direction in contact with the mould
cavity.
ii. Moulding methods.
iii. Mould materials.
iv. Draft allowance is imparted on internal
as well as external surfaces; of course
31.
32.
33. A casting will distort or wrap if :
i. It is of irregular shape,
ii. All it parts do not shrink uniformly i.e., some
parts shrinks while others are restricted from
during so,
iii. It is u or v-shape,
iv. The arms possess unequal thickness,
v. It has long, rangy arms as those of propeller
strut for the ship,
vi. It is a long flat casting,
vii. One portion of the casting cools at a faster
34. A patter is shaken or rapped by striking the same with a
wooden piece from side to side. This is done so that the
pattern a little is loosened in the mold cavity and can be
easily removed.
In turn, therefore, rapping enlarges the mould cavity
which results in a bigger sized casting.
Hence, a –ve allowance is provided on the pattern i.e.,
the pattern dimensions are kept smaller in order to
compensate the enlargement of mould cavity due to
rapping.
The magnitude of shake allowance can be reduced by
increasing the tapper.
35. Moulding sands are classified according to their use.
These are classified and described below :
Green Sand
It is a mixture of silica sand with 18 to 30 percent clay,
having quantity of water 6 to 8 percent. Green sand in its
natural state contains enough moisture to give it
sufficient bonding property. It is soft, light, porous and
retains the shape easily when squeezed in the hand.
Moulds prepared by this sand are known as green sand
moulds which are used for small and medium castings
only.
36. Dry Sand
When moisture from green sand mould is
removed, it is known as dry sand mould and is used
for large size of casting. By drying the mould in
moulding box it becomes stronger and compact.
Facing Sand
It is used directly next to the surface of pattern.
When the mould is poured with the molten metal it
comes directly in contact with the molten metal.
As it is subjected to most severe conditions, it must
possess high strength and refractoriness. It is made
of silica sand and clay in fine powder form.
37. Loam Sand
It is a mixture of clay (about 50%), sand and water
(about 18-20%) to obtain a thin plastic paste which is
used to plaster on moulds with soft bricks and
hardens on drying. This is particularly employed for
loam moulding usually for rough and large castings.
Backing Sand
It is the sand obtained from mould and is used again
and again. Due to its black colour which is due to
burning and addition of coal dust, it is also known as
black sand.
It reduce the cost of moulding.
38.
39. Parting Sand
It is fine sharp dry sand used to keep the green
sand from sticking to the pattern and also to keep
the moulding boxes (drag and cope) separated.
Core Sand
This is silica sand mixed with core oil which is
composed of linseed oil, light mineral oil, resin and
other binding materials. For the sake of economy,
pitch or flours and water may also be used in case
of large cores.
40.
41. A large variety of molding materials is used in foundries for
manufacturing molds and cores. They include molding
sand, system sand or backing sand, facing sand, parting
sand, and core sand. The choice of molding materials is
based on their processing properties. The properties that
are generally required in molding materials are:
42. It is the ability of the molding material to with stand
high temperatures (experienced during pouring)
with out
1. Fusion,
2. Cracking, buckling or scabbing,
3. Experiencing any major physical
change.
Silica sand have high
refractoriness.
43. During pouring and subsequent solidification of a
casting, a large amount of gases and steam is
generated.
These gases are those that have been absorbed by the
metal during melting, air absorbed from the atmosphere
and the steam generated by the molding and core sand.
If these gases are not allowed to escape from the mold,
they would be entrapped inside the casting and cause
casting defects.
To overcome this problem the molding material must be
porous.
Proper venting of the mold also helps in escaping the
gases that are generated inside the mold cavity.
44. • The molding sand that contains moisture is termed as
green sand.
• The green sand particles must have the ability to cling to
each other to impart sufficient strength to the mold.
• The green sand must have enough strength so that the
constructed mold retains its shape.
• Green strength helps in making and handling the
moulds.
45. A mould may either intentionally be dried, or a green
sand mould may lose its moisture and get dried while
waiting for getting poured or when it comes in contact
with molten metal being poured.
The sand thus dried must have dry strength to
1. Withstand erosive forces due to
molten metal,
2. Withstand pressure of molten
metal,
3. Retain its exact shape,and
4. Withstand the metallostatic
pressure of the liquid material.
46. As soon as the moisture is eliminated, the sand would
reach at a high temperature when the metal in the mold is
still in liquid state.
The strength of the sand that is required to hold the shape of
the cavity is called hot strength.
In the absence of adequate hot strength, the mold may
1. enlarge
2. break, erode or
3. get cracked.
47. Collapsibility determines the readiness with which the
molding sand,
1. Automatically gets collapsed after
the casting solidifies, and
2. Breaks down in knock out and
cleaning operations.
If the mould or core does not collapse, it may restrict
free contraction of solidifying metal and cause the
same to tear or crack.
48. • It is the ability of the molding sand to get compacted
to a uniform density.
• Flowability assists molding sand to flow and pack
all-around the pattern and take up the required
shape.
• Flowability increases as clay and water contents
increase.
49. It is the property of molding sand owing to which, it
1. Sticks with the walls of molding boxes,
2. Sticks with gaggers, and
3. Thus makes it possible to mold cope
and drag.
50. • Finer sand mould resist metal penetration and
produce smooth casting surfaces.
• Fineness and permeability are in conflict with
each other and hence they must be balanced for
optimum results.
51. The main ingredients of any molding sand are:
• Base sand,
• Binder, and
• Moisture
52. • Silica sand is most commonly used base sand.
• Other base sands that are also used for making mold are
zircon sand, Chromite sand, and olivine sand.
• Silica sand is cheapest among all types of base sand and
it is easily available.
53. Binders are of many types such as:
1. Clay binders,
2. Organic binders and
3. Inorganic binders
Clay binders are most commonly used binding agents mixed
with the molding sands to provide the strength.
The most popular clay types are:
◦ Kaolinite or fire clay (Al2O3 2 SiO2 2 H2O)
and Bentonite (Al2O3 4 SiO2 nH2O)
◦ Of the two the Bentonite can absorb more
water which increases its bonding power.
54.
55. Clay acquires its bonding action only in the presence of the
required amount of moisture.
When water is added to clay, it penetrates the mixture and
forms a microfilm, which coats the surface of each flake of
the clay.
The amount of water used should be properly controlled.
This is because a part of the water, which coats the
surface of the clay flakes, helps in bonding, while the
remainder helps in improving the plasticity.
57. Moisture content Test
Clay content Test
Chemical composition of sand
Grain shape and surface texture of sand.
Grain size distribution of sand
Refractoriness of sand
Strength Test
Permeability Test
Flowability Test
Shatter index Test
Mould hardness Test.
58. Moisture content Test
CaC2 + 2H2O--->C2H2 + Ca(OH)2
Amount of acetylene released is directly
proportional to moisture content.
speedy moisture teller
59. Dried 50 g sample + 475ml water + 1% NaOH
Floating clay removed
Till clear sand
Dried
Weight
60. • AFS = sum of products / total sum of the percentage of
sand retained on pan and each sieve
Sum of product =
Sum of (
precentage of
sand retained in
each sieve *
factor)
61.
62.
63.
64.
65. • P = VH /ATp
• Where, P = permeability
• V = volume of air passing through the specimen in c.c.
• H = height of specimen in cm
• p = pressure of air in gm/cm2
• A = cross-sectional area of the specimen in cm2
• T = time in minutes.
66. • Sand specimen heated to very high temperatures ranges
depending upon the type of sand.
• The heated sand test pieces are cooled to room temperature
and examined under a microscope for surface characteristics
or by scratching it with a steel needle.
• Sharply silica sand grains easily give way to the needle.
• Thus we evaluating surface characteristics or by scratching it
with a steel needle.
