3. INTRODUCTION
HISTORY
BASIC STEPS OF CASTING
PROCEDURE
SPRUE FORMER AND ITS
ATTACHMENT
CRUCIBLE FORMERS
CASTING RINGS AND LINERS
4. INVESTING PROCEDURE
WAX BURNOUT
CASTING OF ALLOY INTO MOLD
HEAT SOURCE
MACHINES TO INDUCE CASTING FORCE
RECOVERY AND CLEANING OF CASTING
CASTING OF TITANIUM ALLOYS
CASTING OF CASTABLE CERAMICS
CONCLUSION
REFERENCES
5. INTRODUCTION
Casting is an object, formed by the
solidification of a fluid that has been
poured or injected into the mold.(GPT)
Objective of casting procedure is to
provide a metallic duplication of missing
tooth structure with as much accuracy
as possible
7. The meticulous procedure of casting
was used by various craftsmen to
produce jewelry and ornaments. Its
history can be traced back around 3000
B.C. But origin of lost wax technique
when viewed, history makes its
presence in the writings of Theophilus
(11th Century).
8. 1558 Benvenuto Cellini
claimed to have attempted
use of wax and clay for
preparation of castings
1884 Agulihon de saran
used 24K gold to form Inlay.
1897 Phillibrook
described a method of
casting metal filling.
9. Apart from this various studies conducted on the
properties of investment materials and casting
alloys have led a path for a better, practical and
useful processing methods.
1907 Taggart
devised a
practically useful
casting machine.
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1907-LOST WAX PROCESS
1933-Replacement of CO-Cr for gold in RPD
1950-Development of resin veneers for gold
alloys
1959-Porcelain fused to metal technique
1971-The Gold standard
1980-ALL-CERAMIC TECHNOLOGIES
1999-Gold alloy as alternative to Pd-Based
alloys
11. “Lost wax technique”
Surrounding wax pattern
with investment material
Eliminate wax pattern by
heating
Introduce molten metal into
mould through channel
Taggart ( end of
nineteenth century)
14. SPRUING
SPRUE FORMER
SPRUE
SPRUING
The process of attaching a sprue former to the wax
pattern is called as spruing
The sprue former is attached to provide a channel
through which molten alloy reach the mold
15. Types of sprue former
Based on
materials
wax plastic metal
17. SOLID PLASTIC SPRUE FORMER
used for castings of alloys which use 2
stage burn out with Po4 bonded
investment
• Their main disadvantage is its
softening temperature, which is higher
than wax pattern. And may block
escape of wax.
18. Hollow plastic sprues are available that
permit easy escape of gasses.
It can be used in casting FPD in one
piece due to their added rigidity.
19. Metal sprue former
It should be made of non rusting metal
to avoid possible contamination of wax.
They are usually removed from the
investment at the same time as crucible
former is removed.
20. Basic requirements
of sprue former :
Must allow molten wax to
escape from mold.
Must enable molten metal to
flow into mold with as little
turbulence as possible.
Metal within it must remain
molten slightly longer than
alloy that has filled the mold
24. 3) Sprue former direction :
-Should be directed away from any thin
or delicate parts of pattern because
molten metal can fracture investment in
this part.
Should not be attached at 90 degree, it may
lead to turbulence it should be attached 45
degree to bulkiest part of pattern
26. 4) Attachment of sprue :
- Point of attachment should be smooth
to minimize turbulence.
- It is usually flared for high density gold
alloy which acts as same way as
reservoir.
27. 5) SPRUE FORMER LENGTH
- It should be adjusted such that top of wax pattern is with in 6
mm of open end of ring for gypsum investments and
- 3-4 mm for phosphate bonded investments.
28. Wax pattern removal :
- Sprue former should be attached such
that pattern can be removed directly in
the line with its path of withdrawal from
die.
- Any motion that may distort the wax
pattern should be avoided.
29. VENTING
Small auxiliary sprues or vents are
recommended to improve casting of thin
patterns.
- Their action may help gases to escape
during casting or ensure that solidification
begins in critical areas by acting as heat
sink.
30. Reservoir
It is added to sprue network to prevent localize
shrinkage porosity.
Pattern should always solidify first and reservoir
last
31. Because of large
mass of alloy and
heat at centre part
of
ring it remains
molten to furnish
liquid alloy into
mold
as it solidifies
32. Crucible former
The sprue is attached to crucible former which
constitutes the base of casting relation with
casting ring during investing
They form a conical depression in investment,
which guides flow of molten metal..
33. They are available as
Rubber crucible former
Metallic Crucible former
Plastic crucible former
34. Casting ring
They are available as:
1) Shapes - Round
- Oval
2) I) Complete rings - Rigid - Metal (stainless
steel)
- Plastic
- Flexible - Rubber
II) Split rings - Metal
- Plastic
3) I) Cylindrical
II) Conical
35. Considerations in selection of castings
rings
The internal diameter of casting ring should
be 5-10mm greater than the widest
measurement of the pattern and about 6 mm
higher.
For single crown/inlay - small rings as used.
Diameter - 32 mm
For large fixed partial denture – 63mm
37. Usually casting rings are rigid in nature.
Because of this the mold may become
smaller rather than larger due to the
reverse pressure resulting from
confinement of the setting expansion.
To overcome this, flexible rings/ split
rings are used. But the most commonly
used technique is to provide expansion
by lining the ring with a ring liner.
38. Casting ring liners
Casting ring holds the investment in
place during setting and restricts the
expansion of mold.
Provision must be made to permit
investment expansion so a ring liner is
used inside the ring for more
expansion.
39. Materials used as liner
:
1) Traditionally
asbestos (
carcinogenic )
2) Aluminum silicate
ceramic liner
3) Cellulose ( paper )
liner
40. Acts as cushion ( allows mold expansion )
Permits easy removal of investment after
casting.
When ring is carried from furnace to casting
machine, it reduces heat loss, as it is
thermal insulator
41. To ensure uniform expansion, a liner is cut to fit
the inside diameter of the casting ring with no
overlap.
The dry liner is tacked in position with sticky
wax, & then is used either dry or wet.
Wet liner technique- The lined ring is
immersed in water for a time & excess water is
shaken away.
Squeezing the liner should be avoided,
because this leads to variable amounts of
water removal & no uniform expansion.
42. Cellulose liners
Being paper products, are burned away
during the burnout procedure, so a technique
must be found to secure the investment in the
ring
It should be 3.25 mm short of ends of the ring
to keep the investment in contact with ring
after burnout
This also restricts the longitudinal setting and
hygroscopic expansion
43. Ceramic ring liner:
They do not absorb water, but its network
of fibers can retain small amount of water
on its surface/wetting agents can be used
to increase the water sorption on surface .
The binders used in ceramic liner (Ex –
neoprene-latex) can contribute to toxicity
(stimulate fibrosis. act as adsorbent
surface for carcinogenesis).
44. Investing
There are 3 types of investing materials
for dental castings :
1) Gypsum bonded : type 2, 3 & 4 gold
alloys
2) Phosphate bonded : metal ceramic
framework
3) Silica bonded : high melting base metal
alloy in CPD
46. There are 2 techniques :
1) Hand investing
or Brush technique
2) Vaccum technique
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Brush technique
Paint the pattern with surface tension reducer
Add powder & liquid in bowl, & quickly mix by
hand
Attach vaccum hose to bowl, evacuate the bowl
& mechanically spatulate
Coat the entire pattern with investment with
brush
Fill the ring slowly from bottom to upward
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Vaccum technique
Hand spatulate the mix
With crucible former & pattern, attach ring to
mixing bowl
Attach vacuum hose & mix
Invert the bowl & fill the ring under vibration
Remove the vacuum hose before shutting off the
mixer
Remove filled ring & crucible former from bowl
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Remove crucible former
Remove metal sprue if used
Place ring with sprue facing down in furnace on
ceramic tray
Bring furnace to 400 degree C & hold for 30
min
Increase the heat to final burn out temp 650
degree C for gypsum & 700-870 C for
phosphate bonded
51. During burn out, residual carbon may
get trapped in porous investment so it
is advised to begin burn out when
mold is still wet.
When transferring the mold to casting
machine, visually check, it is cherry
red color.
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52. Hygroscopic low-
heat technique:
Used with gypsum
bonded investment
which are allowed to set
under water.
They are used in
casting gold alloys.
The temperature used
in this technique is
4800c for 60-90 mins.
This technique causes
0.55% of expansion
Greater chances of
back pressure porosity
54. This technique can only be used in
hygroscopic low – heat technique because in
high – heat technique, wax pattern will be
melted before most of this expansion takes
place.
This technique results in casting fits that are
more accurate and reproducible than those
produced by either of the commonly used
technique.
55. High heat thermal expansion technique :
Gypsum bonded investment is slowly heated to 6500c -
7000c in 60mins. Then maintained for 15-30 mins.
Phosphate bonded investment is heated after initial slow
raise of temp to 3150c, the temperature is rapidly raised to
750-9000c and maintained for 30 mins
The technique cause 1.33-1.58 % of Thermal expansion
56. Once the 2nd stage of heat application is over,
casting of alloy into mold should be done
immediately.
This prevents cracking of investment and
reheating, which cause change in dimension of
mold (with loss of some expansion previously
obtained) ).
The maximum allowable time between the
completion of burnout and casting of alloy is 1
minute
58. Casting crucible
Clay – for crown and bridge, high noble
metal
Carbon- for noble metal & high fusing
gold alloys
Quartz – high fusing alloys of all types,
Pd, Ni, Co
59. Casting
A) Heat source – to melt the alloy
B) Casting force – to force molten alloy into
mold
Casting force > surface tension of alloy
+ resistance offered by gas
in the mold.
60. Heat Source
A)Torch flame
source of flame Gas air
Gas oxygen
Air acetylene
Oxygen acetylene.
Hydrogen oxygen
GENERATOR
B)Electricity
electrical resistance
melting
Arc melting
Electrical induction
melting
61. Zones of blow-torch flame:
It should be like non luminous brush flame.
Zone 1 – Colorless Zone
Zone 2 - Combustion zone
Zone 3 - Reducing zone
Zone 4 - oxidizing zone
62. 1. Mixing zone – dark in color, no heat present.
2. Combustion zone – green in color, zone of partial
combustion & has oxidizing nature so should be
kept
away from molten metal
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3. Reducing zone – dim blue in color, hottest part of
flame, it is used for fusion of casting alloy.
4. Oxidizing zone – has lower temp. than reducing
zone, it oxidizes the metal so should never be
used for melting alloys
64. The changes seen in metal during fusing are
:
Initially appear spongy later small
globules of fused metal appear later
molten metal flows assuming a spherical
shape at proper casting temperature
the molten alloy is light orange and tend to
spin/follow the flame when the latter is
moved.
At this stage the temperature of molten
65. Flux:
During melting of gold alloys flux may be
added to
1) fusing of metal
2) prevent oxidation
3) porosity
Commonly used fluxes are fused borax
powder ground with boric acid powder.
66. Electrical resistance heated casting
machine :-
An electric heater is an electrical appliance that
converts electrical energy into heat. The heating
element inside every electric heater is simply an
electrical resistor, and works on the principle of Joule
heating: an electric current through a resistor
converts electrical energy into heat energy.
67. It is used to melt ceramic alloys. Here the alloy
is automatically melted in graphite crucible.
It provides best means of temperature control.
It is quite convenient as compared to blow
torch
Advantage : -
Prevents over heating of the alloy
The crucible is always against the casting ring.
So the metal button remain molten slightly
longer and ensures complete solidification.
Precautions :-
Carbon crucible should not be used with
palladium alloys whose melting temperature
0
68. Electrical arc melting: -
It is used to melt higher fusing
alloys.
Electrodes are placed about 5cm
apart.
Safety measures need use of
•Black apron -prevents glare
•Facial viser – protect eyes
•Foot switch – control the current in
electrodes
69. High fusing alloys show rounding off of
corners and signs of collapsing, at
which point they are thrust into the
mould.
The electrodes must be raised from the
crucible before a centrifugal casting
machine is set in .
70. Induction casting :
It is used to melt base metal alloys of high
melting temperature.
The centrifugal casting machine is
controlled by electricity.
71. Principle :-
Induction casting is based on electric
currents in a metal core caused by
induction from a magnetic field.
This apparatus consists of specially
designed crucible around which copper
coil is wound circularly
72. When high density alternating current (of
high frequency 1500 KHz) is passed
through copper coil a magnetic field (I2P) is
set up around the coil.
This AC current also sets up eddy currents
in crucible.
Rapid change of polarity produced causes
the molecules of the alloy to oscillate and
their molecular bonds to break down
73. DIFFERENT TYPES OF
CASTING FORCE
Gravitational force
Vaccum force
Pneumatic (steam/Gas)
Pressure
Centrifugal force
Piston-plunger forces
74. Parts of spring wound centrifugal casting
machine :
It consists two arms one contains the
casting assembly.
75. This arm may be rigid / broken.
The broken arm is commonly used. the
incorporated ‘broken arm principle’
accelerates the effective initial rotation
speed of crucible and casting ring this
increases the linear speed of metal into
the
mold.
The other arm
consists of
appropriate
counter weight
for proper
rotation.
76. Cradle to seat the casting ring.
Bracket to place the crucible against the ring.
Head plate to prevent displacement of casting
ring.
Basic principle :
In broken arm casting machine the metal entry
into the mold is controlled by 2 forces.
Rotational force – which outs initially to force
the metal to the right.
Centrifugal force – attempts to push the alloy
laterally in a direction parallel to the long axis of
rotating arm. This force maintains the constant
force on the molten alloy during rotation.
77. To allow proper orientation of casting ring (consequently
the wax pattern) in the arm of casting machine, a method
of marking the sprue former on the casting ring should be
used.
Locate the pattern in the trailing half of casting ring, 90 to
135° away from the sprue position.
Once the pattern is in position, locate it downward in the
lower outer quarter of the trailing half of the casting ring
and secure it in that position.
J Prosthet Dent
1979;41:531-34
78. A variety of forces act upon a liquid metal subjected to
a
centrifugal force during the casting process:
i. Acceleration force (opposite the direction of
rotation)
ii. Centrifugal force (outward)
iii. Gravitational force (downward)
iv. The resultant force will be in the direction of the
lower outer quarter of the trailing half.
J Prosthet Dent 1979;41:531-34
79. The pressure gradient from the tip of
the casting to the bottom surface is
quite sharp and parabolic in form
Maximum at tip of casting and reaching
zero at the button surface.
More the pressure gradient
Greater the heat transfer
80. Argon gas pressure is used in Ti casting.
Piston –plunger force is used in
compressing
ceramics
81. CLEANING THE CASTING
The noble metal alloy is left in an annealed
condition for burnishing, polishing & similar
procedures.
When water contact with hot investment, a violent
reaction ensues. The investment becomes soft &
granular & the casting is more easily cleaned.
After casting has been
completed, the ring is
removed & quenched in
water as soon as the button
exhibits a dull red glow.
83. PICKLING
50% HCl solution
It aids in removal of any residual investment
as well as of the oxide coating.
The disadvantage of use of HCl is:-
1) The fumes from the acid are likely to
corrode lab. metal furnishings.
2) Health hazard & should be vented via a
fume hood.
84. Sulfuric acid is more advantageous.
Ultrasonic devices are available for cleaning
the casting.
Best method for Pickling is to place the
casting in a test tube or dish & pour the acid
over it.
Pickling solution should be renewed
frequently, since it is likely to become
contaminated.
85. In no case the casting be held with steel tongs
because, when both the casting & tongs come into
contact with pickling solution, it may contaminate
the casting.
The pickling solution
usually contains small
amounts of copper
dissolved from previous
castings.
When steel tongs contact with this electrolyte, a
small galvanic cell is created & copper is deposited
on the casting at the point where tongs grip it.
86. Copper deposition extending into the metal is a future
source for discoloration.
Gold based & palladium based metal ceramic alloys
& base metal alloys are bench cooled to room temp.
before casting is removed from investment.
Precious alloys(Gold-Platinum-Palladium) can be
soaked with hydrofluoric acid.
Nickel Chromium should never be placed in acid
because of high reactivity.
87. Part of the casting is not to be veneered with porcelain,
finished 1st & can be polished to smooth finish by
conventional techniques, but grease based agents
must not contact the surface to which porcelain is
added because, the organic residue could easily lead
to gas bubbles, porcelain detachment & discoloration.
Greasy films are removed by cleaning the casting with
household ammonia followed by, rinsing with distilled
water.
Small nodules & residual oxidation on the porcelain
bearing surface can be removed by rotary carbide burs
& ceramic bound stones.
88. Electrolytic polishing
Electrolytic preparation uses an electrolytic reaction
cell containing a liquid electrolyte with two electrodes:
an anode and cathode.
The sample to be polished/etched forms the anode.
89. Anodic dissolution results in a leveling and
brightening of the specimen surface.
90. TITANIUM CASTING
Titanium casting is highly technique sensitive.
The problem associated with casting Ti is due
to its extreme tendency to interact with
atmospheric components & with the majority
of elements at melting temperature.
91. High melting point (1670ºC), its strong chemical
affinity with O2, N & H.
High reduction capacity.
Low density.
Factors that affect casting of Ti
92. Spruing should be done with 6 gauge
multiple (3) sprues for three unit FPD to
minimize porosity(90% porosity free
castings)
J Prosthet
Dent,1995;73:534-41
95. CASTABLE GLASS CERAMICS
A cast glass ceramic offers unique
advantages to dentist dental
technicians and patients.
They can be used to restore accurate
form and esthetics in complete and
partial coverage restorations.
Spruing :-
Casting ring used – 1 .81 inches long
Sprue size used – 8’ gauge
96. Single sprue for anterior teeth, 2 sprues
for posterior teeth the recommended sprue
length – 3-4mm.
With pattern position 1/8 – ¼ inches from
the top of ring.
A generous reservoir is needed to ensure
that the molten glass is amplified into the
mold during casting.
97. Investing :
Ceramic ring liner is used inside the ring
and invested with phosphate bonded
investment which shows 1.6% thermal
expansion.
Investing done with brush technique
Setting time – 1 hr (in open air)
98. Burn out –
2 stage burnout is usual.
Initial temperature - 4800F for 30min.
Final temperature – 17500F (9500c) for
2 hrs.
99. Casting: -
The crucible is loaded with
4 gram ingot. The casting
temperature is
13580C(24760F).
Molten glass is forced
into the mold cavity
using piston-plunger force
and compressed.
100. Devesting –
Cut approximately ¼ inch from the end of
the ring to remove the casting from the
investment .
The remaining investment is removed
using air abrasive tool and 25m
aluminum oxide at a pressure of 40Psi
while a finger is placed over the margin
areas to prevent chipping.
101. Sprue removal : -
A double sided diamond disk is used to
slowly separate the crown at the
junction of the sprue and crown.
The excess sprue is removed and
finding completed with a white point by
using slow revolutions to avoid
chipping.
At this stage the casting is in a non-
crystalline glass form.
102. Embedding
The bottom of each pocket in the ceramming tray is
covered with the embedment material and vibrated
while the inside of each crown is filled.
One crown is placed in each pocket of tray, covered
completely with additional embedment material and
allowed to set for 15-30 minutes
Ceramming
During this procedure the casting converts into a
form that is 55% crystalline.
Ceramming temperature of 10750C (19600F) a for 6
hours.
Furnace is cooled to 3920F
103. Post-ceram divesting :-
After the tray has cooled to room temperature, the
embedment is broken apart by using finger pressure.
The crown is blasted with 25m aluminum oxide at a
pressure of 40Psi without abrading the margins.
Crown finishing is done with extra fine diamond point
and placed in distilled water for ultrasonic cleaning.
Cooling done to imitate the natural hue and the light
entering the cast glass – ceramic is selectively
filtered by colorcuts incorporated into the exterior
shading porcelains
105. Distortion
Causes :
1. Distortion of wax pattern
Proper handling of wax pattern should be
done.
Thermal changes in different thickness of
pattern can produce distortion
If wax pattern has to be invested after more
than 30 min then store in refrigeration.
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106. 2) Due to hygroscopic or setting
expansion
These expansion movements of
investment produce uneven
movement of proximal walls of
patterns, when it hardens around it.
Gingival margins are forced apart by
expansion while solid occlusal bar of
wax resists expansion.
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107. Thinner the pattern, greater is
distortion.
Lesser the setting expansion, lesser is
distortion.
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108. Surface roughness,
irregularities, discoloration
Surface roughness is defined as
relatively finely spaced surface
imperfections whose height, width &
direction establish the predominant
surface pattern.
Surface roughness is related to
particle size of investment & its ability
to reproduce wax pattern in
microscopic detail.
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109. 2 most common cause of surface
roughness are presence of water
droplets & air bubbles on surface of
pattern.
Surface irregularities refers to surface
imperfections such as nodules that do
not characterize the total of area.
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110. Factors that contribute to it
:
1. Air bubbles –
It may be entrapped during investment
procedure & results in a nodule in
casting.
Best method to avoid it by vacuum
mixing, & applying surface tension
reducing agent.
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111. 2) Water films
appears as minute ridge or veins on
surface
Causes – Pattern is vibrated after
investing or moved.
-Too high L/P ratio
-painting procedure is not
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112. 3) Rapid heating rates :
It results in fins or spines or
characteristic surface roughness can be
seen due to flaking of investment.
Mold should be heated gradually, at
least 60min should elapse from room
temp to 700 degree Celsius.
Greater the bulk of investment, more
slowly it is heated.
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113. 4) Under heating :
Incomplete elimination of wax residue
occur.
Voids or porosities occur when hot
alloy comes in contact with
carbonaceous residue.
Tenacious carbon coating is seen that
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114. 5) Liquid powder ratio :
Higher the L/P ratio, rougher is
surface, so it should be accurate.
If L/P ratio is very low then investment
will be thick & cause air entrapment.
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115. 6) Prolonged heating :
It causes disintegration of investment
& walls of investment are roughened
as result.
Also product of decomposition,
sulphur may contaminate the alloy.
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116. 7) Temperature of alloy :
If alloy is heated to too high temp.
before casting, surface of investment
is likely to be attached & results in
surface irregularities.
Prolonged heating of gold alloy
causes loss of its constituents&
become less fluid.
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117. Alloy should be heated as quickly as
possible until they are fully molten,
should not be overheated.
Time elapsed between heating &
applying casting force should be as
short as possible to avoid any cooling
at this critical stage.
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118. 8) Casting procedure :
Too high pressure during casting can
produce rough surface.
0.10 to 0.14 Mpa of air pressure & 3-4
turns of spring in centrifugal machine
is sufficient.
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119. 9) Foreign body :
Any casting with sharp, well defined
deficiencies indicates presence of
foreign body in mold.
It may occur due to rough crucible
former or careless removal of sprue
former.
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120. 10) Impact of molten alloy :
Direction of sprue should be such that
molten alloy does not strike a wear
portion of mold surface.
Molten alloy may fracture the mold
surface & causes raised area in
casting and goes undetected until
casting does not seat.
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121. 11) Pattern position :
When many patterns are invested together,
they should not be placed too close to each
other.
Many patterns at same plane should be
avoided.
Expansion of wax is much greater than
investment so spaces between them should
not be less than 3 mm.
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122. Porosity
Classification :
1.Solidification defects –
a) Localize shrinkage porosity
b) Micro porosity
c) suck-back porosity
2. Trapped gasses-
a) Pin holes
b) Gas inclusions
c) Subsurface porosity
3. Residual air
a) back pressure porosity
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123. 1) Localise shrinkage porosity :
Occur due to incomplete feeding of
metal during solidification.
1.25% of linear contraction occur in
noble metal alloys when changes from
liquid to solid state.
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124. Porosity in pontic area is due to ability
of pontic to retain heat because of its
bulk & is located in heat centre of ring.
It can be solved by attaching 18 gauge
sprue at surface most distant from
main sprue, extending laterally within
5 mm of ring. It carries heat away from
pontic.
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125. Suck back porosity :
-It occur externally, usually in interior of
crown near area of sprue when a hot
spot is created by hot metal impinging
from sprue channel on point of mold
wall.
-It causes this local region to freeze
last & causes suck back porosity.8/19/2015
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126. It occur at occlusoaxial line angle or
inciso axial line angle that is not well
rounded.
It is solved by flaring the point of
attachment of sprue & by reducing the
mold melt temp. or by placing a Y
shaped sprue with 2 arms wide apart.
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127. 2) Microporosity :
It occur from rapid solidification if mold
or casting temperature is too low.
It is not a serious defect because it
can not be detected until casting is
sectioned.
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128. It also occur from solidification
shrinkage, when solidification is very
rapid for the micro voids to segregate to
liquid pool. Solubility of gasses in liquid
is high so gas is ejected during cooling.
Porosity due to shrinkage- rough voids
Porosity due to gasses- smooth bubbles
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129. 3) Pin holes and gas inclusion porosity
:
These are entrapment of gas during
solidification.
They are same & characterize by
spherical contour but vary in size.
Gas inclusion porosities are larger than
pin hole porosity.
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130. Metals dissolve & liberate gas when
they are melted. On solidification
absorbed gasses are expelled & pin
hole porosity results.
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131. 4) Subsurface porosity :
It occur due to simultaneous
nucleation of sold grains & gases
bubbles at first moment that the metal
freezes at the mold walls.
Controlling the rate at which molten
metal enter the mold can minimize
this. 8/19/2015
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132. 5) Entrapped air or back pressure
porosity:
Occur on inner surface of casting & can
produce large concave depressions.
It is caused by inability of air in the mold
to escape through the pores in
investment or by pressure gradient that
displaces the air pocket towards the end
of investment via molten sprue or button.8/19/2015
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133. It occur because of modern dense
investment, increase in mold density by
vacuum mixing & tendency of mold to clog
with residual carbon.
It is solved by proper burn out, adequate
mold & casting temperature, sufficient high
casting pressure & proper L/P ratio.
Thickness of investment from pattern should
not be more than 6 mm.
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134. 6) Incomplete casting :
It is because molten metal alloy is
prevented from completely filling the
mold.
Factors that lead to this –
a) Venting of the mold
b) High viscosity of fused metal
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135. Insufficient venting is directly related to back
pressure exerted by air in the mold.
So casting pressure should overcome back
pressure, pressure should be applied for 4
seconds.
Mold is filled & metal is solidified in 1 sec or less,
yet it is soft in early stage. So pressure should be
maintained for slightly longer.
It can also occur by incomplete elimination of
wax residue.
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136. 7) Small casting :
If compensation for shrinkage of alloy
is not done, by adequate expansion of
mold cavity, then small casting is
resulted.
Too bright & shiny castings withy short
& rounded margins are formed.
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137. CONCLUSION
Investing & casting, a series of highly
technique sensitive steps, converts the
wax pattern into metal casting. Accurate
& smooth restoration can be obtained by
paying special attention to each step in
the technique.
When initial attempts in casting
produce errors or defects, appropriate
correction must be done at that stage
only.
139. Bert T. Cecconi, D.D.S., M.S.and Kamal Asgar, Ph.D
“Modified hygroscopic gold casting technique”
J Prosthet Dent 1975;33(2):216-220
Ernest Dewald, “ The relationship of pattern position
to the flow of gold and casting completeness”
J Prosthet Dent 1979;41(5):531-534
Asgar K., Arfaei A.H.- “Castability of crowns & bridge
alloys” J Prosthet Dent 1985;54:60-63.
Wayne V Campagni, Michael Majchrowicz, “An
accelerated technique for casting post and core
restorations” J Prosthet Dent 1991;66:155-156.
Editor's Notes
CHANGE COLORS
The process of attaching a sprue former/sprue pin to the wax pattern is called as spruing
Sprue -
The channel left by sprue former following burnout through which molten alloy can reach the mold.
CHANGE PREST
They are preferred for most of the castings because they melt at the same rate as the pattern and thus allow easy escape of the molten wax
SOLID PLASTIC SPRUE FORMER
It softens at higher temperature than wax pattern so it may block the escape of wax and will result in casting roughness.
DIRECT SPRUING :The sprue former provides a direct connection between the pattern area & sprue base or crucible former area.
INDIRECT SPRUING :A connector or reservoir bar is positioned between the pattern & crucible former.
It is common to use indirect spruing for multiple single units& FPD’s.
Should be attached to bulkiest part of pattern away from margin and occlusal contacts.
Normally non centric cusp is used.
Molten metal should flow from thick section towards margin, not reverse
Should not be attached at 90 degree, it may lead to turbulence.
-So it should be attached to 45 degree to bulkiest part of pattern.
5) Sprue former length :
- It depends on length of casting ring.
- For accuracy it should be as close to centre of ring.
- It should be adjusted such that top of wax pattern is with in 6 mm of open end of ring for gypsum investments and 3-4 mm for phosphate bonded investments.
7) Venting :
- Small auxiliary sprues or vents are recommended to improve casting of thin patterns.
- Their action may help gasses to escape during casting or ensure that solidification begins in critical areas by acting as heat sink.
Reservoir
It is added to sprue network to prevent localize shrinkage porosity.
Pattern should always solidify first and reservoir last
Crucible former
The sprue is attached to crucible former which constitutes the base of casting relation with casting ring during investing
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Plastic ring with rubber crucible
formers are used.
The ring is conical in shape with
tapering walls. As the investment sets the investment is tapped out of ring.
Then burnout is done with
out casting ring, this causes
greater expansion.
It is used for traditional gold-
based alloys
Purpose of wax elimination:-
To create a mold space – By wax elimination
To provide thermal expansion of investment.
To remove residual water in investment
eliminates the temperature difference between the molten alloy and investment
Accelerated casting method :
(J Prosthet dent. 66: 155,1991)
This technique uses phosphate bonded investment which sets in 15 mins and then 15 min burn out is done at 8150c.
This method is used for preparing post and core restorations.
The casting produced show similar accuracy and surface roughness as seen with fractional casting method
Zone 1 – Colorless Zone – This zone indicates the mixture of gas and air. No heat is present in this zone
Zone 2 - Combustion zone – green in color - This is an oxidizing zone. Here gases are partially burnt. This zone should be kept away from metal during fusing.
Zone 3 - Reducing zone – dimly blue. It is the hottest part of flame. This area should be in contact with the metal.
Zone 4 - oxidizing zone – outer cone – the combustion occurs with oxygen in air. This part should be never in contact with metal.
Surface of casting appears dark with oxides & tarnish. Such surface film can be removed by a process known as “pickling,” which consists of heating the discoloured casting in an acid.
For gypsum – 50% HCl solution
It aids in removal of any residual investment as well as of the oxide coating.
The disadvantage of use of HCl is:-
1) The fumes from the acid are likely to corrode lab. metal furnishings.
2) Health hazard & should be vented via a fume hood.
Summary
Investing and casting , a series of highly technique sensitive steps,converts the wax pattern into a metal casting. Accurate and smooth restorations can be obtained if the operator pays special attention to each step in the technique.when initial attempts at casting produce errors or defects,appropriate corrective measures must be taken so they do not recur.