CASTING PROCEDURES IN DENTISTRY AND CASTING DEFECTS IN DETAIL

1. CASTING
PROCEDURES AND
DEFECTS
NAMITHA AP
I st year MDS
DEPT.OF PROSTHODONTICS

2. CONTENTS
▪ Introduction
▪ History
▪ Wax pattern
▪ Casting procedure
▪ Spruing the wax pattern
Sprues
Casting rings
Crucible former
Casting ring liner

3. ▪ Investing the wax pattern
Investment materials
Mixing
 Burning out
 Casting the dental alloy
melting the alloy
casting the alloy

4.  Deflasking the cast
 Cleaning the cast
 Sand blasting
 Finishing the framework
 Casting defects
 Conclusion

5. Introduction
▪ Metal casting-a critical
role in the
development and
advancement of
human cultures and
civilizations
▪ Mesopotamia-
birthplace of castings
▪ Copper –first metal to
be casted(low melting
point)

6. ▪ 9000 BCE - Earliest metal objects-
wrought native copper Near East
▪ 5000-3000BCE - Chalcolithic
period:melting of copper;experimentation
with smelting in the near east
▪ 3000-1500BCE -Bronze age:copper and
tin bronze alloys
▪ 2400-2200BCE – copper statue of Pharoah
Pepi I Egypt

7. ▪ 2000 BCE-Bronze age far east
▪ 1500 BCE-Iron age wrought iron ganga valley india
▪ 1100 BCEDiscovery of wrought alloy
▪ 1200BCE-Distillation technique in India
▪ 600 BCE-Iron cast in china
▪ 500 CE-Steel castings in india
▪ 400 CE-Zinc extraction in india

8. History-furnaces
Earliest furnace-simple, easy to operate
Bees wax-patterns
Bellows-blowing air to furnace
Iron age-ceramic ovens
Crucibles and flame ovens for melting of
copper,tin and lead alloys

9. History-molds and patterns
Molds
▪ Sand
▪ Stone
▪ Lime stone
▪ Sun baked clay
Patterns
▪ 1st patterns of casting-
4000 years before at
mesopotamia from
bee wax.
▪ Oldest-Cave of the
treasure (Nahal
Mishmar) hoard in
southern
Israel(Chalcolithic
period4500-3500BCE)

10. History
▪ 11th Century Theophilus  Described lost wax technique, which
was a common practice among the jewelers
▪ 1558  Benvenuto Cellini  claimed to have attempted use of wax
and clay for preparation of castings
▪ 1884  Agulihon de saran used 24K gold to form Inlay.
▪ 1897  Phillibrook described a method of casting metal filling.
▪ 1907  W HTaggart introduced the lost WaxTechnique and casting
machines to the NewYork Odontological group

11. Introduction
▪ Casting is a process by which a wax
patten of a restoration is converted to a
replicate in a dental alloy.
▪ The most common method is lost wax
technique.
▪ The lost wax technique has been used
for centuries but its use in dentistry was
not common until 1907,when
W.H.TAGGART introduced his technique
with the casting machine

12. ▪ Lost wax technique is a process consists of
surrounding the wax pattern with the mold made
of heat resistance investment material,eliminating
the wax by heating and then introducing molten
metal into the mold through a channel called
sprue.

13. Casting - Definition
▪ Something that has been cast in a mold, an
object formed by the solidification of a fluid
that has been poured or injected into a mold.
(GPT9)
▪ The action of poring or injecting a flowable
material in to a refractory mold (GPT9)
▪ An act of forming an object in
mold.(GPT8);the object formed is also
referred to as a casting

14. Wax pattern:
Wax pattern is a detailed model of the final dental
restoration, including all anatomy, contours,
occlusal function & proximal contacts.

15. ▪ Dental waxes are originally polymers consisting of
hydrocarbons & their derivatives (e.g. ether & alcohol) and are
blends of natural and synthetic waxes, natural resins, oils,
foils, and gums
▪ The molecular weight may range from 400-4000.
▪ Dental waxes melt over a range of temperature rather than a
single temperature, because their components consists of a
wide distribution of molecular weight

16. ▪ The American national standards institute /
American dental association(ANSI/ADA)
specification NO:4 waxes used for fabrication of
patterns for dental casting are divided into two
types
TYPE 1: medium wax used in direct techniques;
TYPE 2: soft waxes used in indirect techniques
.

17. STEPS BEFORE WAX PATTERN
• Avoid undercuts-prevent seating
• Accurate impression is made with elastomers
TOOTH/TEETH PREPARATION
• From diestone/a suitable die material/impression is
electroformed
DIE PREPARATION
• Coated/painted over die-spacing for luting cement
• Relief also improves seating of the casting
DIE SPACER

18. Wax pattern
Framework
Precisional dental
casting technique
Spruing the wax pattern
Investing the wax pattern
Burning out the wax pattern(wax elimination)
Casting the dental alloy
Deflasking the casting
Sandblasting the framework
Finishing and polishing the framework
Casting Procedure

19. SPRUING
INVESTING
BREAKOUT
CASTING FROM
INVESTMENT
PICKLE
CASTING

20. Wax pattern
▪ Type II inlay wax/other casting waxes
▪ Take precautions to avoid distortion
▪ Before making pattern apply die lubricant( helps to
separate the wax pattern from die)

21. Spruing the Wax pattern
Sprue:The channel or hole through which plastic/metal is poured or
cast into a gate or reservoir and then into mold(GPT 9)
; A cast metal or plastic that connects casting to the residual sprue
button.(G.P.T-8)
Sprue former or Sprue pin; A wax plastic or metal pattern used to
form channel or channels allow molten metal to flow into a mold
to make casting.(G.P.T-9)
Sprue button; The excess material remaining in the reservoir of the
mold after casting.(G.P.T-9)

22. Spruing the Wax Pattern
Sprue
Lost-head
(reservoir)
Connector
bar

23. Requirements of a sprue
1. The sprue must allow the molten wax to escape from mold.
2. It must enable the molten metal to flow into the mold with as
little turbulence as possible.
3. The metal within it must remain molten slightly longer than the
alloy that has filled the mold.This will provide a reservoir to
compensate for the shrinkage that occur during solidification of the
casting

24. Types of sprue former
Wax
Large
restorations
like RPD
frameworks
Plastic
Smaller
restorations
Metal
Smaller
restorations

25. Wax Sprue former
▪ Most preferred - melt at the same
rate as patterns and thus allow easy
escape of molten wax
▪ melts readily in the normal course
of the wax elimination (burnout)
process
▪ certified by the American Dental
Association -will not leave a residue
of more than 0.1 % of the
specimen's original

26. Plastic Sprue
former
▪ Soften at temperature above the
melting points of inlay waxes
▪ two-stage burnout is
recommended to ensure complete
carbon elimination.
▪ useful when casting fixed partial
dentures (F.P.D) in one piece(added
rigidity minimize distortion).
▪ Hollow plastic sprues are available
that permits easy elimination of
wax

27. Metal sprues
▪ non-resting metal to avoid possible
contamination of wax pattern.
▪ hollow and removed from the
investment at the same time when
the crucible former is removed.
▪ Special care should be taken to
examine the orifice while removing
because small amount of investment
chip-off which result in incomplete
casting.

28. Pre fabricated sprue formers
▪ Spruing and Investing made easier and more
consistent
▪ Have different gauge connector bars to meet the
needs of individual cases
▪ Ex : Ready sprues ,Tri wax system

29. Pre fabricated sprue formers-
range of gauges or diameters
AWG gauge Conductor
diameter in
mm
0000 11.68400
000 10.40384
00 9.26592
0 8.25246
1 7.34822
2 6.54304
3 5.82676
4 5.18922
5 4.62026

30. Functions of sprue former
▪ To form a mount for the wax pattern
▪ To create a channel for elimination of wax during
burn out
▪ Forms a channel for entry of molten alloy
▪ Provides a reservoir of molten metal which
compensates for alloy shrinkage during
solidification

31. Variables and Principles of
Optimal Sprue Design
▪ Sprue formerAttachment
▪ Sprue former selection
▪ Sprue former position
▪ Sprue former Diameter r
▪ Sprue former length
▪ Sprue Former Direction Attachment rue former
Attachment

32. Wax pattern removal
▪ The sprue former should be attached to the wax
pattern with the pattern on the master die
provided that the pattern can be removed directly
in line with its path of withdrawal from the die.
▪ Any motion that might distort the wax pattern
should be avoided during removal.
▪ The gauge selection and design for the sprue
former are often empirical, but optimal
performance during the casting process is based
on the following five general principles.

33. Sprue former Diameter
• Select the gauge sprue former with a diameter that is
approximately the same size as the thickest area of the wax
pattern
• If the pattern is small,the sprue former will also be small.
Because a large sprue former attached to a thin delicate pattern
can cause distortion.
• If the sprue former diameter is too small, this area will solidify
before the casting and localized shrinkage porosity (suck
back)develops.
• 2.5 mm sprue molar and metal ceramic restoration.
• 2 mm sprue premolars and partial coverage crowns

34. Sprue position
▪ A  crucible former
▪ B  sprue
▪ C  wax pattern
▪ D  investment material
▪ E  ring liner
▪ G  thickness of investment at
the top
• Ideal area for the sprue former is
the point of greatest bulk in the
pattern to avoid distorting thin
areas of wax during attachment to
the pattern and to permit the
smooth flow of alloy.
• Sprue formers should be
attached to the least
anatomical area in the wax
pattern i.e. no grooves, cuspal
anatomy, fossae or ridges

35. Sprue former selection
▪ The type of sprue
former selected
influences the burn
out technique used.
Wax sprue formers are
more common than
plastic

36. Sprue former Attachment
▪ Sprue former should be attached to the portion of
the pattern with the largest cross sectional areas.
It’s best for the molten alloys to flow from thick
section to surrounding thin areas.
▪ The Sprue former orientation should minimize the
risk of metal flow on to flat areas of the investment
or small areas such as line angles.
▪ High density gold alloys-flared
▪ Lower density alloys-restricted

37. ▪ The sprue former
should be long enough
to position the pattern
properly in the casting
ring within 6 mm of
the trailing end and
yet short enough so
that the molten alloy
does not solidify
before it fills the mold.
The type of sprue
former selected
influences the burnout
technique used.

38. Sprue former length
▪ The length depends on
the length of the casting
ring
▪ Should be long enough
to properly position the
pattern in the casting
ring within 6mm for GBI
and 3-4 mm for PBI of
the trailing end and
short enough so the
molten alloy does not
solidify before it fills the
mold
Too short
Wax pattern may be
removed from the end
of casting ring
Gases can not be
adequately vented
porosity

40. Sprue Former Directionn
• The sprue former should be
directed away from any thin or
delicate part of the pattern,
because the molten metal may
abrade or fracture investment in
this area and result in a casting
failure.
• If Sprued at a 45 o to the proximal
area a satisfactory casting is
obtained.

41. Reservoir
▪ The reservoir of Spruing system
should be placed in the heat center of
the ring.
▪ This permits the reservoir to remain
molten longer and enables it to
furnish alloys to the patterns until
they complete the solidification
process.
▪ This should have the largest mass at
any part of the sprue system

42. Venting
▪ Small auxiliary sprues or vent have been recommended to improve
casting of thin patterns. Their action may be to help gases escape
during casting or to ensure that solidification begins in critical areas
by acting as heat sink.

43. Spruing Methods-
▪ Direct Spruing
▪ Indirect Spruing
▪ Constricted Spruing

44. Direct Spruing
▪ Flow of the molten metal is straight from
the casting crucible to pattern area in the
ring.
▪ The straight sprue former is attached to
the thickest part of the wax pattern and
other end is to the crucible former
▪ Used for single units and small multi unit
Patterns
▪ Requires less time.
▪ The sprue former can be modified by
placing a ball, or round reservoir, between
the pattern and the button

45. Indirect Spruing
▪ Molten alloy does not flow directly from the
casting crucible into the pattern area in the
heated mold.
▪ With this method of Spruing, the connector
bar (Runner Bar) is 6-gauge or 8-gauge round
wax to which the wax pattern sprues are
attached on one side and with two larger ingot
sprue formers on the other side.
▪ The bar's large volume houses molten metal so
the pattern areas fill with metal first and are
able to draw upon the reservoir if additional
alloy is needed to complete the solidification
process .

46. ▪ Used in multiple single units and FPD
▪ The composition of an alloy will influence the manner in which it fills the mold.
▪ For example, a palladium-silver alloy fills unidirectional, whereasType III gold fills
in a random (scattered) fashion
▪ Offer greater predictability and reliability
▪ Enhanced control of solidification shrinkage

47. Constricted Spruing
▪ Tapering the sprue former at its attachment to the wax pattern rather
than flaring this area is referred to as "constricted spruing"
▪ The taper is to permit the sprue former to function like a true reservoir,
thereby decreasing the likelihood of suck-back porosity.
▪ The constriction may be helpful in the mold-filling process for lower
density base metal alloys.
▪ But, as the density of the metal increases, it is likely to interfere with
mold filling and lead to increased porosity
▪ Therefore, it is recommended that the greater the alloy density, the
greater the sprue -pattern access.

48. 1. Casting ring.
Casting ring hold the investment in place during setting and
restrict the expansion of mould
Metal plastic

49. Metal casting rings
▪ Stainless steel rings
▪ Pyronel alloy metal rings.
Available in two shapes round and oval and
two diameters large & small.

50. ▪ PYRONEL ALLOY METAL RINGS (WHIP MIX
CORPORATION)
▪ Even after repeated heating they remain precision fitting
and stable in size.
▪ They do not flake, corrode or peel away, thus they have
an unusually long life.

51. PLASTIC CASTING RINGS
▪ These are used in ring less casting system
▪ A ring less system that provides maximum expansion of
investment,is available commercially.

52. CRUCIBLE FORMER
▪ The sprue is attached to a crucible former.
▪ Forms the base of the casting ring during investing.
▪ The exact shape of the crucible former depends on
the type of casting machine used.

53. Casting ring liner
▪ With the use of solid metal rings or casting flasks,
provision must be made to permit investment
expansion.
▪ However, the most commonly used technique to
provide investment expansion is to line the walls of
the ring with a ring liner.

54. ▪ Traditionally, asbestos was the material of choice, but it can no longer be used
because its carcinogenic potential makes it a biohazard
▪ non-asbestos ring liner material
1. ceramic (Aluminum silicate)liner
2. A cellulose (paper) liner
3. a ceramic-cellulose combination
– The relative safety of the ceramic ring liners remains uncertain, because
Aluminum silicate also appears capable of producing hazardous-size
respirable particles .

55. FUNCTION
a. Mould expansion
b. Thermal insulator: When the ring is transferred from the
furnace to the casting machines, it reduces loss of heat, as
it is a thermal insulator.
c. Seperating media: Permits easy separation of the
investment from the ring after the casting is over.

56. Placement of liner
▪ Cut to fit the inside diameter of the ring with no overlap.
▪ The dry liner is tacked in position with sticky wax, and it is then
used either dry or wet.
▪ With a wet liner technique, the liner ring is immersed in water for
a time, and the excess water is shaken away.
▪ Squeezing the liner should be avoided, because this leads to
variable amounts of water removal and non-uniform expansion.

57. ▪ A ceramic liner not absorb water like cellulose liner,
its network of fibers can retain water on the surface.
▪ The liner does not only afford greater normal setting
expansion in the investment, but also the absorbed
water causes a semi hygroscopic expansion.
▪ Use of thick liner (not less than 1mm) or 2 liners
provides greater normal and hygroscopic expansion

58. ▪ The expansion of the investment is always greater in
the unrestricted longitudinal direction than in to
radical direction, that is, toward to ring
▪ The liner should be placed 3-4 mm short of the end
of the ring for better uniform expansion and to
decrease distortion of the wax pattern.

59. Casting Ring with Wax Pattern

61. Spruing the wax pattern
Investing the wax pattern

63. Investing the Wax Pattern
Type Melting Range Applications
Gypsum 650- 700º C gold Casting Alloys
Phosphate 850-1100 º C Metal Ceramic Restoration especially
for alloys like gold, platinum ,palladium
cobalt chromium and nickel chromium
Ethyl Silicate 1090 - 1180 º C Base Metal Alloys
Investment needs to expand 1.5 to 2 % before the casting is made to compensate for
metal expansion during melting.
General Formulation for an investment
Refractory FILLER [60-65%] = Quartz, Cristobalite
BINDER (Matrix) [ 30-35%] = Gypsum, Phosphate, Silicate
ADDITIVES (modifiers) [1-5%]

64. Investment Materials - Highlights
▪ Gypsum Bonded
– Used in Conventional Gold Alloys.
– Normal, Hygroscopic andThermal Expansion
▪ Phosphate Bonded
– Metal Ceramic Restoration; HighTemperature Investment
– ceramic substance that hardens at a high temperatures and is responsible
for high temperature strength
▪ Ethyl Silicate Bonded
– Used in high fusing base metal partial denture alloys
– “Green Shrinkage”, occurs in addition to setting shrinkage.”
▪ Titanium Investment
– Newer investments are aimed at casting of titanium and titanium based
alloy.

65. Mixing
Investment can be mixed in 2 ways
1. Hand mix:The liquid is added to a clean, dry mixing bowl, and the powder
is gradually added to the liquid, using the care and caution to minimize air
entrapment. Mixing is formed gently until all the powder has been wet.
2. Vacuum mixing: this is a type of mechanical mixing done under vacuum
created by a vacuum mix machine Removes Air bubbles during mixing

66. Hand Investing
• For investing by hand, the entire
pattern is painted (inside and out) with
a thin layer of investment.
• The casting ring is positioned on the
crucible former, and the remainder of
the investment is vibrated slowly into
the ring.

68. Vacuum Investing
▪ The same equipment used to mix the investment is
employed to invest the pattern under vacuum
▪ Air bubbles are eliminated
▪ Amount of porosity in the investment is reduced
▪ Texture of the cast surface is smoother with better
detail reproduction
▪ Tensile strength is increased
• If Hygroscopic technique is employed, filled casting ring is immediately placed in
37º C water bath.
• InThermal expansion or high heat technique, invested ring is allowed to bench set

69. Casting System
Ring less casting system
• Provides maximum expansion
• Suited for casting alloys that require
greater expansion .
Power cast ring less system
• Consists of three sizes of rings and
formers, preformed wax sprue and shapes,
investment powder, and a special
investment liquid.
• Used for alloys that need more mold
expansion than traditional gold alloys.

70. Investing the Wax Pattern

71. Casting Shrinkage
1. The thermal contraction of the liquid metal between the
temperature to which it is heated and liquidous temperature.
2. The contraction of metal inherent in its change from liquid to
solid state.
3. The thermal contraction of solid metal that occurs down at room
temperature
The alloy and metal shrinks when they change from liquid to solid state

72. Compensation for shrinkage
▪ Setting expansion
▪ Hygroscopic expansion
▪ Thermal expansion
▪ By altering water powder ratio- Lower the liquid powder ratio of
investments greater expansion
▪ Use of 2 ring liners –greater setting and thermal expansion.

73. Spruing the wax pattern
Investing the wax pattern
Burning out the wax pattern

74. BURNOUT
▪ Process of heating an investment mould to eliminate the
embedded wax pattern

75. Objectives of Burnout :
▪ Complete elimination of wax.
▪ To increase the temperature of mold and
investment comparable to fusion temperature of
cast alloy.
▪ For thermal expansion of investment and wax-
pattern mold for compensation shrinkage.
Burn out is done in time – temperature controlled

76. During Burn Out Procedure :
 Water present in pores of investment,
prevents absorption of wax within
mold. Also due to high temperature,
the water vaporizes and flushes out
the wax from the mold.
 Invested ring is held in such a way
that the sprue hole faces down so
that wax can easily flow down.
 In cases of high heat technique 
wax will decompose and form CO or CO2
and these gases will escape through
the pores in the investment

77. ▪ Wax pattern must be completely eliminated to obtain full castings.
▪ If the burn out procedure doesn’t immediately follow the investing
procedure, the investment should be placed at 100% humidity in
humidifier.
▪ The investment shouldn’t be permitted to dry out. Rehydration
won’t replenish all the lost water.

78. Spruing the wax pattern
Investing the wax pattern
Burning out the wax pattern
Casting the dental alloy

79. CASTING / INJECTION MOLDING :
Objectives : To fill the mold with cast
material as completely, efficiently and
quickly as possible.
3 basic steps :
 Fusing the metal alloy
 Heated investment should be carried to the
casting machine
 Forcing / Casting the metal alloy into the
mold.

80. Casting the dental Alloy
The casting technique
• Open flame
Electrically by
• Resistance Melting
• Induction Melting
• Vacuum Arc Melting
• Pneumatic Pressure casting
• Centrifugal casting
Melting the Alloy Casting the Alloy

81. Melting the Alloys
Alloys are melted in one of the four following ways;
1. Alloy is melted in separate crucible by a torch flame
and is cast into mold by centrifugal force.
2. Melted electrically by a resistance heating, then cast
into mold centrifugally by motor or spring action.
3. Melted by induction heating, then cast into mold
centrifugally by motor or spring action.
4. Alloy is vacuum arc melted and cast by pressure in
argon atmosphere .

82. Casting crucibles
▪ It is a vessel or container made of any refractory material used for
melting or calcining any substance that requires a high degree of heat.
(G.P.T.8)

83. Types of Casting crucibles
3 types of casting crucibles are available:-
1) Clay(high noble & noble types)
2) Carbon(high noble alloys& high fusing gold based metal
ceramic alloys)
3) Quartz(high fusing alloys of any type & suited for alloys that
have high melting range & sensitive to carbon contamination).
Preheating the crucible prevents spalling (cracking) and prolongs the crucible's life.

84. FUSING THE METAL ALLOY :
Torch Melting-Choice of Fuel
 Proper Energy source – Gas fuels, Electric source.
GAS FUELS :
 Natural gas + air  supplies the lowest temperature of
all sources. Used for small inlays, and type I and type II
alloys. Temperature produced by this fuel is 2680oC.
 Natural gas and O2  supplies higher temperature used
for extensive alloys. Eg. alloys used for construction of
PFM restorations. Temperature produced by this fuel is
28500C.

85.  Acetylene and O2 mixture : Supplies the
highest temperature that is 31400C.
Too hot for gold alloys
Therefore used for melting base metal
alloys like CO-Cr, Ni-Cr alloys  used
for RPDs.
Co-Cr, Ni-Cr alloys have high fusion
temperature.

86. BLOW TORCH
There are two types of the torches.
1. Multi orifice torch
2.Single orifice torch

87. ▪ The main advantage of multi orifice torch is the
distribution of heat over a wide area for more
uniform heating of a alloy.
▪ The single unit orifice concentrate more heat in
one area

88. Zones of Torch Flame
▪ The first long cone emanating directly from the nozzle is the
zone in which air and gas are mixed before combustion and
no heat is present.
▪ The next cone, green in color and immediately surrounding
the inner cone is combustion zone. Gas andAir are partially
burned. It is oxidizing and it should always be kept away from
the molten metal during fusion.
▪ The next zone dimly blue, is the reducing zone. It’s the
hottest part of the flame and is just beyond the tip of the
green combustion zone.This area should be kept constantly
on the metal during melting.
▪ The outer cone ( Oxidizing Cone) is the area in which
combustion occurs with the oxygen in the air.This portion of
the flame should not be employed to melt the alloy.

90. Electric Melting
Electric resistance melting
▪ Suited for all Gold alloy
▪ Uses carbon crucible providing the reducing
atmosphere throughout the melting regime.
▪ Includes best means of temperature control
Induction melting and Electric Arc Melting
▪ Suited for Cobalt Chromium andTitanium alloys
▪ Less temperature controllable than resistance mode.
Because melting achieved rapidly.

91. Induction Melting Machines
▪ Commonly used with melting of base metal alloys.
▪ Alloy is melted by an induction field that
develops within a crucible surrounded by water-
cooled metal tubing.
▪ Electrical induction furnace is a transformer in
which altering current flows through the primary
winding coil and generates aVariable magnetic
field in the location of the alloy to be melted in a
crucible
▪ Once the alloy reaches the casting temperature it
is forced into the mold by centrifugal force, by air
pressure or vacuum.

92. CARRYING INVESTMENT
CASTING RING TO
CASTING MACHINE
 Using forceps, tongs or tweezers as
quickly as possible.
 The casting ring should be held such
that sprue hole faces down (so that
if remaining wax, it can easily flow
out).
 During this time the color of the
casting should be cherry red.

93. FORCING THE METAL ALLOY INTO THE MOLD
 Resisting forces are present in investment, mold and
molten alloy. These should be overcome by air/gas
pressure, vacuum forces, centrifugal pressure, piston-
plunger forces / for moldable ceramics.
 Casting machines – 3 types :
 Air pressure
 Vacuum casting
 Centrifugal

94. Casting Machines
▪ Casting machines provide the means
for transferring the molten alloy from
the melting crucible to the mold.
▪ All casting machines can be divided
into two general types, Which force the
metal into the mould they are:
1. Centrifugal force type
2. Pneumatic force (or) Air pressure type

95. Centrifugal force Casting Machine
 The machine basically has a strong
spring encased in the base of the
casting machine.
 The spring is wound into tension by
rotating the arms with the weights at
one end and the casting ring at the
other end. In front of the ring is the
crucible in which the gold alloy is
melted. When the spring is released,
the two arms rotate rapidly, and the
molten metal is forced into the mold
by centrifugal force. Pressure of 30-40
Psi is used.

96. Factors Affecting the
Centrifugal Force :
Centrifugal force is
 Directly proportional to
speed
 Directly proportional to
length of arm of casting
machine.
 Directly proportional to
weight of metal

97. Advantages of Centrifugal
casting
▪ Simplicity of design and operations
▪ Able to cast both large and small castings on the same
machine

98. Pneumatic type casting Machines
Steam Pressure type
▪ Very old method still used to a limited extent thorough out
the world today.
▪ Alloy is melted in the sprue-Crucible former part of the mold.

99. Air pressure type
▪ Used for making small castings
▪ Either compressed air or some other gas
such as Carbon dioxide or Nitrogen can be
used to force the molten metal into the
mold
▪ The gas pressure is applied to the molten
metal through a suitable valve mechanism

100. VACUUM CASTING MACHINE
A vacuum is applied through the base beneath
the casting ring
The molten alloy can be drawn into the mold by
suction
It cannot work alone in filling the mold.
Combination can be used :
 Centrifugal Pressure +Vacuum
 Air pressure andVacuum
 Centrifugal + Air pressure +Vacuum
 For titanium and titanium alloys, vacuum
arc heated argon pressure casting
machines are required.

101. Electrical Resistance dental
casting Machines
In this device, current is passed through a resistance-heating
conductor, and automatic melting of the alloy occurs in a
graphite or ceramic
Advantages:
 1. It is especially used for those alloys used for metal ceramic
restoration, which are alloyed with base metals in trace
amounts that tend to oxidize on overheating.

102. For titanium casting
• Process of melting and casting takes
place in an evacuated two chamber
system with a continuous flow of
argon gas
• Titanium ingot is heated in a copper
crucible
• Prevents overheating & reaction

103. Titanium ingot melting-
heated to a temp. of 1600-
17000 c

104. ▪ Titanium is difficult to cast in comparison to the
common alloys. Requires relatively complex and
expensive equipments
▪ High melting point (1671 ºC) and tendency for
contamination in molten state are main problems with
Titanium.
▪ To prevent contamination, Titanium is cast under the
protective atmosphere of argon gas or in the vacuum.

105. ▪ To achieve high melting temperature, arc melting in either
graphite or water cooled copper crucibles are used.
▪ Also, more temperature resistant investments are used. Both
Phosphate bonded silica and magnesia investments produce
good castings.
▪ The casting system force the metal into the mold using either
pressure or centrifugal casting machines.

106. Spruing the wax pattern
Investing the wax pattern
Burning out the wax pattern
Casting the dental alloy
Deflasking the cast

107. Deflasking the casting
Long cooling time

108. Cleaning the Casting
▪ The ring is allowed to cool till the red glow of the metal disappears.
▪ Plunged under running cold water into a large rubber-mixing bowl.
(quenching)
Advantages of quenching
▪ Noble metal alloy is left in an annealed condition for polishing and
similar procedures
▪ When the water contacts the hot investment, the investment
becomes soft and granular and the casting is more easily cleaned
▪ Gypsum bonded investment quickly disintegrate

109. ▪ Phosphate bonded investment do not disintegrate and
must be forcibly removed from the casting ring.
▪ They can be handled as soon as they are sufficiently
cooled under running water

110. Sand Blasting the frame work
▪ After recovery of casting the casting is held in a
sandblasting machine to clean the investment from its
surface
▪ Commercially available abrasive compounds such as
Aluminum oxide, general purpose blasting compounds and
glass beads can remove casting investment and surface
oxides.
▪ 50 µm Grit, non recycled Aluminum oxide abrasive ( White
color) is commonly suggested for air abrading porcelain
bearing surfaces and dental porcelain.

111. Pickling
▪ The surface oxides from the casting are removed by process known as
pickling which consists of heating the discolored casting in an acid
▪ 50% hydrocloric acid is best pickling soluti0n for gypsum bonded
investment
▪ Place the casting in a test tube or dish and the acid over it
▪ HCL is heated, but not boiled with the casting in it because of the
considerable amount of acid fumes involved
▪ After pickling the acid is poured off and casting is removed.
▪ Disadv of HCL- a) fumes from acid corrode laboratrory
metal furnishings
b) fumes are health hazard
▪ A solution of sulfuric acid can also be used

112. Sprueing the wax pattern
Investing the wax pattern
Burning out the wax pattern
Casting the dental alloy
Deflasking the cast
Sandblasting the framework
Finishing the framework

113. Polishing
Separating disc,heatless stone,green,pink,white
stones&rubber wheel are used for polishing

114. Finished frame work

115. CASTING
DEFECTS

118. DISTORTION
CAUSES
 Distortion of the wax pattern- during fabrication
 High co-efficient of thermal expansion of inlay wax causes warpage
of pattern if it is improperly handled.This increases with
 Delay in investing the wax pattern
 Increase in ambient temperature

119. Powder/Water ratio
w/p ratio - Increase setting expansion, hydroscopic expansion thermal
expansion -
Mold expansion too much & casting too large
w/p ratio - Decreased Setting expansion, hydroscopic expansion &
thermal expansion
Mold expansion is less & casting is small
Correct proportion of powder /liquid should be used to prevent this

120. Surface roughness,
irregularities,& discoloration:
 Roughness- finely spaced surface imperfections whose
height, width, & direction establish the predominant
surface pattern.
 Irregularities- isolated imperfections, such as nodules,
that are not characteristic of the entire surface area.

121. Causes:
* Air bubbles on wax pattern
* Water films causing ridges and veins on surface
* Too rapid heating  resulting in fins or spines.
* Underheating causing incomplete elimination of wax.
* Inappropriate water / powder ratio
* Prolonged heating
* Temperature of alloy too high
* Casting pressure too high
* Foreign bodies
* Impact of molten alloy
* Pattern position
* Composition of the investment

122. These are caused due to –
UNDERHEATING – leaves wax residues . These
carbon residues may form a tenacious carbon
coating on the casting
PROLONGED HEATING- decomposes the sulfur
compound in the investment which discolor
the casting and make it brittle.
HIGH SULPHUR CONTENT OF TORCH FLAME
DISCOLORATION

123. POROSITY
Presence of voids or pores within a structures
 Porosity may occur both within the interior region of a
casting and on the external surface.
 External porosity - surface roughness.
 Internal porosity - weakens the casting and if it extends to
the surface, it may be a cause for discoloration.
If it is severe, it can produce leakage at the tooth –
restoration interface and secondary caries may result.

124. Porosities may be classified
(According to the cause) :
I. Solidification defects -
A. Localized shrinkage porosity
B. Microporosity
II. Trapped gases -
A. Pinhole porosity
B. Gas inclusions
C. Subsurface porosity.
III. Residual air.

125. porosity/shrink spot
porosity
 It is caused by the incomplete feeding of
molten alloy during solidification.
 When the alloy solidifies from liquid state, a
shrinkage of at least 1.25% occurs. Thus
during solidification of metal in mold, if
additional molten metal is not available to
compensate for shrinkage, then porosity
occurs.
 It can occur if sprue is thin, then the metal
freezes in the sprue before it does in the
mold. Occur usually in the portion of the
casting that solidifies last- sprue-casting
junction

126. SUCK-BACK POROSITY
 If hot spot is created on in
the mold near to the
sprue this causes the local
region to freeze last and
cause porosity
 Generally occurs at
occluso-cervical line angle
that is not well rounded

127. Microporosity
 It is also caused by solidification shrinkage,
but generally happens in fine grain alloys
when the solidification is too rapid for the
microvoids to segregate.
 This in turn is caused when the mold or
casting temperature being too low.

128. PIN HOLE /GAS INCLUSION POROSITY &
SUBSURFACE POROSITY
 entrapment of gases during solidification
 Both produce spherical defects
 Defect in case of gas inclusion porosity are larger
PINHOLE POROSITY –
many metals in the cast gold alloy especially silver,
copper, palladium and platinum are prone to
dissolve hydrogen and oxygen when they are in
the molten state. As the alloy solidifies these
gases are released.
POROSITY CAUSED BY TRAPPED
GASES

129. GAS INCLUSION POROSITY
Gas that is trapped mechanically by molten metal in the mold / Gas
incorporated during casting procedure. Caused due to –
-Poorly adjusted torch flame
-Use of oxidizing zone of the flame instead of the reducing zone
-PREVENTION
Pre-heating the gold alloy on a graphite crucible (used alloy)
-Correctly adjusting & positioning torch flame during melting

130. Sub-Surface Porosity
 Caused by simultaneous nucleation of solid grains and
gas bubbles
 It can be dimnished by controlling the rate at which
molten metal enters the mold

131. Back Pressure Porosity
 Entrapped air porosity
 Due to inability of air to
escape through mold
 It is found in cavity
surface of a crown or
mesio-occlual-distal
casting
 In case of low
casting/mold
temperature it can occur
on the outer surface

132.  Occurs when the molten alloy has
been prevented from filling the
mold space completely
INCOMPLETE CASTING

133. Inadequate spruing / sprue former too small.
 This can cause premature solidification of alloy
in the sprue former and thus molten alloy cannot
reach all areas of mold.
Sprues blocked with foreign bodies
 Foreign bodies block the flow of molten alloy.
Alloy not hot enough or not sufficiently molten or fluid.
 Results in viscous alloy which cannot wet all areas of mold cavity.
Mold too cold
Insufficient alloy used

134. Conclusion
▪ Investing and casting, a series of highly technique
sensitive steps,converts the wax pattern into a
metal casting.
▪ Accurate and smooth restorations can be obtained
if the operator pays special attention to each step
in the technique.

135. REFERENCES
 SCIENCE OF DENTAL MATERIALS--- ANUSAVICE—11TH EDITION
 CRAIGS’S RESTORATIVE DENTAL MATERIALS 12TH EDITION
 DENTAL LABORATORY PROCEDURES-RHOADS.RUDD.MORROW VOLUME
TWO
 INTRODUCTIONTO METAL CERAMICTECHNOLOGY W. PATRICK NAYLOR
 CONTEMPORARY FIXED PROSTHODONTICS ---STEPHEN F. ROSENSTIEL--
2ND EDITION
 SCHILLINBURG . FUNDEMENTALS OF FIXED PROTHODONTICS