CASTING PROCEDURE

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CASTING PROCEDURE
dr shabeel pn
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Casting procedure

• The lost wax casting technique was introduced by
  TAGGART in 1907

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Steps in casting

• Preparing the wax pattern
• Spruing the wax pattern
• Attaching the sprue to crucible former
• Investing the pattern in the casting ring
• Burnout of the wax
• Casting
• Recovery
• Finishing and polishing

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Preparing the wax pattern

• Prior to casting
• margin of the wax pattern should be readapted
• pattern checked for smoothness, finish and
  contour
• Sprue should be attached to the thickest portion
  of the wax pattern
• wax pattern can be removed from the die using
  sprue
• surfactant should be applied on the wax to obtain
  wetting of the investment
• invested immediately to prevent distortion

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spruing of wax pattern

• Sprue is defined as the channel or hole through
  which plastic or metal poured or cast into gate
  or reservoir and then into mold

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sprue design

• it must allow the molten wax to escape from the
  mold
• enable molten metal to flow into the mold with
  minimal turbulence
• metal within the sprue must remain molten
  slightly longer than the alloy that has filled
  the mold this will act as a reservoir to
  compensate the shrinkage

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material used for sprue

• Wax
• plastic
• Metal

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sprue

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• Wax preferred for most casting because they melt
  at the same rate as the pattern and allow for
  easy escape for molten wax
• Plastic resist distortion rigid,
• may block the escape of wax,
• hollow plastics are available
• Metal non rusting metal should be used,
• removed before casting

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diameter

• should be larger than the thickest portion of the
  wax pattern
• 2.6 mm can be used for most patterns
• 2.0mm for premolar partial veneer restoration
• narrow sprue are sufficient for casting to be
  done on centrifugal machine

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Positioning of the sprue

• Sprue should be attached to the point of greatest
  bulk
• 45 angulation near the bulk of the pattern
• axial wall should have obtuse angle 135
• This prevents air entrapment during investing and
  suck back porosity after casting
• attachment should be flared to prevent turbulence
  during metal flow

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venting

• Small auxiliary sprues or vent should be placed
  to improve the casting
• By allowing the gases to escape

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Attaching the sprue to crucible former

• Crucible former is conical structure with a base
  placed at one end of the ring ,free end of the
  sprue attached to tip of crucible former
• The sprue should be adjusted such that terminal
  end of wax pattern is six mm away from free end
  of the investment

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Investing the pattern in the casting ring

• Ideal properties of investment material
• controllable expansion to compensate for
  shrinkage of cast alloy during cooling
• Produce smooth castings without nodules
• Chemical stability at high casting temp
• Adequate strength to resist casting forces
• Sufficient porosity to allow gas escape
• Easy recovery of casting
• Investment material
• Gypsm bonded and phosphate bonded

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Gypsum bonded

• Alfa hemi hydrate of Gypsum used as binder with
  crystobalite or quartz as refracting material and
  modifiers (coloring matter and reducing agents)
• satisfy most of the requirement for an ideal
  investment material
• Not suitable for casting metal ceramic alloys as
  it is unstable at high temperature
• Three types of expansion
• Setting expansion
• Hygroscopic expansion
• Thermal expansion
• Used for alloys that fuses below 1,975F
• Mainly used with type I, II, III, gold alloys

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Factors increasing the expansion

• Use of full width ring liner
• Prolonged spatulation
• Storage at 100 humidity
• Lower water powder ratio
• use of dry liner
• Use of two ring liners

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Phosphate bonded investment

• Composition binder megnesium oxide
• mono ammonium phosphate
• filler silica ( quartz or crystabolite form)
• They are stable at high temp
• Material of choice for metal ceramic
• They expand rapidly
• Due to
• heat from the setting reaction softens the wax
  and allow freer setting expansion
• The increase strength of the material at high
  temp restrict the shrinkage of the alloy as it
  cools
• The powder mixed with colloidal silica reduces
  the surface roughness of casting and it also
  increases expansion thus expansion can be
  conveniently controlled by diluting the colloidal
  silica slightly with distilled water

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• High noble alloy -gypsum bonded investment (shows
  hygroscopic expansion)
• Base metal alloy phosphate bonded investment
  (shows thermal expansion)

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Ring and ring liners

• Ring liners
• Asbestos (due to health hazard )
• Cellulose Used to aid in uniform expansion of the
  mold
• Wetting the liner increases the hygroscopic
  expansion
• It is coated on the casting ring
• Bench set min for one hour

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Wax burnout

• The removal of wax from the mould usually by heat
• Melted 200c for 30 min
• It does not melt but sublimes directly to gases
  and escapes through the pores of investment
• To burnout remaining traces of wax final burnout
  650c for 45 min
• Gypsum beyond 650c disintegrate investment and
  form sulfur dioxide and dis colour the casting

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CASTING TECHNIQUE

• FAZNA M. ALI
• 1st Batch

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Casting technique

• Setting up the casting machine
• Pre heating the crucible
• Weighing the alloy
• Casting the alloy

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Casting machine
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Setting up the casting machine

• Three clock wise turns and locked in position
  using a pin
• Oxy-acetylene gas is used ordinary alloys
• Oxygen gas is used for metal ceramic alloys

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Pre heating the crucible

• Freezing of the alloy incomplete casting

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Weighing the alloy

• 6 grams for pre-molar retainer castings
• 9 grams for molar retainer castings
• 12 grams pontic castings

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Casting the alloy

• Heated using open flame from torch
• Reducing part of flame is used to prevent
  oxidation
• Flux should be added
• Melts
• If the alloy is ready to cast locking pin is
  released and casting is completed

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recovery

• Removal of residual investment
• Quenching with water gypsum bonded disintegrates
• Using tooth brushes or ultrasonic cleaners
• Phosphate bonded should be removed using sand
  blasting (aluminium oxide)

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Casting deffects

• Distortion
• Surface roughness and irregularities
• Discoloration
• Porosity
• Incomplete casting

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Distortion

• Distortion of wax pattern during fabrication
• Increased with increase in ambient temperature
  and time lag between fabrication and investing

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Prevention

• Proper manipulation of wax
• Avoid stress on pattern during removal
• Immediate investment after removal from die

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Porosity

• Can occur on internal or external surface
• Weakens the casting
• Types A Solidification defects
• B Trapped gases
• C Residual air

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Solidification deffects

• Localised shrinkage porosity molten alloy
  prematurely solidifies in sprue before
  solidifying in mold space
• Causediameter of sprue is small ,lack of
  reservoir ,improper sprue attachment-suck back
  porosity
• Microporositytoo rapid solidification seen as
  small irregular voids
• Avoided by increasing casting temperature,melting
  temperature

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TRAPPED GASES

• Pin hole porosity-tiny spherical voids
• Cause release of entrapped gases while
  solidification
• Gas inclusion porosity-larger spherical voids
• Cause gas inclusion during casting procedure

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• Subsurface porosity due to entrapped gases in
  molten alloy
• Prevention controlling the rate of molten alloy
  entering the mold

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Residual air

• Back pressure porosity
• Causeinability of air to escape through pores in
  the investment ,improper wax elimination ,low
  casting temperature pressure
• Prevention use of porous investment,proper wax
  burn out ,enough casting pressure temperature

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Surface roughness and irregularites

• Surface roughness
• Cause silica particle in investment
• Inaccurate powder liquid ratio ,too rapid heating
  
• Surface irregularities nodules or fins
• Cause air bubble attached to pattern ,water film
  on pattern ,careless removal of pattern
• Prevention correct powder liquid ratio,use of
  mechanical mixer , use of wetting agent

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Discoloration of casting

• Appears dark due to coating of oxides
• Cause under heating ,prolonged heating carbon
  inclusion, high sulfur content of torch flame
• Removed by pickling

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Incomplete casting

• Due to prevention of molten alloy from filling
  the mold space completely
• Cause improper vending of air due to reduced
  casting pressure ,
• Preventionadequate casting pressure ,adequate
  alloy

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Thank You