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CASTING PROCEDURE
Casting procedure
 The lost wax
casting technique
was introduced by
TAGGART in 1907
Steps in casting
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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
Preparing the wax pattern
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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
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”
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
material used for sprue
 Wax
 plastic
 Metal
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
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
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
venting
 Small auxiliary sprues or vent should
be placed to improve the casting
 By allowing the gases to escape
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
a
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
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
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Used for alloys that fuses below 1,975’F
Mainly used with type I, II, III, gold alloys
Factors increasing the expansion
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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
Phosphate bonded investment
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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
 High noble alloy -gypsum bonded
investment (shows hygroscopic
expansion)
 Base metal alloy –phosphate bonded
investment (shows thermal
expansion)
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
Wax burnout
 The removal of wax from the mould usually by heat
 Melted 200’c 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
650’c for 45 min
 Gypsum beyond 650’c disintegrate investment and
form sulfur dioxide and dis colour the casting
CASTING TECHNIQUE
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FAZNA M. ALI
1st Batch
Casting technique
Setting up the casting
machine
Pre heating the
crucible
Weighing the alloy
Casting the alloy
Casting machine
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
Pre heating the crucible
Freezing of the alloy – incomplete casting
Weighing the alloy
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6 grams for pre-molar retainer castings
9 grams for molar retainer castings
12 grams pontic castings
Casting the alloy
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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
recovery
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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)
Casting deffects
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Distortion
Surface roughness and irregularities
Discoloration
Porosity
Incomplete casting
Distortion
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Distortion of wax pattern during
fabrication
Increased with increase in ambient
temperature and time lag between
fabrication and investing
Prevention
Proper manipulation of wax
Avoid stress on pattern during removal
Immediate investment after removal from
die
Porosity
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Can occur on internal or external surface
Weakens the casting
Types :A Solidification defects
B Trapped gases
C Residual air
Solidification deffects
 Localised shrinkage porosity –molten alloy
prematurely solidifies in sprue before solidifying in
mold space
 Cause:diameter of sprue is small ,lack of reservoir
,improper sprue attachment-suck back porosity
 Microporosity:too rapid solidification seen as small
irregular voids
 Avoided by increasing casting temperature,melting
temperature
TRAPPED GASES
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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
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
Cause:inability 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
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
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 ,
Prevention:adequate casting pressure
,adequate alloy
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