casting procedures for cast restorations in dentistry

1. CASTING PROCEDURES
DR.ABHIJIT.PALLEWAR
DEPT OF CONSERVATIVE DENTISTRY &
ENDODONTICS

2. CONTENTS
• Introduction
• History
• Steps in casting procedure
• Sprue formers
• Crucible formers
• Casting rings and ring liners
• Investing procedure

3. • Wax burnout
• Casting of alloys into mold
• Cleaning of casting
• Laws of casting
• Conclusion

4. INTRODUCTION
• Taggart permanently changed the practice of restorative dentistry by
introducing his technique for cast gold restorations.
• Cast dental restorations and appliances have become basic treatment
modalities in dentistry.
• They have several advantages over other types of restorations.
• These are types of restorations that are free of mechanical failure.

5. HISTORY
• This meticulous procedure of casting was used to produce jewelry and
ornaments.
• 3500 B.C. – Egyptians first manufactured refractories which in the form of
glass vessels around a refractory core of mud, sand, and animal dung.
• 11th Century  Theophilus  Described lost wax technique, which was a
common practice in jewelry.
• 1558  B. Cellini  have
attempted use of wax and clay for preparation of castings.

6. • 1897  Phillibrook described a method of casting metal filling
• 1907 – William. H .Taggart introduced the lost wax technique in dentistry.
• 1949 – Moore and Walt developed phosphate bonded investment.
• 1959 - Asgar & Peyton stated that flaring should occur at the sprue/wax
pattern junction
• 1959 - Strickland et al stated the importance of the type, shape location &
direction other than the size of the sprue
• 1959 - Morrison and Warmick reported the findings of ethyl silicate
refractory material for dental use.

7. STEPS IN
CASTING
PROCEDURE

8. Wax Pattern Fabrication
• Fabrication of wax pattern is most empirical of all the steps in cast
fabrication and the cause of most casting failures.
• wax used for fabrication of wax pattern is called as inlay wax.
• A dental wax that can be applied to dies to form direct or indirect patterns
for the lost-wax technique used for casting metals or hot pressing of
ceramics.
• Available in different colors and as pellets, sticks and cakes.

9. Composition Of Inlay Wax
PARAFFIN WAX -40-60 Wt%
GUM DUMMAR
CARNAUBA WAX -25 -30 Wt%
CERESIN – 5Wt%

10. Classification Of Inlay Wax
∆ Acc. to ADA Specification No. 4
Type I : Medium wax employed for direct technique
Type II: Soft wax used for indirect technique

11. Procedures For Wax Pattern Fabrication
• There are four methods for producing wax replicas of cast restorations.
1. Create a wax mass then carve in to the restoration shape using hot, sharp
instrument.
2. Incrementally build up the restoration using wax cones , triangles, drops
etc.
3. Fabricate wax pattern directly intraorally.
4. Last method depends upon preoperative anatomic core.

12. Formulation Of Wax Pattern By
Carving
After lubricating the die, all details are overfilled with
wax, covering all margins
The occlusal, facial, lingual and proximal limits of the
pattern are marked and established
Facial, lingual and occlusal anatomy are created
The occluding components of the pattern are checked
and corrected

13. The proximal contour and anatomy are carved.
The wax components of circumferential tie are
marginated.
The pattern is polished using silk cloth and brushes
The occluding components of the pattern are checked
during static and functional relations

14. Incremental Build-up of
Wax Pattern
Cusps with their tips are built and tried in static
and dynamic occlusal contact
Any deficiencies or interferences are corrected
Triangular ridges of cusps and marginal
ridges are added and tried

15. Facial, lingual and proximal contour and
anatomy are established by addition procedure.
Fossae and grooves are completed and their
occluding parts are checked and corrected
The wax pattern is marginated and the surface
is polished

16. Direct Wax Pattern
A matrix is applied on the tooth and a separating medium
is applied on the cavity details and matrix
An inlay wax stick softened and kneaded at its tip to assure
homogenecity
CLOSED ( STURROCKS ) TECHNIQUE

17. The softened tip is inserted into the cavity and kept there
under pressure
Gross excesses of wax are cut off and the occlusal
anatomy is carved with warm sharp instruments

19. The matrix band is released and removed and the pattern
usually comes out with it
The pattern is replaced into cavity and should be
marginated occlusally, facially and lingually using a hot
tine
Holding the pattern with apical pressure the accessible
areas can be polished
Pass a dental tape or floss interproximally to assure wax
adaptability

20. REMOVING THE PATTERN
FROM CAVITY
 Wax pattern should be removed without distortion and maintaining
the path of removal parallel with the direction of the cavity walls.
 The first choice is to use the sprue former in removing the pattern if
this will not stress one area more than another.
 After attaching the sprue to pattern, it is left for 2-3 min and then
trail removal can be carried out.
 Second choice is to use two fingers on both sides of the pattern and
gently remove in the direction of the path with very light pressure.

21.  This method is only used on dies and is most applicable when the
pattern has two or more sides.
 If this method does not seem to insure even stressing, resort to the
third choice.

22. ◘ A copper staple or a 24 gauze twisted brass wire is inserted into the
occlusal part of the pattern.
◘ For MOD patterns the prongs of the wire are placed near the marginal
ridges and for MO/DO patterns, the prongs are inserted at an acute
angle of 450 near marginal ridge.

23.  Using this staple the wax pattern is lifted up with a direct
pull parallel to cavity walls.
 The staple is removed by holding it with warm pliers to
melt the wax holding the staple.

24. SPRUEING
• SPRUE FORMER: A wax, plastic or metal used to form
the channel or channels which allows molten metal to flow
into a mold to make casting.
SPRUE FORMER

25. • Main objective of sprueing is to create an inlet for the wetting melt into the
investment mold created by the wax pattern.
• This inlet should direct the flow and velocity of cast material so as not
deform any internal details of the mold.
• Attachment of sprue former to the wax pattern should be done while
pattern is on die or tooth in order to assure the confinement of wax pattern.

26. Types And Characteristics Of Sprue Former

27. • Sprue former surfaces should be very smooth to prevent any irregular inlet
walls.
• Sprue former materials should not rust or react with any ingredient of the
environment.
• Sprue formers should be perfectly cylindrical in shape so as to create a
perfectly rounded inlet for the mold.

28. Diameter Of Sprue
• Diameter of the sprue is the most important factor in dictating the speed with
which melt enters and fills the mold.
• Melt velocity is directly proportional to the sprue diameter.
• For most of cast materials diameter of sprue should be greater than the
thickest part of wax pattern.
• Generally sprue former diameter can be between 8-18 gauge ie. (0.8mm –
8mm)
• Usually  for molar and metal ceramic restoration - 10-gauge (2.5mm)
 Premolars and partial coverage restoration - 12-gauge (2.0mm)

29. Length Of Sprue
• Major factor governing the sprue length is the length of the ring.
• Length of sprue should be in such a way that end of wax pattern that is
away from the sprue should be 6mm or 1/8th -1/4th inch from end of the
ring.
• This will allow minimum thickness of investment that can withstand both
melt impact and allow the escape of the mold gases.

30. Location Of Sprue Formers
• Sprue formers should be attached to bulkiest part of wax pattern which
have advantages like:-
1. This will minimize the effect of released residual stresses by heat of
attaching the sprue.
2. This will insure thinner cross-section of the mold will be completely
filled.
3. Melt will always be fluid enough and available until all lesser dimension
sections are completely filled.
• Proximal surfaces are the ideal location for sprue attachment.

31. Angulation Of Sprue Former
• Sprue should never be directed towards thin, delicate cross sections of the
wax pattern.
• The melt flow should not hit these area of the mold at 90° to avoid fracture
failure.
• It should be directed away from or at 45° to these details.

32. Attachment Morphology
• The attachment of sprue former to the wax pattern should be smooth and do
not posses pits or irregularities.
SPRUEING
Direct indirect

33. Reservoir
• Reservoir is a small amount of additional wax added to the sprue former
1-2mm below the wax pattern in order to create an area in the mold.
• It is always indicated when sprue is long and/or thin or for any reason that
could interrupt flow of melt before all mold details are filled

34. Venting
• Vents are the additional sprues placed at thin or thick wax patterns to
improve the quality of the casting.
• In some situations , speed with which mold gases does not escape relative
to the speed the melt is entering , may lead to defects in the mold.
• In these situations or cases require attaching a wax rod to furthest or
close to furthest part of pattern which will stop short of ring surface.
• Gases that doesn’t not escape fast enough ahead of ingressing melt will be
compressed and trapped in these vents.

35. Crucible Former
 They are available as:
Rubber, Metallic and Plastic.
 They are of two types:
1) Steep sided cone- to cast metal using centrifugal casting force
2) Shallow cone- cast metal using stream or air pressure
 Wax pattern with its sprue former is attached to crucible former in a
special locking area.
 The junction between sprue and crucible former should be covered with
wax and carved to obtain proper bulk and flaring.
 Crucible former surface and its junction with sprue former are
meticulously smoothed to create as smooth a crucible as possible.

37. Casting Rings
• Casting rings are usually solid metal rings in
different diameters.
• Casting rings are used to confine the fluid
investment around the wax pattern while the
investment sets.
• These solid metal rings do not allow for
compensation of casting shrinkage.

38. They are available as:
1) Shapes - Round
- Oval
2) Complete rings –
Rigid - Metal
- Plastic
Flexible - Rubber
3) Split rings - Metal
- Plastic

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

40. Casting Ring Liner
 Materials used are:
- Asbestos liner
- Cellulose paper
- Ceramic liner
- Combination of ceramic and cellulose liner
 Uses:
1. Prevents loss of heat while transfering the casting ring from burnout
furnace to casting machine.
2. Allows mold expansion toward periphery of the ring
3. Helps in divesting .

41. Treatment Of Wax Pattern
• Once the sprue is attached to wax pattern,it is carefully removed from
model and attached to crucible former.
• The pattern is cleaned with diluted synthetic detergent or commercially
available wax pattern cleaner to remove any debris and to decrease the
surface tension and increase surface energy of the wax.
• This allows better wetting of pattern by investment material.
• The pattern is left to air dry after which investment is poured in to the ring.

42. INVESTMENT MATERIALS
• There are three main types of investment materials used based
on the binder used or present :
1. Gypsum bonded investment .
2. Phosphate bonded investment .
3. Ethyl silicate bonded investment .

43. COMPOSITION
• GYPSUM BONDED
INVESTMENT
1. Refractory material:-
quartzor crystoballite
65%-75%.
2. Binder:- ἀ -hemihydrate
form of gympsum 25-35
%
3. Chemical modifiers:- 2-
3%
• PHOSPHATE BONDED
INVESTMENT
1. Refractory material:-
silica in form of quartz
or crystoballite 80%
2. Binder:- phosphate
3. Chemical modifiers:-
2-3%
4. Liquid: water or
colloidal silica in water.
• ETHYL SILICATE BONDED
INVESTMENT
1. Refractory material:-
powdered quartz or
crystoballite and
magnesium oxide
2. Binder:- silica gel in
form of sodium silicate
and colloidal silica
(ethyl silicate)
1. Chemical modifiers:- 2-
3%

44. Setting Expansion Of Investment
Material
• Wax shrinkage + casting shrinkage = setting expansion +
hygroscopic expansion + thermal expansion.
• Normal setting expansion: its linear expansion occurs during
normal setting of investment in presence of air.
• Hygroscopic expansion : expansion that occurs when the investment
is allowed to set in presence of water.

45. • Thermal expansion : during burnout procedure for gypsum bonded
investments , the temp. is gradually elevated till 700°C for 20 -30 mins and
for phosphate bonded investments the temp is elevated till 1050° for 20-30
mins. So during this process due to heat , refractory material may undergo
some changes leading to expansion.

46. Methods of Investment
• There are 2 methods of investment :
1. Manual investing
2. Vacuum investing
• MANUAL INVESTING:
 Water and powder are incorporated in the appropriate ratio in a
rubber bowl with a hand spatula using rubbing motion and slowly
rotating the bowl during the mixing period.

47.  Mixing is carried for 1 min and the mix is placed on a vibrating table and
stirred slowly for 30 seconds to remove any entrapped air.
 The investment is carefully applied to the pattern using a small brush
starting at one place and carrying the ivestment forward until the pattern
is completely covered.
 Investment is allowed to set partially and then rest of lined ring is then
filled with investment.

48. Vacuum Investing
• In this technique, the investment is mixed and the
pattern is invested under vacuum.
• The investment and water are measured as usual.
• The mix is placed in a specially constructed mixing
unit and unit is motor driven.
• The air is evacuated by means of a tube attached to
the mixing unit from a vacuum pump.
• During mixing , air is evacuated and possibility of
bubble formation is thereby reduced.

49. Thermal Treatment Of The Investment Wax
Pattern Complex
• Its is also termed as wax burnout or wax elimination procedure.
• Burnout or wax elimination is heating of the casting ring with the
invested wax pattern in a furnace with a temperature- controlled thermostat
until all traces of wax are vaporized or eliminated leaving a cavity or mold
in which molten metal can be cast.
• Once the wax pattern is invested, the investment is allowed to set for
approximately 1 hour for both gypsum and phosphate bonded investments
before the burnout procedure is carried out.

50. Burnout Temperature For Various Investments
Type of
investment
Type of expansion Maximum temp. time
Gypsum bonded hygroscopic 500°C 1 hr
Gypsum bonded Thermal 700°C 1 hr
Phosphate
bonded
Thermal 700°C-1030°C 45 min
Ethyl silicate
bonded
Thermal 1090°C-1180°C -

51. Preparation Of Ring Before Placement In
Furnace
• Before casting ring with invested pattern is
placed in the furnace , the crucible former
and any metal sprue former if used should
be removed carefully.
• For phosphate bonded investments , the
smooth skin that forms on the ring is
removed and all loose particles r debris
present in crucible area are carefully blown
with compressed air.

52. Placement Of Ring In Burnout Furnace
• The invested casting ring is placed in a room
temperature furnace and heated to required
temperature.
• To allow molten wax to flow out, the casting ring is
first placed on a ribbed ceramic tray in the furnace
facing down to drain out wax.
• The temperature is then increased gradually and when
temp. reaches around 300°C the ring is now inverted
with sprue hole placed upward to allow circulation of
oxygen with in oven into mold cavity and react with
wax to form gases.

53. Wax Elimination
• During burnout some of the melted wax gets absorbed by the porous
investment.
• Therefore its is advisable to perform the burnout procedure when the
investment is still wet because water present in pores of investment
prevents absorption of the wax.
• Also when water evaporates it flushes the wax from mold.
• When high heat technique is used the mold temperature generates enough
heat to convert carbon to CO or CO2 which then escapes through pores in
investment.

54. CASTING
• Casting can be described as an object formed by the solidification of a
fluid that has been poured or injected into a mold.
• The procedure involves three steps and each has certain objectives and
goals:-
1. Fusing the alloy or thermoplastically softening the ceramic material.
2. Transporting the thermally treated investment to the casting machine.
3. Forcing the melt into the investment mold.

55. Fusing The Alloy Or Thermoplastically
Softening The Ceramic Material
• Fulfilling these objectives requires :
1. A proper energy source
2. A proper container in which cast material may be fused or softened.
3. A proper environment for casting.
 Proper energy source: there are variety of energy sources but basically
there are two types which are used:
A. Gas fuels
B. Electric energy

56. • GAS FUELS :- these are usually produced by one of three mixture of
gases.
I. Natural gases( mainly propane) and air : this supplies lowest temp of
all sources and is efficient for class I and II cast alloys.
II. Natural gases and oxygen mixture: this supplies high temp and can be
used for class I,II and III cast alloys.
III. Acetylene and oxygen mixture: this the hottest of all gas fuels and can
be used for all types of alloys.

57. • Fusing energy using gas fuels necessitates blow torch.
• Torches are of two types depending upon the tip:
 Single orifice tip:- these concentrate more heat in one area.
 Multiorifice tip: these torches distribute heat over wide area enabling
uniform heating of alloy.
• These torches are used for casting metal ceramic frameworks.
• A typical torch flame is generated either by mixture of natural gas and air,
propane and air ,acetylene and air or acetylene and oxygen.

58. Parts Of The Flame
• When proper proportions of gas and air are mixed the flame produced
shows distinct zones of flame or combustion areas.
• Zone 1: the first long cone emanating directly from nozel of the torch is the
area in which air and gas are mixed just before combustion. Hence no heat
is present in this zone.

59. • Zone 2: the next zone immediately surrounding the inner core which is
green is known as combustion or oxidizing zone. Here gas and air are
partially burned. Care should be taken to keep these two zones away from
alloy during melting.
• Zone 3: the next zone , the reducing zone is dimly blue and located just
outside the green combustion zone. The is the hottest part of the flame and it
should be constantly in contact with the alloy during melting.
• Zone 4: the outermost zone is again oxidizing zone , where combustion
occurs with oxygen in the air. This part of the flame is also not used to melt
the alloy since the temperature is lower than that of reducing zone and it
also oxidizes the alloy.

60. • Electric energy: heat produced from electrical heat sources is the most
controlled, efficient and easily used.
• Its is produced through induction or resistance.
• Induction heat is the most efficient and are only heat sources that can be
used for cast ceramics and class IV alloys.

61. A proper container in which cast material may be fused or
softened: -
• Casting crucibles are the containers in which cast material may be fused or
softened.
• Crucibles should not cause reaction with alloy resulting in contamination of
alloy
• Crucible should withstand the temperature of melting torch and molten
metal.
• Many casting crucibles are available e.g. clay, carbon,quartz and zirconia-
alumina.

62. Type of crucible Type of alloy
Clay High noble and noble alloys
Carbon Noble crown and bridge alloys
Alumina High fusing alloys
Quartz or silica High fusing alloys

63. Transporting the thermally treated investment to the
casting machine.
• The investment mass after heating in an oven has to be carried to the
casting machine while it is at its highest temperature.
• This should be done using forceps, tweezers or specially designed tongs.
• The process has to be performed gently , swiftly and quickly with the
extension of flask for moldable ceramics or crucible part of investment
facing downwards to allow gravitational drop of any loose investment
particles in mold.

64. Forcing the melt into the investment mold.
• This can be achieved using casting machines.
• The function of casting machines is to accelerate the molten metal into
mold by centrifugal force , under air pressure or vacuum or combination of
both.
• All the casting machines operate on the basic principle of centrifugal force
which is used to drive the molten metal into the mold.

65. • Casting machines can be classified as:
1. Centrifugal casting machine
2. Electrical resistance heated centrifugal casting machine
3. Induction casting machine
4. Direct current arc melting machine.
5. Vacuum or pressure assisted casting machine.

66. Centrifugal Casting Machine:
• Casting is done by melting alloy in separate crucible and the molten metal
is forced into the mold by simple centrifugal force.
• The crucible is attached to free arm of the casting machine.
• The casting machine spring is wound and locked.
• Once the heated casting ring is in place and the alloy is melted to casting
temperature, spring is released which initiates rotational motion.

67. Electrical Resistance Heated Centrifugal Casting
Machine.
• The alloy is placed in a graphite or ceramic crucible and heated with the
help of a resistance based heating unit.
• The crucible in this type of machine is located flush with casting ring.
• This allows alloy to stay molten for slightly longer time which ensures
continuous flow of molten metal and proper solidification from tip of
casting to button surface.

68. INDUCTION CASTING MACHINE
• The alloy is melted in an electric induction furnace within the crucible
surrounded by water-cooled metal tubing.
• Once the alloy reaches casting temperature in air or in vacuum, it is cast
into mold by centrifugal force.

69. Direct Current Arc Melting Machine
• The alloy is vacuum melted and cast by pressure in an argon atmosphere.
• A direct current arc is produced between two electrodes , the alloy and
water cooled tungsten electrode.
• Since temperature within the arc exceeds 4000°C the alloy melts very
quickly.
• This has a disadvantage since the alloy can become overheated within
seconds.

70. Vacuum or pressure assisted
casting machine.
• Titanium and its alloys require vacuum arc heating argon pressure casting
machines.
• Melting point of pure titanium is 1671°C and other casting alloys have
liquidous temp below 1500°C.
• To prevent absorption of gases in its molten state titanium is cast in the
protective atmosphere of argon or in vacuum.
• In such high temp melting either a graphite or water cooled crucible is
used.

71. Cleaning the casting from its encasing
investment and surface deposits
• For casting alloys after solidification of the molten alloy, the investment
mass is dropped in to room temperature water.( QUENCHING)
• The water is absorbed into the investment pores will undergo evaporation
within hot mass.
• Steam in large amounts will be produced, crackling the investment into
small pieces and peeling it off the casting.
• Remainder of attached investment can be removed , using a brush or sharp
explorer.

72. • Castings using phosphate bonded investments are not quenched in water
but benched cooled.
• The residual investment is removed from casting using sandblasting with
fine alumina and using an ultrasonic cleaner.
• Once removed casting usually exhibits dark and tarnished surface covered
with oxides.
• The film of oxides can be removed by a process called PICKLING.
• Pickling is a procedure of heating discolored casting in an acid such as
hydrochloric acid( for gypsum bonded investments)
• HCL removes the oxide coating as well as any residual investment.

73. Finishing And Polishing
• Gold being a soft metal need to be finished using slow speed.
• Initial contouring is done with carbide burs, silicon carbide green stones or
heatless polymer stones.
• Finishing is done with aluminum oxide stone or medium grade
impregnated rubber wheels and points( brown and green).
• Final finish is accomplished with finer abrasive impregnated rubber cups ,
wheels and points and polishing is done with a tripoli or rouge with
leather wheels.

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

75. Attach the pattern Sprue former to the thickest part
of the wax pattern
Orient the wax pattern so all the restoration margins will face
the trailing edge when the ring is positioned in the casting machine
Position the wax pattern in a “cold zone” of the investment mold
and the reservoir in the “heat centre” of the casting ring

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

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

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

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

80. Cast toward the margins of the wax patterns
Do not quench the ring immediately after casting

81. REFERENCES
• Text book of materials used in dentistry –s.mahalaxmi
• Text book of operative dentistry –M.A.Marzouk
• Phillips science of dental materials – 1st south Asia
edition
• Text book of Prosthodontics – Shillingburg
• Text book of operative dentistry – Vimal K sikri 4th
edition