Classification of Moulding sand
1: Green sand
• The sand in its natural or moist state is called green sand. It is also called
tempered sand. It is a mixture of sand with 20 to 30 percent clay, having
total amount of water from 6 to 10 percent. The mould prepared with this
sand is called green sand mould, which is used for small size casting of
ferrous and non-ferrous metals.
2: Dry Sand
• The green sand moulds when baked or dried before pouring the molten
metal are called dry sand moulds. The sand of this condition is called dry
sand. The dry sand moulds have greater strength, rigidity and thermal
stability. These moulds used for large and heavy casting.
3: Loam Sand
• A mixture of 50 percent sand grains and 50 percent clay is called loam sand.
It is used for large grey iron casting.
4: Facing Sand
• A sand which is used before pouring the molten metal, on the surface is
called facing sand. It is specially prepared sand from silica sand and clay.
5: Backing or Floor Sand
• A sand used to back up the facing sand and not used next to the
pattern is called backing sand. The sand which have been
repeatedly used may be employed for this purpose. It is also known
as black sand due to its colour.
6: System Sand
• A sand employed in mechanical sand preparation and handling
system is called system sand. This sand has high strength,
permeability and refractoriness.
7: Parting Sand
• A sand employed on the faces of the pattern before the moulding is
called parting sand. The parting sand consists of dried silica sand,
sea sand or burnt sand.
8: Core Sand
• The cores are defined as sand bodies used to form the hollow
portions or cavities of desired shape and size in the casting. Thus
the sand used for making these cores is called core sand. It is
sometimes called oil sand. It is the silica sand mixed with linseed oil
or any other oil as binder.
Coke fired Crucible Furnace
Cupola Furnace
Shell Molding
• A match plate or cope-drag metal pattern is heated and
placed over a box containing sand mixed with thermosetting
resin.
• Box is inverted so that sand and resin fall onto the hot
pattern, causing a layer of the mixture to partially cure on the
surface to form a hard shell.
• Box is repositioned so that loose, uncured particles drop away.
• Sand shell is heated in oven for several minutes for complete
curing.
• Shell mold is stripped from the pattern
• Two halves of the shell mold are assembled, supported by
sand or metal shot in a box, and pouring is accomplished. The
finished casting with sprue is removed.
Advantages
• Smoother cavity surface permits easier flow of
molten metal and better surface finish on casting
• Good dimensional accuracy
• Machining often not required
• Mold collapsibility usually avoids cracks in casting
• Can be mechanized for mass production
Disadvantages
• More expensive metal pattern
• Difficult to justify for small quantities
Investment Casting
• Pattern creation - The wax patterns
are typically injection molded into
a metal die and are formed as one piece.
• Several of these patterns are attached
to a central wax gating system (sprue,
runners, and risers), to form a tree-like
assembly.
• The gating system forms the channels
through which the molten metal will flow
to the mold cavity
Mold creation –
• This "pattern tree" is dipped into a slurry
of fine ceramic particles, coated with more
coarse particles, and then dried to form a
ceramic shell around the patterns and gating
system.
• This process is repeated until the shell is
thick enough to withstand the molten metal
it will encounter.
• The shell is then placed into an oven and the
wax is melted out leaving a hollow ceramic shell
• that acts as a one-piece mold, hence the name
"lost wax" casting.
Pouring
• The mold is preheated in a
furnace to approximately 1000°C
(1832°F) and the molten
metal is poured from a ladle
into the gating system of the
mold, filling the mold cavity.
• Pouring is typically achieved
manually under the force of
gravity, but other methods such
as vacuum or pressure are
sometimes used
Cooling –
• After the mold has been filled, the molten metal is allowed
to cool and solidify into the shape of the final casting.
• Cooling time depends on the thickness of the part,
thickness of the mold, and the material used.
Casting removal –
After the molten metal has cooled, the mold can be broken
and the casting removed.
• The ceramic mold is typically broken using water jets, but
several other methods exist.
• Once removed, the parts are separated from the gating
system by either sawing or cold breaking
Finishing
Often times, finishing operations such as grinding or
sandblasting are used to smooth the part at the gates.
• Heat treatment is also sometimes used to harden the final
part.
Advantages
• – Parts of great complexity and intricacy can be cast
• – Close dimensional control and good surface finish
• – Wax can usually be recovered for reuse
• – Additional machining is not normally required - this
is a net shape process
Disadvantages
• – Many processing steps are required
• – Relatively expensive process
Die Casting
• In Die casting the molten metal is forced to flow into a
permanent metallic mold under moderate to high
pressures, and held under pressure during solidification
• This high pressure forces the metal into intricate details,
produces smooth surface and excellent dimensional
accuracy
• High pressure causes turbulence and air entrapment. In
order to minimize this larger in-gates are used and in
the beginning, pressure is kept low and is increased
gradually
Hot Chamber Casting
Cold Chamber Casting
Centrifugal Casting
General Defects: Misrun
A casting that has solidified before
completely filling mold cavity
Some common defects in castings: (a) misrun
General Defects: Cold Shut
Two portions of metal flow together
but there is a lack of fusion due to
premature freezing
Some common defects in castings: (b) cold shut
General Defects: Cold Shot
Metal splatters during pouring and solid globules
form and become entrapped in casting
Some common defects in castings: (c) cold shot
General Defects: Shrinkage Cavity
Depression in surface or internal void caused by
solidification shrinkage that restricts amount of
molten metal available in last region to freeze
Some common defects in castings: (d) shrinkage cavity
Defects: Sand Blow
Balloon-shaped gas cavity caused by
release of mold gases during pouring
Common defects incastings: (a) sand blow
Sand Defects: Pin Holes
Formation of many small gas cavities at
or slightly below surface of casting
Common defects in castings: (b) pin holes
Comparison of Casting Processes
Manufacturing Technology-I UNIT- I
PART – A (2 Marks)
1. How special forming process is defined?
2. What is metal spinning process? Define casting?
3. When do you make core (or) what is function of core in moulding
sand?
4. Explain the core making process?
5. Write the composition of good moulding sand?
6. What are chaplets?
7. List the factors to be considered in the choice of metal melting
furnace?
8. What are the reasons for the casting defects of cold shuts and
misrun?
9. Name four different casting defects.
10. How casting defects are identified?
Part B (16 Marks)
1. What are the pattern allowances? Explain briefly each. (16)
2. Discuss the properties of moulding sand. (16)
3. Explain the CO2 process of core making state its advantages and
applications.(16)
4. State the different type of mould. Write a short note on „Green sand
mould‟ and shell moulding (16)
5. Write a neat sketch of a cupola, Explain its operate. (16)
6. Explain with a simple sketch how metal is melted in a Electric arc
furnace(16)
7. What are the different types of furnace used in foundry? Describe in detail
with neat sketches any one of them. (16)
8. Explain briefly the various moulding method used in foundries. (16)
9. Enumerate the continuous casting defects and suggest suitable remedies
(16)
10. Explain the various non –destructive inspection methods of cast
products. (16)