This document details the casting process in conservative dentistry, outlining definitions and steps involved in creating dental restorations. It highlights techniques such as lost-wax casting, spruing, and the use of gypsum and phosphate bonded investment materials, alongside casting machines and their operation. The document also addresses desired accuracy, defects prevention, and the types of investments and machines used in the casting procedures.

2. INSTITUTE OF DENTAL
SCIENCE
DEPARTMENT OF CONSERVATIVE
DENTISTRY
Presented by:-
DR. TANMAY SINGH PATHANI {56}
BDS, MPH
SEMINAR ON
INVESTMENT & CASTING

4. •Definition
•Introduction
•Layout of steps involved casting procedure.
•Desired accuracy in casting procedure.
•Spruing
•Casting ring liners
•Investing
•Wax burnout procedure in
•Gypsum bonded investment
•Phosphate bonded investment
•Casting machines and procedure.
•Defects in casting and their prevention.
Contents

6. DEFINITIONS
Casting:
Something that has been cast in a mold.
Or
An object formed by the solidification of a fluid that has been
poured or injected into a mold.
Or
The act of forming an object in a mold.
GPT 8th
edition.

7. Sprue former-
A wax, plastic, or metal pattern used to form the channel
allowing molten metal to flow in the mold to make a
casting.
Investing –
The process of covering or enveloping wholly or in part ,
an object such as denture , tooth, wax form, crown etc.
with a suitable investment material before processing or
casting.

9. INTRODUCTION
The casting process is used to
make dental restorations such as
inlays, onlays, crowns, bridges,
and removable partial dentures

10. • W.H.Taggart introduced lost-wax casting
technique in 1907.
• This was followed by Jameson who
introduced the centrifugal-casting machine
and Solbrig who introduced the steam-
pressure casting machine.

11. centrifugal-casting machine

13. The Layout Of Steps In Making Casting
1. Preparation of the tooth or teeth
2. Making of an impression.
3. Pour gypsum slurry into the impression to make
a positive cast,
4. Make a wax pattern
5. Sprue the wax pattern (fix it in space)

14. 1. Invest the wax pattern.
2. Eliminate the wax pattern by burning the wax out of the
investment in a furnace, thus making the mold.
3. Force molten metal into the mold using one of a variety of
means.
4. Clean the cast.
5. Remove the sprue from the casting.
6. Finish and polish the casting on the die.
7. Cement the finished cast restoration on the prepared

15. The direct technique (in which the pattern is made
on the tooth rather than on the die) if used, steps 2
and 3 are omitted

19. DESIRED ACCURACY OF CASTING
PROCEDURE
The desired accuracy of the casting is about
0.1 %
Therefore, the lost wax procedure requires
specially developed materials that compensate
for the dimensional changes indicated by the
following equation.
SHRIKANGE EXPANSION
(wax+alloy) (wax+setting+hygroscopi
c+thermal)

20. Shrinkage
Wax Direct 0.4%
O'Brien 3rd
editionIndirect 0.2%
Alloys 1.25%-2.5% Craig 11th
edition.

21. Setting
expansion(
%)
Hygroscop
ic
expansion
(%)
Thermal
expansion(
%)
Total
expansion
.(%)
•High heat
(cristobalite)
0.35 .70 1.25(700°C) 1.95
•Hygroscopic
immersion (Beauty
Cast*)
0.30 1.50 .55(480°C) 2.05
•Hygroscopic water
added (Hygrotrol*)
0.75 2 .55(480°C) 2.55
•Phosphate-high
heat
(Ceromigoid *)
0.23-0.50 .35-1.20 1.33-
1.58(700°C)
1.88-2.88
Expansion in Gypsum Bonded Investment.

22. Hi-Temp Carbon-free
investment for high
fusing alloys
1.2%
TE at
1300-1800°F
(700-900°C
.7% Setting
Expansion
Cera-Fina Carbon-free fine
grained investment for
high fusing alloys
1.3% TE at
1300-1800°F
(700-900°C)
.3% Setting
Expansion
WiroFine partial denture
investment material
Linear thermal
expansion [%]
0,8
Bellavest® SH
Graphite-free,
phosphate-bonded
precision casting
investment material
rapidly or
conventionally
heatable casting
investment material
for crowns and bridges
made of precious
metal, non-precious
metal alloys and press
ceramic
50% BegoSol®
1.7% (total
expansion)
80% BegoSol®
2.2%(total
expansion)
Expansion in Phosphate Bonded Investment
Material

24. Die hardener
Oil
Spacer applied

25. Waxing instrument
(PKT set)
Wax pattern

27. SPRUING
PURPOSE-
1. Fix the pattern in space so a mold can be made.
2. To create a channel for elimination of wax during
burnout.
3. To form a channel for the ingress of molten alloy
during casting.
4. To compensate for alloy shrinkage during
solidification.
O'Brien 3rd edition

28. Sprue former

29. 1. Wax sprues are preferred and commonly used as they allow
easy escape of molten wax.

30. 2. Plastic sprues :
• Solid plastic sprues soften at higher
temperature than the wax pattern
• May block the escape of wax because the wax
melts at lower temperature than plastic and
• Thermal expansion of wax is 5 times greater
than plastic which will result in excessive
pressure build up inside the mold during the
burnout before plastic sprue softens resulting
in casting defects
O'Brien 3rd edition

31. 3.Metal sprues can be solid or hollow tubes. Later one is
preferable.
• If metal sprue is used, it should be made of non-
rusting to avoid possible contamination of wax
pattern.
• They removed from the investment at the same time
when the crucible former is removed.
care should be taken to examine the orifice while
removing because small amount of investment chip-
off which result in incomplete casting or inclusion of
foreign object in the casting
O'Brien 3rd edition

32. SELECTION OF SPRUE FORMER
Based on following criteria
DIAMETER- Should be = the thickest portion of wax
pattern
If large- causes distortion
If small- will solidify before casting itself
and cause localized shrinkage
porosity (suck back porosity)
The sprue former should be at least 1. 7 mm(14 guage) in
diameter unless the pattern is extremely small,and sprue
formers up to 2.5 mm(10 guage) in diameter can be used for
very large patterns (bulky) full crown
Murrow and Rudd

33. When small patterns need long sprues,
• wax reservoir placed approximately 1/6 inch (1.5 mm) from
pattern.
• These reservoirs should have diameter greater than thickest
portion of pattern
Murrow and Rudd

34. ATTACHMENT AND ORIENTATION
Should be attached at greatest cross-section
of the pattern
As It is best for molten metal to flow from thick
to thin area i.e. from the marginal ridges
to the gingival margins and not the
reverse.
This also minimizes the risk of turbulence

35. LENGTH-
Sprue former should be long enough so as to properly
position the pattern in casting ring within
6 mm (1/4 inch) of trail end in gypsum bonded
investment and
3 to 4mm (1/8 inch) for phosphate bonded
investment

36. Direction should be 45º to the proximal area
It should be directed away from any thin or delicate
part of investment. Molten alloy may abrade or
fracture investment in this area
DIRECTION

37. CASTING RING LINER
Provides investment expansion.
If not used the mold may become smaller because of reverse pressure
resulting from the confinement of the setting expansion.
Types- Asbestos ( Was used earlier, but is not used now because of
its carcinogenic potential)
Aluminum silicate /ceramic liner
Cellulose liner (it has an advantage that that it can be made wet
with water and allow semi hygroscopic expansion)
Use of two layers of liner allow more expansion.

38. A B C
(A)Casting ring
(B) Ring liner
(C) Base former

39. PLACEMENT
LINER If PLACED 3.25 mm short of ends of the
ring, produces more uniform expansion

40. Placement of the liner within the ring influences expansion of the mold
√ when maximum expansion is required, a liner flush with the open end of
the should be used.
√ A liner that is 3 or 6 mm short of the open end of the ring will produce a
smaller casting. The greater the unlined portion of the ring. the greater the
restrictive effect; thus the distance by which the liner is short should be
carefully controlled

41. INVESTING
Wax pattern should be cleaned of any debris, grease or oil ( a
commercially available wax pattern cleaner or a diluted synthetic
detergent is used)
Any excess liquid is shaken off and pattern is left to air dry
A thin film of cleanser left on pattern reduces surface tension of
wax pattern and permit better wetting of investment
Liquid and powder are mixed with care to not to incorporate air
in the mix. (Vacuum mixing is preferred)

42. Pattern is painted with layer of investment inside and out.
Casting ring is positioned on crucible former and remaining
investment is vibrated slowly in to the ring.
Vacuum mixing results the texture of the cast surface
–smoother with better details
reproduction.
-It also increases the tensile
strength of investment.

43. Vacuum mixing
Investing the wax pattern

44. Investing the wax pattern

45. Care should be taken to not to give excessive vibration while
investing because it may cause solids to settle and may lead to
free water accumulation adjacent to pattern resulting in surface
roughness.

46. Investing the wax pattern
Setting
expansion

47. WAX ELIMINATION AND HEATING
Gypsum bonded and phosphate bonded investment are ready
for burnout after setting of one hour ( They should be kept in 100%
humidity in case if one need to wait).
1. Remove crucible former and any metal sprue.
2. Clean any debris with camel hair brush.
3. Investing rings are placed in a room-temperature furnace
and heated to prescribed maximum temperature.
For GYPSUM BONDED-468ºC for hygroscopic technique
(low heat technique)
650°
C for thermal expansion technique
( high heat technique)

48. RELATED PROBLEMS
Investment decomposition- is seen with gypsum
bonded investment therefore careful temperature
setting should be done.
Melted wax absorbed in investment-
To prevent this begin burnout when mold is still wet
also in high heat technique, high temperature convert
carbon to CO2 or CO which escapes through pores.

49. Heating is done at 500ºc for 60 minute ( It can be done up to
5 hour or longer with little damage)
Expansion is obtained by-
1. 37ºc water bath expands the wax pattern.
2. Warm water entering investment mold adds some hygroscopic
expansion.
3. thermal expansion at 500ºc.
Added expansion for newer noble alloys is obtained by-
1. increasing water bath temperature to 40ºc.
2. using two layers of liners.
3. increasing burnout temperature range of 600ºc to 650ºc.
HYGROSCOPIC LOW HEAT TECHNIQUE
FOR GYPSUM BONDED INVESTMENT

50. This approach depends almost entirely on high heat burnout
to obtain the required expansion and at the same time
eliminating the wax pattern.
Additional expansion is obtained from-
Expansion of pattern from heat produced during
setting.
Warm water entering the mold from wet liner
causing some hygroscopic expansion.
HIGH-HEAT THERMAL EXPANSION TECHNIQUE
FOR GYPSUM BONDED INVESTMENTS.

51. Burning out the wax pattern
… in the furnace
Thermal
expansion
Images from Bego Virtual Academy

52. Mold are placed in furnace at room temperature and
slowly heated to 650ºc to 700ºc in 60 minutes and
held for 15 to 30 minute.
Related problems-
Rapid heating can generate steam which can cause flaking of
mold walls. It may also cause cracking of investment because
outer layer of investment expands thermally resulting
compressive stress in outer layer and tensile stress in
inner layer.
These crack appear from interior outwardly in form of radial
cracks.

53. Sulfer di oxide production
It occurs rapidly over 700*c with reduction of calcium sulfate by
carbon.
CaSo4 + 4C  CaS +4CO
3CaSo4 + CaS  4CaO +4SO2
The sulfur dioxide produced contaminates gold
castings and makes them brittle.
Therefore casting should be made immediately after casting
temperature has been reached to prevent sulfur contamination.

54. PHOSPHATE BONDED INVESTMENT-
• Usual burnout temperature range is from 750º
c
to 900º
c.
• Heating rate is usually slow till 315º
c and is quite
rapid thereafter reaching completion after a hold
at the upper temperature

55. 1.Expansion of wax pattern because setting reaction
raises the mold temperature considerably
2.Setting expansion is higher than in gypsum
bonded because of special liquids (silica sol)
used to enhance such expansion.
3.Thermal expansion is high because of higher
burnout temperature.
PHOSPHATE BONDED INVESTMENTS OBTAIN THEIR
EXPANSION FROM FOLLOWING SOURCE

56. CASTING MACHINES
Several types and designs of casting machines are used to
make dental castings.
All casting machines accelerate molten metal into the mold
either by
1. Centrifugal force or
2. Air pressure.
Numerous modifications and variations of these methods are used
in different machines.
• The selection of the casting and melting techniques is
influenced by type of alloy and restoration to be cast.

57. Phosphate bonded investment material
1 powder
2 debubblizer
3 liquid
1
2
3

58. Three types of casting machines are used
• CENTRIFEUGAL CASTING MACHINES
• INDUCTION MELTING MACHINES
• ELECTRICAL RESISTANCE HEATED CASTING MACHINES

59. CENTRIFEUGAL CASTING MACHINE
A variety of centrifugal machines are available.
• Some spin the mold in a plane parallel to the table top on which the machine is mounted.
• Others rotate in a plane vertical to the table top.
• Some are spring-driven, and
• others are operated by electric power.
• An electric heating unit is attached to some machines to melt the alloys before spinning
the mold to throw in the metal.
• Others have a refractory crucible in which the alloy is melted by a torch before the casting
operation is completed.
All these machines depends on the centrifugal force applied to the molten metal to cause it
to completely fill the mold with properly melted metal

60. Diagram showing the relation between the ceramic crucible used to melt
the alloy and the casting ring in a centrifugal casting machine . Both the
crucible and the ring are placed on the casting machine . Once molten , the
metal is rapidly driven centrifugally out of the crucible and into the mold in
the casting ring . The traverse of the metal takes less than one second .
The oxidized elements and flux , which are less dense ,lag behind the molten
metal as lag.

61. • Broken arm casting machine ( Penwalt / Jelenko )
• Emesco senior casing machine (J.M. Ney company)
Are the examples of centrifugal casing machine and incorporate a “broken arm ”
principal
The melting crucible and mold are positioned at right angle to the spring-propelled
beam of the apparatus.
As the beam begins its rotation the melted alloy , by its inertia, is carried directly toward
the mold.
As centrifugal force is generated by the spinning of the arm the casting flask and the
mold swing into alignment and become subjected to the full centrifugal force.

62. The metal is melted by torch flame in a glazed ceramic crucible
attached to broken arm of crucible.
After the metal has reached the casting temperature the machine is
released and spring triggers the rotational motion.
When metal fills the mold a hydrostatic pressure gradient develops
along the length of casting.
Usually the pressure gradient at the moment before solidification
,begins ,reaches about 0.21 to 0.23 MPa at tip of casting.

63. Methods of Melting the Alloys
Generally two methods are employed for melting of metal alloy
prior to casting:
– Flame / Blow pipe.
– Electrical.
A) FLAME
The fuel employed is a mixture of oxygen-air and acetylene.
The temperature of gas-air flame is influenced by the nature of
the gas and the proportion of gas and air in the mixture.
Care should be taken to obtain a non-luminous flame, with
combustion zones clearly differentiated for melting the alloy.

64. Parts of the flame:
First cone that comes directly from the nozzle
Air and gas are mixed before combustion.
No heat is present in the zone.
• Second cone is the combustion zone
Green and immediately surrounding the inner cone.
Gas and air are partially combusted.
“Oxidizing zone” and
Should be kept away from the metal during fusion (melting).
• Third zone is the reducing zone which is blue.
Hottest part of the flame and is just beyond the tip of Green
combustion zone.
The area should be on metal during fusion.

66. B) ELECTRICAL
There are two methods by which electricity can be employed to melt
the alloy.
• Induction.
• Electric Arc (Resistance).
Although, both the methods work on same principle i.e. heat energy is
produced when electric current is passed through a conductor
depending upon the voltage applied across it.
H = P x t
H = V x I x t (P = V x I)
H = V2
x R x t (I = V x R) Ohms law
Where,
• H = Heat produced in joules.
• P = Powel in watts.
• t = Time in seconds.
• V = Voltage in volts.
• I = Current in amperes.
• R = Resistance in ohms.
The heat energy utilized for melting the alloy.

67. INDUCTION
The apparatus consist of high
frequency induction coil across
which high voltage is applied.
This high frequency coil
surrounds the crucible in which
the alloy / metal pellets are
melted.

68. The energy required to melt the alloy is supplied by a
high frequency generator.
The melting crucible is held in a centrifugal arm, and
during the melting process it is surrounded by the
high frequency generator coil.
The alloy is melted by two ways depending upon the
type of crucible.

69. • In case of graphite crucible the electric energy of
coil is converted to thermal energy which is
utilized by means of conduction radiation
(Graphite is a good conductor of heat and
electricity) by the graphite crucible to melt the
alloy.
• Noble metal alloys are generally melted in
graphite crucible.

70. When ceramic crucibles are used the induction coil
induces the ‘eddy current’ in the alloy itself which
provides energy for the melting of the alloy.
• When the alloy is molten, the centrifugal process is
initiated and the induction coil being lowered.
• The molten alloy is then forced into the casting mold
located behind the melting crucible by means of
centrifugal force
• The ceramic crucibles are generally used for base
metal alloys like nickel-chrome, chrome-cobalt etc.

71. Electric Arc / Resistance
This is a crucible free technique where by the metal / alloy pellet
is directly melted by the heat produced as a result of
resistance offered by alloy to high voltage current.
The apparatus consist of an electrode and a base plate on which
the metal / alloy pellet is placed.
The contact between the electrode terminal and the metal / alloy
pellet is in form of an ‘electric arc’ thus the term ‘electric arc
furnace’.

72. air pressure technique
• The alloy is melted in in the crucible hollow of the ring
followed by applied on the melt.
• Here the alloy is melted by blow-pipe or flame in the
hollow left by the crucible former.
• The high surface tension of the liquid metal prevents it from
falling down in to the sprue channel.
• Once the metal has been melted to the casting temperature,
the air tight piston is applied with 10 to 15 psi air pressure to
force the metal into the mold.

73. ELECTRICAL RESISTANCE HEATED CASTINGELECTRICAL RESISTANCE HEATED CASTING
MACHINEMACHINE
(High frequency centrifugal casting machine)(High frequency centrifugal casting machine)
There is automatic melting of metal in graphite crucible ( this isThere is automatic melting of metal in graphite crucible ( this is
advantageous for metal-ceramic restoration in which traceadvantageous for metal-ceramic restoration in which trace
amount of base metals are prevented from oxidation from torchamount of base metals are prevented from oxidation from torch flameflame
from overheating.from overheating.

74. INDUCTION MELTING MACHINE
Metal is melted by induction field that develops with in the
crucible surrounded by water cooled metal tubing.
Molten metal is forced in to mold by air pressure or both.

75. Fornax®T
• The compact high-frequency induction casting
machine
•
•
• The Fornax® with benchtop design
• induction heating (semiconductor generator)
• casting temperature up to 1550 °C: ideal for all dental
alloys (except titanium)
• Cooling with BEGO circulation unit or through direct
connection to water supply and outlet
• Heating level switch for ceramic or graphite crucible

76. Fundor T Casting machine (BEGO)
Features:
• Casting of all dental alloys (except titanium).
•High degree of safety, by virtue of cover lock.
•Automatic gas lighter as accessory simplifies handling of
the melting torch significantly.

77. High Frequency Centrifugal Casting
Machines
• Degutron (Degusa)Degutron (Degusa)
• The energy required here to melt the alloy is
supplied by a high frequency generator.

78. MEGAPULS 3000 induction casting
machine.
• The machine operates using high
frequency electromagnetic waves
and should not be operated by
anyone with a heart pacemaker.

79. VACUUM PRESSURE CASTING
MACHINES
•Combilador CL-G 77 (Heraeus Kulzer)
Features
• Recommended for casting all types of crown
and bridge work in precious metal alloy.
• Electronic temperature control with digital
display.
• Max.temp. upto 1450°C.

80. TITEC F210 M (OROTIG)

81. It is a semi automatic titanium casting machine
where by the amount of vacuum, Argon-gas
saturation and over pressure levels as well as
the delivery of energy to electric arc to melt the
metal are automatically performed in a
sequence once the machine is started.
The decision to halt the process on the spot
and implement corrections before starting the
process again are in operators hands.

82. • The rotation of the melting / casting chamber from
horizontal to vertical position for the casting to take place,
when the metal (titanium) melting point has been
optimally reached, is also in the operators hand.
• Therefore, both the automation of the machine and the
operators skill go hand in hand to achieve excellent results
constantly.

84. CASTING CRUCIBLES
Clay – used for high noble and noble metal alloys.
Carbon - used for high noble and higher fusing gold based
metal ceramic alloys.
Quartz - recommended for high fusing alloys for any type
specially suited for alloys that have high melting
and are sensitive to carbon contamination.

85. polishing

86. Common causes of failures of casting involves
1. Distortion
2. Surface roughness and irregularities
3. Porosity
4. Incomplete or missing details
.
CASTING DEFECTS

87. Distortion of pattern and dimensional changes
This occurs due to
• High tendency of wax to deform.
• High coefficient of thermal expansion:
35×10-6
/°C.
• Low softening temperature.
• Warping of wax when unrestrained.

88. Distortion
pre investing:-
-temperature at which pattern is fabricated
i.e. lesser the temperature at which pattern is
made ,more are the chances of distortion at high
temp due to release of stresses.
-more the storage time and temperature , more will
be distortion
Pre investing
investing

90. Prevention
• Softening the wax uniformly by heating at 50°C for
atleast 15 mins.
• Reducing the time between wax fabrication and
investing (not more than 30 mins)
• If more than 30m minutes
– store in refrigerator
• Readaptation of the margins after storage.
• Careful removal of the wax pattern from the die
- symmetrical forces

91. DISTORTION DURING INVESTING.
• During pouring of the investment
• Expansion of investment material
• Movement of investing ring before the
hardening of the investing material.

92. PREVENTION
• Gentle pouring
• No movement of ring before hardening of
investment material.

93. Roughness is defined as relatively finely spaced surface
imperfections whose height, width and direction establish the
predominant surface pattern.
Irregularities refers to isolated imperfection such as
nodules that do not characterize the total surface area.
Surface roughness of the casting is invariably greater than that of wax
pattern from which it is made.
SURFACE ROUGHNESS AND IRRAGULARITY

95. Causes
1. Prolonged heating leads to
Disintegration of the investment
Contaminates the casting producing loss of surface
texture.
2. Water powder ratio.
Higher the ratio: the surface of the investment does not
record the fine detail.
3. Casting pressure is too high.
Causes molten metal moves along the mold wall with
pressure leading to minute loss of investment particles/ fine
detail.

96. 4. Foreign bodies.
Pieces of investment if get broken , gets incorporated
into metal resulting in roughness.
5. Reuse of metal alloy.
6. Improper pickling of metal.
Taking gold alloy out from pickling solution with
stainless steel tweezer causes plating on the gold alloy
with base metal and leads to surface roughness.

97. Surface irregularities on an experimental casting .
a). Air bubble
b). Water film
c). Inclusion of foreign body.
Caused by:-

98. NODULES
Small nodules on a casting are caused by air bubbles that becomes
attached to the pattern during or subsequent to the investing
procedure.
• Removal of nodules on margin or on internal surface might cause
marginal discrepancy or alter the fit of the casting, On the
occlusal surface it effects the occlusal anatomy.
• Best method to avoid air bubble is to use vacuum investing
technique and also by using a wetting agent.

99. If the investment become separated from the wax
pattern , water film may form irregularly over
the surface. this type of surface irregularity
appears as minute ridges or veins on the
surface
Too high L:P ratio also produces these surface
irregularities.
Use of wetting agent prevents such irregularities.
WATER FILM

100. Mold Cracking/Fins on Casting
Too rapid heating - unequal expansion of the
investment.
• Outside portion gets heated and expands more
in comparison to inner one.
• Leading to cracks in the investment.
• which in turns produces fins in casting.
• Commonly present in cristobalite investment
which has low inversion temperature.
contd

101. Mold Cracking/Fins on Casting
Pattern position
1. When many patterns are present in same plane.
– Prevention by investing more than one pattern in
different planes.
– Allow 1mm space between the patterns.
2. Patterns placed close to ring wall or near to the ring
end. Space patterns 4 to 5 mm from mold
wall and end.
Investment not thoroughly set
prevention by Spatulate longer, use warmer liquid and allow to set
in warmer room.

102. FOREIGN BODIES
Any casting that shows sharp well defined
deficiencies indicate the presence of some
foreign particle in the mold, such as pieces of
investment and bits of carbon from a flux.
It results in surface roughness and also incomplete
casting areas or surface voids.

103. POROSITY
Depending upon location
External
Internal
Effects of porosity:-
Weakens the alloy
Roughness and discoloration
If severe can lead to tooth surface micro leakage and
secondary caries.

104. i. Solidification defects
(A) Localized shrinkage porosity
(B) Micro porosity
ii Trapped gases
(A) Pin hole porosity
(B) Gas inclusion porosity
(C) Subsurface porosity
iii Residual air
TYPES OF POROSITY

105. Manifests as large irregular void found near the junction of the
sprue and casting.
It is caused by
Incomplete feeding of molten metal during solidification.
Nobel metal alloy contract 1.25% when changes from liquid
to solid.
Therefore there must be continual feeding of molten metal through
the sprue to make up for shrinkage.
If sprue freezes in its cross-section before feeding is
completed- a localized shrinkage void will
occur in the portion of casting which is last to solidify.
LOCALIZED SHRINKAGE POROSITY-

106. Prevention Of Localized Shrinkage Porosity
1. Proper size selection of the sprues
a). Diameter ≥largest thickness of the pattern
size of the sprues varies from 6- 12 guage
– 2.5mm (10 guage)-molars / metal ceramics.
– 2mm (12 guage ) for premolars/ partial coverage.
b). Length wax pattern should be placed 6 mm-3 mm from the top of the ring
,depending on the type of the investing materials.
c). Flaring
– allows even flow of the metal into mold and therefore less porosity
– also acts as reservoir ,and allows continuous feeding of the
molten alloy.

107. In X ,the sprue is attached with rounded corners to avoid
turbulance and the risk of investment fracture.
In Y, the sharp corners of attachment are not desirable.

108. • Indirect spruing
Use of reservoir between wax pattern and sprue base
.recommended in 2 unit /3 unit bridge.

109. Gate Method Of Spruing Large Wax Pattern
Need for following gate technique
• Generally the alloy immediately adjacent to the
walls of the mold solidifies first,
• The exposed surface of the button will solidify
second, And
• The alloy internal to the mold will cool last.

110. • Porosity occurs in the portion of the casting that cools last.
• For large castings such as multiple unit bridges,
the spruing arrangement provide a bulk (or reservoir) of
metal that is external to the dental unit but positioned at the
heat center of the investment mold.
• In the gate sprue technique the runner bar will usually be
the last part to solidify and will contain porosity.

111. Gate Method
Large units (e.g., bridges cast as a single
unit) sprued by the "gate" method, in
which a series of
• Short 8-gauge wax sprue formers are
attached to
• Continuous 6- or 8-gauge "runner bar"
of wax or (preferably) plastic this
runner bar in turn is attached
to
• The crucible former by two or more
large sprue formers,

112. Photo pg 211 RM
This method provides
(1) a minimum of distortion of wax pattern,
(2) a fairly uniform reservoir of metal the entire
casting, and
(3) even distribution of the alloy to all parts of
the casting to minimize porosity.

113. If bulky pontics are placed at the heat
center of the mold or are cast without
a resrvoir within the investment,
porosity will occur in the pontic
itself or at the connector between the
pontic(s) and the abutments,
resulting in a weak bridge
Proper Gate spruing, reservoir
is in the heat center of the
mold

114. Chill vents
Where a dental casting contains both thick and thin elements,
porosity can be minimized by the addition of chill vents that
accelerate the cooling of the pontic relative to the parts of the
casting external to the dental unit itself. These cast extensions
are cut off after retrieval of the casting from the investment.

115. GATE METHOD
• Care must be taken not to distort the large
pattern during handling or by heating and
softening the large bulk of spruing wax because
shrinkage on cooling stresses the pattern.
• The use of rigid plastic sprue formers ·and
runner bars will reduce this problem.

116. SUCK BACK POROSITY
It is a type of localized shrinkage porosity
which occur due to development of hot spot that is
created by hot metal impinging on a point of mold wall. This
hot spot causes the local region to freeze last.
It can be eliminated by-
1. flaring the point of sprue attachment
1. by reducing the mold melt temperature differential
that is lowering the casting temperature by about 30°C.

118. Incorrect solidification sequence
that results in suck back porosity.
the sprue area freeze before the
cusp area of the crown and the
metal in the crown had to feed the
shrinking alloy, causing a void in
the crown.
Correct sequence of solidification
of an alloy in the investment mold
of a full cast crown .the button
should freeze last .this order
allows the molten metal to
compensate for the shrinkage
realized when such increment of
metal goes from the liquid to solid
state

119. PREVENTION
• “Y” shaped sprue.
• Half of the molten alloy enters through
both side,therefore the temperature of
the investment does not rise under the
sprue.

120. Present as fine grain in casted alloy
Also occurs from solidification shrinkage but
Is generally when solidification is too rapid for the
micro voids to segregate to the liquid pool.
MICRO POROSITY
Prevention
By increasing the mold temperature.

122. PIN HOLE POROSITY/ GAS INCLUTION POROSITY
It occurs due to entrapment of gas during solidification.
Metals such as copper and silver dissolve oxygen in large
amount in liquid state.
Molten platinum and palladium have strong affinity for hydrogen
and oxygen.
On solidification absorbed gases are expelled and pin hole porosity
result.
Prevention
1.Premelting alloy in charcoal
2.Properly adjusted blow pipe
(reducing zone of the flame should be
used).

123. Its exact cause is not known.
It is assumed it may be caused by simultaneous nucleation of
solid grains and gas bubbles at the first moment, that the
metal freezes at the mold walls.
Prevention by controlling the rate at which the molten
metal enters the mold.
SUB SURFACE POROSITY

124. ENTRAPPED AIR POROSITY- BACK PRESSURE POROSITY
This is caused by inability of air in mold to escape through the pores in the investment
or
By the pressure gradient that displaces the air pocket towards the end of investment
via the molten sprue and button.
It is seen on inner surface of casting and can produce large
concave depression.
Incidence of air entrapment is increased by
1. Dense modern investment.
2. Dense investment due to vacuum mixing.
3. Clogging of mold with residual carbon
when low heat technique is used.

125. 1.Proper burn out cycle-according to ADA specification no.4,
:melted wax at 500ºC should leave no residue in excess of .10%
Incidence of air entrapment can be
decreased by-
1. proper burnout.
2. Adequate mold and casting
temperature.
3. High casting pressure.
4. Proper L:P ratio.

126. 2. Adequate mold and casting temperature
Temperature differential between mold and
casting should be less.
Lesser differences ,allow the alloy to flow easily, and
the gases are displaced easily.
3. Sufficient high casting pressure.(.10-.14 MPa)
4. Water powder ratio of investment: mixing should be
done according to manufacturer instruction.
Too dense investment: Prevents rescuing of the
gases.
5. The distance of 3-6 mm for phosphate bonded
investment and gypsum bonded investment respectively.

128. It occurs when molten alloy is prevented from
completely filling the mold.
causes are
1. Insufficient pressure to overcome back pressure.
Therefore sufficient pressure should be applied
for at least 4 seconds.
2. Incomplete elimination of wax residue from the mold.
It results in shiny rounded margin which Is caused by
strong reducing atmosphere created by carbon mono oxide le
by residue wax.
Improper burn out of waxes liberates CO2 ,H2O,N2O
INCOMPLETE CASTING

129. The formation of gases and their elimination depends on the
• O2content,
• Burnout temperature
• Time of burnout.
PREVENTION
1. For 500ºCmold temperature and large wax pattern :
Time required 1hr
2. For proper circulation of O2 :for half of the time sprue hole is placed downward,
(melted wax drains down).
Rest half of the time ,it is placed upward, to allow air circulation in the mold cavity.

130. Other causes are
3.Improper venting of gas
manifested as round and dull margins.
4.Casting temperature too low. i.e. , the alloy is
not melted properly
5. Mold temperature too low.

131. FUSION TEMPERATURE OF VARIOUS ALLOYS
• Chrome-Cobalt 870–900°C (1600–1650°F)
• Nickel-Beryllium 815–870°C (1500–1600°F)
• Palladium (2–8% Gold) 700–870°C (1300–1600°F)
• Silver-Palladium 760–815°C (1400–1500°F)
• Ceramic Gold 700–815°C (1300–1500°F) (45% or higher)
• Crown & Bridge Gold 575–700°C (1100–1300°F)

132. PREVENTION
SPRUING:
1. by adding auxillary sprues: which provides a). channel for
leading the molten alloy into the mold cavity.
b). Wax and gas elimination .
c). creates reservoir for molten metal which compensates for alloy
shrinkage.
2. Size of the sprue larger than the thickest part of the wax pattern .
smaller size of the sprue -leads to solidification of the metal in the sprue –
don’t compensate for solidification shrinkage.
3. Length of the sprue- 3-6 mm for phosphate bonded and gypsum bonded.
4. Flaring-acts as reservoir,
Keeps the metal in the molten state for long.

133. Factor which also governs the complete casting
1. The amount of metal used.
High density noble metal alloys
6gms=(4 dwt or pennyweight) is recommended for premolar
and anterior crown
9gms=(6 dwt) for molars
12gms=(8 dwt)=for pontic
1 metal pellet =6 gms.
When casting a restoration that requires 2 dwt of alloy, casting with
8-10 dwt is advisable.
Number of metal pellets required for base metal alloy is less

134. 2.
Casting force –
In the centrifugal machine the force is dependent on
the number of turns.
For
Noble Metal casting - 3 turns
Base Metal alloys - 4-5 turns.

135. Too large castings
Causes
• Excessive expansion of wax
• More than 2 liners used
• Improper positioning of liners.
PREVENTION
Single thickness cellulose liner
Restrict liner 3.25 mm short of the metal ring (allows
controlled longitudinal expansion).

136. TOO SMALL CASTING
Causes
Use of metal ring without liner
Reverse pressure causes decreases in dimension
Prevention :- use liners
• It provides cushioning effect
• If wet , allows hygroscopic
expansion.

137. DISCOLORED CASTING
Black castings are common gypsum bonded investment material
Cause :
1. wax not , completely eleminated
Carbon residues on the mold wall get deposited on the surface of the casting.
2. Disintegration of gypsum bonded investment above 700°C, resulting in release of
SO2and sulfide layer is formed over the alloy which cannot be removed by pickling.
Prevented by
Proper burn out,
Premelting alloy with charcoal.
Properly adjusted torch.

138. Rough Casting Surfaces
Cause Solution
Insufficient spatulation. Spatulate as recommended in instructions to
thoroughly complete all setting reactions.
Replace worn bowl and paddle
-- New equipment improves mixing action.
Rate of burnout too rapid. Decrease heating rate or try two-stage
procedure per instructions.
Overheating alloy melt. Review alloy manufacterer’s directions.
Defective pattern and/or
pattern matieral.
Use only high quality pattern materials. Avoid
was contamination.
Wet pattern. Dry pattern thoroughly after application of
debubblizer

139. Investment Breakdown
Cause Solution
Burnout too rapid Follow burnout procedure as recomended
in instructions.
Use of pre-heated furnace at high
temperature
If pre-heated furnace is used, initial
temperature should be below 500°F
(260°C).
Investment not thoroughly set Spatulate longer, use warmer liquid and
allow to set in warmer room.
Mold overheated Check furnace for temperature calibration
Insufficient setting time. Lengthen the time of bench setting before
mold is placed in oven -- 60 minutes
minimum.
Use of uncoated, solid plastic sprues and
runner bars
A light coat of wax will allow pattern wax
to drain and plastic to expand.

140. Pits And Nodules On Castings
Cause Solution
Inpurities in wax or plastic Be sure that pattern material is free of
foreign matter.
Air bubble entrapped Use wetting agent (SMOOTHEX); dry
thoroughly. Take care during investing to
avoid entrapping air.
Insufficient vacuum during spatulation. A good vacuum of 26-29" Hg (660-740
mm) is recommended for mixing.
Insufficient spatulation Increase spatulation time by 15-
20 seconds
Entrapment of loose investment particles Remove loose investment from sprue hole
before burnout. Avoid sharp edges in sprue
system.

141. Mold Cracking/Fins on Casting
Cause Solution
Too early and/or rapid
burnout
Lengthen the time of bench
setting before mold is
placed in oven, 60 minutes
minimum.
Plastic pattern or sprue
materials with high melting
point may plug sprue hole
during early burnout
Select pattern and sprue
materials that melt easily
and burn out without
difficulty; coat plastic with
wax. Hollow sprues are
preferred
Too many patterns in one Too many patterns in one

142. Mold Cracking/Fins on Casting
Use of excessive casting
pressure.
Reduce pressure (number
of turns).
Liner flush with ring end. Leave adequate space (2-
3 mm) at both ends of the
ring to lock in investment
Air bubble in set mold Avoid air entrapment when
investing.

143. Incomplete Castings
Cause Solution
Incomplete elimination of pattern
materials
Heat soak longer at the recommended
temperature. Clean furnace outlet,
recalibrate.
Insufficient heating of the alloy. Increase temperature of alloy.
Transfer mold to casting machine and cast
at once.
Excessive cooling of mold Casting machine operated with insufficient
pressure or too few turns. Increase
casting pressure; use more turns.
Misalignment of crucible and sprue hole. Position mold with sprue hole aligned with
crucible.

144. Cause Solution
Improper special liquid concentration To increase expansion, increase special
liquid concentration and to decrease
expansion, decrease liquid concentration.
Improper liquid/powder ratio Check liquid/powder ratio and accuracy of
measurements.
Liquid, mix and room temperatures affect
casting dimension
High liquid, room and mix temperatures
give larger castings and vice versa. Normal
range 70-80°F (21°C).
Dry liner inhibits hygroscopic Use liner that readily absorbs water
Pattern deformation. Handle wax pattern with extreme care.
Inaccurate Casting Fit

145. Porosity in Casting
Cause Solution
Incorrect sprueing Review sprueing system
Incomplete burnout Increase burnout time or
temperature to eliminate
pattern material
completely
Entrapment of loose investment particles Remove loose investment
from sprue hole before
burnout.
Alloy that absorbs gas when melted and
releases gas upon solidification, especially
when alloy is overheated
Use fresh alloy or change
method of melting. Adjust
gas mixture. Check
directions for melting of
alloy

146. References
• Anusavice “Skinners science of dental materials”. Tenth Edition.
• Murrow and Rudd
• Shillingburg HT, Jacobi R, Brackett SE. Fundamentals of tooth preparations for cast
metal & porcelain restorations. 1st
ed. Carol stream. Qintessence Publishing Co. Inc –
1991.
• Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. 3rd
ed.
Missouri (CN). Mosby – 2001.
• Craig RG. Restorative Dental Materials. 8th ed.St Louis: CV Mosby Co.1989
• Laboratory manual of the requirements used for casting – BEGO, DENTAURUM,
OROTIG,MEGAPULS 3000.
• Michael J. Lessiter, Ezra L. Kotzin, Timeline Summer 2002,www.castsolutions.com
• http://images.google.co.in/imgres?
imgurl=http://www.edwardfeinbergdmd.com/Short%2520History%2520of
%2520Dentistry/William-Taggert-Casting-machine-
1907.jpg&imgrefurl=http://www.edwardfeinbergdmd.com/history_of_dentistry.htm
&h=256&w=217&sz=16&hl=en&start=32&tbni

147. THANK YOU