dental casting alloys basic science topic for masters in dental surgery

1. DENTAL CASTING
ALLOYS
Presented by;
Dr. Chaithanya S
1st year PG

2. CONTENTS
 Introduction
 History
 Ideal properties
 Composition
 Functional Mechanical properties
 Alloys for All metal prosthesis
 Metal ceramic alloys
 Removable denture alloys
 Alternative technologies for fabricating prosthesis
 Conclusion

3. INTRODUCTION
According to GPT ;
• ” an alloy is a mixture of two or more metals or metalloids that are mutually soluble in each
other in the molten state.
The alloy can be identified as binary, tertiary, quaternary, etc., depending on the number of
constituent metals.
• In dentistry, metals represent one of the three major classes of materials used for the
reconstruction of damaged or missing oral tissues, the other two being polymers and
ceramics.

4. HISTORY OF METALS IN DENTISTRY
Year Event
For 2500 years For 2500 years, gold is the oldest known metal used in dentistry
1789 Jean Darcet introduced low-fusing metal alloy
1847 Platinum–gold alloy consisting of three-fourths gold and one-fourth platinum was introduced
1880 Richmond patented a porcelain tooth soldered to gold backing
1907 Lost wax technique was introduced by W.H. Taggart
1933 Co-Cr was introduced in removable partial dentures, replacing gold
1950 Resin veneers for gold alloys were developed
1959 “Porcelain fused to metal” concept was introduced
1968 Palladium-based alloys were introduced
1971 base metal alloys (Ni-based) were introduced as economical alternative to gold alloys
1980 All-ceramic concept and technology was introduced

5. IDEAL PROPERTIES
1. BIOCOMPATIBILITY
• The alloy must tolerate oral fluids and not release any harmful products into the oral
environment.
2. TARNISH AND CORROSION RESISTANCE
• Can be avoided by use of noble metals that do not react in the oral environment (e.g., gold
and palladium)
• By use of one or more of the metallic elements to form an adherent passivating surface film,
which inhibits any subsurface reaction (e.g., chromium and titanium).

6. 3. THERMAL PROPERTIES
• The melting range must be low enough to form smooth surfaces with the mold wall of the
casting investment
• For metal ceramic prostheses, the alloys must have closely matching thermal expansion
coefficients to be compatible with given porcelains.
4. STRENGTH REQUIREMENTS
• Alloys for metal-ceramic prostheses are finished in thin sections and require sufficient
stiffness to prevent excessive elastic deflection from functional forces, especially when they
are used for long-span frameworks.

7. 5. FABRICATION OF CAST PROSTHESES AND FRAMEWORKS
• The molten alloy should flow freely into investment mold, without any appreciable
interaction with the investment material, and wet the mold surface without forming porosity
– castability
6. PORCELAIN BONDING
• To achieve a sound chemical bond to ceramic veneering materials, the alloy must be able to
form a thin adherent oxide.

8. COMPOSITION
Noble metals
• highly resistant to oxidation and
dissolution in inorganic acids.
• noble metal content of the alloy is at least
25% by weight.
Eg; gold and platinum group of metals
(platinum, palladium, rhodium, ruthenium,
iridium, and osmium)
Base metals
• are combined with these noble metals to
develop alloys with desirable properties for
dental restorations.
Eg; silver, copper, zinc, indium, tin, gallium,
and nickel.

10. CLASSIFICATION
• ALLOY CLASSIFICATION BY NOBLE METAL CONTENT

11. • ALLOY CLASSIFICATION BY MECHANICAL PROPERTIES

12. • ALLOY CLASSIFICATION BASED ON YIELD STRENGTH AND % ELONGATION

13. FUNCTIONAL MECHANICAL
PROPERTIES OF CASTING ALLOYS
1. ELASTIC MODULUS
• For a dental prosthesis, it is equivalent to its flexure resistance.
 In FDP’s;
• a flexing bridge can induce lateral forces on the abutment teeth, resulting in the loosening
of teeth.
• For a metal-ceramic prosthesis, the overlying brittle porcelain will fail catastrophically when
the metal substructure flexes beyond the flexural strength limit of the ceramic.

14.  In RPD’s ;
• major connectors of removable partial dentures, must have enough rigidity to prevent
flexure during placement and function of the prosthesis
• Resistance to flexure also allows clasps to fit into areas of minimal undercuts and still
provide adequate retention

15. 2. YIELD STRENGTH
• Yield strength reflects the capacity of a cast prosthesis to withstand mechanical stresses
without permanent deformation.
• Ideally, the alloys should have a high yield strength, so that a great deal of stress must be
applied before a permanent change in dimensions occurs.
• Generally, alloys with tensile yield strengths above 300 MPa function satisfactorily in the
mouth

16. 3. DUCTILITY
• A reasonable amount of ductility and malleability are essential.
• High ductility means that the amount of deformation that one can produce by adjusting a
prosthesis or by burnishing a cast metal margin plastically is higher for the alloy.
• In order to burnish an alloy, one must exert a high enough stress that is sufficient to exceed
its yield strength.

17. 4. HARDNESS
• A hard restoration surface can also cause excessive wear of the opposing dentition or
restoration(s) and requires more energy in grinding and polishing of the restorations.
• The extremely high hardness of most base metal alloys renders them difficult to cut, grind,
and polish.
• the lower hardness and greater ductility of most noble alloys are major advantages
compared with those of base metal alloys.

18. 5. FATIGUE RESISTANCE
• Most fractures of prostheses and restorations develop progressively over many stress cycles.
• When a removable partial denture is inserted and removed daily, the clasps are strained
elastically as they slide over the undercuts of abutment teeth.
• it initiates cracks from surface flaws of the material. Eventually a crack propagates to a critical
size, and sudden fracture occurs.
• When a comparison made between following cast clasps after a number of constant-
deflection cycles
cobalt-chromium > type 4 gold alloy > Ti-6Al-4V alloy > CP Ti.

19. ALLOYS FOR ALL-METAL PROSTHESIS
These alloys are discussed in three main categories:
noble (includes high noble)
predominantly base metal
CP Ti and titanium alloys

20. GOLD BASED ALLOYS
A/c to ANSI/ADA SP. NO. 5 It is classified as ;
• TYPE I SOFT
Small inlays, Class III and Class V cavities which are not
subjected to great stress. They are easily burnished.
• TYPE II MEDIUM
Inlays subject to moderate stress, thick 3/4 crowns,
abutments, pontics, full crowns, and sometimes
soft saddles.

21. • TYPE III HARD
Inlays, crowns and bridges, situations where there may be great stresses involved. They usually
usually can be age hardened.
• TYPE IV EXTRA-HARD
Inlays subjected to very high stresses, partial denture
frameworks and long span bridges. They can be age hardened.

22. GOLD CONTENT
Traditionally, gold content of dental casting alloys was called
• Karat
• Fineness

23. KARAT
It refers to the parts of pure gold present in 24 parts of alloy,
e.g.
 24 karat gold is pure gold.
 22 karat gold is 22 parts pure gold and 2 parts of other metal.
FINENESS
Fineness of a gold alloy is the parts per thousand of pure gold. Pure gold is 1000 fine.
 Thus, if 3/4 of the gold alloy is pure gold, it is said to be 750 fine.

24. PERCENTAGE COMPOSITION
The percentage composition of gold alloys is preferred over karat and fineness.
 Since 1977, ADA requires manufacturers to specify the percentage composition of gold,
palladium and platinum on all their dental alloy packaging.

25. COMPOSITION OF GOLD ALLOYS;

26. FUNCTIONS OF CONSTITUENT;
1.Gold
 It provides tarnish and corrosion resistance and has a desirable appearance.
It also provides ductility and malleability

27. 2.Copper
It is the principal hardener.
 It reduces the melting point and density of gold.
 If present in sufficient quantity, it gives the alloy a reddish color.
 It also helps to age harden gold alloys.
 In greater amounts, it reduces resistance to tarnish and corrosion of the gold alloy.
Therefore, the maximum content should not exceed 16 percent.

28. 3.Silver
It whitens the alloy, thus helping to counteract the reddish color of copper.
It increases strength and hardness slightly.
In larger amounts, however, it reduces tarnish resistance.

29. 4.Platinum
It increases strength and corrosion resistance.
It also increases melting point and has a whitening effect on the alloy.
It helps reduce the grain size.
Platinum tends to lighten the color of yellow gold-based alloys

30. 5.Palladium
 It is similar to platinum in its effect.
 It hardens and whitens the alloy.
It also raises the fusion temperature and provides tarnish resistance.
 It is less expensive than platinum, thus reducing the cost of the alloy.

31. The minor additions are
6.Zinc
It acts as a scavenger for oxygen (deoxidizing agents).
Without zinc, the silver in the alloy causes absorption of oxygen during melting.
Later during solidification, the oxygen is rejected producing gas porosities in the casting.
7.Indium, tin and iron
 They help to harden ceramic gold-palladium alloys, iron being the most effective.

32. 8.Calcium
It is added to compensate for the decreased CTE that results when the alloy is made silver
free (the elimination of silver is done to reduce the tendency for green stain at the metal
porcelain margin).
9. Iridium, ruthenium, rhenium
They help to decrease the grain size.
They are added in small quantities (about 100–150 ppm).

33. PROPERTIES OF GOLD ALLOYS
 COLOR:
- Can vary from gold to white. It depends on the amount of whitening elements (silver,
platinum, palladium, etc.) present in the alloy
MELTING RANGE:
- Ranges between 920–960 °C

34. DENSITY:
- more number of cast restorations per unit weight can be made from an alloy having a lower
density, than one having a higher density.
• Type III — 15.5 g/cm3
• Type IV — 15.2 g/cm3
YIELD STRENGTH
Type III — 207 MPa
Type IV — 275 Mpa

35. HARDNESS
Type III — 121 MPa
Type IV — 149 Mpa
 ELONGATION
- Type I alloys are easily burnished.
- Age hardening decreases ductility.
Type III—30–40%
Type IV—30–35%

36. MODULUS OF ELASTICITY
- Gold alloys are more flexible than base metal alloys.
TARNISH AND CORROSION RESISTANCE
- Due to their high noble content, Noble metals are less reactive.
 BIOCOMPATIBILITY
- Gold alloys are relatively biocompatible.

37. CASTING SHRINKAGE
- The casting shrinkage in gold alloys is less (1.25–1.65%).
CASTING INVESTMENT
- Gypsum-bonded investments may be used for low fusing gold alloys.

38. HEAT TREATMENT OF GOLD ALLOYS
 Heat treatment of alloys is done in order to alter its mechanical properties.
 Gold alloys can be heat treated if it contains sufficient amount of copper.
 Only Type III and Type IV gold alloys can be heat treated.
There are two types of heat treatment.
1. Softening heat treatment (solution heat treatment /solid solution hardening).
2. Hardening heat treatment (age hardening/ordered solution hardening).

39. SOFTENING HEAT TREATMENT
Softening heat treatment increases ductility, but reduces strength, proportional limit and
hardness (solution heat treatment).
Indications ;
It is indicated for appliances that are to be ground, shaped or otherwise cold worked in or
outside the mouth.

40. Method ;
 The casting is placed in an electric furnace for 10 minutes at 700 °C and then it is
quenched in water.
During this period, all intermediate phases are changed to a disordered solid solution and
the rapid quenching prevents ordering from occurring during cooling

41. HARDENING HEAT TREATMENT (OR AGING)
 Hardening heat treatment increases strength, proportional limit and hardness but decreases
ductility.
It is the copper present in gold alloys which helps in the age hardening process.
Indications
For strengthening metallic dentures, saddles, FDPs and other similar structures before use in
the mouth.
 It is not employed for smaller structures, such as inlays.

42. Method
It is done by ‘soaking’ or aging the casting at a specific temperature (200 -4500c) for a
definite time (15–30 minutes).
It is then water quenched or cooled slowly.
 During this period, the intermediate phases are changed to an ordered solid solution .

43. LOW GOLD ALLOYS
 Also known as ‘economy golds’.
 They are crown and FDP alloys having gold content below 60% (generally in the 42–55%
range). However, gold must be the major element.
reducing gold content increased tarnish and corrosion. This problem was overcome by two
discoveries.
- Palladium made the silver in gold alloy more tarnish resistant. 1% palladium was required for
every 3% of silver.
- The silver-copper ratio had to be carefully balanced to yield a low silver rich phase in the
microstructure.

44. NOTE ;
TECHNIC ALLOY
 This is a gold colored base metal alloy which was frequently misused in India to make all-
metal crowns and FDPs since many years.
 also sometimes referred to as Japanese gold or K-metal.
 contraindicated for any intraoral dental use because of its low strength, low wear resistance
and tendency to tarnish.

45. SILVER – PALLADIUM ALLOYS
These alloys were introduced as a cheaper alternative to gold alloys.
It is predominantly silver in composition. Palladium (at least 25%) is added to provide
nobility and resistance to tarnish.
They may or may not contain copper and gold.
They are white in color.

46. • TYPES:

47. Limitations;
• poor castability because of the lower density and propensity of dissolving oxygen in the
molten-state.
• greater potential for tarnish and corrosion especially Ag-Pd-Cu alloys.

48. METAL-CERAMIC ALLOYS
Metal-ceramic alloys are those alloys that are compatible with porcelain and capable of
bonding to it.
They may be noble metal alloys or base metal alloys.
All have coefficient of thermal expansion (CTE) values which match that of porcelain.

49. REQUIREMENTS OF ALLOYS FOR PORCELAIN BONDING
1. Its melting temperature should be higher than porcelain firing temperatures.
2. It should be able to resist creep or sag at these temperatures.
3. Its CTE should be compatible with that of porcelain.
4. They should be able to bond with porcelain.
5. It should have a high stiffness (modulus of elasticity). Any flexing of the metal framework
may cause porcelain to fracture or delaminate.
6. It should not stain or discolor porcelain.

50. TYPES OF METAL-CERAMIC ALLOYS
High noble alloys
• Gold-palladium-platinum
alloys
• Gold-palladium-silver
alloys
• Gold-palladium alloys
Noble alloys
• Palladium-silver alloys
• Palladium-gallium-silver
alloys
• Palladium-gold alloys
• Palladium-gold-silver
alloys
• Palladium-copper alloys
• Palladium-cobalt alloys
Base metal alloys
• Nickel-chromium alloys
• Nickel-chromium-
beryllium alloys
• Cobalt-chromium alloys
• Pure titanium
• Titanium-aluminum-
vanadium

51. THE HIGH NOBLE (GOLD-BASED) METAL-
CERAMIC ALLOYS
COMMON FEATURES OF HIGH NOBLE (GOLD BASED) ALLOYS
 Cost : These are the most expensive crown and bridge alloys.
 Color : The color can range from white to gold depending on the gold content. The
whitening alloys are palladium and platinum.
 Melting range : from 1149–1304 °C.
 Density : Ranges from 13.5 to 18.3 g/cm³ (depending on the gold content).
 Castability : The high density of these alloys make them easy to cast.

52. GOLD-PALLADIUM-PLATINUM ALLOYS
• The oldest metal ceramic alloy system.
• Very expensive.
• Limited to crowns and 3 unit FDPs

53. GOLD-PALLADIUM-SILVER ALLOYS
• Because of the higher palladium concentrations in Au-Pd-
Ag alloys, the melting ranges are raised above those of the
Au-Pt-Pd alloys
• improved resistance to creep deformation (sag) at elevated
temperatures.

54. GOLD-PALLADIUM ALLOYS
• Excellent castability
• Good bond strength
• Corrosion and tarnish resistance
• Not thermally compatible with high expansion dental porcelain
• High cost

55. PALLADIUM-SILVER ALLOYS
• Their popularity has declined a little because of the greening problems

56. DISCOLORATION OF PORCELAIN BY
SILVER
o Discoloration of the porcelain is seen near the cervical region of the metal-ceramic
prosthesis when silver-containing alloy is used as the substrate
o Color changes included green, yellow-green, yellow-orange, orange, and brown hues
o phenomenon has generally been called “greening.”

57. Cause;
• by the colloidal dispersion of silver atoms entering body and incisal porcelain
• Or the glazed surface from vapor transport or surface diffusion.
Seen in;
• higher-silver-content alloys,
• lighter shades, multiple firing procedures,
• higher temperatures, body porcelain in direct contact with the alloy,
• vacuum firing cycles, and
• with certain porcelains containing lower opacifier and higher sodium contents.

58. PREVENTION OF DISCOLORATION:
• Use of ultra low fusing porcelain or non greening porcelain.
• A pure gold film can be fired on a metal substrate to reduce the surface silver
concentration.
• A ceramic conditioner can be fired as a barrier between the alloy and the porcelain.
• Use of a graphite block routinely to maintain a reducing atmosphere.

59. PALLADIUM-COPPER ALLOYS
• These alloys are technique sensitive. Slight errors can lead to faulty castings
• Copper can cause a slight discoloration of the porcelain because of dark brown oxide layer
formed during oxidation

60. PALLADIUM-COBALT ALLOYS
• They are the most sag resistant of all the noble alloys

61. BASE METAL ALLOYS FOR METAL-
CERAMIC RESTORATIONS
Base metal alloys used for metal-ceramics include
• Nickel-chromium (nickel based) alloys
• Cobalt-chromium (cobalt based) alloys
• Pure titanium
• Titanium-aluminum-vanadium alloys

62. NICKEL-CHROMIUM ALLOYS
• The system contains two major groups:
Beryllium free (class 1)
Beryllium (class 2)
- Of the two, Ni-Cr-Beryllium alloy are generally
regarded as possessing superior properties and
have been more popular

63. NICKEL-CHROMIUM BERYLLIUM FREE ALLOYS
Composition:
• Nickel – 62% to 77%
• Chromium – 11% to 22%
• Boron , iron, molybdenum, Niobium or columbium and tantalum (trace elements).
Limitations;
• Cannot use with Nickel sensitive patients
• May not cast as well as Ni-Cr-Be alloys
• Produces more oxide than Ni-Cr-Be alloys

64. NICKEL-CHROMIUM-BERYLLIUM ALLOY
1. Low density, permits more casting per ounce.
2. High sag resistance
Composition:
• Nickel – 62% to 82%
• Chromium – 11% to 20%
• Beryllium – 2.0%
Limitations;
• Beryllium exposure may be potentially harmful to
technicians and patients

65. TITANIUM AND ITS ALLOYS
It has been adopted in dentistry, because of its excellent biocompatibility, light weight, good
strength and ability to passivate.
USES
In dentistry
1. Metal-ceramic restorations.
2. Dental implants.
3. Partial denture frames.
4. Complete denture bases.
5. Bar connectors.
6. Titanium mesh membranes (Tiomesh) are used in bone augmentation.

66. In surgery
1. Artificial hip joints.
2. Bone splints.
3. Artificial heart pumps.
4. Artificial heart valves parts.
5. Pacemaker cases.

67. PROPERTIES OF TITANIUM
 Resistance to electrochemical degradation
 Good biological response
 Relatively light weight
 Low density (4.5 g/cm3)
 Low modulus (100 GPa)
 High strength (yield strength = 170-480 MPa; ultimate strength = 240-550 MPa)
 Passivity
 Low coefficient of thermal expansion (8.5 x 10–6/°C)
 Melting & boiling point of 1668°C & 3260°C

68. Commercially Pure Titanium (CP Ti):
• It is available in four grades (according to American Society for Testing and Materials ASTM)
which vary according to the oxygen (0.18-0.40 wt.%), iron (0.20-0.50 wt%) and other
impurities.
• It has got an alpha phase structure at
room temperature and converts to beta phase
structure at 883°C which is stronger but brittle.

69. FABRICATION OF TITANIUM RESTORATIONS
Titanium structures can be made by
1. Casting or
2. Machining

70. Casting
• Casting of titanium is a challenge because of its high melting temperature, low density and
high reactivity to atmospheric air.
• Dental castings are made via pressure-vacuum or centrifugal casting methods.
• Investments with high setting expansion are used to compensate for the high casting
shrinkage of titanium.

71. Machining
• Dental implants generally are machined from billet stock of pure metal or alloy
• Dental crowns and FDP frameworks also can be machined from metal blanks via CAD/CAM.
• Another method - electric discharge machining,
which uses a graphite die to erode the metal to
shape via spark erosion

72. CERAMIC VENEERING;
• Special low fusing porcelains with fusing temperatures below 800 °C are used with titanium.
• This is because titanium changes to the β-form (at 883 °C) which is susceptible to oxidation.

73. Advantages
• High strength.
• Light weight.
• Low tarnish and corrosion because of
ability to passivate.
• Can be laser welded.
• Limited thermal conductivity
Disadvantages
• Poor castability.
• Highly technique sensitive.
• Requires expensive machines for casting
and machining.
• Low fusing porcelains (below 800 °C)
required to prevent β phase transformation

74. REMOVABLE DENTURE ALLOYS
ADDITIONAL REQUIREMENTS FOR PARTIAL DENTURE ALLOYS
1.They should be light in weight.
2. They should have high stiffness. The high stiffness prevents the frame from bending under
occlusal forces.
3. They should have good fatigue resistance. Clasps may break after repeated insertion and
removal.
4. They should be economical.
5. They should not react to commercial denture cleansers.

75. The alloys for removable denture use are
1. Cobalt-chromium alloys.
2. Nickel-chromium alloys
3. Aluminium and its alloys
4. Type IV noble alloys
5. Titanium

76. COBALT - CHROMIUM ALLOYS
They possess high strength.
Has excellent corrosion resistance especially at high temperatures, makes them useful for a
number of applications.
These alloys are also known as ‘stellite’ because of their shiny, star-like appearance.
They are bright lustrous, hard, strong and
possess non tarnishing qualities.

77. APPLICATIONS
• Denture base
• Cast removable partial denture framework
• Crowns and fixed partial dentures
• Bar connectors.

78. • According to ADA Sp. No. 14 a minimum of 85% by weight of chromium, cobalt, and nickel
is required.

79. FUNCTIONS OF ALLOYING ELEMENTS
Cobalt hardness, strength and rigidity to the
alloy
Chromium Its passivating effect ensures
corrosion resistance
Nickel It increases ductility.
Molybdenum or tungsten Effective hardeners
Iro, copper and berylium Hardners and refiners
Manganese and silicon Primarily oxide scavengers
Boron Deoxidizer and hardener,
Carbon have a pronounced effect on
strength, hardness and ductility

80. ADVANTAGES AND DISADVANTAGES OF BASE METAL ALLOYS
ADVANTAGES OF BASE METAL ALLOYS
1. Lighter in weight.
2. Better mechanical properties (exceptions are present).
3. As corrosion resistant as gold alloys (due to passivating effect).
4. Less expensive than gold alloys

81. DISADVANTAGES
1. More technique sensitive.
2. Complexity in production of dental appliance.
3. High fusing temperatures.
4. Extremely hard, so requires special tools for finishing.
5. The high hardness can cause excessive wear of restorations and natural teeth contacting the
restorations

82. COMPARISON OF A GOLD ALLOY
AND A BASE METAL ALLOY

83. BIOCOMPATIBILITY
• Biocompatibility of dental alloys are primarily related to elemental release from
these alloys .
• Also influenced by their concentrations and duration of exposure to oral tissues.
• Alloys with high noble content release less mass than alloys with little or no noble
content .

84. • Safety standard for ‘Be’ dust is 2µg/m3 of air for a time weighted 8-hour day. A higher limit
of 25 µg/m3 is allowed for a minimum exposure time of less than 30 minutes.
• Nickel sensitivity – safety standard for pure Ni is 15µg/m3 of air for a 40-hour week.
• To minimize exposure to metallic dust containing Ni or Be , intraoral finishing should be done
with high speed evacuation system and preferably in a wet environment.

85. CLINICAL SELECTION OF
ALLOYS
Factors to be considered
1. Systemic health of the patient
2. Known hypersensitivity to any particular element
3. Physical requirements of alloy to be used .eg: in long span bridges, alloy with
highest elastic modulus is required.
4. Color of the alloy : base metals to be avoided in the smile window of patients
with high lip line

86. TECHNICAL CONSIDERATIONS FOR CASTING ALLOYS
Based on the melting temperatures of the alloys,
high fusing alloys
low fusing alloys.

87. LOW-FUSING ALLOYS
Gold alloys used for all metal restoration are low fusing alloys.
Investment material : Gypsum bonded investments are usually sufficient for the low-fusing
gold alloys.
Melting : The regular gas-air torch is usually sufficient to melt these alloys

88. HIGH-FUSING ALLOYS
The high-fusing alloys include noble metal-ceramic alloys (gold and palladium alloys) as well
as the base-metal alloys
Investment material for noble metal alloys : Phosphate bonded or silica bonded investments
are used for these alloys.
Investment material for base-metal alloys : Phosphate-bonded or silica-bonded investments
are also used for these alloys.
[However, there is one difference. These alloys are very sensitive to a change in their carbon
content. Therefore, carbon containing investments should be avoided when casting base-
metal alloys.]

89. Burnout : A slow burnout is done at a temperature of 732–982 °C.
It is done two hours after investing.
Melting : Oxygen-acetylene torches are usually employed.
Electrical sources of melting such as carbon arcs, argon arcs, high frequency induction, or
silicon-carbide resistance furnaces may also be used.

90. ALTERNATIVE TECHNOLOGIES FOR
FABRICATING PROSTHESIS
Shrinkage of casting alloy in an investment occurs in 3 stages:
 Thermal contraction of liquid metal cooling from its casting temperature to its liquidus
temperature
 Phase change from liquid to solid state
 Thermal contraction of solid metal from solidus temperature to room temperature.
 Technologies are currently available for fabricating metallic prosthesis without challenges of
casting procedures and casting shrinkage .

91. • Most metal prosthesis can be made by one or more of the following methods :
Sintering (diffusion bonding ) of burnished metal foil
CAD-CAM processing of metal blocks
Copy milling of metal blocks
Electroforming of metal copings
3D printing with metal powder followed by sintering

92. CONCLUSION
• Though they are many dental casting alloys we use in routine procedures,
one should always keep in mind the application of basic properties in these
alloys for the best success.

93. REFERENCES
• Phillips’ Science of Dental materials, Kenneth J. Anusavice, Chiayi
Shen, H. Ralph Rawls.—12th ed.
• Craig,s Restorative Dental Materials. John M. Powers, Ronald L,
Sakaguchi.12th edition.
• Manappallil JJ. Basic dental materials. JP Medical Ltd; 2015 Nov 30.

94. THANK YOU