Casting Processes

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Casting Processes
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• Sand casting
• consists of placing a pattern (having the shape
  of the desired casting) in sand to make an
  imprint, incorporating a gating system, filling
  the resulting cavity with molten metal, allowing
  the metal to cool until it solidifies, breaking
  away the sand mold, and removing the casting
• in the USA about 15 million tons of metal are
  cast by sand casting
• used for machine tool bases, engine blocks,
  cylinder heads, and pump houses

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• Sands
• silica sands (SiO2)
• fine round grains can be closely packed and forms
  a smooth mold surface
• good permeability of molds and cores allows gases
  and steam evolved during casting to escape easily
• the mold should have good collapsibility to avoid
  defects in the casting (tearing and cracking)
• ability to with stand high temperatures
• ability to retain shape under the action of metal
  flow
• permeability
• collapsibility

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• Three basic types of sand molds
• green sand (a mixture of sand, clay, and water)
• cold box (various organic and inorganic binders
  are blended into the sand to bond the grains
  chemically for greater strength)
• no-bake molds (a synthetic liquid resin is mixed
  with sand, and the mixture hardness at the room
  temperature

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• Major components of sand molds
• The mold is supported by a flask. Two piece
  molds consist of a cope on top and a drag on the
  bottom. The seam between them is the parting
  line.
• A pouring basin or cup, into which the molten
  metal is poured.
• A sprue, through which the molten metal flows
  downward.
• A gate, which is located at the base of the
  sprue. Molds typically contain a system of gates
  constructed to minimized turbulence in the molten
  metal and control flow so that metal is supplies
  at a rate to adequately supply the critical
  section thickness of the casting. Gating systems
  often include passageways called runners.
• Risers, which supply additional metal to the
  casting as it shrinks during solidification.
• Cores, which are inserts made from sand. They
  are placed in the mold to form hollow regions.
• Vents, which are placed in molds to carry off
  gases produced when the molten metal comes into
  contact with the sand in the molds.

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• Cores
• used to form internal cavities or passages
• they ar placed in the mold cavity before casting
  and are removed from the finished part during
  shakeout and further processing
• they must possess strength, permeability, ability
  to withstand heat, and collapsibility
• they are typically made of sand aggregates
• the core is anchored by core prints (they are
  recesses that are added to the pattern to support
  the core and to provide vents for the escape of
  gasses)
• or by metal supports, known as chaplets

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• Shell mold casting
• produces many types of castings with close
  tolerances and good surface finishes at a low
  cost
• a mounted pattern, made of a ferrous metal or
  aluminum, is heated to 175-370 C, coated with a
  parting agent such as silicone, and clamped to a
  box or chamber containing a fine sand coated with
  a 2.5 - 4.0 thermosetting resin binder
• the sand mixture is blown over the heated
  pattern, coating it evenly
• the assembly is placed in an oven to complete the
  curing of the resin
• the shell is formed by removing the pattern
• two half shells are made and are clamped together
  in preparation for pouring

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• Thin walls (5 - 10mm)
• gases easy to escape
• walls are smooth, less resistance to flow of the
  molten metal, produce castings with sharper
  corners, thinner section
• more economical
• the high quality of the finished casting can
  reduce cleaning, machining, and other finishing
  costs
• can produce complex shapes
• it is also used in producing high precision cores

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• Composite molds
• made of two or more different materials
• utilized in casting complex shapes (impellers for
  turbines)
• molding materials are shells, plaster, sand
  with binder, metal and graphite
• they increase the strength of the mold, improve
  the dimensional accuracy and surface finish, and
  may reduce overall costs

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• Expandable pattern casting (lost foam)
• uses a polystyrene pattern which evaporates upon
  contact with molten metal to form a cavity for
  the casting
• one of the most important casting processes for
  the automobile industry
• the polystyrene pattern is coated with a
  waterbase refractory slurry, dried, and placed in
  a flask. The flask is filled with loose fine
  fine sand. The molten metal is poured into the
  mold.

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• Advantages
• it is relatively simple process (there are no
  parting lines, cores, or riser systems)
• inexpensive flasks are sufficient for the process
• polystyrene is inexpensive (complex shapes and
  various sizes)
• the casting requires minimum finishing and
  cleaning operations
• the process is economical for long production
  runs
• the process can be automated

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• Plaster mold casting
• the mold is made of plaster of paris (gypsum, or
  calcium sulfate) with addition of talc and silica
  flour to improve strength and control the time
  required for the plaster to set
• these components are mixed with water and the
  resulting slurry is poured over the pattern
• after the plaster is set, the pattern is removed
  and the mold is dried
• the mold halves are then assembled to form the
  mold cavity and preheated to about 120 C for 16
  hours
• the molten metal is then poured into the mold

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• Low permeability, the molten metal is poured
  either in a vacuum or under pressure
• material for patterns are aluminum alloys,
  magnesium, zinc, and some copper-base alloys
• because of low thermal conductivity, the castings
  are cooled slowly, yielding more uniform grain
  structure with less warpage and better mechanical
  properties
• high precision casting
• used for casting gears, lock components, valves,
  fittings, tooling, and ornaments

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• Ceramic mold casting
• the slurry is a mixture of fine grained zircon,
  aluminum oxide, and fused silica, which are mixed
  with bonding agents and poured over the pattern,
  which has been placed in a flask
• because of the high temperature resistance, these
  molds can be used in casting ferrous and other
  high temperature alloys

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• Investment casting (lost wax process)
• used during the period 4000-3000 BC
• the pattern is made of wax or plastics

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• Addition of nucleant to the molten metal
• close control of superheat of the molten metal
• control of pouring techniques
• control of cooling rate

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• Vacuum casting
• A schematic illustration of the vacuum casting
  process, or counter gravity low pressure process
  (not to be confused with the vacuum molding
  process) is shown in fig. 5.28. A mixture of the
  fine sand and urethane is molded over metal dies
  and cured with amine vapor. Then the mold is
  held with a robot arm and partially immersed into
  molten metal held in an induction furnace. The
  metal may be melted in air or in a vacuum. The
  vacuum reduces the air pressure inside the mold
  to about two thirds of atmospheric pressure,
  drawing the molten metal into the mold cavities
  through a gate in the bottom of the mold. The
  molten metal in the furnace is at a temperature
  usually 55C above the liquidus temperature
  consequently it begins to solidify within a
  fraction of a second. After the mold is filled,
  it is withdrawn from the molten metal.

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• Permanent mold
• better heat conduction than expendable mold
  (faster cooling that has affect on microstructure
  and grain size)
• mold materials steel, bronze, refractory metal
  alloys, graphite
• core materials shell or no-bake cores, gray
  iron, low carbon steel, hot work die steel
• molds are preheated to facilitate metal flow and
  reduce thermal damage to the dies
• process could be automated for large production
  runs
• process is used mostly for aluminum, magnesium,
  and copper alloys
• good surface quality, close tolerances, uniform
  and good mechanical properties and at high
  production rate
• used to cast automobile pistons, cylinder heads,
  and connecting rods, gear blanks for appliance,
  and kitchenware

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• Slush casting
• after the desired thickness of solidified skin is
  obtained, the mold is inverted or slung, and the
  remaining liquid metal is poured out
• used for small production runs (ornamental and
  decorative objects and toys)
• Pressure casting
• the molten metal is forced upward by gas pressure
  (or by vacuum) into graphite or metal mold

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• Die casting
• developed in the early 1900s
• the molten metal is forced in the die cavity at
  pressures rangint from 0.7 to 700 MPa
• typical parts transmission housing, valve
  bodies, carburetors, motors, business machine and
  appliance components, hand tools, and toys
• Hot chamber process

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• Cold chamber process

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• Process capabilities and machine selection
• die has a tendency to part
• rated according to clamping force
• 25 to 3000 tons
• selected according to die size, piston stroke,
  shot pressure, and cost
• single cavity, multiple cavity, or combined
  cavity
• dies made of hot work die steel
• dies may last half a million shots before wearing

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• Centrifugal casting
• utilizes the inertial forces caused by rotation
  to distribute the molten metal into the mold
  cavities
• True centrifugal casting
• hollow cylindrical parts
• molds are made of steel, iron, or graphite, and
  may be coated with a refractory lining to
  increase mold life
• mold surfaces can be shaped so that pipes with
  various outer shapes, including square or
  polygonal, can be cast
• inner surface remains cylindrical because the
  molten metal is uniformly distributed by
  centrifugal forces

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• Squeeze casting
• solidification of molten metal under high
  pressure
• combination of casting and forging
• high pressure promotes heat transfer, resulting
  in a fine microstructure with good mechanical
  properties and limited microporosity
• made to near net shape with complex shapes and
  fine surface detail, from both ferrous and
  nonferrous alloys

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• Casting techniques for single-crystal components
• used for gas turbine blades usually made of
  nickel-base superalloys
• conventional casting of turbine blades
  (investment with ceramic mold)
• polycrystalline grain structure makes it
  susceptible to creep and cracking along grain
  boundaries under centrifugal force
• directionally solidified blades
• uses a chill plate at one end of the mold
• no transverse grain boundaries, only longitudinal
• single crystal growing
• seed crystal dipped into solution and pulled
  slowly out while being rotated
• floating zone method involves polycrystalline
  silicon resting on a single crystal silicon,
  heated by an induction coil, the single crystal
  grows

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• Rapid solidification
• involves cooling of molten metals at rates as
  high as 106 K/s
• insufficient time to crystallize (amorphous
  alloys or metallic glasses)
• typically contain iron, nickel, and chromium,
  alloyed with carbon, phosphorus, boron, aluminum,
  and silicon
• amorphous alloys exhibit corrosion resistance,
  ductility, and high strength
• useful magnetic properties make them attractive
  for magnetic cores