This document provides information on the casting procedure for dental restorations. It discusses the history of casting, defines key terms, and outlines the basic steps of the casting process including fabrication of the wax pattern, spruing, investing, casting, and finishing. The optimal sprue design is described, noting that the sprue should be attached to the bulkiest part of the pattern away from margins, directed at a 45 degree angle, and of a length that positions the pattern 6mm from the casting ring. Proper sprue design is important to allow gases to escape and molten metal to fully enter the mold cavity.

1. BY ANJU D
I MDS
DEPT OF PROSTHODONTICS
1
CASTING PROCEDURE

2. Contents
 HISTORY
 DEFINITIONS
 BASIC STEPS OF CASTING PROCEDURE
 FABRICATION OF WAX PATTERN
 SPRUING
 SPRUE FORMER AND ITS ATTACHMENT
 CASTING RINGS AND LINERS
 CASTING MACHINES
2

3.  INVESTING PROCEDURE
 CASTING PROCEDURE
 RECOVERY AND FINISHING OF CASTING
 SPECIAL CASTING SITUATIONS
 CASTING DEFECTS
 RULES OF CASTING
 CONCLUSION
 REFERENCE
3

4. 4
INTRODUCTION

5. HISTORY
3000 B.C- Copper
was cast by
Mesopotomians.
2500 B.C -Dental
prosthesis
fabricated from
gold wire was
found in egypt.
500 B.C- Etruscans
made bridges of
soldered gold
bands.
1571- Benvenuto
cellini has done
casting of both
gold and bronze
5

6. 1928 The low heat
casting technique was
published by
Coleman
1930 Carl Scheu, discovered
the phenomena known as
Hygroscopic setting
expansion
1897-Philbrook decsribed a
method of casting metal
into a mold formed from a
wax pattern for restoration
of posterior tooth.
1907 -Taggart -
devised a
practically useful
casting machine.
6

7. 1959 Strickland et al stated the importance of the
type, shape location & direction other than the
size of the sprue
1945 George D. Estes, introduced the vacuum
investing technique to prevent formation of air
bubbles on the surface of the wax.
1959 –Asgar & Peyton stated that flaring
should occur at the sprue/ wax pattern
junction.
7

8. Definitions–GPT9
Casting- 1.Something that has been cast in a mold;an object
formed by the solidification of a fluid that has been cast into
a refractory mold
2.The action of pouring or injecting a flowable material into
a refractory mold
8

9. Casting Ring: A metal or silicone tube in which a refractory mold is
made for casting dental restorations
Crucible : A vessel or container made of any refractory material
(frequently ceramics) used for melting or calcining any substance that
requires a high degree of heat
Crucible Former : The base to which a sprue former is attached
while the wax pattern is being invested in refractory investment; a
convex rubber, plastic, or metal base that forms a concave depression
or crucible in the refractory investment
9

10. Sprue: 1. The channel or hole through which plastic or metal is
poured or cast into a gate or reservoir and then into a mold;
2. The cast metal or plastic that connects a casting to the residual sprue
button
Sprue button: The excess material remaining at the base of the mold
after a dental casting
Sprue former: A wax, plastic, or metal pattern used to form the
channel or channels allowing molten metal to flow into a mold to
make a casting
10

11. Objectives of casting
1) To heat the alloy as quickly as possible to a
completely molten condition.
2) To prevent oxidation by heating the metal with
a well adjusted torch
3) To produce a casting with sharp details by
having adequate pressure to the well melted metal
to force into the mold.
11

12. Sequence of casting
Wax pattern
fabrication
Spruing Investing Burnout
Placement in casting
machine
CastingDivesting
Cleaning and
polishing of casted
restoration
12

13. Fabrication of wax pattern
13

14. Precursor of the finished cast restoration
Requirements of a good inlay wax:
• 1. It must flow readily when heated, without chipping, flaking or losing its
smoothness.
• 2. When cooled, it must be rigid.
• 3. It must be capable of being carved precisely, without chipping, distorting
or smearing.
14
Wax pattern-GPT 9: A wax form that is the positive likeness of an
object to be fabricated

15. •Stresses occur in the inlay wax as a result of the heating and
manipulation of the wax during fabrication of the pattern.
15

16. 16
•Patterns should never be left off the die.
•Invested as soon as possible after
fabrication.

17. 2 TECHNIQUES 17
DIRECT METHOD: INDIRECT METHOD:
Type II inlay wax(soft)
Done on die
Type I inlay wax(soft)
Done in mouth itself

18. The wax pattern in indirect method can be constructed by
1. Dipping method.
2. Addition method.
3. Molten press
method.
4. Injection method
18

19. 1. Dipping method:
. A thin, uniform and adapted layer of wax on the die.
. By dipping the die into wax that has been thoroughly melted
19

20. Addition method:
 By melting the wax and dropping on the die
 Using a heated wax instrument
 Done until complete building of the pattern
 Carved by sharp carver.
Drawback : Stress collected from multiple addition of wax
tends to release with time.
20

21. Different types of waxes used:-
 Special wax for electrical wax heating unit (vario therm),
 Dark blue - Hard and Light blue soft - modelling,
trimming and dipping application
 Lilac-medium hard , red - stress free for cervical margins,
 Green wax-hard for trimming.
 Dip application yellow wax of uniform strength
21

22. Die spacer
 Provide relief space for cement.
 Applied within 0.5mm of the preparation finish line to
provide relief for the cement luting agent.
Example- resins (most commonly used)
 - model paint
 - colored nail polish
 - thermoplastic polymers dissolved in volatile solvents.
22

23. WAX INSTRUMENTS
 P.K.T No.1 and 2 -Wax addition instruments
 P.K.T No.3 -Burnisher
 P.K.T No.4 and 5 -Wax carvers
23

24. 24

25. 25

26. Electric Wax Instruments
 Advantages:
1) Precise temp control of the wax for proper manipulation.
2) Time saving
3) Carbon buildup minimum.
26

27. Wax pattern removal and evaluation
The wax pattern is sufficiently
cooled
Removed using the thumb and
forefinger with light grip pressure.
If not coming out the excess wax at
the margin should be trimmed of.
After removed the patterns is
evaluated
27

28. 28

29. Sprue
Definition: It’s a channel through which molten alloy can reach the
mold in an invested ring after the wax has been eliminated
Role of a sprue :
.Holds the wax pattern to avoid distortion.
.Creates a channel to allow the molten wax to escape from the mold.
.Enables the molten alloy to flow into the mold
29

30. TYPES- WAX,
PLASTIC, METAL
HOLLOW,SOLID
SHAPE –
SINGLE,MULTIPLE
30

31. Types of sprue
1.WAX sprue
 Uses single stage burnout.
 More preferred (because it melts at the same rate as the
pattern)
31

32. Plastic sprues
 In 2 stage burn out
 With Phosphate bonded investment.
 Its softening temperature is higher than wax pattern,may
block escape of wax.
 Undergo expansion before softening
 Used for casting FPD’s because of their high rigidity,
which minimizes distortion.
 May be completely solid (or) hollow plastic,prefabricated
or self made.
32

33. 2 stage burnout
 30 min heat soak @ 8000F
 Oven preheated to this temp burnout for 30min
 Continue burnout @ desired temp
33

34. Metal sprue
 Should be a non-rust metal
 Hollow metallic sprue contact surface area
 Strengthen the attachment between the sprue and pattern.
 Removed from the investment at the same time as the
crucible former.
34

35. Advantages of hollow sprue former
 Increases the contact area
 Holds less heat
35

36. Sprue former/Crucible former
• Sprue is attached to crucible former
• Forms the base of casting in relation with casting ring
during investing.
• Helps by holding sprue in desired ring.
• Available as: Rubber, Metallic and Plastic
36

37.  Basically of 2 types-
a)Steep-sided cone:Centrifugal casting force.
b) Shallow cone: Stream/air pressure
37

38. Spruing
The process of attaching a sprue former/sprue pin to the
wax pattern is called as spruing
Purpose: To provide a channel through which molten alloy
can reach the mold in an invested ring after the wax has
been eliminated..
38

39. Sprue former diameter
 Depends on: -
 The type and size of the pattern.
 Type of casting machine used.
 The dimensions of the casting rings in which casting is
made.
39

40. Sprue former Diameter
• For small inlay 1.5mm
• For large inlay 1.7mm
• For Onlay 2.1mm
• For large pattern 2.6mm
40

41.  Molar around 2.5 mm ( 10 gauge), premolar 2 mm (12
gauge)
 Pre fabricated sprue former are available of gauge from 6
to 18.
 Diameter should be equal to the thickest portion the wax
pattern.
41

42. Large diameter sprue: Improves the flow of molten metal into
the mould.
Less diameter sprue: Causes localized shrinkage porosity
42

43.  A narrow sprue may be useful in air pressure casting
procedure where the metal is melted in conical depression
formed by crucible former.
43

44.  Very short sprue : Porosity in casting at the junction of
sprue and pattern.
 Very long sprue : Sprue solidifies first leading to casting
shrinkage and incomplete casting
44

45. Reservoir:
 • Reservoir is a small amount of additional wax added to
the sprue former 1mm below the wax pattern.
 Placed at the heat center of ring
45
Reservoir

46. 46

47. Spruing metal ceramic units
Small crowns - 8guage wax sprue
 Incsal edge
 Sufficient bulk of sprue – reduce chances of shrinkage
porosity
Large units – gate method of spruing
 Series of 8 guage wax sprue fprmers attached to
continuous 6-8 runner bar
 Plastic sprue preferred
47

48. 48

49. Location of sprue
 Attached to the bulkiest, noncritical part of the pattern.
 Away from margins and occlusal contacts.
 Normally, largest nonfunctional cusp is used.
49

50.  Full veneer crown - Maxillary buccal and mandibular
lingual cusp.
 Partial veneer crown - Cusp that encompasses the
preparation.
Attached to cusp tips near margins
Distortion and restriction of flow of molten metal
50

51. Direction
Attached 45 degrees to the walls of mold
51

52.  The attachment should be smooth
 Do not posses pits or irregularities.
 Irregularities tags of investment fracture by
molten alloy casting failure.
52

53. Types of attachment
Direct In direct
53

54. 54

55. Length of sprue
Should keep the wax pattern 6 mm from the casting ring length.
 Allows gases to escape from end of mold.
 The pattern should be placed as possible to the center of ring
55

56. Venting
 Small auxiliary sprues or vents have been recommended to
improve casting of thin patterns and may helps in : gases
escape during casting.
 Compensate for the shrinkage during solidification
 It is attached to the wax pattern directly opposite to larger
sprue former.
56

57. 57

58. Auxillary sprue
 For large casting an additional auxillary sprue may be
placed for filling the mold.
 Usually 14 to 16 gauge sprue are used
58

59. Casting ring
Casting rings
are used to
confine the fluid
investment
around
Available as:
1) Shapes -
Round Oval
2)Complete
rings –
Rigid Metal
Plastic Flexible
Rubber
3) Split rings -
Metal Plastic
59

60. 1)Internal diameter of casting ring should be 5-10mm greater
than the widest measurement of the pattern
About 6 mm higher.
2) For single crown/inlay - small rings as used. Diameter -
32 mm
3) For large fixed partial denture – 63mm round/oval shaped
casting ring are used
60

61.  Ring less casting system: - plastic rings which is conical in
shape with tapering walls are used.
 Used for traditional gold-base alloys
61

62. Casting ring liner
 Function of casting ring liner:
• Allow uniform expansion.
• In case of wet liner technique- hygroscopic expansion.
• Thickness of the liner should be less than 1mm.
. The expansion seen with 2 liners is greater than one liner
62

63. 63

64. Materials
used are: -
Asbestos
liner
Cellulose
liner
Ceramic
liner
Combination of
ceramic and
cellulose liner
64

65. Preparation of wax pattern
 The wax pattern should be cleaned of any debris, grease,
or oils.
 A commercial pattern cleaner or a diluted synthetic
detergent is used.
65

66.  Cleaning the debris is followed by the application of
Debubblizer.
 Debubblizer :
Citric acid
Sodium 2-phenylpropane-2-sulfonate
Propane-1,2-diol - also known as propylene glycol
66

67.  Bubble free casting with different technique
 17% - open investment
 95% - vacuum investment
67

68. 68

69. Investing the Wax pattern
69

70. Investing procedure
 Investing is the process by which the sprued wax pattern
is embedded in a material called an investment.
An Investment must fulfill these requirements:
 Precise reproduction of wax pattern.
 Sufficient strength to withstand burnout and casting.
 Sufficiently porous to allow escape of gases.
 Expand enough to compensate solidification shrinkage of
alloy.
70

71. 3 types of investments materials available:
 Gypsum bonded investments:
• For conventional casting of gold alloy inlays, • Onlays,
crowns & FPD’s.
 Phosphate bonded investments:
• For metal ceramic restorations
• Pressable ceramics & for base metal alloys.
 Ethyl- silicate bonded investments:
. For casting of removable partial dentures with base metal
alloy.
71

72. Technique
Hand
mixing
Vacuum
mixing
72

73.  After the casting ring has been filled with investment material,
any excess should be removed before the material sets
 The filled ring is now set aside to allow the investment
material to complete its setting reaction & the accompanying
setting expansion.
73

74.  GRINDING THE INVESTMENT
 • Carefully Grinding(on a model trimmer) or scraping the
shiny skin off the end of the investment just prior to burn
out.
 • Removes the impervious layer, opening the pores of the
investment and facilitating gas release as the alloy is cast
into the mold.
74

75.  3mm on each end is left as it serves to lock the investment
within the ring & equalize radial & axial expansion
 Setting is complete in 30-40min.
 Hygroscopic technique is used. - Freshly filled
investment ring is immediately placed into water bath for
30min. & kept at 100ºF(38ºC).
 If burnout is not carried within 1-2hrs, the ring should be
stored in a humidor at 100% humidity, not soaked in water
since excessive hygroscopic expansion may result
75

76. Four mechanisms to produce mold expansion:
 Setting Expansion of Investment
 Hygroscopic Expansion
 Wax Pattern Expansion
 Thermal Expansion
76

77.  Setting Expansion : Result of normal gypsum crystal
growth in air.
 About 0.4% but partly restricted by metal investment ring.
 Hygroscopic Expansion : Max expansion ,water bath at
38°C.
 Water in bath replaces water used by hydration process
 Space between growing crystals is maintained
 Crystals grow longer outward expansion of mold.
 About 1.2 to 2.2% max with expandable ring.
77

78.  When the investment has set, the "skin" at the top of the
ring is trimmed off.
 •The rubber crucible former is removed, and any loose
particles of investment are blown off.
 •The ring is then placed in the furnace for the
recommended burnout schedule
78

79. Thermal Expansion : Investment is heated in burnout oven.
79

80. Burnout procedure
 Once the investment has set for an appropriate period
45min,ready for burnout.
 Crucible former is then carefully removed
 It is advisable to begin the burnout procedure while the
mold is still wet, because water trapped in the pores of
investment reduces the absorption of wax & as water
vaporizes, it flushes wax from mold.
80

81.  Wax elimination or burnout consists of heating the
investment in a thermostatically controlled furnace until all
traces of the wax are vaporized.
 Once the investment is heated during the wax elimination
procedure, heating must be continued, and casting must be
completed.
81

82. Burn out
procedure
Manual
Fully programmable
controls
Semi
automatic
82

83.  Heating of ring should be done slowly.
 - Hygroscopic low-heat technique
 - High-heat thermal expansion technique
83

84. Hygroscopic low-heat technique
 • The temperature used is 500°c for 60-90 mins.
 • Obtain compensation expansion from three sources:
 1. Immersion of investment in 37°c water bath.
 2. The warm water entering the investment mold from the
top adds some of the expansion.
 3. The thermal expansion at 500°c
• This technique causes 0.55% of expansion.
84

85.  Standardized hygroscopic technique was developed for
alloys with high gold content; the newer noble alloy may
require slightly more expansion. This added expansion
may be obtained by making 1 or more of following
changes.
 1. Increasing water bath temperature to 40°C.
 2. Using two layers of liners.
 3. Increasing burnout temperature to a range of 600°C to
650°C.
85

86. High-heat thermal expansion technique
 Gypsum bonded Investment :
 • The investment is slowly heated to 650°c - 700°c in
60mins.
 Maintained for 15-30 mins at this temperature.
 Above 700°c sulfur dioxide - Contaminates gold castings
and makes them extremely brittle
86

87. Phosphate bonded investment :
 • They need higher 2nd stage temperature for total
elimination of wax and revent premature solidification of
higher melting alloys.
 • After initial slow raise of temp to 315°c, the temperature
is rapidly raised to 750-900°C and maintained for 30 mins.
 • The technique cause 1.33-1.58 % of thermal expansion
87

88.  Ethyl silicate bonded investment- 1090 degree c
88

89.  Accelerated casting method (J Prost dent. 66: 155,1991)
• To reduce the total time, Alternative Accelerated casting
technique is proposed that uses phosphate bonded investment
which sets in 15 mins and then 15 min burn out is done at
815°c.
• This method is used for preparing post and core restorations
89

90.  HEAT SOURCES: two basic modes
• Torch flame Gas air torch
 Gas oxygen torch
 Oxy acetylene torch
 Hydrogen oxygen generator
• Electricity
90

91.  Two type of torch tips:
 1. Multi-orifice
 2. Single-orifice
91

92. 92

93.  Zones of the blow touch flame:
 • Zone 1 - colorless zone
 • Zone 2 – Combustion zone
 • Zone 3 - Reducing zone
 • Zone 4 - oxidizing zone
93

94. 94

95.  During melting of the gold alloys flux may be added-
.To minimizing porosity
.To increase fusing of metal
.Prevent oxidation
• Commonly used fluxes are fused borax powder ground with
boric acid power,Charcoal
95

96.  Gas air torch:
• To melt conventional noble metal alloys (used for inlays,
crown and bridge) whose melting points less than 1000°c
Alloy should be approx. 38°C to 66°C above liquidus
temperature
Alloy will become
Red Orange
White
(dull)
White
(Mirror-
like)
96

97.  Gas–oxygen torch:
 Used to melt metal ceramic alloys of higher temperature
up to 1200°c.
 The tip of torch is available as single orifice/multiorifice.
 The oxygen pressure is adjusted to1015 psi.
 The flame is directed onto metal with the nozzle of the
torch about 1.5 cm away from the metal.
 Complete fluid should be obtained within 30 second at
which point the metal is poured into the mould.
97

98.  Oxy-acetylene torch :
 The actual production of flame can be done by adjusting
the pressure and flow of individual gases .
 commonly advised pressure for acetylene nozzle is 3.5
N/cm2 and oxygen nozzle 7-10 N/cm2
 one part of acetylene + 2 and half part of oxygen
98

99.  The best results are obtained when flame is used with a distance of 10cm
between the face of blow torch nozzle and the base of crucible.
 If distance is reduced to -7.5 mm slight porosity
- 5 mm increased porosity due to occluded H2 gas
99

100. Casting crusible
 A pot used to keep metals for melting in a furnace
 Act as a platform on which the heat can be applied to the metal.
1.Clay - High noble & noble
metal types
2. Carbon - High noble alloys
also for higher fusing gold-based
metal ceramic alloys.
100

101. 3. Quartz - Higher fusing, gold
based metal ceramic alloy &
palladium alloys.
4. Zirconia – alumina -High fusing
alloys of any type are sensitive to
carbon contamination. (Alloys like
High Pd,Pd - Ag ,Ni
101

102. 102

103.  The crucibles used with noble metal alloys should not be
used for melting base metal alloy
 Discarded if it contains large amount of oxides and
contaminants from previous metals
103

104. Amount of metal needed 104

105. •Sufficient mass of alloy must be present to
sustain adequate casting pressure –
6gm is typically adequate for premolar and
anterior casting
10gm is adequate for molar casting
12 g is adequate for pontic
105

106. Casting machines 106
The modern dental casting machines came into
existence only in the year of 1907 when Taggart
introduced the pressure-casting machine .

107. Air
pressure
casting
machine.
Torch melting
/Centrifugal
casting
machine.
Electrical
resistance–
heated casting
machine.
Induction
melting
casting
machine.
Vacuum or
pressure
assisted
casting
machine.
Direct-current
arc melting
machine
107

108. Centrifugal casting machine
 Make use of centrifugal force to thrust the liquid metal
into the mold.
 Centrifugal casting is a method of casting parts having
axial symmetry.
108

109. 109
Pouring molten metal into a
cylindrical mold spinning about its
axis of symmetry.
The mold is kept rotating till the
metal has solidified. Mold material
like steels, Cast irons, Graphite may
be used.

110. Air Pressure casting machine
 Alloy is melted in the hollow left by the crucible former by
torch flame and then air pressure is applied through a
piston.
 Carbon dioxide, carbon monoxide or nitrogen gas can be
used.
 Pressure of 10-15 psi is usually applied.
110

111. 111

112. Induction casting machine
 Metal is melted by induction field that develops with in the
crucible surrounded by water cooled metal tubing.
 The crucible surrounded by water cooled metal tubing.
 Molten metal is forced in to mold by air pressure or both.
112

113.  The electric induction furnace is a transformer in which an
alternating current flows through the primary winding coil
and generates a variable magnetic field in the location of
the alloy to be melted in a crucible
 • It is more commonly used for melting base metal alloys
not been used for noble alloy casting as much as other
machines
113

114. Direct current arc melting machine
 The alloy is vacuum melted & cast by pressure in an argon
atmosphere.
 Direct current arc is produced between 2 electrodes Alloy
& water cooled tungsten electrode.
 The temperature within the arc exceeds 4000 ˚c, the alloy
melts very quickly. Disadvantage- alloy can become
overheated
114

115. 115

116. Electrical resistance heated casting machine
 Current is passed through a resistance heating
conductor, and automatic melting of metal in
graphite crucible .
 This is advantageous for metal-ceramic restoration
in which trace amount of base metals are prevented
from oxidation from torch flame overheating.
116

117. Vacuum casting machine
 Vaccum is applied through the base beneath the casting ring
and the molten alloy can be drawn into the mold by Negative
pressure.
 Material is sucked upwards into the mold by a vacuum pump.
 The mold in an inverted position from the usual casting
process, is lowered into the flask with the molten metal
117

118. Other methods of fabrication of restoration
 CAPTEK
 Electroforming
 CAD/CAM
 Copy milling
 Electrical discharge machine
 3Dprinting(additive fabrication)
118

119. Casting force:
 • Casting force > Surface tension of alloy + Resistance
offered by gas in the mold.
 • This can be done by use of following different type of
force • Vacuum force • Air or Gas Pressure • Centrifugal
force
119

120. DIVESTING AND FINISHING
120

121. Quenching
 After casting has been completed, ring is removed &
quenched in water.
Advantages:
 1. Noble metal is left in an annealed enables
burnishing & Polishing.
 2. When water contacts hot investment becomes soft,
granular & casting is more easily cleaned
121

122. Recovery of casting from investment
 Care must be taken to avoid damaging the margin
122
A.Trimming is
done from the
bottom end of
the ring.
B. Investment is
being pushed
out of the
casting ring.
C.The mold is
broken open.
D.Investment is
removed from
the casting.

123. Sandblasting
 The casting is held in a sandblasting machine to clean the
remaining investment from its surface.
123

124. Pickling :
 Surface of the casting appears dark with oxides and tarnish removed
by these process pickling .
 By place the casting in a dish and pour acid over it .
 Heat the acid and don’t boil it .
 Rubber-coated or Teflon tweezers are recommended for this
purpose.
124

125. Type of acids :
 Hydrochloric acid(50%)
 Sulfuric acid (50%)
 Ultrasonic devices
 AQUA REGIA : Hcl + Nitric acid (1:3)
Gold and palladium based metal ceramic alloys and base
metals, these alloys are not generally pickled.
125

126. Removal of sprue
 Remove the sprue with double sided Carborundum
disc/diamond disc
126

127. Finishing
Done to remove small irregularities on the surface
 Fitting surface- Blebs(Extrusions) removed using sharp
chisel or finishing bur
 External surface- Carbide trimming burs
 Try Casting on the die- Look for the Rocking movement
 Marginal Burnishability- To protect friable enamel at the
margins Sliding lap joint at gingival cavosurface margin
127

128. 128

129. Polishing
 Polishing of Gold casting - carbide burs , green stones, pink
stone, tripoli or rouge.
 Polishing of Base metal alloy casting- aluminum oxide, tin
oxide.
 Use
• Slow speed
• Coolant
• Used in slurry
•Decreasing order of abrasive
129

130. CASTING IN SPECIAL SITUATIONS
130

131. Casting Porcelain Fused To Metal Alloys
 Because of high melting temperature of the alloys used in
porcelain fused to metal restorations, special investments and
casting facilities are necessary.
 Calcium sulfate-bonded investment materials and conventional
melting facilities are inadequate
131

132.  Proper spruing for escape of gas.The bar type of spruing
used for casting these alloys
 Disadvantage of the bar type of sprue - increased amount
of alloy required to make castings
 So bar type is not used to make castings of alloys with low
melting ranges and gypsum-bonded investment
132

133.  Other steps in the investing and casting procedure include
 Vacuum investing,
 Careful wax elimination,
 A gas-oxygen torch or other high- temperature melting
facility,
 Centrifugal casting with adequate casting pressure
133

134. Casting Co-Cr and Ni-Cr Alloys and Partial
Denture Frameworks
 High freezing temperatures of base nmetal
alloys, more shrinkage of the alloy must be
compensated
 The extra compensation can be obtained by
(1) Painting a die spacer (varnish) on the die, but
short of the margins, before preparing the
wax pattern
(2) Using two layers of ceramic paper liner in
the investing ring
134

135.  In CPD construction, a suitable cast of refractory material
is used
 A gypsum-bonded investment is used when a low-fusion
point (<13000 C liquidus) alloy is used
 But a phosphate or silica bonded investment is used with
high-fusing point alloys (>13000C solidus)
135

136.  Variables such as mold temperature, temperature of the molten
alloy, and the sprue size and arrangement affect the properties
of the finished casting as much as does the composition.
 Therefore, Co-Cr and Ni-Cr alloys are generally considered
technique-sensitive
 Careful control of manipulative variables in the casting
operations is therefore essential.
136

137.  The alloy melting temperature is an important factor in the
selection and control of the melting and casting equipment
and in the choice of technique and mold equipment used
for the casting of base-metal alloys.
 Only a base-metal alloy that melts below 13000 C can be
cast into a calcium sulfate-bonded investment.
137

138.  Special electric or induction melting facilities or, less
commonly with an oxygen-acetylene torch
 Some investments for cast base-metal alloys lack sufficient
porosity for the rapid escape of gases from the mold cavity
when the hot metal enters.
 Gases may therefore be trapped in the mold cavity and
produce voids and casting defects.
138

139.  Methods such as venting to the surface of the mold to
permit rapid elimination of gases
 When properly designed and cast, the cast base-metal
alloys give acceptable removable partial-denture
restorations.
139

140. Casting Titanium
 Titanium is difficult to cast in comparison with the common dental
casting alloys because it requires relatively complex and expensive
equipment.
 Two problems in casting titanium
1. Its high melting point
2. The tendency for the molten metal to become contaminated.
3. Low density
The MP of commercially pure titanium is 16710 C
140

141.  To prevent absorption of gases, titanium is cast under the
protective atmosphere of argon or in a vacuum.
 To achieve the high melting temperatures, arc melting in
either graphite or water-cooled copper crucibles is used.
 The casting systems force the metal into the mold using
either pressure or centrifugal casting techniques.
141

142. 142

143.  A wax pattern is prepared and sprued, as before, but here
only the more temperature resistant investments can be
used.
 Both phosphate bonded , silica and magnesia investments
produce good castings
143

144. CASTING DEFECTS
144

145.  Any imperfections or irregularities that result in
unsuccessful casting which interferes with the fit of final
restoration or its esthetic and mechanical properties.
145

146. CLASSIFICATION
According to Anusavice
 Distortion
 Surface roughness and irregularities
 Porosity
 Incomplete or missing details
 Based on location
 Internal
 External
146

147.  According to Rosensteil
 Roughness
 Nodules
 Fins
 Incompleteness
 Voids or porosity
 Marginal discrepancy
 Dimensional inaccuracies
147

148. Distortion
 Related to distortion of the wax pattern. Causes:
 Can occur from the time of wax pattern preparation to the
time of investing due to stress relaxation.
 During the investment procedure.
148

149. Minimized by:
 Application of minimum pressure
 Manipulation of wax at high temperature
 Investing pattern immediately
 If storage is necessary, store in refrigerator
149

150. Discoloration
 Over-heating: Attracts oxides
 Under-heating: Incomplete wax elimination
 Carbon Inclusions: Crucible/Investment
 Mixture of Alloys: Re-use of Sprue and button
 Mercury Contamination
 Solder
150

151. Surface roughness & Surface irregularities
 Relatively finely spaced surface imperfections whose
height, width and direction establish the predominant
surface pattern.
 Isolated imperfections such as nodules that are not
characteristic of the entire surface area
 Greater than the wax pattern - the particle size of the
investment and -its ability to reproduce the pattern in
microscopic detail
151

152. Air bubbles
 Small nodules on the casting are caused by air bubbles,
that become attached to the surface during or subsequent
to the investing procedure.
 Prevented By:
 Proper investment technique
 Vibration of mix or by vacuum mixing
 Application of wetting agent properly and correctly –
important that it be applied in a thin layer.
152

153. Water films
 Wax is repellent to water, & If the Investment becomes
separated from the wax pattern, a water film may form
irregularly over the surface.
 Appears as minute ridges or veins on the surface.
 Prevented By: 1.Use of wetting agent
2.Correct L/P ratio (Too high L/P ratio
may produce these irregularAities)
153

154. Rapid heating rates
 It produces Fins or spines on the casting
 Cause: because of flaking of the investment
Prevented by:
 Heat gradually at least 60min from room temperature to
700 c.
 Greater the bulk – more slowly heated
154

155. Under heating
 Incomplete elimination of wax residue
 This factor is mainly important for low heat technique.
155

156. Prolonged heating
 Decomposition or disintegration of the investment & the
walls of the mold are roughened.
 Product of decomposition are sulphour compounds-
contaminates the casting - Why the surface of the casting
does not respond to pickling sometimes
Prevented by-
Thermal expansion technique - the mold should be heated to
the casting temperature & Never higher
156

157. Liquid/Powder Ratio
 The amount of water and powder measure should be
accurate.
 Too little water- investment too thick & cannot be applied
to the wax pattern
 Too much water- making investment easier but reproduces
poor casting.
machine.
157

158. Casting pressure
 To high pressure – rough surface of the casting
 To low pressure – incomplete casting
 Average – 0.10 to 0.14 Mpa in an air pressure machine
and - 3 to 4 turns of the spring in centrifugal casting
158

159.  Foreign bodies
 Shows sharp, well defined deficiencies
 May be: -
Pieces of the investment
Bits of the carbon from the flux
Sulfur components from
Decomposition of the gypsum investment
High sulfur content torch flame
159

160. Pattern position
 Should not place too close together
 Should not place many patterns in same plane
 Space between the pattern is atleast 3mm
160

161. Impact of metal alloy
Cause:
 Direct impact of molten alloy on the weak portion of the
mold surface, may fracture or abrade the mold surface
regardless of its bulk.
Prevented by:
 Proper spruing.
 Placement of sprue at 45 degree
161

162. Carbon inclusions
 Carbon from carbon crucible, carbon containing
investment, improperly adjusted torch - absorbed by the
alloys during casting – Carbides ,Carbon inclusions
162

163. Porosity
 Classified as follows:
I. Solidification defects
A. localized shrinkage porosity
B. B. Micro porosity
II. Trapped gases
A. pin hole porosity
B. gas inclusion porosity
C. sub surface porosity
III. Residual air
163

164. Localised shrinkage porosity
 It is caused by premature termination of the molten metal
during solidification. It mainly occurs at sprue-casting junction.
Cause:
 Diameter is too narrow
 Long thin sprue
 Transfer of heat center
 Absence of reservoir
 Direction of sprue at 90 degree
164

165. 165

166. Prevented by
 Using sprue of correct thickness
 Attach sprue to the thickest portion of the wax pattern
 Flaring the sprue at the point of attachment
 Placing reservoir close to the attachment
166

167. Suck back porosity
 A hot spot is created by the hot metal impinging on the
mold wall near the sprue - freeze last
167

168.  Often occurs at occlusoaxial or incisoaxial line angle
PREVENTED BY
 Flaring the point of sprue attachment
 Reducing the temperature between the mold & molten alloy
 Sprue placed at 45 degree
168

169. Pin hole or gas inclusion porossity
 Spherical contour
 Gas inclusion porosities are much larger than pin hole
porosity.
Causes
 Metals dissolve gases when molten, expelled during
solidification.
Eg- copper & silver dissolves oxygen platinum & palladium
dissolves hydrogen
169

170.  Gas occluded from a poorly adjusted torch flame
 Use of oxidizing zone rather than reducing zone.
 Casting is usually black, do not clean easily on pickling
Can be reduced not avoided
170

171. Sub surface porosity
Causes
 Simultaneous nucleation of solid grains and gas bubbles at
the first moment that the alloy freezes at mold walls
Prevented by
 Controlling the rate at which the molten metal enters the
mold.
171

172. Back pressure porosity
 Some times referred to as entrapped-air porosity.
 Outer surface of the casting
 When the casting or mold temperature is low
 Solidification occurs before the trapped air can escape.
172

173.  Causes
Inability of the air in the mold to escape through the pores in
the investment
 Prevented by:
Proper burnout
Sufficiently high casting pressure
Adequate L/P ratio
Thickness of investment between tip of pattern and end of
ring is not greater than 6mm.
173

174. Incomplete casting
 Factors that inhibit the mold filling is:
1. In sufficient venting
2. In sufficient casting pressure, pressure should be applied
atleast for 4 sec
3. Incomplete elimination of wax
4. Lower L/p ratio
5. Viscosity of the fused metal
174

175. Casting is both an art and science governed by numerous rules, or
“laws”.
 • Based on earlier work of Ingersoll & Wandling (1986), W.
Patrick Naylor formulated an expanded set of 17 separate
recommendations for Spruing, investing, burnout, and melting
and casting procedures.
 • Collectively these guidelines are referred to as the laws of
casting.
175

176. RULES OF CASTING
1. Attach the pattern sprue former to the thickest part of thr
wax pattern
2. Orient wax patterns so all the restoration margins will
face the trailing edge when the ring is placed in the
casting machine
3. Position the wax pattern in a “cold zone” of the
investment mold and the reservoir in the “heat centre” of
the casting ring
176

177. 4. A reservoir must have sufficient molten alloy to
accommodate the shrinkage that occurs within the
restorations
5. Do not cast a button if a connector (runner) bar, or other
internal reservoir, is used
6. Turbulence must be minimized, if not totally eliminated
177

178. 7. Select a casting ring of sufficient length and diameter to
accommodate the patterns to be invested
8. Increase the wettability of the wax patterns
9. Weigh any bulk investment and measure the investment
liquid for a precise powder-liquid ratio
178

179. 10. Eliminate the incorporation of air in the casting
investment and remove the ammonia gas by product of
phosphate-bonded investments by mixing under vacuum
11. Allow the casting investment to set completely before
initiating the burnout procedure
12. Use a wax elimination (burnout) technique that is specific
for the type patterns involved (wax versus plastic) and
recommended for the particular type of casting alloy selected
179

180. 13. Adequate heat must be available to properly melt and cast
the alloy
14.When torch casting, use the “reducing zone” of the flame
to melt the alloy and not the oxidizing zone
15. Provide enough force to cause the liquid alloy to flow
onto the heated mold
180

181. 16. Cast toward the margins of the wax patterns
17. Do not quench the ring immediately after casting
181

182. REVIEW OF LITERATURE
The effects of sprue design on the roughness and
porosity of titanium castings
Improvements in the degree of roughness and porosity of titanium
crown castings were the result of the double sprue design. The
disadvantage of casting lower density metal, such as titanium, with
centrifugal casting machines can be coun- teracted by higher rotational
speed or by using special sprue design
(J Prosthet Dent 1997;78:400-4.)
182

183. Evaluation of three variables affecting the casting of
base metal alloys
 Effect of venting on the ability to make castings without voids or
other defects.
 The use of vents in conjunction with sprues of sufficient width gave
consistently accept- able castings, whereas identical patterns without
vents resulted in extensive defects in the castings.
 The results also confirm that an adequately sized sprue is required
for successful castings, since the 1 mm sprue gave defective
castings, even with the concomitant use of vents.
 JPD APRIL 1980 VOLUME 43 NUMBER 4
183

184. The effect of sprue attachment design on castability and
porosity
 It has been implied that the sprue attachment designs
empirically observed to provide acceptable castability can
be uniformly applied to all alloys.
 JPD APRIL 1989 VOLUME 61 NUMBER 4
184

185. Effects of sprue design, casting machine, and heat
source on casting porosity
 Excellent castings can be fabricated with any of the casting
machines and heat sources tested. The results also demonstrated that
casting failures occur, even when a technique with a generally high
degree of success is used.
 Sprue design is more critical by far than either the type of casting
machine or the source of heat. The sprue should allow the molten
alloy to fill the casting void quickly and not allow the metal to
solidify prematurely.
 JPD JULY 1984 VOLUME 52 NUMBER 1
185

186. The effect of investment material type on the
contamination zone and mechanical properties of
commercially pure titanium castings
 According to the results of this study, the extent of the contamination
zone as well as the yield strength and percentage elongation of the
cpTi castings were significantly affected by the type of the
investment material.
 JPDVolume 94, Issue 6, December 2005
186

187. Conclusion
187

188. References
 K. J. Anusavice, Phillips Science of Dental Materials, 11th edition
 Introduction to metal ceramic technology, W.P. Naylor
 Craig R.G, Restorative Dental Materials;10th edition
 Rudd And Morrow, Dental Laboratory Procedures- Removable
Partial Denture, 2nd edition
 John J. Manappallil, basic dental materials, 2nd edition
 Shillinburg, fundamentals of fixed Prosthodontics
 Contemporary Fixed Prosthodontics Stephen F. Rosenstiel, 4th
Edition.
 Materials used in dentistry – S Mahalaxmi
188

189. 189