INDIAN INSTITUTE OF TECHNOLOGY DELHI

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INDIAN INSTITUTE OF TECHNOLOGY DELHI
INVESMENT CASTING

• Submitted to
  Submitted by
• Prof. MP Gururajan
  Emrati Kumari
• 
  (2008AMD3240)

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INTRODUCTION

• Investment Casting-
• One of the oldest known metal-forming Technique.
• Today high-technology waxes, refractory material
  and specialist alloys used.
• Casting allow the production of components with
  accuracy, repeatability, versatility and
  integrity in a variety of metals and
  high-performance alloys.
• Utilized it when complex detail, undercut or
  non-machinable features and accurate parts are
  required.
• It requires best significant lead time and is
  best suited for low volume production rate.

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Manufacturing Process

• Step I tooling-
• Investment Casting is to create a die and form of
  part to the required specifications.
• The die is used to inject paraffin wax, which is
  used to create the complex shapes required
  manufacture component or part.
• Die is attached to a central sprue. Multiple die
  may be attached to same sprue. The sprue is a
  channel which distribute the molten wax into each
  of the die or forms.
• Tool is constructed from Aluminum or hot die
  steel material.
• For most applications single/multi cavity dies
  are used for high production quantities.

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.

• Step II Shell Building-
• The wax assembly is now dipped into a ceramic
  slurry, to obtain the shell thickness desired.
• A layer of fine sand (zircon) is added on top of
  each ceramic layer.
• This process repeated until the desired shell or
  casting form is created.
• Step III -
• After the shell/casting form is created, the wax
  is removed.
• This is where the term Lost-wax process comes
  form.
• This leaves an impression or casting form which
  will create the desired investment casting part,
  which will be filled with the desired metal or
  material.
• Before beginning the casting, shell must be
  heated up in a furnace so they do not fail due to
  differential temperature introduced poring in the
  molten metal or material.

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• Step IV -
• The casting metal or material is poured into the
  pre-heated ceramic shell.
• The casting material fills each part and feature
  of the investment casting shell.
• The individual cast part will be removed after
  the mold cools and the shell is removed.
• The shell is typically removed by water blasting,
  through alternative methods are available.
• They are still attached to the sprue assembly or
  each other.
• Individual parts are removed by cold-break
  (dipping in liquid nitrogen and breaking the
  parts off with hammer and chisel) or with
  alternative manufacturing machine tools.

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• Step V -
• The rough cast parts are now separated with each
  other and the sprue.
• First the gate or the place where parts was
  connected to the sprue, must be removed.
• Step VI -
• Investment Casting yield exceedingly fine quality
  products made off all type of metals.
• It has special application in fabrication very
  high temperature metal, specially those which can
  not cast in metal or plaster molds and those
  which are difficult to machine or work.

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Type of material for which the process is used

• A pattern of the component to be cast is produced
  by injection-molding special waxes into a metal
  die.
• Pre-formed ceramic cores can be included in the
  wax pattern as it is molded, which can create
  intricate hollows within the finished casting.
• As many as several hundred patterns may be
  assembled into a tree around a wax runner system
  (riser and sprue). Once a tree has been
  assembled, a pour cup is attached.
• Shell for cast
  turbocharger rotor

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• The completed tree is dipped or inverted by hand
  or via robotic control into a ceramic slurry of
  ethyl silicate, colloidal silica.
• Fine sand is applied to the inverted tree in a
  fluidised bed, rain tower sander or by hand.
• During primary coat the sand will be typically be
  a zircon-based, as zirconium is less likely to
  react with the molten metal when poured into the
  shell.
• The stuccoed tree is then allowed to dry before
  re-dipping in slurry and applying secondary coats
  of mullite, molochite, chamotte or fused silica
  refractory material.
• This process is repeated until the shell is thick
  enough to withstand te mechanical shock of
  receiving the molten metal. Dry times range from
  24 to 48 hours, and total production from two
  days to one week.

View of the ceramic impression in a turbocharger
shell
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• After the shell has been constructed, the wax is
  removed in an autoclave or furnace (hence, the
  lost-wax process).
• Most shell failures occur at this point, as the
  fragile stuccoed shell is subjected to extremes
  of temp. and in an autoclave pressure.
• The shell is fired at temp. of around 1100 C to
  induce chemical and physical changes in the set
  refractory materials forming a ceramic shell.
• This leave a ceramic impression of the part to be
  cast.
• Most foundries remove the shells from the furnace
  while still hot and pour the molten metal into
  the ceramic shell.
• Various methods of pouring the molten metal
  include vacuum casting, anti-gravity casting,
  tilt casting, gravity pouring, pressure assisted
  pouring, centrifugal casting.

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• After the molten metal cools, the shell is
  removed. This is generally done with water jets,
  vibration, grit blasting or chemical dissolution.
• The cooled parts are removed from the tree by
  sawing them free or by dipping them in liquid
  nitrogen and breaking them off with a hammer and
  chisel.
• Many cast parts require grinding of the gate and
  runner bar attachments.
• Because molten metal cools slowly, it does not
  finish as hard as some forging and machining
  processes.
• Cast parts often are subsequently hardened by
  heat treatment and surface hardening.
• The parts are inspected by eye or in special
  cases by X-ray at the foundry or by specialty
  firms.

Completed turbocharger rotor
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A property of the material is affect by the
process-

• Thermophysical properties of wax-
• The linear deformation of the sample, which was
  measured as a
• function of temperature, was converted to
  thermal expansion and
• density data.
• At temperatures below 55 C, the wax behaved like
  a hard paste or solid.
• As the wax was heated above 60 C, the wax lost
  its strength, became very soft, and flowed out
  into the surrounding silica powder. The density
  at 110 C was obtained using a hydrometer.

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Advantages-

• Investment casting is a special propose
  manufacturing process, use in jewelry.
• Through investment casting we can produce complex
  parts. Example-turbine blades, aerospace, power
  generation, complete aircraft door frames.
• It gives extremely good surface finish 80 to 120
  micron inches.
• It is also widely used by firearms manufacturers
  to fabricate firearm receivers, triggers,
  hammers, and other precision parts at low cost.
• Greate design freedom.
• Feasibility of replacing two or more fabricated
  sub-assembling by a single piece casting.
• Lighter, Stronger components may be designed,
  without a mismatch giving an improved aesthetic
  appearance.
• The process permits a high level of consistency,
  batch to batch.

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• Compared to other conventional way of
  manufacturing casting, with lost wax process
  close dimensional tolerance of 0.13mm to
  0.25mm.
• With the ability to produce special alloys to
  customers' requirements, the choice of
  metallurgical specifications is virtually
  unlimited.
• The light stressed wax impression dies have a
  very long life and are not costly considering
  design complexity.
• If required, minor design modification may be
  made economically without going for few tooling.
• A high level of metallurgical integrity and
  Strength can be achieved in the lost wax process.

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Disadvantages-

• Investment casting cost is high.
• Investment casting requires specialized
  equipment.
• In this costly refractories and binders.
• Many operations to make a mold so it is time
  taking process around two to one week.
• Some time leave occasional minute defects like
  macro and micro defects, includng shrinkage
  porosity, grain size and the extent of surface
  (100 microm) of the casting solidifies after the
  bulk.

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