Text Book:

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Metal Casting Processes

• CHAPTER 11
• PART II

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Metal Casting Process

• Investment Casting
• Vacuum Casting
• Permanent-Mold Casting
• Slush Casting
• Pressure Casting
• Die Casting
• Centrifugal Casting

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Investment Casting

• 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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Investment Casting Process
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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 - cont
• 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 CASTINGIn 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. KNOCKOUTWhen the metal has cooled and
  solidified, the ceramic shell is broken off by
  vibration or water blasting.

• 7. CUT OFFThe parts are cut away from the
  central sprue using a high speed friction saw.
• 8. FINISHED CASTINGSAfter minor finishing
  operations, the metal castings--identical to the
  original wax patterns--are ready for shipment to
  the customer.

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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-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

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Vacuum Casting

1. Mixture of fine sand and urethane is molded over
   metal dies a cured with amine vapor
2. The mold is partially immersed into molten metal
   held in an induction furnace
3. The metal is melted in air or in a vacuum
4. The molten metal is usually 55 C above the
   liquidus temperature begins to solidify within
   a fraction of a second

• 5. Alternative to investment, shell-mold, and
  green-sand casting
• 6. Relatively low cost

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Permanent-Mold Casting

• 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

• 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
• 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

Pressure Casting
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Pressure Casting

• (a) The bottom-pressure casting process utilizes
  graphite molds for the productin 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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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.
• 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

• 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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Various types of cavities in a die casting die.

1. Single cavity die
2. Multiple cavity die

• c) Combination die
• d) Unit die

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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

• Utilizes the inertial forces caused by rotation
  to distribute the molten metal in to the mold
  cavities
• First used in the 1800s
• 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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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.

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Squeeze Casting
(c) Close die and apply pressure

• (a) Melt Metal

(d) Eject squeeze casting and charge melt stock
and repeat cycle
Sequence of operations in the squeeze-casting
process. This process combines the advantages of
casting and forging.
(b) Pour molten metal into die
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Single Crystal Casting of Turbines blades

• Fig Methods of casting turbine blades. (a)
  directional solidification (b) method to produce
  a single-crystal blade and (c) a single-crystal
  blade with construction portion still attached.

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Single Crystal-Casting

• Fig Two methods of crystal growing (a) crystal
  pulling and (b) the floating zone method. Crystal
  growing is especially important in the
  semiconductor industry.

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Melt Spinning

• Fig Schematic illustration of melt-spinning to
  produce thin strips of amorphous metal.

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Types of Melting Furnaces

• Fig Two types of melting furnaces are used in
  foundries (a) crucible and (b) cupola

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THE END