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Dr. Kriti trehan
MDS 1st year
13/3/18
Casting procedures
CONTENTS
 Introduction
 Steps in casting procedure
 Wax pattern removal
 Sprue formers
 Crucible formers
 Casting rings and ring liners
 Investing procedure
 Wax burnout
 Casting of alloys into mold
 Casting of titanium alloys
 Cleaning of casting
Introduction
 Casting is one the most widely used methods for
fabrication of metallic restorations.
 The lost wax casting technique was first described at the
end of 19th century as a means of making dental castings.
 The process consists of surrounding the wax pattern with a
mold made of heat resistant investment material,
eliminating the wax by heating and then introducing the
molten metal into the mold through a channel called
“Sprue”.
• In Dentistry, the resulting casting must be a highly accurate
replica of wax pattern with surface details & accurate
dimensions.
• Small variations in investing or casting can significantly
affect the quality of final restoration.
• Successful casting depends on accuracy & consistency of
technique.
• We are going to know the exact influence of each variable in
the technique & to make rationale changes to modify the
technique according to need.
1. Wax pattern removal
2. Spruing
3. Investing
4. Burnout
5. Casting
Steps in casting procedures
Wax patterns
 First procedure in the casting of an inlay or crown for the
lost-wax process is the preparation of a dental wax pattern.
 Direct Wax Technique – Pattern made within the tooth.
Indirect Wax Technique – Pattern prepared within a die.
 INLAY WAX – Specialized dental wax applied on the die
surface for preparation of direct & indirect patterns
 American National Standards Institute / American Dental
Association Sp No 4:
o Type I – Medium Wax – Direct techniques
o Type II – Soft Wax – Indirect techniques
INDIRECT TECHNIQUE DIRECT TECHNIQUE
REMOVAL OF WAX PATTERN
Direct pattern:
 Sprue former  Attached to the pattern  Removed directly
in line with its path of withdrawl .
 Hook it with an explorer point and rotate it out of the cavity .
 In a MOD use a staple pin . Fasten it and insert a floss and
hook it .
Indirect patterns:
 The wax should be allowed to cool
thoroughly before the pattern is
removed from the die.
 A constant light grip is maintained
on the pattern by the thumb and
forefinger of one hand while
pressure is applied against them
with the thumb and forefinger of the
other hand, which also holds the
die
Sprue
 When the wax pattern has been completed and its margin
has been reflowed, it is carefully evaluated for smoothness,
finish, and contour.
 The pattern is inspected under magnification, and any
residual flash is removed. A sprue is attached to the
pattern, then removed from the die and fitted to a crucible
former.
Sprue : The channel or hole through which plastic or
metal poured or cast into gate or reservoir and then
into mold.
1. Basic requirements of sprue:
a) Must allow the molten wax to escape from the mould.
b) Sprue must enable the molten metal to flow into the mould with as
little turbulence as possible.
c) Metal must remain molten slightly longer than the alloy that has
filled the mould. This provides a reservoir to compensate for the
shrinkage that occurs during solidification of the casting.
2. Types of sprue :
 The sprue can be wax, plastic, or metal.
a) Wax sprues are preferred for most castings because they
melt at the same rate as the pattern and thus allow easy
escape of the molten wax.
b) Solid plastic sprues soften at a higher temperature than the
wax pattern and may block the escape of wax, resulting in
increased casting roughness.
 However, plastic sprues can be useful when casting fixed
partial dentures (FPDs) in one piece because their added
rigidity minimizes distortion. Also, hollow plastic sprues are
available that permit the escape of wax.
c) If a metal sprue is used, it should be made of nonrusting
metal to avoid possible contamination of the wax.
 Metal sprues are often hollow to increase contact surface
area and strengthen the attachment between the sprue and
pattern.
 They are usually removed from the investment at the same
time as the crucible former.
 Special care must be taken to examine the orifice for small
particles of investment that may break off when such a
sprue is removed because these can cause an incomplete
casting if undetected.
3. Sprue former diameter:
 The diameter and length of the sprue former depends on:
 The type and size of the pattern.
 The type of casting machine to be used.
 The dimensions of the casting rings in which casting is
made.
 Pre fabricated sprue former are available in a wide variety of
gauge from 6 to 18.
 The diameter of sprue should be equal to the thickest
portion the wax pattern.
 Usually for molar and metal ceramic restoration - 10-gauge
(2.5mm)
 Premolars and partial coverage restoration - 12-gauge
(2.0mm)
 Large diameter sprue: this improves the flow of molten metal
into the mould.
 Small diameter sprue: this area will solidify before the
casting itself and localized shrinkage porosity (“suck-back”
porosity) may develop.
4. Length of the sprue former
 Should be long enough to properly position the pattern in
the casting ring within 6 mm of the end of the ring yet short
enough so that the molten alloy doesn’t solidify before it fills
the mold . ( 6mm – Gypsum bonded investments & 3-4mm –
Phosphate bonded )
 Average sprue length – Large inlay – 4-5mm
Small inlay – 3-4mm
 Short sprue – Moves the pattern more away from the end of
the ring and the gases cannot be adequately vented.
 Long sprue – Solidify before the mold causing shrinkage
porosity.
5. Sprue former location:
 The sprue should be attached to the bulkiest part of the
pattern, away from margins and occlusal contacts.
 Normally the largest noncentric cusp is used.
 The point of attachment should permit a stream of metal to
be directed to all parts of the mold without having to flow
opposite the direction of the casting force.
 Full veneer crown - sprue is attached to Maxillary buccal and
mandibular lingual cusp.
 Partial veneer crown - sprue is attached to cusp that
encompasses the preparation.
6. Sprue former direction:
 Should be directed away from any thin or delicate parts of
pattern - molten metal may abrade or fracture investment in
this area.
 It should be attached 45 degrees to the walls of mold, which
decreases the turbulence of molten alloy.
7. Sprue former attachment :
 The attachment of sprue former to the wax pattern should be
smooth and do not posses pits or irregularities.
 Irregularities produces tags of investment which is prone for
fracture by molten alloy leading to casting failure.
 The sprue former connection to the wax pattern is generally
flared for high-density gold alloys but often restricted for
lower-density alloys.
 This design minimizes the risk of turbulence. Also, the
orientation of the sprue former should minimize the risk for
metal flow onto flat areas of the investment or onto small
areas such as line angles.
8. Reservoir:
 The reservoir is placed approximately 1.5mm from the
pattern .
 Function – Prevents localized shrinkage porosity, because of
the large mass of alloy and position in the heat centre of the
ring , the reservoir will remain molten to furnish liquid alloy
into the mold as it solidifies .
Reservoir
9. Venting :
• Small auxiliary sprues or vents have been recommended to
improve casting of thin patterns and may help in :
 Escape of gases during casting.
 Compensate for the shrinkage during solidification.
 Solidification begins in critical areas by acting as a heat
sink.
10. Spruing technique
DIRECT INDIRECT
• The sprue former
provides a direct
connection between the
pattern area and the
sprue base or crucible
former area.
• A connector or reservoir
bar is positioned
between the pattern and
the crucible former
Crucible former
 The sprue is attached to crucible former which constitutes
the base of the casting ring during investing.
 It also helps by holding sprue in desired ring.
 Crucible formers are basically of 2 types:
a) Steep-sided cone: used with metal when casted using
centrifugal casting force.
b) Shallow cone: used to cast metal using stream/air
pressure.
 They are available as-
 Rubber crucible former
 Metallic Crucible former
 Plastic crucible former
 The exact shape of the crucible former depends on the type
of casting machine used.
 With most modern machines, the crucible former is tall to
allow use of a short sprue and allow the pattern to be
positioned near the end of the casting ring.
 They form a conical depression in investment, which guides
flow of molten metal.
 It should be clean and petroleum is applied to prevent
formation of rough investment tag.
 Then the end of sprue former is passed into the hole and
held in position till the molten wax sets.
Casting ring
 Casting rings are used to confine the fluid investment around the
wax pattern while the investment sets.
 It also allow the hardened investment to be safely handled during
burnout and casting.
 The internal diameter of casting ring should be 5-10mm greater
than the widest measurement of the pattern and about 6 mm
higher.
 For single crown/inlay - small rings are used, diameter - 32 mm
 For large fixed partial denture – 63mm round/oval shaped casting
ring are used.
SHAPE
•Round
•Oval
COMPLETE
RING
•Rigid : Metal ,
plastic
•Flexible:
Rubber
SPLIT
RING
• Metal
• Plastic
Types of casting ring
RING LESS CASTING SYSTEM
 Ringless Casting System is designed to increase
productivity by achieving consistently accurate results
without the time-consuming steps associated with the use of
metal casting rings.
 Utilizes durable, reusable plastic rings that are tapered
allowing for unimpeded expansion of investment and easy
removal of mold prior to burnout.
 This allows for quick and easy divesting after casting while
reducing clean-up chores.
 Investment expansion is easier to control and not limited to
the thickness of a ring liner.
Casting ring liners
 The most commonly used technique to provide room for
investment expansion.
 Triple fold function : - Freedom to expand which would
otherwise be restricted by the ring .
 Helps to offset the contraction of the more rapidly cooling ring
while the gold alloy is being melted .
 In a wet liner certain amount of hygroscopic expansion is
afforded and a thicker liner provides even greater semi
hygroscopic expansion.
• Asbestos liner: Asbestos is refractory to high temperature,
they show a sufficient amount of water absorption.
• There are 3 types of asbestos
i. White asbestos (least toxic) this type is used in dentistry.
ii. Blue asbestos (most toxic)
iii. Brown asbestos(Intermediately toxic).
 Asbestos is no longer used in dentistry because of its
carcinogenic and toxic potential.
• Cellulose liner: This material shows adequate water
absorption.
• It is burnt during burnout procedure.
• To keep the investment in contact with ring after burnout,the
liner is kept 3mm short of ring ends.
• This also restricts the longitudinal setting and hygroscopic
expansion.
• Ceramic ring liner They are basically alumino-silicate
fibrous material.
• They do not absorb water to large extent, but its network of
fibres can retain small amount of water on its surface.
• They are refractory to high temperature.
• The binders used in ceramic liner (Ex – neoprene-latex) can
contribute to toxicity (stimulate fibrosis/ act as adsorbent
surface for carcinogenesis).
• They posses fibers of length 5.3-17.8 mm. Diameter 0.2-
0.97 mm
 Disadvantages: Burned away during casting , difficult to
secure in place.
 Absorb materials much less than asbestos , and their
combination with some gypsum bonded investments will
produce fins .
 Not compatible with phosphate bonded investments.
Manipulation of a casting ring liner
 Cut the liner to fit the inside diameter of the casting ring , no
over lap.
 Dry liner technique – Dry liner is tacked in position with
sticky wax .
 Wet liner technique –Lined ring is immersed in water and
excess water is shaken away .
 Avoid touching or adapting with fingers - reduce the
cushioning effect.
 Attach the liner firmly to the ring by wax to prevent it from
“riding up” during investing and inadvertently affecting the
size of the casting
Investing
 Investment materials are:
 Gypsum bonded investment
 Phosphate bonded investment
 Ethyl silicate bonded investment
The process of covering or enveloping an object such
as a denture, tooth, wax form, crown, with a suitable
investment material before processing, or casting
Investing procedure :
• The wax pattern should be cleaned of any debris, grease, or
oils.
• A commercial wax pattern cleaner or a diluted synthetic
detergent may be used.
• Any excess liquid is shaken off, and the pattern is left to air-
dry while the investment is being prepared.
• The thin film of cleanser left on the pattern reduces the
surface tension of the wax and permits better “wetting” of the
investment to ensure complete coverage of the intricate
portions of the pattern
• Vacuum mixing of investment materials is highly
recommended for consistent results in investing and casting
with minimal surface defects, especially when phosphate-
bonded investments are used.
Vacuum investing machines.
The Whip Mix combination unit Girrbach Vacumat
• Air bubbles that remain in the mix, even with vacuum mixing,
can be entrapped on flat or concave surfaces that are not
oriented suitably for air evacuation.
• Tilting the ring slightly aids in releasing these bubbles so that
they can rise to the surface.
• Excessive vibration should be avoided because it can cause
solids in the investment to settle and may lead to free-water
accumulation adjacent to the wax pattern, resulting in
surface roughness.
• Excessive vibration can also dislodge small patterns from
the sprue former , resulting in a miscast.
• If the hygroscopic technique is employed, the filled casting
ring is immediately placed in a 37 °C water bath with the
crucible former’s side down.
• For the thermal expansion or high-heat technique, the
invested ring is allowed to bench set undisturbed for the time
recommended by the manufacturer.
Wax elimination
 Wax elimination or burnout consists of heating the investment
in a thermostatically controlled furnace until all traces of the
wax are vaporized.
 Once the investment has set for an appropriate period 1 hour
it is ready for burnout.
 The crucible former is then carefully removed and the
invested rings are placed in a room temperature furnace and
heated to the prescribed maximum temperature.
Manual
Semiauto-
matic
Fully
Programm-
able controls
Burnout ovens
• For gypsum-bonded investments, this temperature can be
either 500 °C for the hygroscopic technique or 700 °C the
thermal expansion technique.
• With phosphate-bonded investments, the maximum
temperature setting may range from 700 °C to 1030 °C,
depending on the type of alloy selected.
• It is also advisable to begin the burnout procedure while the
mold is still wet. Water trapped in the pores of the
investment reduces the absorption of wax, and as the water
vaporizes, it flushes wax from the mold.
 When the high-heat technique is used, the mold temperature
generates enough heat to convert carbon to either carbon
monoxide or carbon dioxide. These gases can then escape
through the pores in the heated investment.
 Hygroscopic low-heat technique
 This technique obtains its compensation expansion from
three sources:
1) 37 °C water bath expands the wax pattern,
2) The warm water entering the investment mold from the top
adds some hygroscopic expansion
3) The thermal expansion at 500 °C provides the needed
thermal expansion.
 The molds should remain in the furnace for at least 60 minutes,
and they may be held up to 5 hours longer with little damage.
 Even though the mold is usually held at this temperature for 60
to 90 minutes, sufficient residual fine carbon may be retained to
reduce the venting of the mold.
 Because of this potential for reduced venting, back-pressure
porosity is a greater hazard in the low-heat technique.
 Advantages:
i. Less investment degradation
ii. A cooler surface for smoother castings
iii. The convenience of placing the molds directly in the 500°C
furnace.
iv. The last benefit makes it possible to keep one or more
furnaces at the burnout temperature so that molds may be put
in as they are ready.
 The standardized hygroscopic technique was developed for
alloys with a high gold content; the newer noble alloys may
require slightly more expansion.
 This added expansion may be obtained by making one or
more of the following changes:
 Increasing the water bath temperature to 40 °C
 Using two layers of liner
 Increasing the burnout temperature to a range of 600 °C to
650 °C
• In the low-heat casting technique, the alloy should also be
cast soon after removal of the ring from the oven; otherwise
a significant variation from the desired casting dimensions
may occur.
High-heat thermal expansion technique :
 Gypsum bonded Investment : •
o These casting investments are relatively fragile and require
the use of a metal ring for protection during heating.
o The molds are usually placed in a furnace at room
temperature, slowly heated to 650 °C to 700 °C in 60
minutes, and held for 15 to 30 minutes at the upper
temperature.
o Too rapid a heating rate may also cause cracking of the
investment. In such a case, the outside layer of the
investment expands much more than the center sections.
 Investment decomposition and alloy contamination is
related to a chemical reaction between the residual
carbon and calcium sulfate binder.
 The reduction of calcium sulfate by carbon takes place
rapidly above 700 °C in accordance with the following
reactions.
 The sulfur dioxide as a product of this reaction
contaminates gold castings and makes them extremely
brittle.
• CaSO4 + 4C= CaS + 4CO
• 3CaSO4 + CaS → 4CaO + 4SO2
 Therefore , after the casting temperature has been reached,
the casting should be made immediately.
 Maintaining a high temperature for a considerable length of
time may result in sulfur contamination of the casting and
also in a rough surface on the casting because of the
disintegration of the investment.
 A few gypsum investments, some with a considerable
amount of cristobalite, are now offered for use with a much
more rapid burnout procedure.
 Phosphate bonded investment :
o They need higher 2nd stage temperature for total
elimination of wax and to prevent premature solidification of
higher melting alloys.
o After the temperature reaches 400 °C, the rate of heating
can be safely increased. After burnout, usually at a final
temperature of 700 °C to 1030 °C depending on the alloy
melting range, the casting is made.
o Because the entire process involving phosphate investments
takes a long time, the demand for time-saving changes is
strong.
Accelerated casting method :
 Conventional casting techniques require considerable time,
typically 1 hour bench set for the investment and 1 to 2 hours
for the wax elimination.
 Accelerated casting procedures have been proposed that
reduce this time to 30 to 40 minutes .
 Initially suggested as a way to make cast post-and-core
restorations in a one-visit procedure.
 The technique uses a phosphate-bonded investment that is
given approximately 15 minutes bench and a 15-minute wax
elimination by placing the ring in a furnace preheated to 815° C
(1500° F).
CASTING
Casting procedure: It is a process of obtaining a metallic
duplicate of a missing tooth structure by pouring molten metal
into a mold of a required form & allowing it to solidify to obtain
a metallic duplicate.
Something that has been cast in a mold; an
object formed by the solidification of a fluid that
has been poured or injected into a refractory
mold.(GPT-9)
CASTING MACHINES
 Alloys are melted in one of the four following ways:
Gas/Air (Most Common) Resistance
Gas/Oxygen Induction
Air /Acetylene Direct Current Arc
Oxygen /Acetylene
TORCH ELECTRICAL
Types of torch flame :

Gas air torch :
 Gas-air torch is used to melt conventional noble metal alloys
(used for inlays, crown and bridge) whose melting points
less than 1000⁰c.
Gas oxygen torch
 Used to melt metal ceramic alloys of higher temperature up
to 1200⁰c. The tip of torch is available as single
orifice/multiorifice the oxygen pressure is adjusted to 10-15
psi.
 The flame is directed onto metal with the nozzle of the torch
about 1.5 cm away from the metal.
 Complete fluid should be obtained within 30 second at
which point the metal is poured into the mould.
Air acetylene & oxygen acetylene gas
 These were designed mainly for Cobalt chromium base
alloys.
 The actual production of flame can be done by adjusting the
pressure and flow of individual gases .
 One part of acetylene + 2 and half part of oxygen
Advantage : Hottest flame hence faster burnout .
Disadvantages : Excessive heat may distill lower melting
components .
 Overheating – gases to dissolve in the casting – porosity
 Highly technique sensitive
 The best results are obtained when flame is used with a
distance of 10cm between the face of blow torch nozzle and
the base of crucible.
 When the reducing zone is in contact, the surface of the gold
alloy is bright and mirror like.
 When the oxidizing portion of the flame is in contact with
alloy there is a dull film developed over the surface
TORCH MELTING OF NOBLE METAL ALLOY
 The alloy is melted in a separate crucible by a torch flame
and cast into the mold by centrifugal force.
 Temperature of gas-air flame is influenced by :
 Nature of the gas
 Proportion of gas and air in the mixture.
• Two types of flames can be obtained with a casting torch, the
air supply for the lower flame is excessive, so that incomplete
combustion and a lower temperature will result.
• The upper brush flame indicates the proper adjustment for
maximal efficiency and temperature.
 The parts of the flame can be identified by the conical areas:
 The first long cone emanating directly from the nozzle is the
zone in which the air and gas are mixed before combustion.
No heat is present in this zone.
 The next cone, which is green and immediately surrounding
the inner cone, is known as the combustion
zone. Here, the gas and air are partially burned.
This zone is definitely oxidizing and
should always be kept away from the
molten alloy during fusion.
 The next zone, dimly blue and located just beyond the tip of
the green combustion zone, is the reducing zone.
 This is the hottest part of the flame, and it should be kept
constantly on the alloy during melting.
 The outer cone (oxidizing zone) is the area in which
combustion occurs with the oxygen in the air.
 Under no circumstances should this portion of the flame be
used to melt the alloy. Not only is its temperature lower than
that of the reducing zone but it also oxidizes the alloy
 The alloy first appears to be
spongy and then small globules of
fused alloy appear. The molten
alloy soon assumes a spheroidal
shape.
 At the proper casting temperature,
the molten alloy is light orange and
tends to spin or follow the flame
when the latter is moved slightly.
 At this point, the alloy should be approximately 38 °C to 66
°C above its liquidus temperature. The casting should be
made immediately when the proper temperature is reached.
Flux:
 Flux for gold crown and bridge alloys to aid in minimizing
porosity.
 When properly used, the flux increases the fluidity of the
alloy and the film of flux formed on the surface of the molten
alloy helps prevent oxidation.
 Reducing fluxes are excellent for cleaning old alloy, a better
flux for the casting procedure may be made from equal parts
of fused borax powder ground with boric acid powder.
 The boric acid aids in retaining the borax on the surface of
the alloy. The flux is added when the alloy is completely
melted and should be used with both old and new alloy.
 Old sprues and buttons from the same alloy may be recast if
they are not contaminated.
B. Electrical resistance–heated casting machine :
 During electrical melting of alloys heat energy is produced
when electric current is passed through a conductor
depending upon the voltage applied across it.
 The alloy is melted electrically by a resistance heating .
 Current is passed through a resistance heating conductor,
and automatic melting of the alloy occurs in a graphite or
ceramic crucible.
 A carbon crucible should not be used in the melting of high-
palladium alloys, palladium-silver alloys, nickel-chromium
alloys, or cobalt chromium base metal alloys.
 This is an advantage, especially for
alloys such as those used for metal-
ceramic prostheses, which are alloyed
with base metals in trace amounts that
tend to oxidize on overheating.
 Another advantage is that the crucible
in the furnace is located flush against
the casting ring. Therefore, the alloy
button remains molten slightly longer,
again ensuring that solidification
progresses completely from the tip of
the casting to the button surface.
C. 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. It is more commonly used for melting
base metal alloys.
k
D. 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.
E. Vacuum- or pressure-assisted casting machine
 First evacuate the melting chamber to reduce oxidation.
 Apply air pressure uniformly about the casting ring forcing
molten alloy into mold.
 Vacuum is applied to the bottom of
the mold.
 Molten alloy is “PUSHED & SUCK
ED” into the mold by gravity or vacuum.
 Used for titanium and titanium alloys.
CASTING CRUCIBLES:
 The melting of alloy requires a crucible to act as a platform
on which the heat can be applied to the metal.
 Generally four types of casting crucibles are available: clay,
carbon, quartz, and zirconia-alumina.
 Traditionally a wet lining of asbestos sheet was used on casting
crucible. The moistened asbestos sheet provides a clean and
good surface on which the alloy could be melted.
 Advantages is, prevent alloy contamination with oxides and
residuals that may be present in the crucible.
 Carbon crucibles can be used not only for high noble crown
and bridge alloys but also for the higher-fusing gold-based
metal-ceramic alloys.
 Carbon crucibles should not be used in melting of high
palladium, palladium silver alloys (to be melted above
1504⁰c) and also with nickel chromium/cobalt chromium
base metal alloys
 Clay crucibles are appropriate for
many of the crown and bridge
alloys, such as the high noble and
noble types.
 Crucibles made from alumina,
quartz, or silica are recommended
for high-fusing alloys of any type.
These are especially suited for
alloys that have a high melting
temperature or those that are
sensitive to carbon contamination.
Casting force
 Casting force > surface tension of alloy + resistance offered by
gas in the mold .
 This can be done by use of following different type of force:
 Vacuum force
 Air or Gas Pressure
 Centrifugal force
 Sufficient mass of alloy must be present to sustain adequate
casting pressure
 6g is typically adequate for premolar and anterior casting
 10g is adequate for molar casting
 12 g is adequate for pontic
Cleaning the casting
Quenching
 When a type III or IV gold alloy has been cast and it has
solidified, the ring should be quenched in water as soon as
the button exhibits a dull-red glow.
 Two advantages are gained in quenching:
a) The noble metal alloy is left in an annealed condition for
burnishing, polishing, and similar procedures .
b) When the water contacts the hot investment, a violent
reaction ensues, resulting in a soft, granular investment that
is easily removed.
Divesting:
•Removal of Investment /
Recovery of Casting.
• Hold the end of the ring
for about ¼ inch.
• Bulk – finger pressure.
• Sprue is removed from
the restoration using a
carborundum separating
disk/ abrasive disk
mounted in a hand piece.
Pickling
 Surface of the casting appears dark with oxides and tarnish.
Such a surface film can be removed by a process called
pickling.
Solutions used:
 50% HCl
 Phosphoric acid,
 Hydrofluoric acid
 Advantages of HCl: Aids in removal of residual investment
as well as oxide coating.
 Disadvantages: Likely to corrode laboratory metal
furnishings and fumes are health hazard
 Method of cleaning :
a) Place the casting in test tube or dish and pour acid over it.
b) Heating the casting and then dropping into the pickling
solution.
 Gold and palladium based metal ceramic alloys and base
metals, these alloys are not generally pickled if they are
bench cooled.
 Pickling solution should be renewed frequently, since it is
likely to become contaminated.
 Precious alloys(Gold-Platinum-Palladium) can be soaked
with hydroflouric acid .
 Nickel Chromium should never be placed in acid because of
high reactivity.
 Ultrasonic devices are useful for cleaning the casting, as are
commercial pickling solutions made of acid salts. Abrasive
blasting devices are also useful for cleaning the surface of
castings.
 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.
 Recovery and cleaning of the casting are more difficult when
a phosphate-bonded investment is used because such
materials do not contain the soft gypsum products.
 Also, the particles usually include large grains of quartz.
 In some instances, such as with gold-containing alloys, the
investment adheres rather tenaciously, usually requiring
cleaning in an ultrasonic device.
 Neither the phosphate binder nor the silica refractory is
soluble in HCl or H2SO4.
Sand blasting :
 Casting is held in a sand blasting machine to clean the
investment from the surface.
 Base metal alloys require a light sandblasting, usually with
fine alumina.
 Chromium-based partial dentures are usually sandblasted to
remove the investment. Acid should not be used for cleaning
base metal alloys
Finishing and polishing :
 Brown or White Al2O3 stones are used.
 Rag or felt wheels impregnated with abrasives are used in
the initial phase of this stage.
 Final polishing is achieved using various oxides of tin and
aluminium used in conjunction with a small rag or chamois
buffing wheel, followed with an iron oxide rouge.
 Residual traces of rosin or waxlike matrix from oxides
 Polishing compound remover followed by a hot, soapy water
rinse.
References
1. Anusavice K.J.-“Phillips’ Science of Dental materials”
11th edition , 2003.Pg 319-335.
2. Contemporary fixed prosthodontics rosenstiel,land ,
fujimoto- 5 th edition , Pg 601- 618.
3. Removable partial prosthodontics , Mc Cracken’s, 13th
edition, Pg 259 to 265.

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Casting procedures

  • 1. Dr. Kriti trehan MDS 1st year 13/3/18 Casting procedures
  • 2. CONTENTS  Introduction  Steps in casting procedure  Wax pattern removal  Sprue formers  Crucible formers  Casting rings and ring liners  Investing procedure  Wax burnout  Casting of alloys into mold  Casting of titanium alloys  Cleaning of casting
  • 3. Introduction  Casting is one the most widely used methods for fabrication of metallic restorations.  The lost wax casting technique was first described at the end of 19th century as a means of making dental castings.  The process consists of surrounding the wax pattern with a mold made of heat resistant investment material, eliminating the wax by heating and then introducing the molten metal into the mold through a channel called “Sprue”.
  • 4. • In Dentistry, the resulting casting must be a highly accurate replica of wax pattern with surface details & accurate dimensions. • Small variations in investing or casting can significantly affect the quality of final restoration. • Successful casting depends on accuracy & consistency of technique. • We are going to know the exact influence of each variable in the technique & to make rationale changes to modify the technique according to need.
  • 5.
  • 6. 1. Wax pattern removal 2. Spruing 3. Investing 4. Burnout 5. Casting Steps in casting procedures
  • 7.
  • 8. Wax patterns  First procedure in the casting of an inlay or crown for the lost-wax process is the preparation of a dental wax pattern.  Direct Wax Technique – Pattern made within the tooth. Indirect Wax Technique – Pattern prepared within a die.  INLAY WAX – Specialized dental wax applied on the die surface for preparation of direct & indirect patterns  American National Standards Institute / American Dental Association Sp No 4: o Type I – Medium Wax – Direct techniques o Type II – Soft Wax – Indirect techniques
  • 10. REMOVAL OF WAX PATTERN Direct pattern:  Sprue former  Attached to the pattern  Removed directly in line with its path of withdrawl .  Hook it with an explorer point and rotate it out of the cavity .  In a MOD use a staple pin . Fasten it and insert a floss and hook it .
  • 11. Indirect patterns:  The wax should be allowed to cool thoroughly before the pattern is removed from the die.  A constant light grip is maintained on the pattern by the thumb and forefinger of one hand while pressure is applied against them with the thumb and forefinger of the other hand, which also holds the die
  • 12. Sprue  When the wax pattern has been completed and its margin has been reflowed, it is carefully evaluated for smoothness, finish, and contour.  The pattern is inspected under magnification, and any residual flash is removed. A sprue is attached to the pattern, then removed from the die and fitted to a crucible former. Sprue : The channel or hole through which plastic or metal poured or cast into gate or reservoir and then into mold.
  • 13. 1. Basic requirements of sprue: a) Must allow the molten wax to escape from the mould. b) Sprue must enable the molten metal to flow into the mould with as little turbulence as possible. c) Metal must remain molten slightly longer than the alloy that has filled the mould. This provides a reservoir to compensate for the shrinkage that occurs during solidification of the casting.
  • 14. 2. Types of sprue :  The sprue can be wax, plastic, or metal. a) Wax sprues are preferred for most castings because they melt at the same rate as the pattern and thus allow easy escape of the molten wax.
  • 15. b) Solid plastic sprues soften at a higher temperature than the wax pattern and may block the escape of wax, resulting in increased casting roughness.  However, plastic sprues can be useful when casting fixed partial dentures (FPDs) in one piece because their added rigidity minimizes distortion. Also, hollow plastic sprues are available that permit the escape of wax.
  • 16. c) If a metal sprue is used, it should be made of nonrusting metal to avoid possible contamination of the wax.  Metal sprues are often hollow to increase contact surface area and strengthen the attachment between the sprue and pattern.  They are usually removed from the investment at the same time as the crucible former.  Special care must be taken to examine the orifice for small particles of investment that may break off when such a sprue is removed because these can cause an incomplete casting if undetected.
  • 17. 3. Sprue former diameter:  The diameter and length of the sprue former depends on:  The type and size of the pattern.  The type of casting machine to be used.  The dimensions of the casting rings in which casting is made.  Pre fabricated sprue former are available in a wide variety of gauge from 6 to 18.  The diameter of sprue should be equal to the thickest portion the wax pattern.
  • 18.  Usually for molar and metal ceramic restoration - 10-gauge (2.5mm)  Premolars and partial coverage restoration - 12-gauge (2.0mm)  Large diameter sprue: this improves the flow of molten metal into the mould.  Small diameter sprue: this area will solidify before the casting itself and localized shrinkage porosity (“suck-back” porosity) may develop.
  • 19. 4. Length of the sprue former  Should be long enough to properly position the pattern in the casting ring within 6 mm of the end of the ring yet short enough so that the molten alloy doesn’t solidify before it fills the mold . ( 6mm – Gypsum bonded investments & 3-4mm – Phosphate bonded )
  • 20.  Average sprue length – Large inlay – 4-5mm Small inlay – 3-4mm  Short sprue – Moves the pattern more away from the end of the ring and the gases cannot be adequately vented.  Long sprue – Solidify before the mold causing shrinkage porosity.
  • 21. 5. Sprue former location:  The sprue should be attached to the bulkiest part of the pattern, away from margins and occlusal contacts.  Normally the largest noncentric cusp is used.
  • 22.  The point of attachment should permit a stream of metal to be directed to all parts of the mold without having to flow opposite the direction of the casting force.  Full veneer crown - sprue is attached to Maxillary buccal and mandibular lingual cusp.  Partial veneer crown - sprue is attached to cusp that encompasses the preparation.
  • 23. 6. Sprue former direction:  Should be directed away from any thin or delicate parts of pattern - molten metal may abrade or fracture investment in this area.  It should be attached 45 degrees to the walls of mold, which decreases the turbulence of molten alloy.
  • 24. 7. Sprue former attachment :  The attachment of sprue former to the wax pattern should be smooth and do not posses pits or irregularities.  Irregularities produces tags of investment which is prone for fracture by molten alloy leading to casting failure.  The sprue former connection to the wax pattern is generally flared for high-density gold alloys but often restricted for lower-density alloys.  This design minimizes the risk of turbulence. Also, the orientation of the sprue former should minimize the risk for metal flow onto flat areas of the investment or onto small areas such as line angles.
  • 25. 8. Reservoir:  The reservoir is placed approximately 1.5mm from the pattern .  Function – Prevents localized shrinkage porosity, because of the large mass of alloy and position in the heat centre of the ring , the reservoir will remain molten to furnish liquid alloy into the mold as it solidifies . Reservoir
  • 26. 9. Venting : • Small auxiliary sprues or vents have been recommended to improve casting of thin patterns and may help in :  Escape of gases during casting.  Compensate for the shrinkage during solidification.  Solidification begins in critical areas by acting as a heat sink.
  • 27. 10. Spruing technique DIRECT INDIRECT • The sprue former provides a direct connection between the pattern area and the sprue base or crucible former area. • A connector or reservoir bar is positioned between the pattern and the crucible former
  • 28. Crucible former  The sprue is attached to crucible former which constitutes the base of the casting ring during investing.  It also helps by holding sprue in desired ring.  Crucible formers are basically of 2 types: a) Steep-sided cone: used with metal when casted using centrifugal casting force. b) Shallow cone: used to cast metal using stream/air pressure.
  • 29.  They are available as-  Rubber crucible former  Metallic Crucible former  Plastic crucible former
  • 30.  The exact shape of the crucible former depends on the type of casting machine used.  With most modern machines, the crucible former is tall to allow use of a short sprue and allow the pattern to be positioned near the end of the casting ring.  They form a conical depression in investment, which guides flow of molten metal.  It should be clean and petroleum is applied to prevent formation of rough investment tag.  Then the end of sprue former is passed into the hole and held in position till the molten wax sets.
  • 31. Casting ring  Casting rings are used to confine the fluid investment around the wax pattern while the investment sets.  It also allow the hardened investment to be safely handled during burnout and casting.  The internal diameter of casting ring should be 5-10mm greater than the widest measurement of the pattern and about 6 mm higher.  For single crown/inlay - small rings are used, diameter - 32 mm  For large fixed partial denture – 63mm round/oval shaped casting ring are used.
  • 32. SHAPE •Round •Oval COMPLETE RING •Rigid : Metal , plastic •Flexible: Rubber SPLIT RING • Metal • Plastic Types of casting ring
  • 33. RING LESS CASTING SYSTEM  Ringless Casting System is designed to increase productivity by achieving consistently accurate results without the time-consuming steps associated with the use of metal casting rings.  Utilizes durable, reusable plastic rings that are tapered allowing for unimpeded expansion of investment and easy removal of mold prior to burnout.  This allows for quick and easy divesting after casting while reducing clean-up chores.  Investment expansion is easier to control and not limited to the thickness of a ring liner.
  • 34.
  • 35. Casting ring liners  The most commonly used technique to provide room for investment expansion.  Triple fold function : - Freedom to expand which would otherwise be restricted by the ring .  Helps to offset the contraction of the more rapidly cooling ring while the gold alloy is being melted .  In a wet liner certain amount of hygroscopic expansion is afforded and a thicker liner provides even greater semi hygroscopic expansion.
  • 36. • Asbestos liner: Asbestos is refractory to high temperature, they show a sufficient amount of water absorption. • There are 3 types of asbestos i. White asbestos (least toxic) this type is used in dentistry. ii. Blue asbestos (most toxic) iii. Brown asbestos(Intermediately toxic).  Asbestos is no longer used in dentistry because of its carcinogenic and toxic potential.
  • 37. • Cellulose liner: This material shows adequate water absorption. • It is burnt during burnout procedure. • To keep the investment in contact with ring after burnout,the liner is kept 3mm short of ring ends. • This also restricts the longitudinal setting and hygroscopic expansion.
  • 38. • Ceramic ring liner They are basically alumino-silicate fibrous material. • They do not absorb water to large extent, but its network of fibres can retain small amount of water on its surface. • They are refractory to high temperature. • The binders used in ceramic liner (Ex – neoprene-latex) can contribute to toxicity (stimulate fibrosis/ act as adsorbent surface for carcinogenesis). • They posses fibers of length 5.3-17.8 mm. Diameter 0.2- 0.97 mm
  • 39.  Disadvantages: Burned away during casting , difficult to secure in place.  Absorb materials much less than asbestos , and their combination with some gypsum bonded investments will produce fins .  Not compatible with phosphate bonded investments.
  • 40.
  • 41. Manipulation of a casting ring liner  Cut the liner to fit the inside diameter of the casting ring , no over lap.  Dry liner technique – Dry liner is tacked in position with sticky wax .  Wet liner technique –Lined ring is immersed in water and excess water is shaken away .  Avoid touching or adapting with fingers - reduce the cushioning effect.  Attach the liner firmly to the ring by wax to prevent it from “riding up” during investing and inadvertently affecting the size of the casting
  • 42.
  • 43. Investing  Investment materials are:  Gypsum bonded investment  Phosphate bonded investment  Ethyl silicate bonded investment The process of covering or enveloping an object such as a denture, tooth, wax form, crown, with a suitable investment material before processing, or casting
  • 44. Investing procedure : • The wax pattern should be cleaned of any debris, grease, or oils. • A commercial wax pattern cleaner or a diluted synthetic detergent may be used. • Any excess liquid is shaken off, and the pattern is left to air- dry while the investment is being prepared. • The thin film of cleanser left on the pattern reduces the surface tension of the wax and permits better “wetting” of the investment to ensure complete coverage of the intricate portions of the pattern
  • 45. • Vacuum mixing of investment materials is highly recommended for consistent results in investing and casting with minimal surface defects, especially when phosphate- bonded investments are used. Vacuum investing machines. The Whip Mix combination unit Girrbach Vacumat
  • 46.
  • 47.
  • 48. • Air bubbles that remain in the mix, even with vacuum mixing, can be entrapped on flat or concave surfaces that are not oriented suitably for air evacuation. • Tilting the ring slightly aids in releasing these bubbles so that they can rise to the surface. • Excessive vibration should be avoided because it can cause solids in the investment to settle and may lead to free-water accumulation adjacent to the wax pattern, resulting in surface roughness. • Excessive vibration can also dislodge small patterns from the sprue former , resulting in a miscast.
  • 49. • If the hygroscopic technique is employed, the filled casting ring is immediately placed in a 37 °C water bath with the crucible former’s side down. • For the thermal expansion or high-heat technique, the invested ring is allowed to bench set undisturbed for the time recommended by the manufacturer.
  • 50. Wax elimination  Wax elimination or burnout consists of heating the investment in a thermostatically controlled furnace until all traces of the wax are vaporized.  Once the investment has set for an appropriate period 1 hour it is ready for burnout.  The crucible former is then carefully removed and the invested rings are placed in a room temperature furnace and heated to the prescribed maximum temperature.
  • 51.
  • 53. • For gypsum-bonded investments, this temperature can be either 500 °C for the hygroscopic technique or 700 °C the thermal expansion technique. • With phosphate-bonded investments, the maximum temperature setting may range from 700 °C to 1030 °C, depending on the type of alloy selected. • It is also advisable to begin the burnout procedure while the mold is still wet. Water trapped in the pores of the investment reduces the absorption of wax, and as the water vaporizes, it flushes wax from the mold.
  • 54.  When the high-heat technique is used, the mold temperature generates enough heat to convert carbon to either carbon monoxide or carbon dioxide. These gases can then escape through the pores in the heated investment.  Hygroscopic low-heat technique  This technique obtains its compensation expansion from three sources: 1) 37 °C water bath expands the wax pattern, 2) The warm water entering the investment mold from the top adds some hygroscopic expansion 3) The thermal expansion at 500 °C provides the needed thermal expansion.
  • 55.  The molds should remain in the furnace for at least 60 minutes, and they may be held up to 5 hours longer with little damage.  Even though the mold is usually held at this temperature for 60 to 90 minutes, sufficient residual fine carbon may be retained to reduce the venting of the mold.  Because of this potential for reduced venting, back-pressure porosity is a greater hazard in the low-heat technique.  Advantages: i. Less investment degradation ii. A cooler surface for smoother castings iii. The convenience of placing the molds directly in the 500°C furnace. iv. The last benefit makes it possible to keep one or more furnaces at the burnout temperature so that molds may be put in as they are ready.
  • 56.  The standardized hygroscopic technique was developed for alloys with a high gold content; the newer noble alloys may require slightly more expansion.  This added expansion may be obtained by making one or more of the following changes:  Increasing the water bath temperature to 40 °C  Using two layers of liner  Increasing the burnout temperature to a range of 600 °C to 650 °C • In the low-heat casting technique, the alloy should also be cast soon after removal of the ring from the oven; otherwise a significant variation from the desired casting dimensions may occur.
  • 57. High-heat thermal expansion technique :  Gypsum bonded Investment : • o These casting investments are relatively fragile and require the use of a metal ring for protection during heating. o The molds are usually placed in a furnace at room temperature, slowly heated to 650 °C to 700 °C in 60 minutes, and held for 15 to 30 minutes at the upper temperature. o Too rapid a heating rate may also cause cracking of the investment. In such a case, the outside layer of the investment expands much more than the center sections.
  • 58.  Investment decomposition and alloy contamination is related to a chemical reaction between the residual carbon and calcium sulfate binder.  The reduction of calcium sulfate by carbon takes place rapidly above 700 °C in accordance with the following reactions.  The sulfur dioxide as a product of this reaction contaminates gold castings and makes them extremely brittle. • CaSO4 + 4C= CaS + 4CO • 3CaSO4 + CaS → 4CaO + 4SO2
  • 59.  Therefore , after the casting temperature has been reached, the casting should be made immediately.  Maintaining a high temperature for a considerable length of time may result in sulfur contamination of the casting and also in a rough surface on the casting because of the disintegration of the investment.  A few gypsum investments, some with a considerable amount of cristobalite, are now offered for use with a much more rapid burnout procedure.
  • 60.  Phosphate bonded investment : o They need higher 2nd stage temperature for total elimination of wax and to prevent premature solidification of higher melting alloys. o After the temperature reaches 400 °C, the rate of heating can be safely increased. After burnout, usually at a final temperature of 700 °C to 1030 °C depending on the alloy melting range, the casting is made. o Because the entire process involving phosphate investments takes a long time, the demand for time-saving changes is strong.
  • 61. Accelerated casting method :  Conventional casting techniques require considerable time, typically 1 hour bench set for the investment and 1 to 2 hours for the wax elimination.  Accelerated casting procedures have been proposed that reduce this time to 30 to 40 minutes .  Initially suggested as a way to make cast post-and-core restorations in a one-visit procedure.  The technique uses a phosphate-bonded investment that is given approximately 15 minutes bench and a 15-minute wax elimination by placing the ring in a furnace preheated to 815° C (1500° F).
  • 62. CASTING Casting procedure: It is a process of obtaining a metallic duplicate of a missing tooth structure by pouring molten metal into a mold of a required form & allowing it to solidify to obtain a metallic duplicate. Something that has been cast in a mold; an object formed by the solidification of a fluid that has been poured or injected into a refractory mold.(GPT-9)
  • 63. CASTING MACHINES  Alloys are melted in one of the four following ways: Gas/Air (Most Common) Resistance Gas/Oxygen Induction Air /Acetylene Direct Current Arc Oxygen /Acetylene TORCH ELECTRICAL
  • 64. Types of torch flame : 
  • 65. Gas air torch :  Gas-air torch is used to melt conventional noble metal alloys (used for inlays, crown and bridge) whose melting points less than 1000⁰c. Gas oxygen torch  Used to melt metal ceramic alloys of higher temperature up to 1200⁰c. The tip of torch is available as single orifice/multiorifice the oxygen pressure is adjusted to 10-15 psi.  The flame is directed onto metal with the nozzle of the torch about 1.5 cm away from the metal.  Complete fluid should be obtained within 30 second at which point the metal is poured into the mould.
  • 66. Air acetylene & oxygen acetylene gas  These were designed mainly for Cobalt chromium base alloys.  The actual production of flame can be done by adjusting the pressure and flow of individual gases .  One part of acetylene + 2 and half part of oxygen Advantage : Hottest flame hence faster burnout . Disadvantages : Excessive heat may distill lower melting components .  Overheating – gases to dissolve in the casting – porosity  Highly technique sensitive
  • 67.  The best results are obtained when flame is used with a distance of 10cm between the face of blow torch nozzle and the base of crucible.  When the reducing zone is in contact, the surface of the gold alloy is bright and mirror like.  When the oxidizing portion of the flame is in contact with alloy there is a dull film developed over the surface
  • 68. TORCH MELTING OF NOBLE METAL ALLOY  The alloy is melted in a separate crucible by a torch flame and cast into the mold by centrifugal force.  Temperature of gas-air flame is influenced by :  Nature of the gas  Proportion of gas and air in the mixture. • Two types of flames can be obtained with a casting torch, the air supply for the lower flame is excessive, so that incomplete combustion and a lower temperature will result. • The upper brush flame indicates the proper adjustment for maximal efficiency and temperature.
  • 69.  The parts of the flame can be identified by the conical areas:  The first long cone emanating directly from the nozzle is the zone in which the air and gas are mixed before combustion. No heat is present in this zone.  The next cone, which is green and immediately surrounding the inner cone, is known as the combustion zone. Here, the gas and air are partially burned. This zone is definitely oxidizing and should always be kept away from the molten alloy during fusion.
  • 70.  The next zone, dimly blue and located just beyond the tip of the green combustion zone, is the reducing zone.  This is the hottest part of the flame, and it should be kept constantly on the alloy during melting.  The outer cone (oxidizing zone) is the area in which combustion occurs with the oxygen in the air.  Under no circumstances should this portion of the flame be used to melt the alloy. Not only is its temperature lower than that of the reducing zone but it also oxidizes the alloy
  • 71.  The alloy first appears to be spongy and then small globules of fused alloy appear. The molten alloy soon assumes a spheroidal shape.  At the proper casting temperature, the molten alloy is light orange and tends to spin or follow the flame when the latter is moved slightly.
  • 72.  At this point, the alloy should be approximately 38 °C to 66 °C above its liquidus temperature. The casting should be made immediately when the proper temperature is reached. Flux:  Flux for gold crown and bridge alloys to aid in minimizing porosity.  When properly used, the flux increases the fluidity of the alloy and the film of flux formed on the surface of the molten alloy helps prevent oxidation.  Reducing fluxes are excellent for cleaning old alloy, a better flux for the casting procedure may be made from equal parts of fused borax powder ground with boric acid powder.
  • 73.  The boric acid aids in retaining the borax on the surface of the alloy. The flux is added when the alloy is completely melted and should be used with both old and new alloy.  Old sprues and buttons from the same alloy may be recast if they are not contaminated.
  • 74.
  • 75. B. Electrical resistance–heated casting machine :  During electrical melting of alloys heat energy is produced when electric current is passed through a conductor depending upon the voltage applied across it.  The alloy is melted electrically by a resistance heating .  Current is passed through a resistance heating conductor, and automatic melting of the alloy occurs in a graphite or ceramic crucible.  A carbon crucible should not be used in the melting of high- palladium alloys, palladium-silver alloys, nickel-chromium alloys, or cobalt chromium base metal alloys.
  • 76.  This is an advantage, especially for alloys such as those used for metal- ceramic prostheses, which are alloyed with base metals in trace amounts that tend to oxidize on overheating.  Another advantage is that the crucible in the furnace is located flush against the casting ring. Therefore, the alloy button remains molten slightly longer, again ensuring that solidification progresses completely from the tip of the casting to the button surface.
  • 77. C. 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. It is more commonly used for melting base metal alloys.
  • 78. k
  • 79. D. 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.
  • 80. E. Vacuum- or pressure-assisted casting machine  First evacuate the melting chamber to reduce oxidation.  Apply air pressure uniformly about the casting ring forcing molten alloy into mold.  Vacuum is applied to the bottom of the mold.  Molten alloy is “PUSHED & SUCK ED” into the mold by gravity or vacuum.  Used for titanium and titanium alloys.
  • 81. CASTING CRUCIBLES:  The melting of alloy requires a crucible to act as a platform on which the heat can be applied to the metal.  Generally four types of casting crucibles are available: clay, carbon, quartz, and zirconia-alumina.  Traditionally a wet lining of asbestos sheet was used on casting crucible. The moistened asbestos sheet provides a clean and good surface on which the alloy could be melted.  Advantages is, prevent alloy contamination with oxides and residuals that may be present in the crucible.
  • 82.  Carbon crucibles can be used not only for high noble crown and bridge alloys but also for the higher-fusing gold-based metal-ceramic alloys.  Carbon crucibles should not be used in melting of high palladium, palladium silver alloys (to be melted above 1504⁰c) and also with nickel chromium/cobalt chromium base metal alloys
  • 83.  Clay crucibles are appropriate for many of the crown and bridge alloys, such as the high noble and noble types.  Crucibles made from alumina, quartz, or silica are recommended for high-fusing alloys of any type. These are especially suited for alloys that have a high melting temperature or those that are sensitive to carbon contamination.
  • 84. Casting force  Casting force > surface tension of alloy + resistance offered by gas in the mold .  This can be done by use of following different type of force:  Vacuum force  Air or Gas Pressure  Centrifugal force  Sufficient mass of alloy must be present to sustain adequate casting pressure  6g is typically adequate for premolar and anterior casting  10g is adequate for molar casting  12 g is adequate for pontic
  • 85. Cleaning the casting Quenching  When a type III or IV gold alloy has been cast and it has solidified, the ring should be quenched in water as soon as the button exhibits a dull-red glow.
  • 86.  Two advantages are gained in quenching: a) The noble metal alloy is left in an annealed condition for burnishing, polishing, and similar procedures . b) When the water contacts the hot investment, a violent reaction ensues, resulting in a soft, granular investment that is easily removed.
  • 87. Divesting: •Removal of Investment / Recovery of Casting. • Hold the end of the ring for about ¼ inch. • Bulk – finger pressure. • Sprue is removed from the restoration using a carborundum separating disk/ abrasive disk mounted in a hand piece.
  • 88. Pickling  Surface of the casting appears dark with oxides and tarnish. Such a surface film can be removed by a process called pickling. Solutions used:  50% HCl  Phosphoric acid,  Hydrofluoric acid  Advantages of HCl: Aids in removal of residual investment as well as oxide coating.  Disadvantages: Likely to corrode laboratory metal furnishings and fumes are health hazard
  • 89.  Method of cleaning : a) Place the casting in test tube or dish and pour acid over it. b) Heating the casting and then dropping into the pickling solution.  Gold and palladium based metal ceramic alloys and base metals, these alloys are not generally pickled if they are bench cooled.  Pickling solution should be renewed frequently, since it is likely to become contaminated.  Precious alloys(Gold-Platinum-Palladium) can be soaked with hydroflouric acid .
  • 90.  Nickel Chromium should never be placed in acid because of high reactivity.  Ultrasonic devices are useful for cleaning the casting, as are commercial pickling solutions made of acid salts. Abrasive blasting devices are also useful for cleaning the surface of castings.  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.
  • 91.  Recovery and cleaning of the casting are more difficult when a phosphate-bonded investment is used because such materials do not contain the soft gypsum products.  Also, the particles usually include large grains of quartz.  In some instances, such as with gold-containing alloys, the investment adheres rather tenaciously, usually requiring cleaning in an ultrasonic device.  Neither the phosphate binder nor the silica refractory is soluble in HCl or H2SO4.
  • 92. Sand blasting :  Casting is held in a sand blasting machine to clean the investment from the surface.  Base metal alloys require a light sandblasting, usually with fine alumina.  Chromium-based partial dentures are usually sandblasted to remove the investment. Acid should not be used for cleaning base metal alloys
  • 93.
  • 94. Finishing and polishing :  Brown or White Al2O3 stones are used.  Rag or felt wheels impregnated with abrasives are used in the initial phase of this stage.  Final polishing is achieved using various oxides of tin and aluminium used in conjunction with a small rag or chamois buffing wheel, followed with an iron oxide rouge.  Residual traces of rosin or waxlike matrix from oxides  Polishing compound remover followed by a hot, soapy water rinse.
  • 95.
  • 96. References 1. Anusavice K.J.-“Phillips’ Science of Dental materials” 11th edition , 2003.Pg 319-335. 2. Contemporary fixed prosthodontics rosenstiel,land , fujimoto- 5 th edition , Pg 601- 618. 3. Removable partial prosthodontics , Mc Cracken’s, 13th edition, Pg 259 to 265.

Editor's Notes

  1. In dentistry, successful results depend on careful handling of the wax. The practitioner must understand that every defect or void in the wax will appear in the casting
  2. WHY 6mm ? Provides adequate bulk of investment to withstand force of inrushing gold, yet still allows gases to escape from the end of the mold . More than 6 mm – The gold will solidify before the entrapped air can escape , resulting in rounded margins , incomplete casting , or mold fracture
  3. Example spruing on the cusp tip can give good results, but spruing on the proximal contact may produce a casting that is too wide mesiodistally and too short occlusocervically.
  4. Should not be attached at a right angle to a broad flat surface- lead to turbulence within mold cavity & porosity.
  5. Flaring of the sprue former may act in much the same way as a reservoir, facilitating the entry of the fluid alloy into the pattern area
  6. Diameter should be greater than the bulkiest portion of the sprue .
  7. It is common to use indirect spruing for multiple single units and fixed partial dentures, although several single units can be sprued with multiple direct sprue formers.
  8. Carcinogenic , biohazard .
  9. Carcinogenic , biohazard .
  10. They show potential for development of Mesothelioma
  11. They show potential for development of Mesothelioma
  12. Insert the sprue into the hole in the crucible former, holding it with forceps (Fig. 22-10, C. It should now be luted into place with wax and the junction between sprue and crucible should be smoothed. Use of a surfactant greatly enhances wetting of the pattern during investing (Fig. 22-10, D). 5. Line the casting ring, keeping it flush with the open end, and moisten the liner (Fig. 22-10, E). 6. Place the ring over the pattern to ensure it is sufficiently long to cover the pattern with about 6 mm of investment (Fig. 22-10, F). If necessary, the sprue may be shortened, or a longer ring may be chosen as an alternative.
  13. Carcinogenic , biohazard .
  14. Good results are possible with brush application of vacuum-mixed investment or when the investment is poured into the ring under vacuum.
  15. Good results are possible with brush application of vacuum-mixed investment or when the investment is poured into the ring under vacuum.
  16. Good results are possible with brush application of vacuum-mixed investment or when the investment is poured into the ring under vacuum.
  17. Good results are possible with brush application of vacuum-mixed investment or when the investment is poured into the ring under vacuum.
  18. .
  19. The temperature setting is more critical with gypsum-bonded investments than for the phosphate type because the gypsum investments are more prone to investment decomposition. During burnout, some of the melted wax is absorbed by the investment and residual carbon produced by ignition of the liquid wax becomes trapped in the porous investment.
  20. Reducing fluxes containing powdered charcoal are often used, but small bits of carbon may be carried into the mold and cause a deficiency at a critical margin
  21. The temperature of the gas-air flame is greatly influenced by the nature of the gas and the proportions of gas and air in the mixture. Considerable care should be taken to obtain a nonluminous brush flame with the different
  22. The device has become popular in the casting of jewelry, but it has not been used as much as the other two techniques for noble alloy castings.
  23. The device has become popular in the casting of jewelry, but it has not been used as much as the other two techniques for noble alloy castings.
  24. Crown and bridge alloys with a high palladium content—such as palladium-silver alloys for metal-ceramic copings and any of the nickel-based or cobalt-based alloys—are included in this category
  25. Crown and bridge alloys with a high palladium content—such as palladium-silver alloys for metal-ceramic copings and any of the nickel-based or cobalt-based alloys—are included in this category
  26. Crown and bridge alloys with a high palladium content—such as palladium-silver alloys for metal-ceramic copings and any of the nickel-based or cobalt-based alloys—are included in this category
  27. Crown and bridge alloys with a high palladium content—such as palladium-silver alloys for metal-ceramic copings and any of the nickel-based or cobalt-based alloys—are included in this category
  28. The temperature of the gas-air flame is greatly influenced by the nature of the gas and the proportions of gas and air in the mixture. Considerable care should be taken to obtain a nonluminous brush flame with the different
  29. The temperature of the gas-air flame is greatly influenced by the nature of the gas and the proportions of gas and air in the mixture. Considerable care should be taken to obtain a nonluminous brush flame with the different
  30. Cobalt cr allooys are usaaly divested and are not pckled.
  31. The temperature of the gas-air flame is greatly influenced by the nature of the gas and the proportions of gas and air in the mixture. Considerable care should be taken to obtain a nonluminous brush flame with the different
  32. The temperature of the gas-air flame is greatly influenced by the nature of the gas and the proportions of gas and air in the mixture. Considerable care should be taken to obtain a nonluminous brush flame with the different
  33. The pickling solution usually contains small amounts of copper dissolved from previous castings. When the steel tongs contact this electrolyte, a small galvanic cell is created and copper is deposited on the casting at the point where the tongs grip it. This copper deposition extends into the alloy and is a future source for discoloration in the area
  34. The temperature of the gas-air flame is greatly influenced by the nature of the gas and the proportions of gas and air in the mixture. Considerable care should be taken to obtain a nonluminous brush flame with the different
  35. The temperature of the gas-air flame is greatly influenced by the nature of the gas and the proportions of gas and air in the mixture. Considerable care should be taken to obtain a nonluminous brush flame with the different
  36. The temperature of the gas-air flame is greatly influenced by the nature of the gas and the proportions of gas and air in the mixture. Considerable care should be taken to obtain a nonluminous brush flame with the different
  37. The temperature of the gas-air flame is greatly influenced by the nature of the gas and the proportions of gas and air in the mixture. Considerable care should be taken to obtain a nonluminous brush flame with the different