Introduction to Casting processes

1. CASTING
M.SIVAKUMAR

2. DEFINE CASTING
 Casting means pouring molten metal into a refractory mould
with a cavity of the shape to be made, and allowing it to
solidification. The solidified object is called as casting.
 Application of casting are cylinder, blocks, piston, piston ring,
wheels, housings, etc.
Advantage Disadvantage
• Molten metal can flow in any
small section.
• Any intricate shape can be
made.
• It is possible to cast any almost
material (Ferrous and non
ferrous )
• Casting is cooled uniformly, so
there is no directional
solidification.
• Poor surface finish.
• Dimensional inaccuracy.
• Defect arising out of the
moisture present in the casting.

4. CASTING TERMS
1.Flask: A metal or wood frame,
without fixed top or bottom, in
which the mold is formed.
drag - lower molding flask,
cope - upper molding flask,
cheek - intermediate molding flask
used in three piece molding.

5. CASTING TERMS:
2. Pattern: It is the replica of
the final object to be made.
The mold cavity is made
with the help of pattern.
3. Parting line: This is the
dividing line between the
two molding flasks that
makes up the mold.
Pattern

6. 4. Pouring basin: A small funnel shaped cavity at the top of the mold
into which the molten metal is poured.
5. Sprue: The passage through which the molten metal, from the
pouring basin, reaches the mold cavity. In many cases it controls the
flow of metal into the mold.

7. 6. Runner: The channel through which the molten metal is
carried from the sprue to the gate.
7. Riser: A column of molten metal placed in the mold to
feed the castings as it shrinks and solidifies. Also known
as feed head.
8. Gate: A channel through which the molten metal enters
the mold cavity.

8. 9. Core: A separate part of the mold, made of sand and
generally baked, which is used to create openings
and various shaped cavities in the castings.
10.Chaplets: Chaplets are used to support the cores
inside the mold cavity to take care of its own weight
and overcome the metallostatic force.
11. Vent: Small opening in the mold to facilitate escape
of air and gases.

9. APPLICATIONS OF CASTING:
 Transportation vehicles
 Turbine vanes
 Power generators
 Railway crossings
 Agricultural parts
 Aircraft jet engine parts
 Sanitary fittings
 Communication, Construction and Atomic Energy
applications, etc..

10. FUNCTIONS OF PATTERNS:
 A Pattern prepares a mould cavity for the purpose of
making a casting.
 A Pattern may contain projections known as core prints
if the casting requires a core and need to be made
hollow.
 Patterns properly made and having finished and
smooth surfaces reduce casting defects.
 Properly constructed patterns minimize overall cost of
the casting.

11. The pattern material should be:
1. Easily worked, shaped and joined.
2. Light in weight.
3. Strong, hard and durable.
4. Resistant to wear and abrasion .
5. Resistant to corrosion, and to chemical reactions.
6. Dimensionally stable and unaffected by variations in
temperature and humidity.
7. Available at low cost.

12. MATERIALS FOR MAKING PATTERNS:
WOOD
METAL
PLASTIC
PLASTER
WAX

13. TYPES OF PATTERNS:
Single piece pattern.
Split pattern
Loose piece pattern
Match plate pattern
Sweep pattern
Gated pattern
Skeleton pattern
Follow board pattern
Cope and Drag pattern

14. (a)Split pattern
(b) Follow-board
(c) Match Plate
(d) Loose-piece
(e) Sweep
(f) Skeleton
pattern

15. TYPES OF PATTERN ALLOWANCES
The various pattern allowances are:
1. Shrinkage allowance
2. Machining allowance
3. Draft allowances
4. Distortion allowance
5. Shake allowance.
VIDEO

16. SAND CASTING
 Sand casting is a cast part produced by forming a
mold from a sand mixture and then pouring molten
liquid metal into the cavity in the mold. The mold is
then cooled until the metal has solidified
 Most widely used casting process, accounting for a
significant majority of total tonnage cast
 Nearly all alloys can be sand casted, including metals
with high melting temperatures, such as steel, nickel,
and titanium
 Castings range in size from small to very large
 Production quantities from one to millions

17. STEPS IN SAND CASTING
1. Pour the molten metal into sand mold CAVITY
2. Allow time for metal to solidify
3. Break up the mold to remove casting
4. Clean and inspect casting
 Separate gating and riser system
5. Heat treatment of casting is sometimes required to
improve metallurgical properties

18. SAND CASTING PRODUCTION SEQUENCE
Figure: Steps in the production sequence in sand casting.
The steps include not only the casting operation but also
pattern-making and mold-making.

19. MAKING THE SAND MOLD
 The cavity in the sand mold is formed by packing sand
around a pattern, then separating the mold into two
halves and removing the pattern
 The mold must also contain gating and riser system
 If casting is to have internal surfaces, a core must be
included in mold
 A new sand mold must be made for each part produced

20. FOUNDRY SANDS
Silica (SiO2) or silica mixed with other minerals
 Good refractory properties - capacity to endure high
temperatures
 Small grain size yields better surface finish on the cast
part
 Large grain size is more permeable, allowing gases to
escape during pouring
 Irregular grain shapes strengthen molds due to
interlocking, compared to round grains
 Disadvantage: interlocking tends to reduce
permeability

21. BINDERS USED WITH FOUNDRY SANDS
 Sand is held together by a mixture of water and
bonding clay
 Typical mix: 90% sand, 7% clay and 3% water
 Other bonding agents also used in sand molds:
 Organic resins (e.g , phenolic resins)
 Inorganic binders (e.g , sodium silicate and
phosphate)
 Additives are sometimes combined with the mixture to
increase strength and/or permeability

22. TYPES OF SAND MOLD
Green sand
Dry sand
Facing sand
Parting sand
Backing sand

23. DESIRABLE MOLD PROPERTIES
Refractoriness
Permeability
Green strength
Dry strength
Hot strength
Collapsibility
Adhesiveness
Cohesiveness

24. THE MOLD IN CASTING
 Mold or mould cavity is a container with cavity whose
geometry determines part shape
 Actual size and shape of cavity must be slightly
oversized to allow for shrinkage of metal during
solidification and cooling
 Molds are made of a variety of materials, including sand,
plaster, ceramic, and metal

25. OPEN MOLDS AND CLOSED MOLDS
Two forms of mold: (a) open mold, simply a container in the shape of
the desired part; and (b) closed mold, in which the mold geometry is
more complex and requires a gating system (passageway) leading
into the cavity.
Cavity is open to atmosphere
Cavity is closed

26. TWO CATEGORIES OF CASTING PROCESSES
1. Expendable mold processes – uses an
expendable mold which must be destroyed to
remove casting
 Mold materials: sand, plaster, and similar
materials, plus binders
2. Permanent mold processes – uses a permanent
mold which can be used over and over to produce
many castings
 Made of metal (or, less commonly, a ceramic
refractory material)

27. HEATING THE METAL
 Heating furnaces are used to heat the metal to
molten temperature sufficient for casting
 The heat required is the sum of:
1. Heat to raise temperature to melting point
2. Heat to raise molten metal to desired
temperature for pouring

28. POURING THE MOLTEN METAL
 For this step to be successful, metal must flow into all
regions of the mold, most importantly the main cavity,
before solidifying
 Factors that determine success
 Pouring temperature
 Pouring rate
 Turbulence
 Pouring temperature should be sufficiently high in order
to prevent the molten metal to start solidifying on its way
to the cavity

29. POURING THE MOLTEN METAL
Pouring rate should neither be high (may stuck the
runner – should match viscosity of the metal) nor very
low that may start solidifying on its way to the cavity
Turbulence should be kept to a minimum in order to
ensure smooth flow and to avoid mold damage and
entrapment of foreign materials. Also, turbulence causes
oxidation at the inner surface of cavity. This results in
cavity damage and poor surface quality of casting.

30. SOLIDIFICATION: PURE METALS
- Because of the chilling action of the
mold wall, a thin skin of solid metal is
initially formed at interface immediately
after pouring.
- The skin formed initially has equi-axed,
fine grained and randomly oriented
structure. This is because of rapid
cooling.
- As freezing proceeds, the grains grow
inwardly, away from heat flow direction,
as needles or spine of solid metal.
-

31. RISER
It is a reservoir in the mold which is a source of liquid metal to
compensate for shrinkage of the part during solidification
Most metals are less dense as a liquid than as a solid so
castings shrink upon cooling, which can leave a void at the
last point to solidify. Risers prevent this by providing molten
metal to the casting as it solidifies, so that the cavity forms
in the riser and not in the casting

32. RISER DESIGN
 Riser is used to compensate for shrinkage of part during
solidification and later it is separated from the casting and
re-melted to make more castings
 There could be different designs of riser:
- Side riser: Attached to the side of casting through a channel
- Top riser: Connected to the top surface of the casting
- Open riser: Exposed to the outside at the top surface of
cope- Disadvantage of allowing of more heat to escape
promoting faster solidification.
- Blind riser: Entirely enclosed within the mold.

33. Casting defects