Lecture 5

• Dr. Ahmed Khalifa

• ahmed.khalifa@mhiet.edu.eg
Special Casting Processes
• Shell Casting
• Investment Casting
• Centrifugal Casting
• Die Casting
• Ceramic Mould
• CO2 Moulding
 SHELL CASTING
• Preparation of Metal Match plate pattern
• Mix the investment material
• Heat the pattern
• Invest the pattern
• Curing the shell
• Remove the shell
• Assemble the shells
• Pour the metal
• Remove the casting
 Shell Molding / Croning or C-
process
• A heated (200 ºC to 300
ºC ) metal pattern is
covered with a mixture of
fine sand and 5 – 10 % of
thermosetting phenolic
resin( phenol
formaldehyde) which acts
as binder
• To prevent the sticking of
shell with the pattern a
release agent Silicone is
sprayed over the hot
pattern.
• This causes a skin of
about 3.5 mm (0.125
in) of sand/plastic
mixture to adhere to
the pattern.
• The shell thickness
will depend upon the
time of contact of
mixture with the
heated pattern
•Once more heated for around 420 ºC
for 3 min for curing of shell
•This skin is removed from the
pattern to form the "shell mold".
• The two halves of the
shell mold are
secured together and
the metal is poured in
the shell to form the
part.
• Once the metal
solidifies, the shell is
broken.
Shell Molding
 Advantages
• Better surface finish,
• Better dimensional tolerances,
• Higher throughput due to reduced cycle
times.
• Less sand is used
• This process can produce complex parts
with good surface finish 1.25 µm to 3.75
µm and dimensional tolerance of 0.5%
• A fairly high capital investment is
required, but high production rates can be
achieved. The process overall is quite
cost effective due to reduced machining
and cleanup costs.
 Dis advantages
• High pattern cost and resin cost
• Un economical for small runs
• Size of the casting is limited
• Serious dust and fume problems
 Shell Molding
• The materials that can be used with this process
are cast irons, and aluminum and copper alloys.
• Typical parts made with this process are
connecting rods, gear housings, lever arms etc.
Shell-Mold Casting

Figure 12-19 (Top) Two

halves of a shell-mold
pattern. (Bottom) The two
shells before clamping,
and the final shell-mold
casting with attached
pouring basin, runner, and
riser. (Courtesy of Shalco
Systems, Lansing, MI.)
 INVESTMENT CASTING
• Investment casting is also called lost-wax
process or precision Investment Casting
• metals that are hard to machine or fabricate are
good candidates for this process.
• The mold is made by making a pattern using
wax or some other material that can be melted
away.
• Term investment derives from the fact that the
pattern is invested with the refractory material
 INVESTMENT CASTING
• This wax pattern is dipped in refractory slurry, which
coats the wax pattern and forms a skin.
• Slurry – Fine silica sand + water/ethyl silicate or
Gypsum solution
• This is dried and the process of dipping in the slurry
and drying is repeated until a robust thickness is
achieved.
• After this, the entire pattern is placed in an oven and
the wax is melted away.
• This leads to a mold that can be filled with the molten
metal.
• Because the mold is formed around a one-piece
pattern, (which does not have to be pulled out from the
mold as in a traditional sand casting process), very
intricate parts and undercuts can be made
 Step by step procedure
• Make a master pattern of the part to be
cast (metal easily machined such as
brass, Aluminum alloy, alloy of tin lead
bismuth)
• Making a master dies
 Schematic illustration of investment casting
1. WAX INJECTION : Wax replicas of the
desired castings are produced by
injection molding. These replicas are
called patterns.

2. ASSEMBLY : The patterns are

attached to a central wax stick, called
a sprue, to form a casting cluster or
assembly.

3. SHELL BUILDING : The shell is built by immersing

the assembly in a liquid ceramic slurry and then
into a bed of extremely fine sand. Up to eight
layers may be applied in this manner.

4. DEWAX : Once the ceramic is dry, the wax is

melted out, creating a negative impression of the
assembly within the shell.
 • 5. CONVENTIONAL CASTING
In the conventional process, the shell is filled
with molten metal by gravity pouring. As the
metal cools, the parts and gates, sprue and
pouring cup become one solid casting.

• 6. KNOCKOUT
When the metal has cooled and solidified,
the ceramic shell is broken off by vibration or
water blasting.

• 7. CUT OFF
The parts are cut away from the central sprue
using a high speed friction saw.

• 8. FINISHED CASTINGS
After minor finishing operations, the metal
castings--identical to the original wax patterns--
are ready for shipment to the customer.
Casting with expendable mould:
Investment Casting
 Advantages and Limitations
• Parts of greater complexity and intricacy can be cast
• Close dimensional control ±0.075mm
• Superior surface finish
• Un machinable Alloys can be cast
• The lost wax can be reused
• Additional machining is not required in normal
course
• Preferred for casting weight less than 5 kg,( weighing
1g – 35Kg)
• Maximum dimension less than 300 mm, Thickness
is usually restricted to 15mm
 High-melting point alloys
• Al, Cu, Ni, Carbon and alloy steels, tool steels etc.
are the common materials

• Not a cheap process

CENTRIFUGAL CASTING
 CENTRIFUGAL CASTING
In centrifugal casting, a permanent mold is rotated
about its axis at high speeds (300 to 3000 rpm) as the
molten metal is poured.
The molten metal is centrifugally thrown towards the
inside mold wall, where it solidifies after cooling.
The casting is usually a fine grain casting with a very
fine-grained outer diameter, which is resistant to
atmospheric corrosion, a typical situation with pipes.
The inside diameter has more impurities and
inclusions, which can be machined away.
• Only cylindrical shapes can be produced with this
process. Size limits are upto 3 m (10 feet) diameter
and 15 m (50 feet) length.
• Typical materials that can be cast with this process
are iron, steel, stainless steels, and alloys of
aluminum, copper and nickel.
• Two materials can be cast by introducing a second
material during the process.
• Typical parts made by this process are pipes, boilers,
pressure vessels, flywheels, cylinder liners and other
parts that are axi-symmetric.
 Semi centrifugal Casting Process

(a) Schematic illustration of the semi centrifugal casting process. Wheels with spokes
can be cast by this process. (b) Schematic illustration of casting by centrifuging. The
molds are placed at the periphery of the machine, and the molten metal is forced into
the molds by centrifugal force.
 DIE-CASTING
• In Die-casting the metal is injected into
the mold under high pressure of 10-
210Mpa
• The mould used for making a casting is
permanent, called Die.
• Hot Chamber
• Cold Chamber
Cold-chamber – molten metal is poured in to
the injection chamber & the shot chamber is
not heated
• In a cold chamber process, the molten
metal is ladled into the cold chamber for
each shot. There is less time exposure of
the melt to the plunger walls or the
plunger. This is particularly useful for
metals such as Aluminum, and Copper
(and its alloys) that alloy easily with Iron at
the higher temperatures.
Hot-chamber - involves the use of a piston to push
molten metal in to the die cavity

• In a hot chamber process the pressure chamber is

connected to the die cavity is immersed permanently
in the molten metal. The inlet port of the pressurizing
cylinder is uncovered as the plunger moves to the
open (unpressurized) position. This allows a new
charge of molten metal to fill the cavity and thus can
fill the cavity faster than the cold chamber process.
The hot chamber process is used for metals of low
melting point and high fluidity such as tin, zinc, and
lead that tend not to alloy easily with steel at their
melt temperatures.
 Die Casting
• Parts made from here range from:
– Hand tools
– Building Hardware
– Automobile
– Telecommuniction
– Toys
– Appliance components
• 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
Cycle in Hot Chamber Casting
Hot chamber Die-casting process
• 1. The die is closed and
the piston rises, opening
the port and allowing
molten metal to fill the
cylinder.

• 2. The plunger moves

down and seals the port
pushing the molten metal
through the gooseneck
and nozzle into the die
cavity, where it is held
under pressure until it
solidifies.
• 3. The die opens and the
cores, if any, retract. The
casting remains in only one
die, the ejector side. The
plunger returns, allowing
residual molten metal to
flow back through the
nozzle and gooseneck.

• 4. Ejector pins push the

casting out of the ejector
die. As the plunger
uncovers the filling hole,
molten metal flows through
the inlet to refill the
gooseneck, as in step (1).
Cycle in Cold Chamber Casting
 Cold-Die casting process

• 1. The die is closed and the

molten metal is ladled into the
cold-chamber shot sleeve.

• 2. The plunger pushes the

molten metal into the die cavity
where it is held under pressure
until solidification.
 • 3. The die opens and the
plunger advances, to ensure
that the casting remains in the
ejector die. Cores, if any,
retract.

• 4. Ejector pins push the

casting out of the ejector die
and the plunger returns to its
original position.
Process Capabilities and Machine Selection
– Dies are rated according to their clamping force that is needed
– Factors involved in selection of die cast machines are
• Die size
• Piston stroke
• Shot pressure
• Cost
– Die-casting dies
• Single cavity
• Multiple-cavity
• Combination-cavity
• Unit dies
– Ratio of Die weight to part weight is 1000 to 1
– Surface cracking is a problem with dies due to the hot metal that
is poured in to them
– Has ability to produce strong high- quality parts with complex
shapes
– Good dimensional accuracy and surface details
 Various types of cavities in a die casting die.

a) Single – cavity die

b) Multiple – cavity die

c) Combination die

d) Unit die
Permanent mould casting: Die casting

Graphite+oil
•800 ton hot chamber die casting machine, DAM 8005. This is the largest
hot chamber machine in the world and costs about $1.25 million.
General Configuration of a Die Casting
Machine
Advantages of Die Casting

 High production rates

 Closer dimensional
tolerances
 Superior surface finish
 Improved mechanical
properties
Ceramic-Shell Casting
– Variation of the investment-casting process
– Uses same type of wax or plastic pattern as investment casting
– Patten is then dipped into fluidized bed of
• Fine- grained fused silica
• Zircon flour
– Pattern is then dipped into coarser grained silica to build up additional
coatings and proper thickness to withstand the thermal shock of pouring
– The rest of the procedure follows the investment casting process

Fig : Investment casting of an integrally cast rotor for a gas turbine. (a) Wax pattern assembly. (b) Ceramic
shell around wax pattern. (c) Wax melted out and the mold is filled under a vacuum, with molten super alloy.
(d) The cast rotor produced to net or near-net shape.
 Ceramic mold casting
• This process is expensive, but can
eliminate secondary machining operations.
Typical parts made from this process
include impellers made from stainless
steel, bronze, complex cutting tools,
plastic mold tooling.
 CO2 Moulding
• A sand molding technique and uses sand
grain in which is mixed a solution of
sodium silicate that acts to bind the sand
particles.
• CO2 gas is used to harden the sand after
the mould has been prepared.
• H2O+Na2SiO3+ CO2 →Na2CO3+SiO2
Comparison of Casting Processes