In-detail explanation of casting procedures, casting machines and casting defects in dentistry.

1. CASTING PROCEDURES
Presented by:
Dr. Jehan Dordi
I year MDS
1

2. CONTENTS
• Introduction
• Sprue and Spruing
• Crucible Former
• Casting Ring
• Casting Ring Liner
• Investing
• Shrinkage Compensation
• Burnout Procedure
• Casting machines
• Casting Crucibles
• Flux in Casting
• Heat treatment
2

3. • Casting Procedures
• Divesting & Cleaning the casting
• Finishing and polishing
• Degassing Ceramo-metal casting
• Casting defects
• Review of Literature
• References
3

4. INTRODUCTION
4

5. Terminology
• Casting: It is defined as something that has been cast in a mold; an object formed by the
solidification of a fluid that has been cast into a refractory mould (GPT-9).
5

6. 3000 B.C- Copper was cast by Mesopotamian's
2500 B.C- Dental prosthesis fabricated from gold wire was found in Egypt
500 B.C.- Etruscans made bridges of soldered gold bands
1571- Benevento had done casting of both gold and bronze
1897- Phil brook described a method of casting metal in mould formed from a wax
pattern for posterior tooth
1907- Taggart devised a practically useful casting machine
1928- The low heat casting technique was published by Coleman
1930- Carl Scheu, discovered the phenomena known as Hygroscopic Setting Expansion
1945- George D. Estes introduced the vacuum investing technique to prevent formation
of air bubbles on surface of wax
1959- Strickland et al stated importance of type, shape & direction other than size of
sprue
1959- Peyton stated tar flaring should occur at the sprue/ wax pattern junction 6

7. The Lost Wax Process
• The casting procedure by lost wax technique
was introduced by Dr. William H. Taggart.
• He introduced this technique and the casting
machine in 1907 before the New York
Odontological Group .
• It soon led to the casting of inlays, onlays,
clowns, fixed partial dentures (TPDs), and
frameworks for removable partial dentures.
Blowpipe
Mould
Compressed air cylinder
7

8. Steps In Casting Procedure
• Tooth/ teeth preparation
• Impression
• Die Preparation
• Wax pattern fabrication
• Spruing
• Investing procedure
• Burn out procedure
• Casting
• Divesting & Cleaning of the casting
• Finishing of the casting
8

9. SPRUE & SPRUING
9

10. Sprue
• It is the channel or hole through which plastic or metal is poured or cast into a gate or reservoir
and then into a mould. (GPT-9)
Wax pattern attached to the crucible former with a sprue ready for investing. A ring liner is in place. 10

11. Types of Sprue
• According to material used:
• Wax sprue former (for large restorations E.g.. RPD
frameworks)
• Plastic/Resin sprue former (smaller restorations)
• Metal sprue ( smaller restorations E.g.. Crown)
• According to process:
• Prefabricated
• Custom made
• According to shape:
• Solid
• Hollow (Round)
• According to the diameter of teeth:
• For molars: 2.5mm (10-gauge)
• For anteriors and pre-molars: 2.0mm (12-gauge)
11

12. Materials Used In Sprue
• Special wax supplied as rope form (rolls) of different diameters (gauges) for selection.
• Hollow stainless steel wires of different diameters and lengths.
• These are to be coated with thin layer of inlay wax by dipping in molten wax.
• This is done for easy removal of sprue former after setting of investment.
12

13. Length & Diameter of The Sprue
• The length of the sprue former should be adjusted so that the pattern is approximately 6 mm
from the open end of the ring.
• The sprue former should be atleast 1.7mm in diameter unless the pattern is extremely small.
• Sprue formers upto 2.5mm in diameter is used for very large patterns and bulky full crowns.
13

14. Sprue Former Direction
• It is attached at 45 degrees to the walls
of the mould, which decreases the
turbulence of molten alloy.
14

15. Spruing Principles And Technique
• The first step in the production of refractory mould is termed as spruing the pattern, whereby the
wax pattern is attached to conical base by extension of wax, plastic, or metal.
• This connector is known as sprue former, which should be properly selected in respect to size
and configuration.
15

16. Location Of Sprue
• Sprue is attached to the bulkiest, non-critical part of the pattern.
• Away from the margins and occlusal contacts.
• Normally it is attached to largest non-functional cusp used.
Wrong Correct
16

17. Types of Spruing
• Direct spruing: Sprue former provides direct connection between
pattern area and the crucible former.
• A basic weakness of direct spruing is the potential for suck back
porosity at the junction of restoration and the sprue.
• Indirect spruing: It uses the same basic principal of spruing. But the
only difference lies in attachment of 3 running horizontal bars. The
whole indirect sprue complex consists of 3 parts:
1. Feeder sprue
2. Horizontal running bar
3. 6 or 8 gauge sprue former
17

18. Spruing The Pattern
• Selection of sprue former depends on
1. type and size of the pattern and
2. the size of the ring used.
• Long and thin sprue formers cause internal porosity.
• This maybe overcome by using a reservoir or larger sprue.
• Reservoir must have greater bulk than the adjacent portion of the pattern into which the sprue
former is inserted.
Reservoir
18

19. Wax sprue-former length is checked before attachment to
ensure that pattern is at least 3 to 6 mm from end of ring.
19

20. • Any overheating of the wax could distort the pattern and hence hollow metal/plastic sprue
formers are preferred, since they hold less heat than a solid sprue former.
• The sprue former should be smooth and produce no sharp angles at the juncture of the wax
pattern or the crucible former.
Sprue former placement should provide smooth-flowing
entry of gold into mold (arrows), with no sharp angles.
Sprue former
20

21. • Prefabricated sprue formers are available in a variety of
sizes and configurations.
• These plastic sprue formers are more rigid and
decreases the chances of distorting the pattern.
• A three-unit bridge and sprues with the “gate”
technique, using prefabricated plastic formers is
particularly useful for long-span multiple castings.
Prefabricated plastic sprue formers
Three-unit bridge and sprues with "gate" technique,
using prefabricated plastic sprue former
21

22. • Poor sprue designs can result in poor castings.
• The sprue former must provide a proper in gate,
allowing for smooth unobstructed flow of the
molten gold into the mould.
Poor sprue designs such as these can lead to casting failure.
22

23. Spruing Metal-Ceramic Units
• For small crowns a short, 8-gauge wax sprue
former should be placed on the incisal edge so
that the alloy may flow directly into the thinnest
section of the casting.
• The investment mold is generally about 1000° F
(540°C) cooler than the molten alloy, and
solidification occurs very quickly.
Small units are easily cast by attaching short 6- or 8-
gauge wax sprue former at incisal tip of wax pattern.
23

24. • Movement of the liquid metal to the most distal margins
of the casting must not be obstructed.
• There should always be sufficient bulk of the sprue to
allow for contraction of the metal during solidification
without introduction of shrinkage porosity.
• For larger castings (e.g., posterior crowns or small bridge
units) an additional auxiliary sprue may be placed to
facilitate filling the mould.
Incomplete casting caused by excessively thin
wax patterns and obstruction of gold flow in
facial area of coping.
24

25. • Small-diameter primary or auxiliary sprue formers attached to thicker sections of
the casting will often cause shrinkage porosity.
(A) Small primary sprue formers can create shrinkage porosity in casting.
(B) Very thin auxiliary sprues can create regions of porosity in thicker sections of larger castings.
25

26. • Large units are best sprued by the “gate”
method, whereby a series of short 8-gauge wax
sprue formers are attached to a continuous 6- or
8-gauge “ runner bar’ of wax or (preferably)
plastic.
1. "Gate” spruing.
A. Six- or eight-gauge sprue formers
B. runner bar, 6-gauge or larger
C. feeder sprues
D. crucible former.
2 and 3, Four-unit bridge to be cast using prefabricated plastic "gate”
sprue former. These rigid patterns help minimize wax distortion during
investing procedures. 26

27. • This runner bar in turn is attached to the crucible former by two or more large sprue formers,
providing:
1. A minimum of distortion of the wax pattern,
2. A fairly uniform reservoir of metal near the entire casting, and
3. Even distribution of the alloy to all parts of the casting to minimize porosity.
• For large castings, it is critical that the spruing arrangement provide a reservoir of metal that is
external to the dental unit but positioned at the heat center of the investment mould.
• Generally the alloy immediately adjacent to the walls of the mould will solidify first, the
exposed surface of the button will solidify second, and the alloy internal to the mold will cool
last. 27

28. • If bulky pontics are placed at the heat center of the
mould without a reservoir, porosity is very likely to
occur in the pontic itself, resulting in a weak bridge.
• Where a dental casting contains both thick and thin
elements, porosity can be minimized by the addition
of chill vents.
• Chill vents accelerate the cooling of the pontic
relative to the parts of the casting external to the
dental unit itself.
Addition of chill vents to bulky dental units may
speed solidification and minimize porosity in the
thick pontic or connector areas.
Venting
28

29. Spruing Of Cast Partial Framework Pattern
• Spruing for casting of removable partial dentures involves a large quantity of metal, so special
attention must be given to points of attachment, directions of the sprues, and the direction of
flow of the molten metal.
• A heavy sprue is an additional aid for offsetting the shrinkage.
• It acts as a reservoir for the casting because its bulk remains molten longer than the rest of the
casting and supplies molten metal to those areas that solidify and shrink first.
29

30. • Mandibular casts are usually sprued through a
hole in the center of the cast because it is easier
to attach the sprue leads.
• Maxillary casts are sprued the same way if the
shape of the casting permits; otherwise, they are
sprued from above.
• Main and auxiliary sprues are attached to the
central sprue approximately 7 mm below the tip
of the central sprue.
Wax pattern, not cast, is at least 7 mm from casting ring.
A, Maxillary wax-up showing clearance.
B, Mandibular wax-up centered in ring.
30

31. Procedure:
• Check the size of the investment cast in the investment
flask.
• When the wax pattern, is centered in the ring there should
be at least 7-mm clearance between the wax pattern and
the side of the ring.
• Cut a 7-mm hole through the base of the cast in the center
of the wax-up.
• The hole can be started with a bur and completed with a
knife blade.
31

32. • Soften half a sheet of baseplate wax.
Lay a straight hand-piece fissure bur
on the edge of the wax, and roll the
wax in a rod large enough to just pass
through the hole in the base of the
cast.
A, Straight hand-piece fissure bur is laid on edge of half sheet of softened baseplate wax
B, Wax is rolled around it to form rod large enough to just pass through hole in cast
C, Wax sprue cone completed.
32

33. • Push the wax rod through the base of the
refractory cast until it protrudes from the
other side about 10 mm.
• Seal the rod to the cast with wax on both sides
of the cast.
• The rod protruding through the bottom of the
cast will act as a handle during the investment
procedure.
33

34. • Use 8 or 10 gauge round wax for sprue leads on
mandibular casts, and lay them directly on the surface of
the cast.
• Attach one end to the central sprue and the other to the
bottom edge of the major connector.
• An auxiliary lead is usually made of 14-gauge round
sprue wax and is attached to the central sprue on the
same level as the main sprues.
• It should run as straight as possible and attach to the
minor connector on the molar just below the occlusal rest
or to the distal portion of the denture base.
34

35. • Six-gauge half-round wax is most often used for sprue
leads when spruing maxillary cast wax-ups.
• Reinforce and smooth all junctions between the sprues
and the framework patterns, and they are ready for
investing
35

36. CRUCIBLE FORMER
36

37. 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. (GPT-9)
• The sprue is attached to a crucible former, usually made of rubber, which serves as a base for
the casting ring during investing.
• The exact shape of the crucible former depends on the type of ring and casting machine used.
• With most modern machines, the crucible former is tall, to allow use of a short sprue and also to
enable the pattern to be positioned near the end of the casting ring.
Rubber crucible formers and
corresponding casting rings.
37

38. CASTING RING
38

39. Casting Ring
• Casting ring is a metal or silicone tube in which a refractory
mold is made for casting dental restorations (GPT-9).
• Investment material is poured in the casting ring and allowed
to set around wax pattern.
• Types of the casting ring:
1. Rubber ring
2. Metal ring
39

40. TYPES OF RING
ACCORDING TO
SHAPE
Round
Oval
COMPLETE RING
Rigid
Flexible
SPLIT RING
Metal
Plastic
40

41. CASTING RING LINER
41

42. Casting Ring Liner
• Casting ring liners are the materials used to line casting ring so that during heating and
expansion of the investment the compression of the liner should free the investment from the
restraint of the ring. It is placed on inner side of the casting ring.
• Types of Casting ring liner:
1. Asbestos
2. Non asbestos
i. Fibrous ceramic aluminosilicate
ii. Cellulose
iii. Ceramic- Cellulose
42

43. • The functions of liners are as follows:
• To allow enough lateral expansion of mould (investment), during setting or heating.
• Longer longitudinal expansion is reduced by adhesion of investment at the ends (3mm
space). This minimizes the distortion.
• When the ring is transferred from furnace to the casting machine it reduces the heat loss as it
a thermal insulator.
• Permits easy separation of the investment from the ring after casting process is over.
43

44. • Formerly asbestos sheet liners were laid inside
the casting ring leaving about 3mm at the ends.
• Due to health hazards of inhaling asbestos dust
or vapours which is carcinogenic, now cellulose
or alumina silicate ceramic liners are used.
Casting Ring Liner
44

45. • If wetted these liners provide some extra water for the Hygroscopic Setting Expansion
(HSE) of mould. Two or three layers give higher HSE of the mould laterally.
• Split casting rings or plastic casting rings are used to get adequate lateral expansions
specially for high fusing alloy.
• In base metal alloy casting procedures ring-less casting investment is done.
45

46. Ring-Less Casting System
• A ring-less casting system provides maximum expansion of
the investment.
• This system, is called the Power Cast ring-less system
(Whip-Mix Corporation, Louisville, USA), consists of three
sizes of rings and formers, preformed wax sprues and shapes,
investments powder and a special investment liquid.
• The tapered plastic rings allow for removal of the investment
mould after the material has set.
• This system is suited for casting of alloys that require greater
mould expansion than traditional gold based alloys.
Crucible formers and cone-shaped plastic rings for a
ringless investment technique in casting. The crucible
former and plastic ring are removed before wax
elimination, which leaves the invested wax pattern. The
systems are designed to achieve expansion that is
unrestricted by a metal ring.
46

47. INVESTING
47

48. Investing
• Investing: the process of covering or enveloping, wholly or in part, an
object such as a denture, tooth, wax form, crown, etc., with a suitable
investment material before processing, soldering, or casting. (GPT-9)
48

49. 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.
49

50. Preparatory Steps For Investing
• First of all secure the wax pattern that is made.
• A thin layer of special surfactant or wetting agents is applied, thin
layer should be maintained which otherwise collect air bubble from
the mix and cause nodule of casting.
• Cleaning the wax pattern of debris or oil is done by surfactants
which are available as:
i. Pattern Cleanser
ii. Diluted synthetic detergent solution
iii. Debbublizer
• The detergent material left behind reduces surface tension and
causes better wetting. 50

51. Investing Technique
• There are two methods of investing
1. Hand mixing investing method
2. Vacuum mixing and investing
51

52. Hand Mixing Investing Method
• Required amount of distilled water (mixture of
water + colloidal silica gel for phosphate bonded
investment) is taken in a clean flexible bowl.
• Weighed amount of powder is added and mixed
thoroughly.
• When a brush or small instrument is used to apply
the investment material to the pattern, the wet
material should be gently applied, pushed ahead of
the applicator.
(A)Paintbrush or small instrument is used to apply initial
layer of investment over pattern; this procedure decreases
chances of bubble entrapment inside pattern.
(B), Remaining investment is then poured into ring, carefully,
so as not to create voids in ring. 52

53. • When the pattern is completely covered in this manner, the remaining investment is poured
into the ring carefully to avoid entrapping air, and the investing procedure is completed.
• A small amount of mix is painted on the surface of wax pattern for better wetting and then
the remaining mix is vibrated into casting ring to fill completely.
• Too much vibration causes sedimentation of larger investment particles, leaving thin mix
close to the pattern, which may cause rough surface.
53

54. Vacuum Mixing And Investing
• Parts of machine:
1. Special investment mixing closed bowl,
2. Connection to evacuating pump,
3. A mixing pad connected to rotating shaft of an
electric motor and
4. A side platform for resting the casting ring is used.
• The proportioned material is taken in it, the speed and
time of spatulations are adjusted to get mix of
reproducible consistency.
54

55. • After the casting ring has been filled with the investment material, any excess
should be removed before the material sets.
• The ring is now set aside to allow the investment material to complete its setting
reaction and the accompanying setting expansion.
55

56. Grinding the Investment
• Carefully grind (on a model trimmer) or scrap 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.
• 3mm on each end is left as it serves to lock the investment within the ring & equalize radial &
axial expansion.
56

57. • The setting is completed in 30 to 40 minutes, at
which time it may be placed in a burnout oven to
eliminate the wax and obtain the remaining required
thermal expansion. (According to “Philips Science
of Dental Materials” the setting time is
approximately one hour.)
Before it starts to set, investment is wiped flush with
top of ring. Any scraping attempted after set may
fill in pores and inhibit gas escape from mold
during casting.
57

58. • When a hygroscopic technique is used, the
freshly filled investment ring is immediately
placed into a water bath for 30 minutes and kept
at 100° F (38° C).
• If metal sprue is used, it is pulled out easily, as
exothermic reaction softens the wax coating.
• The crucible formed is carefully cleaned by
removing the loosely adhering investment
particles which otherwise may fall into sprue and
block it.
Hygroscopic technique relies mainly on setting expansion to gain
proper expansion of mold. After being filled, ring is immediately
placed in constant temperature water bath at 100° F (38° C) for
at least 30 minutes.
58

59. SHRINKAGE COMPENSATION
59

60. Shrinkage compensation
• The molten alloys used for dental restorations shrink upon solidification: gold alloys by
approximately 1.5% and nickel-chromium alloys by as much as 2.4%.
• If the mold is not made correspondingly larger than the original wax pattern, the resultant
casting will be that much smaller.
• For crowns, therefore, it is necessary to compensate for the solidification shrinkage of the
specific alloy used by expanding the mold enough to at least equal the shrinkage.
Four mechanisms to produce mold expansion:
1. Setting Expansion of Investment
2. Hygroscopic Expansion
3. Wax Pattern Expansion
4. Thermal Expansion
60

61. • Setting Expansion: It is result of normal gypsum crystal growth. The expansion probably is
enhanced by silica particles in the investment interfering with the forming crystalline structure
of the gypsum, causing it to expand outward It occurs in small amount i.e. about 0.4% but is
partly restricted by metal investment ring.
• Hygroscopic Expansion: Maximum amount of expansion is by hygroscopic expansion. Water
bath at 38°C is used, water in bath replaces water used by hydration process. Space between
growing crystals is thus maintained. Crystals grow longer in outward direction causing
expansion of mould. About 1.2 to 2.2% hygroscopic expansion takes place. More controlled
amounts of hygroscopic expansion may be achieved by adding a measured amount of water to
the setting investment
61

62. • Wax pattern expansion: Expansion of the wax pattern while the investment is still fluid occurs
when the wax is warmed above the temperature at which it was formed. The low-temperature
burnout technique employs a combination of wax pattern expansion and thermal expansion of
the mold. After the investment filled ring is removed from a 100°F (38°C) water bath, the ring is
heated to only 900°F (482°C) before casting to produce the additional expansion needed.
• Thermal expansion: Thermal expansion of the investment occurs when the investment is
heated in the burnout oven. The investment around the wax pattern is allowed to harden in air at
room temperature, and then it is heated to approximately 1200° F (650°C). At this temperature,
the investment and metal ring expand enough to compensate for the shrinkage of the gold alloy.
• When the investment has set, the layer 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.
62

63. BURNOUT PROCEDURE
63

64. Burnout procedure
• Elimination of the wax pattern from the mould of set
investment material is referred to as the burnout.
• Ideally the casting ring containing the investment
should be placed into the oven approximately 1 hour
after investing the wax pattern.
• If the burnout procedure must be delayed for several
hours or overnight, it is advisable to place the
invested pattern into a humidor to prevent excessive
drying.
Investment rings should be placed in humidor to
prevent drying if it is anticipated that pattern will
not be burned out for several hours or overnight.
64

65. • To facilitate a rapid and clean burnout, the ring may
be placed on a raised object within the oven.
• A burnout started prematurely may cause fine cracks
in the gypsum-bonded investment.
• Once the heating is initiated, the casting should be
completed without permitting the mold to cool.
Porcelain tooth placed in bottom of oven provides good
support for one edge of ring, allowing molten wax and
gases to escape freely.
Cracks, caused by expanding steam, occur when damp investment
is heated too rapidly. This is major cause of fins on castings
65

66. • The mold should be placed in an oven preheated to approximately 900° F (480° C), held at
that temperature for 20 minutes, and the temperature then raised slowly to 1290° F (700° C)
and held for 30 minutes.
• Prolonged heating above 1290° F should be avoided.
• Gypsum-bonded investments will break down at this temperature, and the Sulphur and
chlorine will contaminate the gold alloy as it enters the mould.
• With a quartz investment, the rate of heating should be carefully controlled, since rapid
heating will produce a larger casting than will slow heating.
66

67. CASTING MACHINES
67

68. Classification Of Casting Machines
• Based on methods of casting the machines are:
1. Centrifugal force type
2. Air pressure type
3. Vacuum type
• Based on heating system employed:
1. Torch melted
2. Induction melted
3. Arc melted
68

69. Casting Machines
• Alloys are melted in one of the four following ways,
depending on the available types of casting machines:
1. The alloy is melted in a separate crucible by a torch
flame and cast into the mold by centrifugal force.
2. The alloy is melted electrically by a resistance heating
or induction furnace and then cast into the mold
centrifugally by motor or spring action.
Centrifugal casting machine, spring wound.
Spring-wound casting machine with electrical
resistance melting furnace.
69

70. 3. The alloy is melted by induction heating and
then cast into the mold centrifugally by motor
or spring action.
4. The alloy is vacuum-arc melted and cast by
pressure in an argon atmosphere.
Induction melting casting machine with induction coil
Vacuum-arc Melting Machine
70

71. Torch Melting/Centrifugal Casting Machine
• The casting machine spring is first wound
from two to five turns.
• The alloy is melted by a torch flame in a
glazed ceramic crucible attached to the
“broken arm” of the casting machine.
• The broken arm feature accelerates the
initial rotational speed of the crucible and
casting ring, thus increasing the linear speed
of the liquid casting alloy as it moves into
and through the mold.
• Once the metal has reached the casting
temperature and the heated casting ring is in
position, the machine is released and the
spring triggers the rotational motion
71

72. Counter Weight
Tongs
Broken arm
Crucible former
Platform for
casting ring
Hinge for rotation
72

73. Zones of Flame
• Mixtures of natural gas and air, or oxyacetylene gases are used.
• Zone 1 - Non combustion zone/ Mixing Zone
• Zone 2 - Combustion zone
• Zone 3 - Reducing zone
• Zone 4 - oxidizing zone
73

74. Electrical Resistance–Heated Casting Machine
• In this device, current is passed through a resistance
heating conductor, and automatic melting of the alloy
occurs in a graphite or ceramic crucible.
• This is an advantage, especially for alloys such as those
used for metal-ceramic prosthesis, which are alloyed with
base metals in trace amounts that tend to oxidize on
overheating.
74

75. Induction Melting Machine
• With this unit, the alloy is melted by an induction
field that develops within a crucible surrounded by
water-cooled metal tubing.
• 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.
• Once the alloy reaches the casting temperature in
air or in vacuum, it is forced into the mold by
centrifugal force, air pressure, or vacuum. 75

76. Direct-Current Arc Melting Machine
• The direct-current arc is produced between two
electrodes: the alloy and the water-cooled tungsten
electrode.
• The temperature within the arc exceeds 4000 °C and
the alloy melts very quickly.
• This method poses a high risk of overheating the
alloy, and damage may result after only a few
seconds of prolonged heating.
76

77. Vacuum or Pressure-Assisted Casting Machine
• For this machine, the molten alloy is heated to the
casting temperature, drawn into the evacuated mold
by gravity or vacuum, and subjected to additional
pressure to force the alloy into the mould.
• For titanium and titanium alloys, vacuum arc heated-
argon pressure casting machines are required.
77

78. CASTING CRUCIBLE
78

79. Casting Crucible
• It is 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. (GPT- 9)
79

80. • Following are the types of crucible used for casting:
1. Clay crucible (for many of the crown and bridge alloys, such as
the high noble and noble types)
2. Carbon crucible (For high noble crown and bridge alloys but
also for the higher-fusing gold-based metal-ceramic alloys)
3. Quartz crucible (for high-fusing alloys of any type)
4. Ceramic crucible (For palladium-silver and nickel or cobalt
based alloys)
80

81. FLUX IN CASTING
81

82. Flux in casting
• A reducing flux should be used in melting the alloy.
• The flux (50% boric acid powder and 50% fused borax) increases the fluidity and reduces the
potential for oxidation.
• Flux maybe added to:
1. To minimizing porosity.
2. To increase fusing of metal.
3. Prevent oxidation.
82

83. HEAT TREATMENT
83

84. Heat Treatment
• To adequately resist excessive wear, restoration must be returned to a hardened state prior to its
clinical use.
• The physical properties, specifically the mechanical properties of some gold alloys, may be
modified by heat treatment.
• Gold alloys containing 7% or more copper will respond to heat treatment by changes in
microstructure, and consequently in their handling properties.
84

85. • Softening heat treatment consists of heating the alloy to 1290° F (700° C) and maintaining that
temperature for approximately 15 minutes, followed by quenching in room-temperature water.
• Hardening heat treatment may be conducted in either of two ways:
1. Heat soaking at a constant temperature of approximately 840° F (450° C) for 15 minutes
2. Low cooling from 840° to 480° F (450° to 250° C) over a period of 15 minutes, followed
by quenching in water.
85

86. CASTING PROCEDURES
86

87. Casting Procedures: Low-fusing Gold Alloys
• To melt low-fusing gold alloy for the casting procedure, three methods are available:
1. Compressed air and gas,
2. Oxygen and gas, or
3. An electric casting machine.
87

88. • When a torch is used flame cone should be
approximately 40 mm.
• The tip of the reducing cone is used to make a rapid
and clean melt.
• Electric furnace casting machine eliminates the need
for a torch and the judgment required in heating the
gold alloy. It is well controlled, accurate, and precise.
Properly adjusted torch.
(A) Reducing zone is used to melt gold. (B)Oxidizing
zone is cooler than reducing zone and will not heat
gold to proper casting temperature.
Electric casting machine
88

89. • If the investment cools prior to the casting, the mold
will become smaller through cooling contraction of
the investment.
• Castings produced with large amounts of alloy will
usually produce better detail and definition than
minimum amounts of metal used.
• If melted in a reducing flame, molten gold alloy that
has reached the proper temperature for casting will
appear shiny with a mirror like surface.
When gold has reached proper temperature for casting, it
will be elliptic with well-rounded edges, yellow-orange in
color, and will appear to be “spinning.”
89

90. • When the alloy has solidified and cooled for approximately 1 minute, the investment may be
placed into room-temperature water, and the alloy quenched.
• This will aid in removing the adhering investment, which can be cleaned from the casting with
a laboratory brush, toothbrush, or an air abrasive.
Pencil-type abrasive blasters can be used to remove
investment and oxides from casting without appreciable
danger of damaging margins.
90

91. Casting High-Fusing Metal-Ceramic Alloys
• Casting of porcelain-bonding alloys requires that the metal be heated approximately 212° F
(100° C) higher than its upper melting point.
• This is generally in the range of 2200° to 2500° F (1200° to 1370° C).
• These temperatures can be achieved with
1. A gas-oxygen torch or
2. By induction heating.
91

92. Induction Casting
• Induction casting can be used for alloys of any composition and provides for excellent control of
the casting temperature.
• The alloy is rapidly melted in a carbon crucible, heated by electrical induction.
• The atmosphere is reducing, and at casting the molten alloy temperature is centrifugally cast
into the mold.
• The apparatus for induction casting is expensive.
92

93. Torch Casting
• All alloys can be cast with a gas-oxygen or oxy-acetylene torch.
• The casting of porcelain-bonding alloys differs from casting low-fusing gold alloys in the
following respects:
1. No asbestos is used in the high-heat crucibles, since it may react with the alloy constituents at
high temperatures.
2. The technician should have one high-heat crucible for each brand of alloy being cast in order
to avoid cross-contamination.
3. No casting flux is used on porcelain-bonding alloys, since flux may alter its composition and
diminish the bond strength. This is particularly true of gold alloys.
4. The visual clues for assessing casting temperatures are different from low-fusing gold alloys,
since high-fusing alloys are heated white-hot before they are released into the mold.
93

94. 5. Greater casting pressures (i.e., the number of turns of a centrifugal machine) are generally
required to force the porcelain-bonding alloy into the mold.
• When a multiple-sprue technique is used, the inlets should be aligned for the fastest and most
uniform distribution of alloy within the mold.
• The sprue and button should be sufficient to provide for cooling dynamics that do not lead to
shrinkage porosity.
94

95. • High-fusing gold alloys will melt into a pool that is cast
when it has reached a white-hot, almost vaporous state.
• Nonprecious nickel-chromium alloys are generally
supplied as sharp cornered ingots of various shapes and
are heated until the corners round over.
When gold alloy is ready to cast it will be white hot,
forming smooth oxide-free elliptical pool.
A, Base metal ingot— beginning of melt with gas-oxygen
flame.
B, Ingot continues to melt.
C, Base metal ingot ready to cast. Corners and sharp edges
have rounded.
D, Base metal ingot overheated, produces changes in
composition and excessive oxidation.
95

96. • All alloys should be melted as quickly as possible to
avoid excessive oxidation.
• High-fusing gold alloys are least sensitive to
overheating or prolonged heating.
• After casting, the machine should be allowed to spin
freely until it has nearly stopped .
• It may then be braked still, and the ring removed and
promptly quenched in water.
• The investment is initially removed by carving and
breaking it off the casting, taking care not to damage
the casting.
Investments must be carefully broken away from
casting. Attention to proper investing and casting
procedures will allow for clean separation of casting
from investment
96

97. Casting of Titanium alloys
• Advantages of titanium and titanium alloys include excellent
biocompatibility and corrosion resistance, which results from
the presence of a thin, adherent, passivating surface layer of
titanium dioxide.
• The dental casting of titanium and titanium alloys poses special
problems because of the high melting point of titanium
(1668°C) and its strong tendency to oxidize and react with other
materials.
• Titanium dental casting machines that represent a substantial
expense must provide either a vacuum environment or an argon
atmosphere.
• Both vacuum/argon pressure and centrifugal casting machines
have been developed, and both argon-arc melting and induction
melting have been used to fuse titanium and titanium alloys.
97

98. • Patterns for casting are coated, and investments must be used to provide the appropriate
expansion.
• Reaction of titanium or titanium alloy with the investment (and perhaps with the residual
atmosphere in the casting machine) results in a very hard near-surface region (termed α case)
that can exceed 50 μm in thickness.
• Currently, titanium castings of clinically acceptable accuracy can be produced, whereby the
marginal fit of cast complete crowns is superior to that for titanium crowns milled by the
computer-aided design/computer assisted manufacturing (CAD/CAM) technique.
98

99. CLEANING THE CASTING
99

100. Quenching
• After the casting has solidified, the ring is
removed and quenched in water as soon as the
button exhibits a dull-red glow.
• Two advantages are gained in quenching:
1. The noble metal alloy is left in an annealed
condition for burnishing, polishing, and similar
procedures, and
2. When the water contacts the hot investment, a
violent reaction ensues, resulting in a soft,
granular investment that is easily removed.
100

101. Pickling
• Often the surface of the casting appears dark with oxides and tarnish, such surface can be
removed by a process known as pickling.
• Pickling consists of heating the discolored casting in an acid.
• One of the best pickling solutions for gypsum-bonded investments is a 50% hydrochloric
acid solution.
• Nonprecious nickel-chromium alloys must never be placed in acid because of their high
reactivity.
• The hydrochloric acid aids in the removal of any residual investment, as well as of the oxide
coating.
101

102. • 'The best method for pickling is to place the casting in a test tube or dish and to pour the acid
over.
• The pickling solution should be renewed frequently, because it is likely to become contaminated
after reusing the solution several times.
• In no case should the casting be held with steel tongs so that both the casting and the tongs come
into contact with the pickling solution, because this may contaminate the casting.
• When the steel tongs contact electrolyte, a small galvanic cell is created and copper is deposited
on the casting at the point where the tongs grip it which becomes future source for discoloration
in the area.
102

103. Sandblasting
• The process of altering the surface of a material through the use of abrasive particles
propelled by compressed air or other gases. (GPT-9).
• It can be done using a high speed sandblaster.
• Most common medium for sandblasting in dentistry is aluminum oxide.
• The fine aluminum oxides are recycled within the sandblasting machine and replaced after
30- 40 minutes of total accumulated operation time.
• 60-80 psi of force is exerted by the machine.
103

104. • Provides mechanical retention between porcelain-metal by removing the casting oxides as well
as proper sandblasting of the alloy with 100-150 microns aluminum oxide results to mechanical
bonding.
• Also provides micromechanical retention of denture base to framework.
• Two types of sandblasting
1. Automatic sandblaster
2. Blast cabinets
104

105. Cleaning The Casting
• The first step is to thoroughly clean the surfaces of all
residual investment.
• Abrasive blasting is effective for this.
• The blasting medium can become embedded in the
surface and be a source of contamination if not
thoroughly removed.
• Traces of investment can easily be ground into the
metal surface and act as sources of gas at the
porcelain -metal interface when porcelain is added.
Residual investment
(I) is easily removed from casting
(C) by use of Al203 or glass bead powders in pencil-
tip air-abrasive units. Margins are easily protected by
using this type apparatus .
105

106. • Resin-bound abrasives (e.g., some carborundum or paper
disks) can contaminate the metal surface with resin,
leading to severe bubbling when porcelain is added.
• Once the porcelain-bearing surface is clean, it may
optionally be sandblasted with a clean 25 to 50 µm
aluminum oxide abrasive to texturize it.
Contamination of metal surface by finishing with
resin bound abrasives can lead to severe bubbling
in porcelain. This layer must be removed with
hydrofluoric acid and metal refinished with no
contaminating, ceramic-bound abrasives.
106

107. • This will increase the area of bonding and improve
the wettability of the metal.
• Surface roughness is not necessary to achieve a
satisfactory porcelain-to-metal bond, since the main
mode of attachment is chemical, not mechanical
• The casting can now be cleaned in distilled water in
an ultrasonic cleaner.
• When the metal surface is absolutely clean — free of
oxides, investment and organic films — it is ready for
the next stage of metal conditioning (finishing).
Aluminum oxide air- abrasion of metal surface after
finishing with ceramic -bound stones will produce uniform
texture and improve wettability by opaque porcelain.
107

108. FINISHING OF CASTING
108

109. Objectives And Procedures
• The objectives and procedures for finishing
are different for each part of the cast
restoration.
• Zone 1 is the internal margin
• Zone 2 the internal surface
• Zone 3 the sprue
• Zone 4 the proximal contacts
• Zone 5 the occlusal surface
• Zone 6 the axial walls and
• Zone 7 the external margins.
109

110. Zone 1: Internal Margin
Objective:
• To minimize any dissolution of the luting agent, a 1-
mm wide band of metal must be closely adapted to
the tooth surface.
• A discrepancy within this zone can significantly
reduce a restoration’s longevity.
• Good adaptation is obtained by careful reflowing of
the wax pattern.
Reflowing of the wax pattern. The objective is to create a
well-adapted 1-mm zone to prevent cement dissolution.
Proper reflowing before investing is essential.
110

111. Procedure for Zone-1
• If a defect occurs in the marginal area, the restoration must be remade.
• Even small nodules can prevent a casting from seating completely.
• Small nodules, if far enough away from the margin itself, can be removed under a binocular
microscope with exceptionally cautious use of small rotary instruments (e.g., a No. 1/2 round
bur).
111

112. Zone 2: Internal Surface (Intaglio)
Objective:
• No contact should exist between the die and the internal surface (intaglio) of the casting.
• A uniform space of 25 to 35 μm is necessary for the luting agent to spread evenly.
• Any contacts must be identified and relieved by careful selective grinding of the internal
surface.
112

113. Procedure for Zone-2
• Casting’s internal surface should, be examined for nodules
before the restoration is seated on the die.
• Even a very small nodule can result in significant increase of the
marginal gap width.
• Any significant force will abrade or chip the die, so that the
casting will seat on the die but will not seat fully on the prepared
tooth.
113

114. • Once the casting has been adjusted, determining
the exact location of the nodule is no longer
possible.
• Therefore, the nodule should be removed
entirely in one step, rather than through
sequential relief of the internal surface.
• Several agents are commercially available to
facilitate identification of the seating interference
between the casting and the die. When a nodule is removed, removal of slightly more than the
defect ensures complete seating of the restoration.
114

115. A, Incomplete seating.
B, Liquid marking agent.
C, A thin coat is applied to the internal surface and air dried.
D, The casting is gently returned to the die.
E, The area of interference is identified.
F, Nodules are best removed with a small round bur.
G, Seated casting.
A B C D
EFG
115

116. Zone 3: The Sprue
Objective
• To re-establish proper coronal structure and function, the sprue must be sectioned, and the
casting must be recontoured in the area of its attachment.
Procedure for Zone- 3:
• Once the fit of the casting has been verified on the die and it has been found to be acceptable,
the sprue is sectioned, and the area of its attachment to the casting is reshaped.
116

117. (A)Attached sprue to casting
(B)cut around the sprue and then twist it off.
(C)With multiple castings made simultaneously, care
must be taken not to damage the margin inadvertently.
(D)Disks and stones are used for gross recontouring.
B
C
A
D
117

118. Zone 4: Proximal Contacts
Objective
• The proximal contact areas are adjusted in the laboratory so that they will be correct (or slightly
too tight) when the casting is evaluated in the patient’s mouth.
118

119. Procedure for Zone-4
• The proximal contacts on the stone cast can be minimally
relieved by careful scraping with a scalpel.
• When proximal contacts are adjusted, placing a thin
articulating film between adjacent castings or between the
casting and the adjacent tooth is helpful.
• Doing this allows the areas where binding contact occurs
to be adjusted through selective adjustment where
markings result.
119

120. A, Thin articulating film interposed between a metal-ceramic restoration and the adjacent tooth.
B, The area of contact that prevents complete seating is readily apparent.
C, Articulating film is used to detect the location of an excessive proximal contact on cast metal.
A B C
120

121. Zone 5: Occlusal Surface
Objective
• Occlusal contacts are re-established in static and dynamic relationships to the opposing arch.
• Obtaining accurate and stable contacts does not require highly polished metal occlusal surfaces;
a satin finish is acceptable.
• Occlusal form must ensure positional stability and satisfy all functional requirements.
121

122. Procedure for Zone-5
• The occlusal contacts are checked with thin articulating film to
ensure that they match the design in the waxing stage.
• If an occlusal contact is heavy in wax, it springs back slightly
when the articulator is opened and produces an occlusal
prematurity in the casting.
A and B, Occlusal prematurity's are
generally the result of excessively heavy
contact on the wax pattern.
B
A
122

123. • Occlusal adjustments can be performed with flame shaped
finishing burs or diamond bur.
• The correct technique for occlusal adjustment is to re-develop the
anatomy of the entire ridge or cusp rather than grinding only the
point of interference.
• Simultaneously, any nodules can be removed, and grooves can be
defined with a finishing bur or a small round bur.
A, Occlusal adjustment is readily accomplished
with a pointed diamond or carbide bur.
B, The grooves and fissures are concurrently
refined.
B
A
123

124. • Before starting any adjustment, the practitioner should
use a thickness gauge on the metal.
• If only minimum clearance was established at the tooth
preparation stage, indiscriminate adjustment leads to
inadequate thickness of the casting.
As occlusal adjustments are made
(A), the residual thickness is continually monitored with an
appropriately designed thickness gauge
(B). For structural durability, metal thickness of less than
1.0 mm is inadequate and is the result of insufficient
occlusal reduction.
124

125. • If the wax pattern has been carefully
finished, a smooth casting results, and
removing surface oxides with a soft wire
brush wheel is sufficient.
• The surface can then be polished with
rouge on a soft brush wheel (which
removes only 5 μm from the surface of
the casting.
A, Initially a wire brush is used on the occlusal surfaces.
B, A fine-grit sandpaper disk is applied for removing pits and irregularities from the axial walls
C A rubber wheel is then used on the axial walls.
D, Castings, after polishing with buffing compound, immediately before clinical evaluation.
E The completed castings immediately before cementationE
DC
BA
125

126. Zone 6: Axial Walls
Objective
• When axial wall finishing is completed, the walls should be smoothly contoured and highly
polished, enabling the patient to perform optimum plaque control.
126

127. Procedure for Zone-6
• Surface defects are removed by grinding with
abrasive particles bound into a grinding stone
or rubber wheel, on a paper disk, or applied
as an abrasive paste.
• A sequence of progressively finer grades is
used to attain the desired surface.
(A)Carborundum disks and stones of varying degrees of coarseness are
typically used first;
(B)these are followed by garnet paper and sandpaper disks,
(C)rubber points and white Arkansas stones,
(D)and rubber wheels and points, along with small carbide burs for
removing nodules.
C
B
D
A
127

128. • The most efficient method of polishing
is to use a sequence of progressively
finer abrasives each removing the
scratches made by the previous grade.
• Time is wasted if the progression to a
finer grade abrasive is too rapid
because the coarser grits remove
material much more efficiently.
A, Assorted abrasives, sandpaper disks, rubber points, and polishing wheels.
B, Instruments used range from small carbides (for removing nodules) and a steel wire brush (for occlusal surface
smoothing) to buffing wheels and compounds.
C, A coarse wheel is used to true and thin the edge of a rubber wheel.
D, Buffing compounds applied on a wheel or bristle brush.
A
D
B
C
128

129. Zone 7: External Margins
Objective
• Margin finishing is crucial for a restoration’s longevity and therefore merits special attention.
• The objective of all cast restoration finishing is a highly polished metal surface without ledges
or steps as the transition is made from restoration to unprepared tooth.
• Failure to accomplish this leads to compromised plaque control.
129

130. Procedure for Zone-7
• Where access allows, cavo-surface margins
should be finished directly on the tooth.
A Supragingival margins allow access for finishing the
restoration directly on the tooth.
B, Fine-grit white stone lubricated with petroleum jelly.
C, Rubber point.
D, Completed restoration.
A
C D
B
130

131. A. When subgingival margins do not allow access, final finishing is performed on the die. During final
polishing, the margin is carefully supported with a finger.
B. Carefully rubbing a smooth instrument along the length of the margin (burnishing).
C. Gently brushing a fine-grit stone over the surface to remove casting roughness Using a soft rubber
wheel or point.
CBA
131

132. DEGASSING CERAMOMETAL
CASTINGS
132

133. Degassing Ceramometal Castings
• After the coping or framework is cleaned, it should be subjected to a degassing or surface –
conditioning treatment.
• During degassing the unit is heat treated to remove contaminants from the surface or possibly to
remove entrapped gas from the alloy .
• In addition, a thin oxide film is formed on the surface. This film is essential to porcelain
bonding.
• Precious gold alloys should be rapidly heated in air to 1950° F (1065° C ), held there 5 to 10
minutes , and rapidly cooled to room temperature.
133

134. • The unit will have darkened due to surface oxidation .
• Semiprecious and base metal alloys are generally heated to 1950° F , and held at this
temperature in a 25- to 28- inch (64- to 70- cm) mercury vacuum for 5 to 10 minutes.
• At this temperature the vacuum can be broken and the unit then cooled quickly. This procedure
will produce a thin but sufficient oxide coating on the metal surface.
• A second aluminium oxide (Al2O3) abrasive blasting of the surface is recommended for base
metal alloys to remove excessive oxidation.
134

135. CASTING DEFECTS
135

136. Casting Defects
• Defects in castings can be classified under four headings:
1. Distortion;
2. Surface roughness and irregularities;
3. Porosity and
4. Incomplete or missing detail
136

137. Distortion
• Distortion of the casting is probably related to a
distortion of the wax pattern, it can be minimized by
proper manipulation of the wax and handling of the
pattern.
• The setting and hygroscopic expansions of the
investment may produce a non-uniform expansion of
the walls of the pattern.
• Factors that affect distortion are:
1. The configuration of the pattern,
2. The type of wax, and
3. The thickness influence the distortion that occurs
• Distortion increases as the thickness of the pattern
decreases, the lower is the setting expansion of the
investment, the less lower is distortion.
137

138. Surface Roughness, Irregularities,
And Discoloration
• Surface roughness is defined as relatively finely spaced surface
imperfections whose height, width, and direction establish the
predominant surface pattern.
• Surface irregularities are isolated imperfections, such as nodules,
that are not characteristic of the entire surface area.
• Improper technique can lead to a marked increase in surface
roughness as well as to the formation of surface irregularities.
138

139. • Surface discoloration and roughness can result from
sulfur contamination.
• The interaction of the molten alloy with sulfur produces
a black or gray layer on the surface of gold alloys that is
brittle and does not clean readily during pickling.
A black-coated noble metal alloy casting resulting from
sulfur contamination or oxidation during melting of the
alloy.
139

140. Air Voids
• Small nodules on a casting are caused by air
bubbles that become attached to the pattern during
or subsequent to the investing procedure.
• The best method to avoid air bubbles is to use the
vacuum investing technique.
• A wetting agent may be helpful in preventing the
collection of air bubbles on the surface of the
pattern.
• It is best to air-dry the wetting agent, because any
excess liquid dilutes the investment, possibly
producing surface irregularities on the casting.
140

141. Water Films
• Wax is repellent to water; if the investment becomes
separated from the wax pattern in some manner, a
water film may form irregularly over the surface.
• If the pattern is slightly moved, jarred, or vibrated
after investing or if the painting procedure does not
result in intimate contact of the investment with the
pattern, such a condition may result.
• If too little water is used, the investment may be un-
manageably thick, so that it cannot be properly
applied to the pattern.
• A wetting agent is useful for the prevention of such
irregularities. 141

142. Foreign Bodies
• When foreign substances get into the mold a surface
roughness may be produced
• A rough crucible former with investment clinging to it may
roughen the investment on its removal so that bits of
investment are carried into the mold with the molten alloy.
• Usually contamination results not only in surface roughness
but also in incomplete areas or surface voids.
• Any casting that shows sharp, well-defined deficiencies
indicates the presence of some foreign particles in the mold,
such as pieces of investment and bits of carbon from a flux.
• Bright-appearing concavities may be the result of flux being
carried into the mould with the metal.
142

143. Impact Of Molten Alloy On The Mold Wall
• The direction of the sprue former should be such that the molten gold alloy does not impact a
weak portion of the mould surface.
• Sometimes the abraded area is smooth, so that it cannot be detected on the surface of the
casting.
• Such a depression in the mould is reflected as a raised area on the casting, which prevents
complete seating of the casting.
• It can be avoided by proper spruing so as to prevent the direct impact of the molten metal at an
angle of 90° to the investment surface.
143

144. Pattern Position
• If several patterns are invested in the same ring, they should not be placed too close together.
• Likewise, positioning too many patterns in the same plane in the mould should be avoided.
• The expansion of wax is much greater than that of the investment, and this may cause
breakdown or cracking of the investment if the spacing between patterns is less than 3 mm.
144

145. Carbon Inclusions
• Carbon from a crucible, an improperly adjusted torch, or a carbon-containing investment, can be
absorbed by the alloy during casting.
• These particles may lead to the formation of carbides or even create visible carbon inclusions.
145

146. Porosity
• Porosity may occur both within the interior region of a casting and in the external surface which
results in surface roughness.
• Internal porosity weakens the casting and also cause discoloration.
• If severe, it can cause plaque accumulation at the tooth-restoration interface, and secondary
caries or periodontal disease may result within the adjacent tooth structure.
• Although porosity in a casting cannot be prevented entirely, it can be minimized by the use of
proper techniques.
146

147. Porosities in noble metal alloy castings are classified below.
• Solidification defects
A. Localized shrinkage porosity
B. Microporosity
C. Suck-back porosity
• Trapped gases
A. Pinhole porosity
B. Gas inclusions
C. Subsurface porosity
D. Back pressure porosity
147

148. Localized Shrinkage
• Generally caused by premature termination of the flow of
molten metal during solidification.
• Continual feeding of molten metal through the sprue must
occur to make up for the shrinkage of metal volume during
solidification.
• Localized shrinkage generally occurs near the sprue-casting
junction.
• If the sprue freezes in its cross section before this flow is
completed, a localized shrinkage void will occur in the last
portion of the casting that solidifies.
Localized shrinkage porosity in pontic of
three-unit bridge caused by delayed
solidification and lack of a chill-set sprue.
148

149. Microporosity
• Microporosity occurs from solidification shrinkage.
• It occurs when the solidification is too rapid for the micro-voids
to segregate to the liquid pool.
• This premature solidification causes the formation of small,
irregular voids.
• Such phenomena can occur from rapid solidification if the mold
or casting temperature is too low.
149

150. Pinhole- Porosity
• Oxygen is dissolved by some of the metals, while they are in the
molten state.
• During solidification, the gas is expelled to form blebs and pores
in the metal.
• The porosity that extends to the surface is usually in the form of
small pinpoint holes.
• When the surface is polished, other pinholes appear.
150

151. Gas Inclusion Porosity
• The gas inclusion porosities are usually much larger than pinhole porosity.
• Larger spherical porosities can be caused by gas occluded from a poorly adjusted torch flame or
by use of the mixing or oxidizing zones of the flame rather than the reducing zone.
• If the alloy has been used before, these types of porosities can be minimized by pre-melting and
correctly adjusting and positioning the torch flame during melting.
151

152. Subsurface Porosity
• They may be caused by the simultaneous nucleation of solid grains
and gas bubbles at the first moment that the alloy freezes at the
mould walls.
• This type of porosity can be diminished by controlling the rate at
which the molten metal enters the mould.
152

153. Suck-Back Porosity
• It occurs externally, usually in the interior of a crown near the area
of the sprue, if a hot spot has been created by the hot metal
impinging from the sprue channel on a point of the mould wall.
• The entering metal impinges onto the mold surface at this point
and creates a higher localized mold temperature in this region,
known as a hot spot.
• This hot spot causes the local region to freeze last and results in
what is called suck-back porosity.
• Suck-back porosity often occurs at an occluso-axial line angle or
inciso-axial line angle that is not well rounded.
153

154. • A hot spot may retain a localized pool of molten metal after other
areas of the casting have solidified.
• This, in turn, creates a shrinkage void or suck back porosity.
• This type of porosity defect can be eliminated by flaring the point
of sprue attachment and reducing the mold-melt temperature
differential—that is, lowering the casting temperature by about
30 °C.
154

155. Back-Pressure Porosity
• Entrapped air bubbles on the inner surface of the casting, are
sometimes referred to as back-pressure porosity.
• This is caused by the inability of the air in the mold to escape
through the pores in the investment or by the pressure gradient
that displaces the air pocket toward the end of the investment
via the molten sprue and button.
• Frequently found at the cavity surface of a crown or mesio-
occlusal-distal casting.
• It can be prevented by use of the dense modern investments,
by an increase in mold density produced by vacuum investing,
and by the tendency for the mold to clog with residual carbon
when the low-heat technique is used.
155

156. Incomplete Casting
• The obvious cause is that the molten alloy has been prevented in some
manner from completely filling the mould.
• At least two factors that may inhibit the ingress of the liquefied alloy
are insufficient venting of the mold and high viscosity of the fused
metal.
• The first consideration, insufficient venting, is directly related to the
back pressure exerted by the air in the mould.
• If the air cannot be vented quickly, the molten alloy does not fill the
mold before it solidifies.
• These failures are usually exemplified by rounded, incomplete
margins. 156

157. • A second common cause of an incomplete casting is
incomplete elimination of wax residues from the mold.
• If too many products of combustion remain in the mold, the
pores in the investment may become filled, so that the air
cannot be vented completely.
Incomplete casting resulting from incomplete wax
elimination is characterized by rounded margins and
shiny appearance.
157

158. SUMMARY OF CASTING
DEFECTS
158

159. Type of Failure Procedural Errors Solution
Suck-back porosity Sprue attached at a right angle to
the wax pattern, creating a
“hot spot”
Sprue should always be attached at
the bulkiest portion of the wax pattern
directed at 45° to the surface
Improper margin fit Improper handling of wax pattern
during removal from the oral
cavity
Hard wax can be used for wax pattern
fabrication.
Careful handling of the pattern during
removal along the path of insertion,
taking care not to disturb the margins
Air bubble in the
Casting
Air bubble entrapment during
investing procedure
Prevented by carrying out the mixing
and investing procedure under
vacuum
159

160. Type of Failure Procedural Errors Solution
Water film Pattern is slightly moved, jarred, or
vibrated after investing
Prevented by proper use of a wetting
agent and proper care of the invested
casting ring
Contamination of the alloy Reaction of the alloy with crucible
used
Crucible unable to withstand the
temperature
Choose appropriate crucible for each
alloy
Microporosities Solidification occurs too rapidly
(when the mold or casting
temperature is too low)
Proper temperature to be maintained
160

161. Type of Failure Procedural Errors Solution
Pits in casting • Debris in mould
• Dirty wax
• Loose debris in crucible
• Mould temperature too hot
• Join ingate, sprue former, and
pattern with continuous smooth
surface with no jagged areas of
investment to be broken off and
pushed into mold ahead of alloy
• Use only clean, new wax for
patterns and sprue formers
• Use clean crucible for each casting
• Lower burnout temperature since
too high temperature causes
investment breakdown, producing
weak surface easily abraded by
alloy
Fins on castings • Over vibration during investment
or disturbing ring during set
• Heating ring too rapidly causes
moisture in mold to form steam
and rupture out mold
• Avoid over vibration and place
investment ring in area free from
disturbances until investment is
completely set
• Heat mold slowly.
161

162. Type of Failure Procedural Errors Solution
Localized shrinkage porosity &
Incomplete casting
• Using a sprue with a very small
diameter
• Discrepancy in temperatures
between the casting ring/ mold and
the molten alloy
• Insufficient casting pressure during
the casting process
• Incomplete elimination of wax and
incomplete venting
• Higher viscosity of the molten alloy
Larger sprue diameter to allow proper
flow of molten metal into the mold
• Sprue to be attached at the bulkiest
portion of the wax pattern directed at
45° to the surface
• Proper balance between mold and
molten alloy temperatures
• Pressure on the arm of the casting
machine should be maintained for at
least 4 sec to allow the alloy to fill the
mould
• A vent of 0.5 mm in diameter should
be provided to allow escape of gases
• Proper casting temperature of the
molten metal to be maintained during
casting
Contamination of the casting The casting held with steel tongs
during pickling
Use of steel tongs should be avoided
162

163. Type of Failure Procedural Errors Solution
Rounded margins • Incomplete burnout of wax
pattern
• Insufficient heating of alloy
before casting
• Margins melted while
attaching pattern to sprue or
former
• Improper diameter/length of
sprue restricts flow of alloy
into mold; metal freezes
before margins are complete
• Heat soak mould for 1 hour
at minimum of 950° F (510°
C) to ensure elimination of
carbon residue
• Heat alloy to 150° F (57° C)
above fusion temperature at
moment of casting alloy
• Keep pattern and sprue
former in horizontal position
when attaching, so that heat
rising from instrument does
not soften pattern
• Average casting should have
10-gauge sprue
approximately 6 mm long
163

164. Type of Failure Procedural Errors Solution
Rough surface on casting • Excess moisture on pattern
• Water-powder ratio too high
in mixing investment
• Too much casting pressure,
causing investment
breakdown from force of
alloy
• Prolonged overheating of
gypsum- bound investment
• Remove all moisture from
pattern, sprue former, and
ring
• Use correct water-powder
ratio
• Do not wind machine too
tightly
• Use correct heating cycle for
burnout procedure; do not
heat above 1290° F (700° C)
164

165. REVIEW OF LITERATURE
165

166. Lombardas P, Carbunaru A, McAlarney ME, Toothaker RW, of Dental S, Oral
Surgery NY. Dimensional accuracy of castings produced with ringless and
metal ring investment systems. The Journal of prosthetic dentistry. 2010 Jul
1;84(1):27 31.
• Lombardas et al did a study to determine “Dimensional accuracy of castings produced with
ringless and metal ring investment systems”
• This study compared the vertical margin accuracy of lost wax castings produced with the
conventional casting technique using a metal ring and a technique that uses a ringless system.
• From copings fabricated on a metal die,
1. 30 castings were produced from a high palladium alloy for metal ceramic restorations
2. 10 castings were cast with phosphate-bonded investment with the ringless technique
3. 10 were cast with the phosphate-bonded investment with a metal ring, and
4. The final 10 were cast using Hi-temp (Whip Mix) phosphate-bonded investment with a metal
ring.
166

167. • The internal surface of the castings were not modified before seating with finger pressure. For
vertical margin discrepancy measurements, an optical microscope at a magnification of 100×
was used. Data were analyzed with 1-way ANOVA (repeated measures) and the Student test.
• When following the manufacturers’ recommendations, the castings of the ringless technique
provided less vertical margin discrepancy (mean value 181 ± 71 μm) than the castings produced
with the conventional metal ring technique (290 ± 87 μm and 291 ± 88 μm). The difference was
significant (P<.001)
167

168. • Within the conditions of this study, the following conclusions were drawn:
1. The vertical margin discrepancy of the ringless group for the buccal, the lingual, and the distal
sites were significantly less than that of the 2 ring groups (P<.001).
2. There was no significant difference of the vertical margin discrepancy between the 2 metal
ring groups.
3. There was no significant difference in the vertical margin discrepancy at the buccal, lingual,
mesial, and distal surfaces within the same group.
4. The ringless technique was clinically acceptable and can be used for the fabrication of fixed
prosthodontic restorations.
168

169. Prabhu KG, Eswaran MA, Phanikrishna G, Deepthi B. Sprue design
alterations and its effect on the properties of base metal alloy castings: An in
vitro study. Journal of pharmacy & bioallied sciences. 2015 Aug;7(Suppl
2):S524.
• Prabhu et al did a study on to evaluate “Sprue design alterations and its effect on the properties
of base metal alloy castings: An in vitro study.”
• To study the effect of various sprue designs on the properties of base metal alloy castings. The
base metal alloys are extensively used for their excellent properties such as an increase in
hardness, high melting range, high elastic modulus, its compatibility with ceramic material and
low cost.
• However, to improve the properties of the base metal alloys is leading to various modifications
in their fabrication procedure – which include sprue designs and their mode of attachment to the
wax pattern.
169

170. • Study compared the effect of three sprue designs on the properties of mass and micro-hardness
of base metal alloy castings. viz.
1. Conical sprue,
2. Cylindrical sprue and
3. Bottleneck sprue
• A prefabricated wax mesh pattern was selected for the fabrication of the alloy test samples in the
study.
• The cylindrical sprue design was connected to the mesh pattern with a straight attachment.
• The bottleneck sprue design was connected to the mesh pattern with a constricted attachment.
• The conical sprue design was connected to the mesh pattern with a flared attachment.
• In this manner, ten samples for each of the three different sprue designs were prepared.
170

171. • Results: The obtained value for mass and microhardness were subjected to statistical analysis.
ANOVA test was performed to determine the difference between the sprue designs.
• Conclusion: The bottleneck sprue, conical sprue, and cylindrical sprue designs did not exert any
apparent influence on the mass and microhardness.
171

172. Earnshaw R. The effect of casting ring liners on the potential expansion of a
gypsum-bonded investment. Journal of dental research. 2010 Nov;67(11):1366-
70.
• Earnshaw R. did a study to determine “The effect of casting ring liners on the potential
expansion of a gypsum-bonded investment”
• A study was conducted on cellulose paper, ceramic paper and asbestos paper of the effects on
the setting and subsequent thermal expansion of a gypsum-bonded cristobalite casting
investment.
• Thermal expansion measurements were made on the same specimens that were produced during
the setting expansion tests.
• Control specimens setting against a smooth dry surface showed a total expansion of 1. 7%.
Specimens setting against dry ceramic liners had similar total expansions, in the range 1. 6 to 1.
7%.
• Specimens setting against either of the wet lining materials showed an increased total expansion
(in the range 2.2 to 2.3%), by virtue mainly of a large increase in setting expansion.
172

173. • Dry asbestos and dry cellulose liners gave higher expansions than pre-wetted ones, since they
abstracted water from the mix, reducing its effective W/P ratio (giving a thicker mix), and then
functioned as wet liners.
• These results suggest that, at least as far as potential investment expansion is concerned, wet
cellulose liners have an effect similar to that of the traditional wet asbestos liners.
• Dry ceramic liners give a much lower investment expansion, and when these liners are used, an
investment with an increased measured expansion could be an advantage.
173

174. Baskaran BE, Prabhu KG, Prabhu R, Krishna GP, Eswaran MA, Gajapathi B.
Casting made simple using modified sprue design: An in vitro study. Indian
Journal of Dental Research. 2014 May 1;25(3):340.
• Baskaran et al did a study on “Casting made simple using modified sprue design: An in vitro
study”.
• Regardless of the alloy used for casting, the casting technique should yield a casted alloy, which
should possess sufficient mass, surface hardness and minimal porosity after casting.
• Twenty patterns for casting were made from three-dimensional printed resin pattern simulating a
3 unit FPD and casted using modified sprue technique.
• Later test samples were cemented sequentially on stainless steel model using pressure indicating
paste and evaluated for vertical marginal gap in eight predetermined reference areas.
174

175. • Marginal gap were measured in microns using Video Measuring System. A portion of the axial
wall of the cast abutments depicting premolar and molar were sectioned and embedded in
acrylic resin and tested for micro hardness and porosity.
• The results obtained for marginal gap, micro hardness, and porosity of all test samples were
tabulated, descriptive statistics were calculated and the values were found to be within the
clinically acceptable range.
• Thus new sprue technique can be an alternative and convenient method for casting which would
minimize metal wasting and less time consuming.
175

176. REFERENCES
• Kenneth J. Anusavice. Philip’s Science of Dental Material. 11th Edition. St. Louis, Missouri:
Saunders Elsevier; 2003.
• Rosenstiel, Land, Fujimoto. Contemporary Fixed Prosthodontics. 5th Edition. St. Louis
Missouri: Saunders Elsevier; 2016.
• Herbert T Shillingburg, David Sather, Joseph Cain, Luis Blanco. Fundamentals of Fixed
Prosthodontics. 4th Edition. Hanover Park, Illinois, USA: Quintessence Publishing Co, Inc;
2012.
• Robert Marrow, Kenneth Rudd, John Rhoads. Dental Laboratory Procedures Volume II. 2nd
Edition. St. Louis, Missouri: Mosby; 1986
• V Shama Bhat, Nandish BT. Science of Dental Materials. 2nd Edition. Delhi: CBS Publishers;
2013
• S Mahalaxmi. Materials used in dentistry. 1st Edition. Haryana: Wolters Kluwer Health; 2013
176

177. • Robert Marrow, Kenneth Rudd, John Rhoads. Dental Laboratory Procedures Volume III. 2nd
Edition. St. Louis, Missouri: Mosby; 1986
• Lombardas P, Carbunaru A, McAlarney ME, Toothaker RW, of Dental S, Oral Surgery NY.
Dimensional accuracy of castings produced with ringless and metal ring investment systems.
The Journal of prosthetic dentistry. 2010 Jul 1;84(1):27 31.
• Earnshaw R. The effect of casting ring liners on the potential expansion of a gypsum-bonded
investment. Journal of dental research. 2010 Nov;67(11):1366-70.
• Prabhu KG, Eswaran MA, Phanikrishna G, Deepthi B. Sprue design alterations and its effect on
the properties of base metal alloy castings: An in vitro study. Journal of pharmacy & bioallied
sciences. 2015 Aug;7(Suppl 2):S524.
• Baskaran BE, Prabhu KG, Prabhu R, Krishna GP, Eswaran MA, Gajapathi B. Casting made
simple using modified sprue design: An in vitro study. Indian Journal of Dental Research. 2014
May 1;25(3):340.
177

178. THANK YOU
178