Casting
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Casting since about 4000 BC…
Ancient Greece; bronze
statue casting circa 450BC
Iron works in early Europe,
e.g. cast iron cannons from
England circa 1543
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Casting
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A fabrication process whereby a totally molten metal is
poured into a mold cavity having the desired shape; upon
solidification, the metal assumes the shape of the mold but
experiences some shrinkage.
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Casting techniques are used when
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The finished shape is so large or complicated that any
other method would be impractical
2.
A particular alloy is so low in ductility that forming by
either hot or cold working would be difficult
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In comparison to other fabrication processes, casting
is the most economical.
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Outline
• Metal Casting Processes
– Sand Casting
– Investment Casting
– Vacuum Casting
– Permanent-Mold Casting
– Slush Casting
– Pressure Casting
– Die Casting
– Centrifugal Casting
– Continuous Casting
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Sand Casting
• A two-piece mold is formed by packing sand around a
pattern that has the shape of the intended casting.
Large parts, e.g., auto engine blocks
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PATTERNS AND CORES
a full-sized model of the part, enlarged to account for shrinkage and
machining allowances in the final casting
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PATTERNS AND CORES
Acore is a full-scalemodel of the interior surfaces of the part.
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MOLDS AND MOLD MAKING
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In making the mold, the grains of sand are held together by a
mixture of water and bonding clay. A typical mixture (by volume) is
90% sand, 3% water, and 7% clay.
Other bonding agents can be used in place of clay, including
organic resins (e.g., phenolic resins) and inorganic binders (e.g.,
sodium silicate and phosphate). Besides sand and binder,
additives are sometimes combined with the mixture to enhance
properties such as;
strength and/or permeability of the mold
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MOLDS AND MOLD MAKING
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Strength
Permeability.
thermal stability
Collapsibility
reusability
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MOLDS AND MOLD MAKING
• Green sand molds.
• A dry-sand mold-organic binders rather than
• clay, and the mold is baked in a large oven at temperatures
ranging from 200oC to 320oC
• [8 from Groover].
• skin-dried mold-drying the surface of a greensandmold toadepthof 10 to25mm
• No-bake molds- Chemicals.
furan resins (consisting of
furfural alcohol, urea, and formaldehyde), phenolics, and alkyd oils. Nobake molds are growing in popularity due to their good dimensional control
in high production applications.
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THE CASTING OPERATION
• Pouring Solidification and Cooling.
• Casting Design and Problems in Casting?
• How can we minimize the core removal
difficulties.
• Design of riser and pouring system for an
aluminum sheer of 5*50*100
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Vacuum Molding
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EXPANDED POLYSTYRENE PROCESS
The expanded polystyrene casting process uses a mold of sand
packed around a polystyrene foam pattern that vaporizes when the
molten metal is poured into the mold.
The foam pattern includes the sprue, risers, and gating system, and it may
also contain internal cores (if needed), thus eliminating the need for a
separate core in the mold.
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Investment Casting
In investment casting, a pattern made of wax is coated with a refractory material to
make the mold, after which the wax is melted away prior to pouring the molten
metal. The term investment comes from one of the less familiar definitions of the
word invest, which is ‘‘to cover completely,’’ this referring to the coating of the
refractory material around the wax pattern
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Also called lost-wax process
First used 4000 – 3000 BC
The pattern is made of wax or of a plastic by molding or rapid
prototyping techniques
Term investment derives from the fact that the pattern is invested
with the refractory material
Need careful handling because they are not strong enough to withstand
the forces involved in mold making
Wax can be recovered and reused
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PLASTER-MOLD AND CERAMICMOLD CASTING
• Additives such as talc and silica flour are mixed with the
plaster to control contraction and setting time, reduce
cracking, and increase strength.
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Investment Casting Process
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Investment Casting
The mold is then
placed in a steam
autoclave for dewaxing.
High
Pressure
Steam
Recovered
Wax
Residual wax
is then fired
from the
shell.
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Investment Casting
Mold is
preheated, and
molten metal is
poured into the
mold cavity.
After cooling, the
ceramic shell is
chipped or
blasted off, and
the metal part
attached to
gating system
remains.
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Investment Casting
CAD Solid Model
Polycarbonate
Pattern
Aluminum
Casting
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Investment Casting
• One-piece mold
– Dried in the air
– Heated to 90 – 175 C
– Held inverted for 12 hrs to melt out wax
– The mold is then heated to 650 – 1150 C for about
4 hrs depending on the metal to be cast to drive
off the water of crystallization
– After the metal has been poured the mold is
broken up and the cast is removed
– A number of patterns can be joined to make one
mold called a tree which increases production rate
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One-piece mold - con’t
– Small parts
• The tree can be inserted on to a flask and filled with
slurry investment
• The investment is then placed into a chamber and
evacuated to remove air bubbles
• Next it is placed in a vacuum drawing machine to produce
fine detail
– Not a cheap process
– Produces fine details
– Good surface finish
– Few or no finishing operations
– Can produce intricate parts from parts weighing 1g – 35Kg
• Ex : Investment die casting examples
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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.
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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.
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6. KNOCKOUT
When the metal has cooled and solidified,
the ceramic shell is broken off by
vibration or water blasting.
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7. CUT OFF
The parts are cut away from the central sprue
using a high speed friction saw.
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8. FINISHED CASTINGS
After minor finishing operations, the metal
castings--identical to the original wax
patterns are ready for shipment to the
customer.
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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 rotor24
produced to net or near-net shape.
Investment Casting
(lost-wax casting)
• The pattern is made from a wax or
Plaster die formed
around wax prototype
plastic that has a low Tm. Around
the pattern is poured a fluid
slurry, which sets up to form a
solid mold or investment.
• The mold is then heated, such that the pattern melts and is
burned out, leaving behind a mold cavity having the desired
shape.
• This technique is employed when high dimensional accuracy,
reproduction of fine detail, and an excellent finish are required
(in jewelry and dental crowns and inlays, and blades for gas
turbine and jet engine impellers)
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Turbine Blades Made of Ni-based Superalloys
• The materials for turbine blades are required to perform
at higher and higher temperatures.
• The use of advanced nickel-based alloys, together with
innovative cooling design
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Vacuum Casting
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Mixture of fine sand
and urethane is molded
over metal dies a cured
with amine vapor
The mold is partially
immersed into molten
metal held in an
induction furnace
The metal is melted in
air or in a vacuum
The molten metal is
usually 55 C above the
liquidus temperature –
begins to solidify within
a fraction of a second
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Alternative to investment,
shell-mold, and green-sand
casting
Relatively low cost
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Permanent-Mold Casting
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Called hard-mold casting
Two halves of a mold are made from materials such as iron, steel,
bronze, or other alloys
The mold cavity and gating system are machined in to the mold
Sand aggregate are placed in to the mold prior to casting for producing
cavities
Typical core materials are
– Oil-bonded or resin-boned sand
– Plaster
– Graphite
– Gray iron
– Low-carbon steel
– Hot-worked die steel
Mold cavity surfaces are coated with refractory slurry to increase the
life of the mold every few castings
Mechanical ejectors are used to remove complex parts
Can produce high production rates
Good surface finish
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Slush Casting
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Molten metal is poured into the metal mold
A desired thickness of the solidified skin is obtained
The remaining metal is poured out
The mold halves are then opened and the casting is removed
Pressure Casting
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Used a graphite or metal mold
Molten metal is forced into the mold by gas pressure
The pressure is maintained until the metal solidifies in the mold
Used for high-quality castings
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Pressure Casting
(a)
(b)
The bottom-pressure casting process utilizes graphite molds
for the production of steel railroad wheels.
(b) Gravity pouring method of casting a railroad wheel. Note
that the pouring basin also serves as a riser.
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Die Casting
• Further example of permanent-mold casting
• Molten metal is forced into the die cavity at pressures ranging
from .7MPa – 700MPa
• Parts made from here range from:
– Hand tools
– Toys
– Appliance components
• There are two basic types of die casting machines
– Hot-chamber - involves the use of a piston to push molten
metal in to the die cavity
– Cold-chamber – molten metal is poured in to the injection
chamber & the shot chamber is not heated
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Die Casting
– The liquid metal is forced into a
mold (die) under pressure and at a
relatively high velocity, and allowed
to solidify with the pressure
maintained.
– A two-piece permanent steel mold is employed; when clamped
together, the two pieces form the desired shape.
– When complete solidification has been achieved, the mold pieces
are opened and the cast piece is ejected.
– Rapid casting rates are possible, making this an inexpensive
method; a single set of molds may be used for thousands of
castings.
This technique lends itself only to relatively small
pieces and to alloys of low melting points such as Zn,
Al, and Mg
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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.
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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.
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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).
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Cold-Die Casting Process
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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.
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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.
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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
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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.
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Centrifugal Casting
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Utilizes the inertial forces caused by rotation to distribute
the molten metal in to the mold cavities
First used in the 1800’;s
Three types of centrifugal casting
– True centrifugal casting
– Semi centrifugal casting
– Centrifuging
• Schematic illustration of the centrifugal casting
process. Pipes, cylinder liners, and similarly shaped
parts can be cast with this process.
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Casting Defects
• Discontinuities in castings
that exhibit a size, shape,
orientation, or location that
makes them detrimental to
the useful service life of
the casting
• Some casting defects are
remedied by minor repair or
refurbishing techniques,
such as welding
• Other casting defects are
cause for rejection of the
casting
(Metallurgy, by B. J. Moniz, American
Technical Publishers, Inc., 1994)
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Casting Defects - Metallic Projections
• Metallic Projections: fins (flash), swells, and scabs
– Fins are excessive amounts of metal created by solidification into
the parting line of the mold
Fins are removed by grinding or sandblasting
– Swells are excessive amounts of metal in the vicinity of gates or
beneath the sprue
– Scabs are surface slivers caused by splashing and rapid
solidification of the metal when it is first poured and strikes the
mold wall
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Casting Defects — Cavities
• Blowholes, pinholes, shrinkage cavities, & porosity
– Blowholes and pinholes are holes formed by gas entrapped during
solidification
– Shrinkage cavities are cavities that have a rougher shape and
sometimes penetrate deep into the casting
Shrinkage cavities are caused by lack of proper feeding or nonprogressive solidification
– Porosity is pockets of gas inside the metal caused by micro-shrinkage,
e.g. dendritic shrinkage during solidification.
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Casting Defects — Discontinuities
• Cracks in casting and are caused by hot tearing, hot cracking, and
lack of fusion (cold shut)
– A hot tear is a fracture formed during solidification because
of hindered contraction
– A hot crack is a crack formed during cooling after
solidification because of internal stresses developed in the
casting
– Lack of fusion is a discontinuity caused when two streams of
liquid in the solidifying casting meet but fail to unite
Rounded edges indicate poor contact between various metal
streams during filling of the mold
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Casting Defects — Defective Surfaces
• Casting surface irregularities that are caused by incipient
freezing from too low a casting temperature
• Wrinkles, depressions and adhering sand particles
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Casting Defects — Inclusions
• Particles of foreign material in the metal matrix
• The particles are usually nonmetallic compounds but may be
any substance that is not soluble in the matrix
– Slag, dross, and flux inclusions arise from melting slags,
products of metal treatment, or fluxes
They are often deep within the casting
– Mold or core inclusions come from sand or mold dressings
and are usually found close to the surface
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THE END
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