Casting Procedures and Defects

PROCEDURES
AND
DEFECTS
1
Dr.T.Vivek , I MDS ,Department of Conservative Dentistry And
Endodontics, JSS Dental Hospital, Mysore.

CONTENTS
•INTRODUCTION
•HISTORY
•DEFINITIONS
•LOST WAX TECHNIQUE
•FABRICATION OF WAX PATTERN
•WAX PATTERN REMOVAL
•EVALUATION OF WAX PATTERN
•SPRUING THE WAX PATTERN
•VENTING
•RESERVOIR2

•CRUCIBLE FORMER
•CASTING RINGS AND LINERS
•RINGLESS CASTING SYSTEM
•INVESTMENT PROCEDURES
•COMPENSATION FOR SHRINKAGE
•CASTING PROCEDURES- Burn Out
Casting Machines
•RECOVERY AND FINISHING OF CASTING
•CASTING DEFECTS
•CONCLUSION
3

INTRODUCTION
 The restoration of lost tooth structure, the replacement of teeth
by means of fixed and removable partial denture prosthesis,
and full mouth restorations, in fact the practice of dentistry with
our present-day concept would virtually be impossible without
the dental casting process.
 The casting procedure is used to make dental restorations such
as Inlays,Onlays,Crowns,post &core and ceramics.Because
castings must meet stringent dimensional requirements,the
casting process is extremely demanding.
 In dentistry the resulting casting must be an accurate
reproduction of wax pattern in both surface details and overall
dimension.Small variation in investing or casting can
significantly effect the quality of the final restoration.
4

HISTORY
 3000 B.C Copper was cast by Mesopotomians.
 2500 B.C Dental prosthesis fabricated from gold
wire was found in egypt.
 500 B.C Etruscans made bridges of soldered gold
bands.
 1571 Benvenuto cellini has done casting of both
gold and bronze
 1897 Philbrook decsribed a method of casting metal
into a mold formed from a wax pattern for restoration of
posterior tooth.
5

HISTORY
 1907 William Taggart introduced “LOST WAX
TECHNIQUE.” But he was unable to overcome the problem of
casting shrinkage.
 1908 Lane proposed the idea of casting into an enlarged
mold which he achieved by using an investment containing
high % of silica, heated upto 6500c. He was thus first to
introduce mold expansion as a compensation technique.
 1910 Van horn discovered the value of wax expansion.
 1930 Carl Scheu, discovered the phenomena known as
Hygroscopic setting expansion.
6

DEFINITIONS
• The technique is so named because wax pattern of a
restoration is invested in a investment material, then the
pattern is burned out (lost) to create a space into which
molten metal is placed or cast.
• The mold channel through which molten metal or
ceramic flows into the mold cavity.
• The base to which sprue former is attached while
the wax pattern is invested in refactory investment
7

DEFINITIONS
• Surrounding the wax pattern with a material that
can accurately duplicate its shape and anatomic
features.
• It is the process of heating an investment mold
to eliminate the embedded wax or plastic
pattern.
• It is the process in which molten material is
thrown into a mold and allowed to harden into
the shape of the mold.
8

DEFINITIONS
• Process of rapidly cooling the hot casting by
dipping it in water bath.
• Process of removing the investment from a
casting.
• Removal of surface film containing oxides,which
involves heating the casting in an acid.
9

LOST WAX TECHNIQUE
10
Quenching and Pickling
Molten metal is cast into the void created by the wax pattern and the sprue
Invested pattern is heated until all the remnants of wax are burned away
The pattern and sprue are invested
All aspects of final restoration are incorporated into the wax pattern
Wax pattern is first formed on a die or directly on the tooth.
Sprue is removed and the casting is polished and delivered to the patient.

FABRICATION OF WAX PATTERN
 The wax pattern is the precursor of the finished cast
restoration that will be placed on the prepared tooth.
 A few extra minutes spent on the wax pattern can often
save hours that might be spent correcting the casting.
Requirements of a good inlay wax:
1. It must flow readily when heated, without chipping, flaking
or losing its smoothness.
2. When cooled, it must be rigid.
3. It must be capable of being carved precisely, without
chipping, distorting or smearing.
11

DISTORTION
•Stresses occur in the inlay wax as a
result
of the heating and manipulation of the
wax during fabrication of the pattern.
•Wax “relaxes” as these stresses are
released.
To Minimise the distortion :
•Patterns should never be left off the
die.
•Invested as soon as possible after
fabrication.
12

Two fundamental ways to prepare a wax pattern
for a dental restoration:
 DIRECT METHOD:
In this method pattern is prepared on tooth in the
mouth.
Type I wax(medium) is formulated for making
direct wax pattern.
13

INDIRECT METHOD:
 In this method a die of tooth is first made,
and the pattern is made on the die.
 Type II inlay wax(soft) is used.
 The die is lubricated with a wetting agent.
 Die spacer is used.
The wax pattern is constructed by:
 Dipping method:
 To develop a thin, uniform and adapted layer
of thin wax on the die. This is done by dipping the
die into wax that has been thoroughly melted.
14

Addition method:
 This is done by melting the wax and dropping it on the die
using a heated wax instrument until complete building of
the pattern, and then carved by sharp carver.
 Drawback : Stress collected from multiple addition of wax
tends to release with time and subsequently distort the
wax pattern.
No. 7 Waxing spatula for initial coping
15

WAX INSTRUMENTS
P.K.Thomas No.1 and 2 Wax addition instruments
P.K.Thomas No.3 Burnisher
P.K.Thomas No.4 and 5 Wax carvers
16

Electric Wax Instruments
Advantages: 1) Precise Temperature control of the wax for proper manipula
2) Carbon buildup can be kept to a minimum.
17

WAX PATTERN REMOVAL
There is a definite possibility of stressing the wax pattern
while removing it from the die or the tooth, so an effort
should be made to make these stresses even.
 Along with the sprue former.
 Explorer tip.
 Staple like wire( MOD pattern).
 Small loop of gold zephyr wire.
Indirect Finger PressureWashed Rubber DamDirect finger pressure
18

EMERGENCE PROFILE- Stein and Kuwata
The line between the base of gingival
sulcus and height of contour
described as emergence profile, which is
a straight line which faciliates, access for
oral hygiene measures
21

SPRUING THE WAX PATTERN
The purpose of the sprue former, or sprue pin, is to provide a channel
through which molten alloy can reach the mold in an invested ring after the
wax has been eliminated.
Basic requirements of a SPRUE:
 It must allow the molten wax to escape from the mold.
 It must enable the molten metal to flow into the mold with as little
turbulence as possible.
 The metal within it must remain molten slightly longer than the alloy that
has filled the mold. This provides a reservoir to compensate
for the shrinkage that occurs during solidification of
the casting.
22

Materials Used for Sprue
WAX METAL PLASTIC
•Commonly used
•Most preferred
•Easy to manipulate
•Easy to burnout
•Lacks Rigidity
•Stronger than a wax sprue
•Mostly used for small inlays
•They are often Hollow
•It cannot be burned out,
removed along with crucible
•Has the rigidity of the metal
•Minimised Distortion
•May block the escape of the
wax
•Hollow plastic sprues are
available that allow the
escape of the wax.
23

• The gauge selection and design for the sprue former are
often empirical, but the optimal performance during the
casting process is based on the following general principles
:
 Sprue Diameter
 Sprue Position
 Sprue Attachment
 Sprue direction
 Sprue Length
 Venting
 Reservoir
24

SPRUE DIAMETER
• It should be of approximately the same size as the thickest
area of the wax pattern
Too large- Distortion
Too small- Premature solidification
Sprue former Diameter
• For small inlay - 1.5mm
• For large inlay - 1.7mm
• For Onlay - 2.1mm
• For large pattern - 2.6mm
10
14
25

SPRUE LOCATION
Site of prefer placement are on the:
• Bulkiest Non-functional cusp tip (Non-critical part)
• Marginal ridge
• Proximal contact
26

SPRUE
ATTACHMENT
• It should be attached to the portion of the pattern with
largest cross sectional area
• The sprue’s point of attachment to the pattern should be
smooth to minimize Turbulence.
• Generally it must be Flared- Facilitating the entry of the
alloy.
27

SPRUE DIRECTION
• The sprue former should be directed away from any thin or
delicate parts of the pattern, because the molten metal may
abrade or fracture the investment in this area.
• SHOULD NOT BE ATTACHED AT A RIGHT ANGLE- TURBULENCE
UNEVEN EXPANSION
SUCK BACK
POROSITY
So, the sprue former is attached at an angle (450C)
to allow the molten metal to flow freely to
all the portions of the mold.
Mold Impedance to flow Mold
450
900
28

SPRUE LENGTH
• Depends on the length of the casting ring.
Too short- Gases cannot be adequately vented
Too long- Pattern will be too close to the ring,and the
molten alloy may break through the investment during
casting.
Therefore the sprue length should be adjusted so that the
top of the wax pattern is within 6mm (1/8”-1/4”) from the
open end.
29
BACK UP INVESTMENT

VENTING
• In some situations, there is some doubt about the speed
with which the mold gases will escape relative to the
speed the melt is entering. This could be due to:
 Considerable thickness of Investment walls surrounding
the pattern.
 High density of Investment.
• Small auxillary sprues or vents have been recommended
to improve the casting of thin patterns.
• A wax rod is attached to the farthest part of the wax
pattern.
30

• Their action may help gases escape during casting and
ensure that solidification begins in critical areas by acting
as HEAT SINK
• Gases that will not escape fast enough ahead of
ingressing metal will be compressed and trapped in these
vents.
31

RESERVOIR
• Patterns can be sprued :
Direct - Connection between the pattern and the crucible
former
Indirect - A reservoir is placed between pattern and crucible.
It should be at a distance of 1-2mm from the wax pattern.
It prevent localized shrinkage porosity in the casting.
The resulting solidification shrinkage occurs in the reservoir bar and not
in the prosthesis.
32

CRUCIBLE FORMER
• The crucible part of the investment is funneled shaped
component that is connected to the sprued wax pattern.
• It comes in different Materials, Sizes, inclination, diameter.
• The deeper the crucible is and more the inclined its walls
are, the more velocity will be imparted to the melt on its
way to the mold.
Crucible for moldable ceramics are metallic and have
the
exact dimensions as the pellet of the raw ceramic which
will
be injected.
33

CASTING RING AND LINERS
Casting ring is use to invest the wax pattern in the investment
medium.
Types
Metal rings.
Rubber rings.
Split rings.
With the use of solid metal rings or casting flasks, provision
must be made to permit investment expansion.
The mold may actually become smaller rather than larger
because of the reverse pressure resulting from the
confinement of the setting expansion.
This effect can be overcome by using a split ring or flexible
rubber ring that permits the setting expansion of the
investment. 34

Setting Expansion
Metal Rings Confinement Reverse Pressure
Mold become small
SOLUTION : Use of Split Ring
Flexible Rubber Ring
Ring liners
35

• A resilient liner is placed inside the ring to provide a buffer of
pliable material against which the investment can expand to
enlarge the mold.
• NO LINER- Expansion is towards the center of the mold thus
distorting the casting
Types
Asbestos Ring Liner- No longer used (Carcinogenic)
Cellulose paper
Fibrous ceramic aluminous silicate
Ceramic cellulose combination.
NON-ASBESTOS
GC New Casting Ring liner
Nobil-liner
36

Ring liners are placed to ensure uniform expansion in form
of
• Normal setting expansion.
• Semi hygroscopic expansion- Wet liner.
Minimum Thickness of the
liner should be 1mm.
37

• The liner should fit the inside diameter of the casting ring
with no overlap.
Liner 3.25mm short of
Casting ring- provides a “lock”
38

RINGLESS CASTING SYSTEM
• This system provides maximum expansion of the
investment.
• The tapered plastic rings allow easy removal of the
investment after the material has set.
• It is suited for the casting of alloys that require greater
mold expansion than traditional gold-based alloys.
PowerCast Ringless system( Whip-mix Corporation) 39

Preparation of wax pattern
The wax pattern should be cleaned of any debris,
grease, or oils.
A commercial pattern cleaner or a diluted synthetic
detergent is used.
40

Cleaning the debris is followed
by the application of Debubblizer.
The pattern should be invested
as soon as possible, after it is
removed from the die, and it
should not be subjected to warm
environment during this interval.
In any case it should not stand
more than 20-30 min before
been invested.
41

Investing is the process by which the sprued wax pattern is
embedded in a material called an investment.
An Investment must fulfill these requirements:
 The investment must be able to withstand the heat of the
burnout and the actual casting of the molten alloy.
 It must confirm to the pattern in a way
such that the size and surface details
are exactly reproduced.
 It must expand sufficiently to
compensate for the solidification shrinkage of the alloy.
43

• In general, there are 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. 44

• Gypsum bounded is further classified by ADA
specification no 2 (casting investment) into two
types depending upon the expansion required to
compensate for the contraction of the molten gold
alloys during solidification
• TYPE I: THERMAL EXPANSION TYPE
For casting of inlays and crowns
• TYPE II: HYGROSCOPIC EXPANSION TYPE
For casting Inlays , Onlays and Crowns
• TYPE III: Used in construction of partial dentures with
gold alloy.
45

GYSPUM BONDED INVESTMENT
BINDER
Calcium Sulphate
hemihydrate
• 25-45%
Refractory
filler(Silica)
• Quartz
• Cristobalite
• Trydimite
• Fused Quartz
46

REDUCING
AGENTS
• Carbon or powdered
copper – Provide
NON-OXIDISING
atmosphere
BORIC
ACID/
NaCl
• Regulate the setting
expansion and
setting time
• Prevent detrimental
shrinkage of
gypsum
OXALATES
• Prevent Sulphur
contamination
MODIFIERS
47

• Mainly used in casting Gold alloys with melting ranges
below 10000c
• When quartz ,tridymite or cristobalite is heated, a change
in crystalline form occurs at a transition temperature
characteristic of the particular form of silica.
 Quartz 5750c B-Quartz
 Cristobalite 200-2700c B-Cristobalite
The density decreases as the  form changes to the B
form with a resulting increase in volume that is
evidenced by a rapid increase in the linear expansion .
48

PHOSPHATE BONDED INVESTMENT
BINDER
Magnesium
Oxide(base)
Ammonium
phosphate(acid)
Refractory
filler(Silic
a)- 80%
• Quartz
• Cristobalite
• Trydimite
• Fused Quartz
OTHER
AGENTS
Colloidal
Silica
50

Basic
reaction
Polymerisation
Heated about 300c
ammonia is released
Complete Heating 1040c
NH4H2PO4+MgO+5H20 NH4MgPO4+6H2O
Mg2. P2O7 + 2NH3 + 13H20
Forming Colloidal multimolecular
aggregate around excess MgO and fillers
Mg3(P2O4)2
crystaline form
51

Ethyl silicate bonded investments
• This type of investments loosing popularity because of
the more complicated and time consuming procedures
involved, but it is still used in the construction of the high
fusing base metal partial dentures alloys.
• In this case, the binder is a silica gel that inverts to silica
(cristobalite) on heating. Several methods may be used
to produce silica or silicic acid gel binder.
1.
Magnesium Oxide is added to increase strength
Na SiO2 + acid or acid salt bonding silicic acid
gel
53

Aqueous solution of colloidal silica + ammonium chloride silica gel
Hydrolysing ethyl silicate + HCl + ethyl alcohol+water colloidal sol of polysilicic acid
A coherent gel of polysilicic acid then forms which is
accompanied by a setting shrinkage.
This soft gel is dried at a temperature below 168 C (334F).
during drying, the gel loses alcohol and H2O to form a
concentrated hard gel---green shrinkage and it occurs in
addition to the setting shrinkage.
54

BRUSH TECHNIQUE
When the investment reaches the level of the pattern, tilt the ring several times to
cover and uncover the pattern, thereby minimizing the possible entrapment of air.
Coat the entire pattern with investment, pushing the material ahead of the brush from
a single point.
Attach the vacuum hose to the bowl, evacuate the bowl, and mechanically spatulate.
Add investment powder to the liquid in the mixing bowl
In this technique, pattern is first painted with surface tension reducer; the surface must
be wet completely.
55

GRINDING THE INVESTMENT
• Carefully Grinding(on a model trimmer) or scraping the
shiny skin off the end of the investment just prior to burn
out.
• Removes the impervious layer, opening the pores of the
investment and facilitating gas release as the alloy is cast
into the mold.
58

COMPENSATION FOR
SHRINKAGE
Wax Pattern
Expansion-Negligible
Setting Expansion
Thermal Expansion
1-2%
Casting Ring
Liners
Ringless Casting
System
Hygroscopic
1.2-2.2%
Normal
0.4%
WAX SHRINKAGE= 0.35% GOLD ALLOY SHRINKAGE= 1.25% BASE-METAL ALLOY= 2.4%
•Wet Liner
•2 Liners
Controlled water
added technique
59

• For gypsum bonded investment
– 500°C for hygroscopic technique.
– 700°C for thermal expansion technique.
• Phosphate bonded
– 700°C to 1030°C.
61

HYGROSCOPIC LOW HEAT TECHNIQUE
• Obtains compensation expansion from 3 sources.
37°C water bath expands wax pattern.
Warm water entering the investment mold from top
adds some hygroscopic expansion.
Thermal expansion at 500°C provides needed thermal
expansion.
62

ADVANTAGES
• Less investment degradation.
• Cooler surface for smoother castings.
• Convenience of placing molds directly in 500°C
furnace.
• Care taken for sufficient burnout time.
• The molds should remain in furnace for atleast 60
min.
DISADVANATAGE:
– Back pressure porosity great hazard in low heat technique.
63

 Standardized hygroscopic technique was developed
for alloys with high gold content; the newer noble
alloy may require slightly more expansion. This
added expansion may be obtained by making 1 or
more of following changes.
1. Increasing water bath temperature to 40°C.
2. Using two layers of liners.
3. Increasing burnout temperature to a range of
600°C to 650°C.
64

HIGH-HEAT EXPANSION TECHNIQUE
• Depend almost entirely on high-heat burnout to obtain
the required expansion, while at the same time
eliminating the wax pattern.
• Additional expansion:
– Slight heating of gypsum investments on setting. Thus
expanding the wax pattern.
– Water entering from wet liner adds a small amount of
hygroscopic expansion to the normal setting
expansion.
65

GYPSUM INVESTMENTS
• Fragile and require use of metal ring.
• Slowly heated to 600°C to 700°C in 60 minutes and held for 15
to 30 minutes at the upper temperature.
• Slow Rate of heating:
– Smoothness.
– Good Overall dimension of investment.
• Too rapid heating:
• Cracking of investment (outside layer expands >center
section).
• Radial cracks interior to outward.
• Casting with fins or spines.
66

• Investment decomposition and alloy contamination is
related to the chemical reaction between the residual
carbon and sulfate binder.
• Reduction of calcium sulfate by carbon takes place
rapidly above 700°C.
CaSO4 + 4C  CaS + 4CO
3CaSO4 + CaS  4CaO + 4SO2
• This reaction occurs when gypsum investment are
heated above 700°C in presence of carbon. SO2 as a
product of this reaction contaminates gold castings and
makes them extremely brittle.
67

Burn Out of Phosphate Bonded
• Obtain their expansion from following sources.
– Expansion of the wax pattern.
– Setting expansion (because of liquid used).
– Thermal expansion
• Phosphate investments much harder and stronger than
gypsum investments.
• Burnout temperature range from 750°C to 1030°C.
• It is necessary to use a slow heating rate to prevent
cracking.
Two stage burnout
-Placed directly in furnace at 200-3000C
-Held for 20-30 min. then cast
68

• Alloys are melted in one of the 4 following ways:
In a separate crucible by a torch flame, and cast
centrifugally.
Electrically by a resistance heating or induction furnace.
Centrifugally by motor or spring action.
By induction heating. Centrifugally by motor or spring
action.
Vacuum arc melted. Cast by pressure in argon
atmosphere.
70

Parts of Torch Flame
 Melted by placing on inner sidewall of crucible.
 Fuel used -Natural gas(Propane)+ Oxygen Gold
Alloys
-Artificial gas and air.
- Acetylene + Oxygen air
 Non-luminous brush flame with different
combustion zones should be obtained.
 Mixing zone
 Combustion zone.
 Reducing zone.
 Oxidizing zone.
71

• Gas-air torch is used to melt conventional noble metal
alloys (used for inlays, crown and bridge) whose melting
points less than 1000⁰c
• Alloy should be approx. 38°C to 66°C above
liquidus temperature
It will become
Red

Orange

White (dull)

White (Mirror-like)
•When the gold is orange, transfer the ring from the
furnace to the cradle of the casting machine
•When the molten alloy surface is shiny and
shimmering-It indicates the alloy is not contaminated
with oxides
AIR GAS
72

USE OF FLUX FOR GOLD ALLOYS
• Minimize porosity
• To increase the fluidity of the metal i.e so as to increase
the flow of the molten alloy.
• Film of flux formed over surface of
the alloy prevents oxidation
Example:
1. powdered charcoal
2. Fused borax powder with boric acid powder
73

ELECTRICAL
There are two methods by which electricity can be employed
to melt the alloy.
Induction.
Electric Arc (Resistance).
Both the methods work on same principle i.e. heat energy is
produced when electric current is passed through a
conductor
depending upon the voltage applied across it.
74

frequency
induction coil across which high voltage is applied. This
high
frequency coil surrounds the crucible in which the alloy /
metal
pellets are melted.
Graphite is a good conductor of
heat and electricity.
 It transmits the heat produced
by the high frequency
induction coil, to the metal /
alloy pellets in the crucible.
 This process is the indirect
heating of the pellets in the
graphite crucible.
 Used for Noble metal alloys.
 Ceramic is a bad conductor
of heat and electricity.
 The surrounding high
frequency coil induces
intrinsic Currents in the alloy
/ metal pellets, which
produce heat required to melt
the alloys / metal pellets.
 used for base metal alloys
Graphite crucible Ceramic crucible
75

Electric Arc / Resistance
 This is a crucible free technique where by the metal /
alloy
pellet is directly melted by the heat produced as a result of
resistance offered by it to high voltage current.
 The apparatus consist of an electrode and a base plate on
which the metal / alloy pellet is placed.
 The contact between the electrode terminal and the metal
/alloy pellet is in form of an ‘electric arc’ thus the term
‘electric arc furnace’.
76

CENTRIFUGAL CASTING MACHINE
Casting machine spring wound from 2
to 5 turns
Alloy is melted by a torch flame in a
glazed ceramic crucible attached to the
“broken-arm” of casting machine.
Torch flame is generated from a gas
mixture of propane and air.
Machine is released and spring triggers
the rotation motion.
As the metal fills the mold, a hydrostatic
pressure gradient develops along the
length of the casting.
Broken arm feature accelerates the
initial rotational speed of the
crucible and casting ring.
Casting pressure = 30-40psi
77

The metal / alloy pellets are placed
in the crucible.
Flame/blowpipe is use to heat
the alloy/metal in the crucible.
Once the alloy / metal is in a molten
form.
The flame is removed and the
broken-arm is released. 78

ELECTRICAL RESISTANCE-HEATED CASTING MACHINE
• Current is passed through a resistance heating conductor,
and automatic melting of the alloy occurs in a graphite or
ceramic crucible.
• Advantages:
– For metal ceramic prosthesis.
– Base metals in trace amounts that tend to oxidize on
overheating.
– Crucible located flush against casting ring.
79

INDUCTION MELTING MACHINE
• Alloy is melted by induction field that develops within a
crucible surrounded by water cooled metal tubing.
• The electrical 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 crucible.
• Once the alloy reaches casting temperature in
air/vacuum it is forced into mold by centrifugal force by
air pressure, or by vacuum.
• More commonly used for melting base metal alloys.
81

DIRECT CURRENT ARC MELTING MACHINE
• Direct current arc is produced between two electrodes-
the alloy and water cooled tungsten electrode.
• The temperature between the arc exceeds 4000°C.
• Has high risk of overheating.
83

GAS PRESSURE
• This is introduced over the melt, in a closed
compartment
• Gas used- Carbon dioxide
Carbon monoxide Inert gases
Nitrogen
• The gas should be chosen so that the cast material has
no affinity towards it.
• The Casting pressure … 15-20psi
84

VACUUM OR PRESSURE ASSISTED CASTING
MACHINE
• Molten alloy is heated to casting temperature drawn into
the evacuated mold by gravity or vacuum and subjected to
additional pressure to force the alloy into the mold.
• Used for titanium and titanium alloys. Under vacuum arc
heated-argon pressure casting machine.
• Casting pressure used here is about
0.1 Mpa(15psi) which is very less.
• So vacuum pressure is always used in
combination with other methods.
85

•STEAM PRESSURE
•Moist Asbestos pad
86

RECOVERY AND FINISHING
OF CASTING
87

This is the process of rapidly cooling the hot casting by
dipping it in water.
After the casting has solidified the ring is removed and
quenched in water as soon as the button has lost its glow.
After the casting has solidified the ring is removed and
quenched in water as soon as the button exhibits a dull
yellow glow. The ring in quenched in
cold water.
QUENCHING
88

Advantages of Quenching
When the water contact hot investment, a violent
reaction
ensues(Thermal shock), resulting in as soft, granular
investment that is easily removed.
The noble metal alloy is left in an annealed condition for
burnishing and polishing.
89

PICKLING
The surface of casting appears dark with oxide and tarnish. Such
a surface film can be removed by the process called Pickling.
which consist of heating the discolored casting in a acid.
Used for cleaning gold castings.
Pickling solutions used :
 50% Hydrochloric acid.
 50% Sulfuric acid + Pottasium dichromate
 Hydrofluoric acid- For phospahte bonded invested
 AQUA REGIA : Hcl + Nitric acid (1:3)
90

Precautions to be taken
Ultrasonics –prosthesis is sealed in
teflon container.
Casting should not be held with steel
tongs
Always use plastic covered pliers to
introduce and remove the casting from
the pickling solution
When steel tongs come into contact
with gold in strong solution,
electrodeposition may occur on the
surface of the casting
91

SANDBLASTING
Gold based and palladium based metal ceramic and
base metal alloy are bench cooled to room temperature
before casting is removed from investment and then
subjected to sandblasting.
Recycled Aluminium oxide(50µ)with pressure of 100psi
Acid is NOT used for BASE METAL
ALLOYS.
92

REMOVAL OF SPRUE
Remove the sprue with double sided
Carborundum disc/diamond disc.
93

FINISHING
This is done to remove small irregularities on the surface
 Fitting surface- Blebs(Extrusions) removed using sharp chisel
or finishing bur
 External surface- Carbide trimming burs
 Try Casting on the die- Look for the Rocking
movement
 Marginal Burnishability- Ball/Beaver
To prevent cement line exposure
To protect friable enamel at the margins
Sliding lap joint at gingival cavosurface margin
94

POLISHING
Remove surface scratches
Obtain smooth shiny surface
Seat casting
Polishing of Gold casting is done with
carbide burs , green stones, pink stone,
tripoli or rouge.
Polishing of Base metal alloy casting is
done with aluminum oxide, tin oxide.
Note:
•Use Slow speed
•Coolant
•Used in slurry
•Decreasing order of abrasive
95

A mole on the chin, a dimple on the cheek, imperfections have
long been considered signs of beauty and individuality. Not so in
the field of dental sciences and specifically casting technology.
 Any imperfections or irregularities that result in
unsuccessful casting which interferes with the fit of final
restoration or its esthetic and mechanical properties.
 Defects in casting can be classified under four headings.
(1) Distortion
(2) Surface roughness and irregularities
(3) Porosity
(4) Incomplete or missing detail
97

DISTORTION
 Distortion
 Impression making- The impression(Alginate) must be poured
immediately within 30 min.
 Rubber base impressions can be poured even after a day ..!!
Warm wax thoroughly
before creating pattern.
Invest it quickly
•Change in the shape of the casting
•Mainly incorporated in the wax pattern stage
98

DISCOLORATION:
 Over-heating: Attracts oxides
 Under-heating: Incomplete wax elimination
 Carbon Inclusions: Crucible/Investment
 Mixture of Alloys: Re-use of Sprue and button
 Mercury Contamination
 Solder
99

SURFACE ROUGHNESS AND
IRREGULARITIES
 Surface roughness is defined as relatively
finely spaced surface imperfections that
predominant surface pattern.
 Surface irregularities are isolated
imperfections, such as nodules, that are not
characteristic of the entire surface area
100

•Air Bubbles
•Water film-Excess
Vibration
•Foreign body- Broken
Investment Pieces,Bits of
the carbon from the flux
•Higher L:P ratio- Weak
Investment
101

 Rapid Heating of the mold- FINS
 High temperature of molten alloy
 Casting Pressure : Too high a pressure during casting
produces a rough surface on the casting. A gauge
pressure of 0.10 to 0.14 Mpa (15 to 20 psi) in an air
pressure casting machine or three to four turns of the
spring in an average type of centrifugal casting
machine(30-40psi) is sufficient
102

 Composition of the Investment
The ratio of the binder to the quartz influence, the
surface texture of the casting. In addition, a coarse silica
causes more surface roughness
 Multiple Patterns:
Should not be placed too close together
Should not place many patterns in same plane
Space between the pattern is atleast 3mm
103

POROSITIES
I. Solidification defects :
A. Localized shrinkage porosity
B. Suck-back porosity
C. Microporosity
II. Trapped gases :
A.Pinhole porosity
B.Gas inclusion porosity
C.Subsurface porosity
III. Residual air : Back pressure porosity
104

PROBLEM
SOLIDIFICATION DEFECTS
LIKELY CAUSES REMEDIES
Localized shrinkage porosity
•Premature termination of
flow of molten metal.
•It mainly occurs at sprue-casting
junction.
Use of reservoir.
Increase the sprue diame
Suck-back porosity •Hot spot created by hot
metal impinging on point
on mold surface.
•It often occurs at OCCLUSOAXIAL
OR INCISOAXIAL LINE ANGLE
•Flare the point of
attachment of sprue.
•Lowering the casting
temperature by about 30°C
Micro Porosity
Small Irregular
•Rapid solidification of the mold
•Casting temperature is too low.
Increase the mold
or casting temperature
105

TRAPPED
GASES
PROBLEM LIKELY CAUSES REMEDIES
•poorly adjusted torch flame, or
the use of the mixing or oxidizing
zones
• Gas mechanically trapped
by molten metal in mold.
•Gas incorporated during
casting procedures.
•Absorbed gases are expelled
on solidification.
Pin hole Porosity
Small Spherical
Gas inclusion
Porosity
Large Spherical
Sub Surface
Porosity
•Simultaneous nucleation
of solid grains and gas
bubbles.
•Rate of entering of
molten metal in mold.
•Correctly adjusting
and positioning the
torch during melting.
•Prevent oxidation
of alloys-Flux
106

RESIDUAL AIR
PROBLEM LIKELY CAUSES REMEDIES
Back Pressure
Porosity
•Inability of gases in mold
to escape.
•Pressure gradient that displace
air towards the end of
investment.
•Tendency for mold to clog with
residual carbon.
•Proper venting.
•Place pattern no more than
6-8mm from the end
of the casting ring
•Sufficient casting
pressure.
•Proper burn out
temperature.
107

INCOMPLETE CASTING
Rounded and incomplete
margins- Insuffecient casting
pressure
Rounded and shiny- Incomplete wax elimina
108
Insuffecient alloy

Review Of Various Studies…..
Surface Porosity of Different Investment Materials with
Different Mixing Techniques
Ahmed A et al. Al–Rafidain Dent J. 2009; 9(2): 307–314
Conclusion: Manual technique showing higher value of
surface area and number of porosities than the Vacuum
technique.
109

The effect of sprue attachment design on castability and
porosity.
Verrett RG, Duke ES. J Prosthet Dent. 1989 Apr;61(4):418-
24.
Conclusion: Flared and straight sprue attachments
optimized castability and minimized porosity than compared
to abrupt constriction, and gradual constriction
110

The effect of casting ring liners on the potential
expansion of a gypsum-bonded investment.
Earnshaw R J Dent Res. 1988 Nov;67(11):1366-70
Conclusion: Wet lining materials showed an increased
total expansion (in the range 2.2 to 2.3%), where as dry
surface showed a total expansion of 1.7%
111

Effect of wax melting range and investment liquid
concentration on the accuracy of a three-quarter
crown casting
Michio Ito et al,The Journal of Prosthetic Dentistry Volume
87, Issue 1, January 2002, Pages 57–61
Conclusion:
1) The higher the softening temperature, the larger the
casting shrinkage.
2) Casting shrinkage was smaller with the use of 100%
special liquid.
112

Effects of rapid burnout type gypsum-bonded investment on performance of
castings.
Murakami S et al, Dent Mater J.1994 Dec;13(2):240-50.
Rapid burnout type gypsum-bonded investment materials were developed to shorten
the time required for dental casting procedures
Conclusion:
1)When the investment block was rapidly heated at 7000 C, no fractures were
observed in the rapid burnout type investments with one exception, while a
conventional cristobalite investment broke into pieces shortly after being placed in
the furnace
2) Casting fins were sometimes induced only for the material which showed
fracturing on rapid heating
3) The 30 min-setting expansion was significantly different among the materials
although there were no differences in thermal expansion, and the material showing
greater 30 min-setting expansion was efficient to obtain better fit of the crown as in
113

Dimensional accuracy of castings produced with
ringless and metal ring investment systems
Pelopidas Lombardas The Journal of Prosthetic Dentistry
Volume 84, Issue 1,July 2000, Pages 27–31
Conclusion: The ringless technique may produce
accurate castings when compared to metal rings
114

Effect of internal microblasting on retention of cemented
cast crowns.
Randolph P. O'Connor. The Journal of Prosthetic Dentistry
Volume 64, Issue 5, November 1990, Pages 557–562.
This investigation measured the retention of type III cast gold
crowns after cleansing with pickling solution (group I) or
microblasting with 50-micron aluminum oxide (group II) or 50-
micron glass beads (group III)
Conclusion: Microblasting the internal surface of cast crowns
with 50-micron aluminum oxide significantly improved the
retention of castings luted with zinc phosphate cement.
115

Measurement of paint-on die spacers used for casting
relief
W.V. Campagni The Journal of Prosthetic Dentistry
Volume 47, Issue 6, June 1982, Pages 606–611
Three paints used for die spacing were measured for film
thickness. Two of the materials are marketed as die
spacers, and the third is sold as a model airplane paint.
Conclusion:
The three die spacing materials were capable of achieving
clinically acceptable relief in the range of 20 to 40 μ by
using a variable number of coats.
116

CONCLUSION
117
Investing and casting, a series of highly technique sensitive
steps,coverts the wax pattern into a metal casting.
Accurate and smooth restorations can be obtained if the
operator pays special attention to each step in the technique
When initial attempts at casting produce errors or defects,
appropriate corrective measures must be taken so that they
do not recur.
Thus a thorough and in-depth knowledge of defects,their
etiology and prevention will go a long way in avoiding these
undesirable imperfections allowing the fulfillment of dreamOf
every dentist and desire of every individual- The ideal
restoration, and perfect rehabilitation yielding Patient function

Casting Procedures and Defects

More Related Content

What's hot (20)

Similar to Casting Procedures and Defects (20)

Recently uploaded (20)

Casting Procedures and Defects