Primary forming process casting

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Primary forming process (casting)

• A casting is produced by pouring molten metal
  into a mould cavity and allowing it to solidify.

• The mould cavity is the shape of the required
  component.

• Vital factors in determining the outcome are
• Fluidity (easy to flow)
• Fusibility (low melting point)

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Casting

• A fabrication process whereby 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.
  
• Casting techniques are used when
• The finished shape is so large or complicated
  that any other method would be impractical.
• A particular alloy is so low in ductility that
  forming by either hot or cold working would be
  difficult.
• In comparison to other fabrication processes,
  casting is the most economical.

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Classification of casting process

• Sand Casting

• Investment Casting

• Permanent Mould Casting

• Die Casting

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Sand casting
The traditional method of casting metals is in
sand moulds and has been used for many years.
A two-piece mold is formed by packing sand
around a pattern that has the shape of the
intended casting.
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The major features of sand moulds

• The flask (cope and drag)

• A pouring cup

• A sprue

• Risers

• Cores

• Vents

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Sequence of operations for sand 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

• The pattern is made from a wax or 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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Sequences involve in 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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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

• 1. The die is closed and the piston rises,
  opening the port and allowing molten metal to
  fill the cylinder. Pressure range up to 35 MPa

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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Hot chamber die casting
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 chamber die casting
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. Pressures ranges from 20 to 70
MPa.
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Cold chamber die casting

• 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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Adv/disadv of different casting process
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Casting defects

• Various defects can develop in manufacturing
  processes depending on factors such as materials,
  part design, and processing techniques.

• While some defects affects only the appearance of
  the parts made, others can have major adverse
  effects on the structural integrity of the parts.

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Casting defects - fins

• 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

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Casting defects - swells

• Swells are excessive amounts of metal in the
  vicinity of gates or beneath the sprue

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Casting defects - scabs

• 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- blowholes/pinholes

• Blowholes, pinholes, shrinkage cavities,
  porosity
• Blowholes and pinholes are holes formed by gas
  entrapped during solidification

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Casting defects- shrinkage

• Shrinkage, which causes dimensional changes, is
  the result of the following three sequential
  events
• Contraction of the molten metal as it cools prior
  to solidification.
• Contraction of the metal during the phase change
  from liquid to solid.
• Contraction of the solidified metal (the
  casting) as its temperature drops to ambient
  temperature.

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Casting defects- shrinkage cavities

• 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 non-progressive solidification

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Casting defects- porosity

• Porosity is pockets of gas inside the metal
  caused by micro-shrinkage during solidification.

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Casting defects- lack of fusion

• 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- hot tear, hot crack

• 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- 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

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Casting defects- hot crack

• A hot crack is a crack formed during cooling
  after solidification because of internal stresses
  developed in the casting

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Casting defects- defective surfaces

• Casting surface irregularities

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Casting defects- inclusions

• Particles of foreign materials in the metal matrix

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Casting design guidelines
Account for shrinkage - geometry - shrinkage
cavities
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Casting design guidelines
(a) avoid sharp corners (b) use fillets to
blend section changes smoothly (c1) avoid rapid
changes in cross-section areas
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Casting design guidelines
Avoid large, flat areas - warpage due to
residual stresses (why?)
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Casting design guidelines
Avoid rapid changes in cross-section areas If
unavoidable, design mold to ensure - easy metal
flow - uniform, rapid cooling (use chills,
fluid-cooled tubes)
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Casting design guidelines

• Provide drafts and tapers
• easy removal, avoid damage
• along what direction should we taper ?

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References

• S. Kalpakjian, S.R. Schmid Manufacturing
  Engineering Technology, 5th edition,
  Prentice-Hall International, 2006.
• E. Paul Degarmo, J. R. Black, R. A. Kohser
  Materials and Processes in Manufacturing, 9th
  edition, John Wiley Sons, Inc, 2003.
• R. L. Timings, S. P. Wilkinson Manufacturing
  Technology, 2nd edition, Pearson Education
  Limited, London, 2000.