Metal Casting,Comparison of Casting Processes,Production steps in sand casting including pattern making and mold making,Patterns,Casting Defects

1. Lecture 3
Metal Casting

2. TABLE 11.1
Process
Advantages
Limitations
Sand
Some finishing required;
somewhat coarse finish; wide
tolerances.
Shell mold
Good dimensional accuracy and
surface finish; high production
rate.
Part size limited; expensive
patterns and equipment
required.
Expendable pattern
Summary of
Casting
Processes
Almost any metal cast; no limit
to size, shape or weight; low
tooling cost.
Most metals cast with no limit
to size; complex shapes
Patterns have low strength and
can be costly for low quantities
Plaster mold
Intricate shapes; good
dimensional accu- racy and
finish; low porosity.
Limited to nonferrous metals;
limited size and volume of
production; mold making time
relatively long.
Ceramic mold
Intricate shapes; close
tolerance parts; good surface
finish.
Limited size.
Investment
Intricate shapes; excellent
surface finish and accuracy;
almost any metal cast.
Part size limited; expensive
patterns, molds, and labor.
Permanent mold
Good surface finish and
dimensional accuracy; low
porosity; high production rate.
High mold cost; limited shape
and intricacy; not suitable for
high-melting-point metals.
Die
Excellent dimensional accuracy
and surface finish; high
production rate.
Die cost is high; part size
limited; usually limited to
nonferrous metals; long lead
time.
Centrifugal
Large cylindrical parts with
good quality; high production
rate.
Equipment is expensive; part
shape limited.

3. Basic Features

Pattern and Mould
◦ A pattern is made of wood or metal, is a replica of the
final product and is used for preparing mould cavity
◦ Mould cavity which contains molten metal is
essentially a negative of the final product
◦ Mould material should posses refractory
characteristics and with stand the pouring temperature
◦ When the mold is used for single casting, it made of
sand and known as expendable mold
◦ When the mold is used repeatedly for number of
castings and is made of metal or graphite are called
permanent mould
◦ For making holes or hollow cavities inside a casting,
cores made of either sand or metal are used.

4. 
Melting and Pouring
◦ Several types of furnaces are available for
melting metals and their selection
depends on the type of metal, the
maximum temperature required and the
rate and the mode of molten metal delivery.
◦ Before pouring provisions are made for
the escape of dissolved gases. The gating
system should be designed to minimize
the turbulent flow and erosion of mould
cavity.The other important factors are the
pouring temperature and the pouring rate.

5. 
Solidification and Cooling
◦ The properties of the casting significantly
depends on the solidification time cooing rate.
◦ Shrinkage of casting, during cooling of solidified
metal should not be restrained by the mould
material, otherwise internal stresses may
develop and form cracks in casting.
◦ Proper care should be taken at the design stage
of casting so that shrinkage can occur without
casting defects.

6. 
Removal, Cleaning, Finishing and
Inspection
◦ After the casting is removed from the mould it
is thoroughly cleaned and the excess material
usually along the parting line and the place
where the molten metal was poured, is removed
using a potable grinder.
◦ White light inspection, pressure test, magnetic
particle inspection, radiographic test,
ultrasonic inspection etc. are used

7. Classification of casting processes

8. Open and Closed Mould

9. Sand Casting (Expandable-mould, Permanentpattern Casting)

10. Pattern geometry

11. Use of chaplets to avoid shifting of cores
Possible chaplet design
and casting with core

12. Production steps in sand casting
including pattern making and mold
making

13. Patterns

Variety of patters are used in casting and
the choice depends on the configuration of
casting and number of casting required
◦
◦
◦
◦
◦
◦
◦
◦
Single-piece pattern
Split pattern
Follow board pattern
Cope and drag pattern
Match plate pattern
Loose-piece pattern
Sweep pattern
Skeleton pattern

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

15. Pattern allowances
Shrinkage allowance
 Draft allowance
 Machining allowance
 Distortion allowance


16. Moulding Materials
Major part of Moulding material in sand casting are
1.
2.
3.
70-85% silica sand (SiO2)
10-12% bonding material e.g., clay cereal etc.
3-6% water
Requirements of molding sand are:
(a)
(b)
(c)
(d)
Refractoriness
Cohesiveness
Permeability
Collapsibility
The performance of mould depends on following
factors:
(a) Permeability
(b) Green strength
(c) Dry strength

17. Effect of moisture, grain size and shape
on mould quality

18. Melting and Pouring

The quality of casting depends on the method of melting. The
melting technique should provide molten metal at required
temperature, but should also provide the material of good
quality and in the required quantity.
Pouring vessels

19. 
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


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Molten metal is prevented from oxidation by covering the molten
metal with fluxes or by carrying out melting and pouring in
vacuum
Ladles which pour the molten metal from beneath the surface
are used
The two main consideration during pouring are the temperature
and pouring rate
Fluidity of molten metal is more at higher temperature but it
results into more amount of dissolved gases and high
temperature also damage the mould walls and results into poor
surface quality of the casting
To control the amount of dissolved gases low, the temperature
should not be in superheated range
In ferrous metals, the dissolved hydrogen and nitrogen are
removed by passing CO. In non-ferrous metals, Cl, He, or Ar
gases are used.
Therefore, fluidity and gas solubility are two conflicting
requirements. The optimum pouring temp. is therefore decided
on the basis of fluidity requirements. The temp. should be able
to fill the whole cavity at the same time it should enter inside the
voids between the sand particles.

20. 

Cooling rate depends on casting material and configuration. It
also depends on volume and surface area of the casting also.
The pouring rate should be such that solidification does not start
and the cavity is completely filled without eroding mould surface
and undue turbulence.

21. The Gating System
1.
2.
3.
4.
Minimize turbulent flow so that absorption
of gases, oxidation of metal and erosion of
mould surfaces are less
Regulate the entry of molten metal into
the mould cavity
Ensure complete filling of mould cavity,
and
Promote a temperature gradient within the
casting so that all sections irrespective of
size and shape could solidify properly

22. The Gating System
A: pouring basin
 B: Weir
 C: Sprue
 D: Sprue well
 E: Runner
 F: Ingates
 G: Runner break
up
 H: Blind
 J: Riser


23. Use of chills

24. Cooling and Solidification
Pure metal
Alloy

25. Mechanism of Solidification
Pure metals solidifies at a constant temp. equal to
its freezing point, which same as its melting point.
 The change form liquid to solid does not occur all
at once. The process of solidification starts with
nucleation, the formation of stable solid particles
within the liquid metal. Nuclei of solid phase,
generally a few hundred atom in size, start
appearing at a temperature below the freezing
temperature. The temp. around this goes down and
is called supercooling or undercooling. In pure
metals supercooling is around 20% of the freezing
temp.
 A nuclease, more than a certain critical size grows,
and causes solidification.


26. 


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By adding, certain foreign materials (nucleating agents) the
undercooling temp. is reduced which causes enhanced
nucleation.
In case of pure metals fine equi-axed grains are formed near
the wall of the mold and columnar grain growth takes place
upto the centre of the ingot.
In typical solid-solution alloy, the columnar grains do not extend
upto the center of casting but are interrupted by an inner zone
of equiaxed graines.
My adding typical nucleating agents like sodium, magnesium or
bismuth the inner zone of equiaxed grained can be extended in
whole casting.

27. Solidification Time
Once the material cools down to freezing
temperature, the solidification process for
the pure metals does not require a
decrease in temperature and a plateau is
obtained in the cooling curves, called
thermal arrest. The solidification time is total
time required for the liquid metal to solidify.
 Solidification time has been found to be
directly proportional to volume and
inversely proportional to surface area.


28. Location of Risers and Open and Closed
Risers
•Top riser has the
advantage of
additional pressure
head and smaller
feeding distance over
the side riser.
•Blind risers are
generally bigger in
size because of
additional area of
heat conduction.

29. Why Riser?



The shrinkage occurs in three stages,
1. When temperature of liquid metal drops from
pouring to zero temperature
2. When the metal changes from liquid to solid
state, and
3. When the temperature of solid phase drops
from freezing to room temperature
The shrinkage for stage 3 is compensated by
providing shrinkage allowance on pattern, while
the shrinkage during stages 1 and 2 are
compensated by providing risers.
The riser should solidify in the last otherwise liquid
metal will start flowing from casting to riser. It
should promote directional solidification. The
shape, size and location of the risers are important
considerations in casting design

30. Cleaning and Finishing
1.
2.
3.
4.
5.
Casting is taken out of the mould by shaking and
the Moulding sand is recycled often with suitable
additions.
The remaining sand, some of which may be
embedded in the casting, is removed by means
of Shot blasting.
The excess material in the form of sprue,
runners, gates etc., along with the flashes
formed due to flow of molten metal into the gaps
is broken manuaaly in case of brittle casting or
removed by sawing and grinding in case of
ductile grinding.
The entire casting is then cleaned by either shot
blasting or chemical pickling.
Sometimes castings are heat treated to achieve
better mechanical properties.

31. Casting Defects
Defects
may occur due to one or more
of the following reasons:
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Fault in design of casting pattern
Fault in design on mold and core
Fault in design of gating system and riser
Improper choice of moulding sand
Improper metal composition
Inadequate melting temperature and rate
of pouring

32. Classification of casting defects
Surface
Defect
Blow
Scar
Blister
Drop
Scab
Penetration
Buckle
Casting defects
Internal
Defect
Blow holes
Porosity
Pin holes
Inclusions
Dross
Visible
defects
Wash
Rat tail
Swell
Misrun
Cold shut
Hot tear
Shrinkage/Shift

33. Surface Defects


These are due to poor design and quality of sand
molds and general cause is poor ramming
Blow is relatively large cavity produced by gases
which displace molten metal from convex
surface. Scar is shallow blow generally occurring
on a flat surface. A scar covered with a thin layer
of metal is called blister. These are due to
improper permeability or venting. Sometimes
excessive gas forming constituents in moulding

34. 



Drop is an irregularly-shaped projection on the cope
surface caused by dropping of sand.
A scab when an up heaved sand gets separated
from the mould surface and the molten metal flows
between the displaced sand and the mold.
Penetration occurs when the molten metal flows
between the sand particles in the mould. These
defects are due to inadequate strength of the mold
and high temperature of the molten metal adds on it.
Buckle is a vee-shaped depression on the surface of
a flat casting caused by expansion of a thin layer of
sand at the mould face. A proper amount of volatile
additives in moulding material could eliminate this
defect by providing room for expansion.

35. Internal Defects
The internal defects found in the castings are mainly due to
trapped gases and dirty metal. Gases get trapped due to hard
ramming or improper venting. These defects also occur when
excessive moisture or excessive gas forming materials are
used for mould making.
 Blow holes are large spherical shaped gas bubbles, while
porosity indicates a large number of uniformly distributed tiny
holes. Pin holes are tiny blow holes appearing just below the
casting surface.
 Inclusions are the non-metallic particles in the metal matrix,
Lighter impurities appearing the casting surface are dross.


36. Visible Defects

37. Insufficient mould strength, insufficient metal, low pouring
temperature, and bad design of casting are some of the
common causes.
 Wash is a low projection near the gate caused by erosion of
sand by the flowing metal. Rat tail is a long, shallow, angular
depression caused by expansion of the sand. Swell is the
deformation of vertical mould surface due to hydrostatic
pressure caused by moisture in the sand.
 Misrun and cold shut are caused by insufficient superheat
provided to the liquid metal.
 Hot tear is the crack in the casting caused by high residual
stresses.
 Shrinkage is essentially solidification contraction and occurs
due to improper use of Riser.
 Shift is due to misalignment of two parts of the mould or
incorrect core location.


38. Casting with expendable mould:
Investment Casting

39. Advantages and Limitations

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Parts of greater complexity and intricacy can be
cast
Close dimensional control 0.075mm
Good surface finish
The lost wax can be reused
Additional machining is not required in normal
course
Preferred for casting weight less than 5 kg,
maximum dimension less than 300 mm,
Thickness is usually restricted to 15mm
Al, Cu, Ni, Carbon and alloy steels, tool steels
etc. are the common materials

40. Permanent mould casting: Die casting
Graphite+oil

41. General Configuration of a Die Casting
Machine

42. 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 ingates are used and in the
beginning pressure is kept low and is
increased gradually


43. Cycle in Hot Chamber Casting

44. Cycle in Cold Chamber
Casting

45. Centrifugal Casting
•A permanent mold made of metal or ceramic is rotated at
high speed (300 to 3000 rpm). The molten metal is then
poured into the mold cavity and due to centrifugal action the
molten metal conform to the cavity provided in the mould.
•Castings are known for their higher densities in the outer
most regions.
•The process gives good surface finish
•Applications: pipes, bushings, gears, flywheels etc.

46. Comparison of Casting Processes