CASTING PROCESS · 2020-03-17 · Introduction Casting is the process by which a wax pattern of a...
Transcript of CASTING PROCESS · 2020-03-17 · Introduction Casting is the process by which a wax pattern of a...
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CASTING PROCESS
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Contents
Introduction Preparation of a die
Wax Pattern fabrication
Sprue Former
Reservoir
Casting Ring liner
Investing
Burnout
Lost wax Technique
Crucibles
Casting
Casting machines
Cleaning of casting
Take home message
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The casting method consists of forming a wax pattern , surrounding it with investment material , and later heating the investment mold to remove the wax before casting the molten metal into the mold .
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Introduction Casting is the process by which a wax
pattern of a restoration is converted to a
replicate in a dental alloy .
The casting process is used to make dental restorations such as:
Inlays and onlays
crowns, bridges
removable partial dentures.
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Introduction In dentistry the resulting casting must be an accurate reproduction of the wax pattern in both surface details and overall dimension.
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Reproducing the wax up in metal
with predictable results has always
been a challenge.
Small variation in investing or casting
can significantly effect the quality of
the final restoration.
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Definition
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“ Casting is the process by which a
wax pattern of a prepared tooth is
fabricated and converted to its
metallic replica”(Rosenteil)
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STEPS IN MAKING A
RESTORATION
CAST
1 . TOOTH PREPARATION PREPARATION
2 . IMPRESSION
/CAVITY
3. 4. 5. 6.
DIE PREPARATION WAX PATTERN FABRICATION SPRUING INVESTING
7. BURNOUT
8. CASTING
9. CLEANING & POLISHING 18
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Die
Defined as the positive reproduction of the form of the prepared
tooth in any suitable substance in which inlays, crowns & other
restorations are made.
•
IDEAL REQUIREMENTS
Accurate reproduction of the fine details
Dimensional accuracy
Good strength & hardness
Ease of use
Abrasion resistance
Relatively inexpensive
•
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•
•
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•
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PREPRATION OF MASTER DIE :
Commonly used die materials-
Type IV gypsum product - 0.1%
(setting expansion)
Type V gypsum product
expansion)
- 0.3 %( setting
Disadvantage- Susceptibility to abrasion
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Means to increase abrasion resistance of die :
-Silver plating
-Coating hardener
- Adding
gypsum
surface with die
die hardener to
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Die spacer Used to provide relief space for cement.
Applied within 0.5mm of the preparation finish line to provide relief
for the cement luting agent.
Example- resins (most commonly used)
- -
-
model paint
colored nail polish
thermoplastic polymers dissolved in volatile solvents.
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Diestone/investment combination
In this, die material and investment medium have
a comparable composition
Divestment – a commercially available gypsum bonded material .
Divestment is mixed with colloidal silica and die is
made from this mix & wax pattern is constructed.
Advantage- It eliminate possibility of of wax pattern while removing it from
distortion die .
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OTHER DIE MATERIALS
Amalgam
Acrylic resins (shrinkage 0.6%)
Polyester resin
Epoxy resins (less shrinkage then
acrylic resins0.1to0.2%)
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Wax pattern fabrication
- It is contouring of wax
pattern into desired shape and
form.
- Shortest time
should elapse
between the time
the pattern is
removed from the
die & the time it is
invested.
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Wax Technique
A) DIRECT B) INDIRECT
Wax pattern is made
directly inside the
mouth, as in inlay case.
Wax pattern is
the die.
made on
Type-1 medium wax. Type -2 soft wax.
Exceptionally demanding procedure.
More commonly used
technique.
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Sprue & Sprue former
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Sprue Part of casting that acts as a
channel for the molten metal to flow into the mold cavity after the wax has been eliminated.
.
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PURPOSE: To form a mount for the wax pattern & fix the pattern in space so a mold can be made.
To create a channel for elimination of wax during burn out.
To form channel for ingress of molten alloy To compensate for alloy shrinkage
during solidification.
.
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Wax pattern attached to the crucible former with a sprue ready for investing. A ring liner is in place.
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Spruing of wax pattern :
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FUNCTION:
Facilitate flow of molten metal from crucible to mold.
Store additional metal & prevent shrinkage porosity.
May be used as handle to remove wax pattern.
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Types of Sprue
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wax, plastic and metal ------ main sprue wax or plastic ------accessory sprue only. Wax sprue :melt at the same rate as the pattern and so allow an easy escape of the molten wax. plastic sprue :higher temperature than the wax pattern which may cause the interruption of the escape of wax resulting in roughness of the casting. It is mainly used for casting fixed partial dentures due to its rigidity. Metal sprue : are often a hollow sprue which provides a stronger attachment to the pattern and less heat transfer to wax pattern, so causes less distortion of the wax pattern.
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Sprue
DIAMETER LOCATION LENGTH ATTACHMENT
DIRECTION
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Diameter of the sprue former the diameter of the sprue should be larger than the thickest part of the wax pattern rate of flow of the molten alloy into the mold cavity is controlled by: 1- the diameter of the sprue 2- the pressure of the casting machine 3- the density of the molten alloy This diameter will also ensure a reservoir during solidification [2.5 mm] sprue --------molar and metal ceramic, [2.0 mm] sprue -------- premolars and partial coverage restoration.
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Location of the sprue former The sprue former should be attached to the bulkiest portion of the wax pattern to minimize the distortion of the wax pattern because the sprue is away from the fine margins. The sprue former should be attached at an angle to allow the molten alloy to flow freely to all portions of the mold (fine margins) without flowing in an opposite direction of the casting force.
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If the sprue is directed at right angle to the mold , a hot spot may be created at this point . This will lead to the fact that the alloy adjacent to it molten after the rest of the casting has been solidified, causing "suck-back porosity ” .
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Multiple sprue formers When 2 units or more are being cast together, each is joined to a
runner bar. A single sprue is used to feed the runner bar. In case of 2 units, they may be cast with a runner bar or each fed from a separate sprue.
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Venting It is recommended to improve casting of thin patterns .
A vent is needed to help gases escape during casting or to ensure that alloy solidification started by acting as a
heat sink .
This vent is made of small auxiliary sprue formers .
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Sprue former
The sprue is attached to a crucible former, usually made of rubber, which constitutes the base of the casting ring during investing.
May be metal , plastic or rubber
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. also referred to as a sprue former
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Crucible formers/ Sprue 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
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RESERVOIR
Piece of wax attached to the sprue about 1mm away from the pattern ,as a enlarged round mass or a
connector bar between the wax pattern sprue former.
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ImporImporttaancence
Importance :
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SURFACE TREATMENT OF COMPLETED WAX PATTERN
Before the wax pattern is invested, it should be cleaned of any debris, grease, oils and separating medium.
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Casting ring & liner The casting ring serves as a container
for the investment while it sets & restricts
the mold. setting expansion of
Metal casting
ring
Sprue base or
Crucible former
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Ring liner
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Casting Ring They are available as-
1) Shapes - Round - Oval
2) Complete rings I) - Rigid
- Metal (stainless steel)
- Plastic
II) Flexible - Rubber Split rings
I) metal
II) plastic
3)
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Casting Ring Considerations :
1) The internal diameter of casting ring should be 5-10mm greater than the widest measurement of the pattern mm higher.
and about 6
2) For single crown/inlay - small rings as used. Diameter - 32 mm
3) For large fixed partial denture – 63mm round/oval shaped casting ring are used
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CASTING RING LINERS
NON ASBESTOS LINERS
CELLULOSE PAPER AL-SILICATE CERAMIC
ASBESTOS LINERS
Asbestos:- carcinogenic potential makes it a biohazard.
Functions of a liner Affords greater normal expansion in the
investment The absorbed water causes a semi hygroscopic
expansion Thickness – not less than 1mm
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Liner technique A)DRY LINER TECHNIQUE - tacked in
position with sticky wax. B)WET LINER TECHNIQUE - lining ring is
water is shaken immersed in water & excess off.
The liner is cut to fit the inside diameter of the casting ring with no overlap and 3mm
short of the top and bottom of the ring , this
serves to lock the investment within the ring
& equalize expansion.
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Casting ring liner
Wet dry
Uniformly wet
Min imal th icknes s not< "J rnrn Avoid squezing
Helps cause
semihygroscopic
expansion
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RINGLESS CASTING TECHNIQUE
With the use of higher-strength, phosphate- Bonded investments, the ringless technique has
become quite popular.
The method uses a paper or plastic casting ring
unrestricted expansion. and is designed to allow
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The crucible former and plastic ring are removed
before wax elimination, leaving the invested wax pattern.
The systems are designed to achieve expansion that
is unrestricted by a metal ring.
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The position of the pattern in the casting ring affects expansion:
single crown : should be positioned within the ring equidistant from its walls.
fixed prostheses : are being cast as one piece, greater accuracy will be achieved if the pattern is placed near the center of a large or special oval ring rather than near the edge of a smaller ring .
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Investing
Process by which the sprued wax pattern is embedded in a material called an investment.
OR
A ceramic material that is suitable for forming mould into which a metal or alloy is cast.
a
The operation INVESTING
of forming a mold is known as
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Components of investment materials
• All the investment materials are composed of :
• 1-Refractory material .
• 2-Binder .
• 3-Modifiers
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1-Refractory materials : 65%
• A material that will not decompose or disintegrate
on heating such as : silica
• Quartz and cristobalite
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• ROLE OF REFRACTORY MATERIAL
• -Resist high temperature .
• -Produce thermal expansion.
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2-Binder : 30%
• -Since the refractory material do not form a coherent mass some kind of binders are needed to bind the silica particles .
• -Also binder give more strength and rigidity to the investment .
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3-Modifiers: 5%
• Such as boric acid, copper powder, sodium chloride and graphite,
• FUNCTIONS OF MODIFIERS
• -Provide non oxidizing atmosphere in the mold(e.g. carbon & copper powder act as reducing agents when gold alloys are cast .
• -Dissolve black color from the surface .
• -Regulate setting time and setting expansion . e.g. Boric acid.
• -Coloring agents are also added .
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Types of investment materials according to the
type of the binder:
• Gypsum bonded investment material
• Phosphate bonded investment material
• Silicate bonded investment material
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Gypsum bonded material :
• Refractory material : Silica is present in one of
its allotropic forms quartz or cristobalite.
• The binder : Is α-calcium sulphate hemihydrate
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Gypsum bonded investment :
• It used for casting gold alloys.
• Chemicals and modifiers are added such as graphite or
charcoal to reduce any oxides formed on the metal ,
boric acid is added to prevent shrinkage of gypsum
during heating
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Properties of the gypsum bonded investment :
• 1) Dimensional changes :
• A-setting expansion:
Expansion occurs by out word thrusting of growing crystal
of gypsum.
• B-hygroscopic expansion:
Obtained by allowing the gypsum to set under water.
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Hygroscopic expansion can be controlled by :
1. Time of immersion
2.W/P ratio of the original mix
• Decrease W/P >> hygroscopic expansion will
increase
3.Silica content, silica size and shape : increase
silica will decrease hygroscopic expansion
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C- Thermal expansion : It is due to the displacive
transformation of silica
D- shrinkage on heat :This occurs due to the dehydration
of gypsum Shrinkage eliminated by adding small quantity
of NaCl or boric acid .
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2- Strength :Gypsum bonded investment materials has sufficient strength to withstand the driving force of molten gold alloy during casting . 3- Porosity :gypsum bonded investment materials have adequate porosity to prevent back pressure porosity and allow escape of air from the mold 4-fineness :the investment which contains finer particles of silica and calcium sulfate hemihydrate give smooth surface on the finished casting .
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Limitation of using gypsum investment :
• Used with gold alloy only ???
• Because gypsum bonded investment when heated until 700 c give sufficient thermal expansion which compensate shrinkage of gold alloy so it is suitable for using with gold alloy
• Also gypsum has sufficient strength which is similar to the strength of gold alloy which makes gypsum withstands the force of molten gold alloy when entering the mould .
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Phosphate bonded investment
• Refractory material : silica
• Binder : magnesium ammonium phosphate
• Result from reaction between ammonium hydrogen
phosphate and magnesium oxide
• Water which result from reaction reduce viscosity of the mix
• Excess of ammonium hydrogen phosphate react with silica
during setting and increase investment strength
• Mix with silica sol or water
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• When mix with silica sol used with base metal alloy
• When mix with water used with gold alloy
•
• Can with stand tem 1000 to 1100 c
• Casting ring with gold alloy is metal
• Casting ring with bas metal alloy is rubber
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Properties of phosphate investment
• 1) Dimensional changes
• Setting expansion
• Hygroscopic expansion
• Thermal expansion
• Thermal shrinkage
• 2) strength
• 3) porosity
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Silica bonded investment
• Refractory material : silica
• Binder: polysilicic acid
• Steps of formation:
• Hydrolysis to ethyl silicate in presence of HCL and give silicic acid and alcohol.
• Gelation silicic acid bind silica particles together and then accumulate to form polysilicic acid gel (setting shrinkage)
• Dryness: on heat, loss to water and alcohol and silica particles tightly packed together (green shrinkage)
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Propoperties
• Dimensional changes
• Setting shrinkage
• Green shrinkage
• Thermal expansion
• Porosity
• strength
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While the wax pattern is air drying, the appropriate amount of distilled
water (Gypsum Bonded
investments), colloidal silica special
liquid (Phosphate Bonded
investments) is dispensed.
Powder should be weighed before
mixing it with liquid.
The liquid is added to clean dry
mixing bowl, and the powder is
gradually added to the liquid using
care and caution to minimize air
entrapment.
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Paint off technique
Wet investment material is gently painted over a
complex wax pattern by
the use of sable hair brush,
covering it completely .
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Poured into the ring thinly from a height (20-30)cm at
a slight angle from
bottom to the top of a ring
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Allow the investment (45 – 60) min before
to set
burnout procedure
commences
Place invested pattern in a
humidator if burnout process is delayed.
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METHODS OF INVESTING: A) HAND INVESTING
B) VACCUM INVESTING
Advantages of vacuum investing The amount of Porosity in the reduced
investment is •
The texture of cast surface is smoother with better detail reproduction The tensile strength is greatly increased
•
•
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Brush technique of investing
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Vaccum Investing
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•When the investment has set, the "skin" at the top of the ring
is trimmed off. •The rubber crucible former is removed, and any loose particles
of investment are blown off. •The ring is then placed in the furnace for the recommended burnout schedule
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BURN OUT Elimination of the wax
pattern from the mold of set investment is referred to as a
burnout.
Ring may be placed on a raised object within the oven
to completely eliminate the
wax & form a cavity into
which the molten metal is
cast. Oven is preheated to approx. 400 degree C for 20 mins. Temperature raised slowly to 700 degree C for 30 mins.
•
•
•
•
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For gypsum investment. 500 degree c - hygroscopic expansion
tech.
700 degree c - thermal expansion
tech.
b) For phosphate investment 700 - 1030
degree c
a)
c) Ethyl silicate degree c
bonded investment- 1090
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The ring should be maintained long
enough at the maximum temperature
(“heat soak”( to minimize a sudden drop
in temperature upon removal from the
oven.
Such a drop could result in an
incomplete casting because of excessively
rapid solidification of the alloy as it enters
the mold.
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If materials used during the casting process didn't shrink or expand, the size of the final cast
restoration would be the same as the original wax
pattern. The management of dimensional changes is
complex, but can be summarized by the equation:
wax shrinkage + metal shrinkage = wax
expansion + setting expansion +
hygroscopic expansion + thermal expansion
.
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Casting Casting of an alloy into the mold space
uses 2 basic requirements:
A) Heat source – to melt the alloy
B) Casting force – to force molten alloy
into mold
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Casting force > surface tension of alloy
+ resistance offered by gas in
mold
the
This can be done by use
different type of force-
Vacuum force
Air or Gas Pressure
Centrifugal force
of following
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Melting Melting Melting
temp temp
temp
of of
of
pure gold –1063⁰c gold base
alloy-924-960⁰c metal alloy-1155-1304⁰c
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A) Heat Source: Different types of materials and method are used as heat source to melt alloy.
1)
Two basic modes are
Torch flame–
Gas air
Gas oxygen
Air acetylene
Oxygen acetylene.
by using
hydrogen oxygen generator
Electricity 2)
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METHODS OF MELTING ALLOY
A)TORCH MELTING
-For low temperature metals
-Mixture of natural/artificial gas, oxygen/tank gas –oxyacetylene
B) ELECTRIC MELTING -For higher temperature
metals.
electric resistance melting, induction melting.
Less faster than electric heating but more faster than resistance heating.
melts alloy faster,& can
be easily over heated
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Two type of torch tips: 1.Multi-orifice
2.Single-orifice
Zones of the blow torch flame:
Zone 1
Zone 2
Zone 3
Zone 4
- colorless zone /Non
– Combustion zone
– Reducing zone
- oxidizing zone
combustion zone
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Melting methods 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
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Gas oxy 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 to10- 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.
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Oxy acetylene torch : The actual production of flame can be done by adjusting the pressure and flow of individual gases . commonly advised pressure for acetylene nozzle is 3.5 N/cm2 and oxygen nozzle 7-10 N/cm2
one part of acetylene + 2 and half part of oxygen 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. If distance is reduced to –
- 7.5 mm -slight porosity - 5 mm -increased porosity
gas due to occluded H2
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CRUCIBLES :
The Melting of alloy requires a crucible to act
be as a platform on which the heat
Carbon
can
applied to the meQtaual.rtz Clay
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1. Clay - High noble & noble metal types
2. Carbon - High noble crown and bridge alloys also for higher fusing gold-based
metal ceramic alloys.
3. Quartz - Higher fusing, gold based metal ceramic
alloy & palladium alloys.
4. Zirconia – alumina -High fusing alloys of any type :
specially for alloys that have temperature or are sensitive
a high melting to carbon contamination.
,Ni based or Co based (Alloys like High Pd,Pd - Ag
are included in this category)
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• The crucibles used with noble metal alloys should
not be used for melting base metal alloy
•Crucible should be discarded if it contains large
amount of oxides and contaminants from
metals
•Sufficient mass of alloy must be present
adequate casting pressure---
previous
to sustain
6gm is typically adequate for premolar casting
10gm is adequate for molar casting
and anterior
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Casting machines Air pressure casting machine.
Torch melting /Centrifugal casting machine.
Electrical resistance–heated casting machine.
Induction melting casting machine.
Vacuum or pressure assisted casting
machine.
Direct-current arc melting machine.
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15 p u ua y ppl ed
Air Pressure casting machine
Alloy is melted in the hollow left by the crucible former by torch flame and then air pressure is applied through a piston. Carbon dioxide, carbon monoxide or nitrogen
gas can be used.
Pressure of 10- si is s ll a i .
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Vaccum casting machine Vaccum is applied through the base beneath the casting
ring and the molten alloy can be drawn into the mold by
NEGATIVE PRESSURE. In this case, the material is
sucked upwards into the mold by a vacuum pump. The
mold in an inverted position from the usual casting
process, is lowered into the flask with the molten metal.
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CENTRIFUGAL CASTING MACHINE
This machine makes use of centrifugal force to thrust the liquid metal into the mold.
Centrifugal casting is a method of casting parts having axial
symmetry. The method involves pouring molten metal into a
cylindrical mold spinning about its
axis of symmetry. The mold is kept rotating till the metal has
solidified. Mold material like steels, Cast irons, Graphite may be used.
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Centrifugal casting is carried out
as follows: •The mold wall is coated by a
refractory ceramic coating •Starting rotation of the mold at
a predetermined speed. •Pouring a molten metal directly into
the mold •The mold is stopped after the casting
has solidified. •Extraction of the casting from the mold.
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Direct current
The alloy is
vacuum melted &
cast by pressure in
an argon
atmosphere. Direct current arc
is produced between
2 electrodes Alloy & water cooled
tungsten electrode.
The temperature within the arc exceeds 4000 ˚c, the alloy melts very quickly.
Disadvantage- alloy can become overheated.
arc melting machine
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ELECTRICAL RESISTANCE HEATED CASTING MACHINE
-There is automatic melting of metal in graphite
crucible .
-This is advantageous for
metal-ceramic
restoration in which trace
amount of base metals
are prevented from
oxidation from torch flame
from overheating.
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INDUCTION MACHINE
CASTING
Metal is melted by induction field that develops with in
the crucible surrounded by water cooled metal tubing.
Molten metal is forced in to mold by air pressure or
both.
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Vacuum
or pressure- assisted
casting machine Titanium & its alloys require vacuum arc heating argon pressure casting machines.
The melting point of
commercially pure titanium 1671˚c. In such high temperature , either a graphite or water cooled copper crucible is used.
To prevent absorption of gases in its molten state, titanium is cast in the protective atmosphere of argon or in vacuum.
is
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DIVESTING
“It refers to removal of casting from
the investment mold “
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Recovery of a casting from phosphate-bonded investment. A, Trimming is done from the bottom end of the ring.
B, Investment is being pushed out of the casting ring.
C, The mold is broken open.
D, Investment is removed from the casting.
Care must be taken to avoid damaging the margin
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•
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Sandblasting : The casting is held in a sandblasting
machine to
investment
clean the remaining
from its surface.
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Cleaning the casting
After the casting has solidified the ring removed and quenched in water. This
leaves the cast metal in annealed
condition resulting in a porous, soft,
granular investment that is easily
removed .(Gold alloys)
is •
Often the surface of casting appears dark •
with oxides and tarnish, such a film can be removed by process called pickling.
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Pickling Heating a discolored casting in an acid.
Mask the dark/tarnished appearance Solution used-
1.(50%) dil HCL,
2. (50%) dil Sulphuric acid,
of adherent oxide.
Others –ultrasonic device.
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Disadvantages of hydrochloric acid : It is a health hazard Fumes from the acid are likely to corrode the clinic
laboratory metal furniture
and
Dilute hydrochloric acid should not be used unless necessary neutralizing solutions are immediately at
It causes irreversible tissue injury.
hand
Ultrasonic pickling can be carried out while the prostheses is sealed in a Teflon container
Best method of pickling- the casting is placed in
tube and acid is poured
a test
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After cleaning of casting :
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Trimming The casting is
& polishing : trimmed , shaped and smoothen
burs or stones. with suitable The sprue is sectioned off with a cutting disc. White stone ,rubber wheels, rubber disks, and
fine grit agents
are included in the finishing and polishing
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Inspection & finishing of casting
A) Inner surface ( which will be in contact with the prepared surface of the tooth)should be carefully
examined under higher magnification & illumination for
any discrepancy.
B) Tiny air bubbles in the investment create very minute
nodules on the inner surface, which interfere with the fitting of the casting.
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C) Improper coating of inner surface of a narrow wax
pattern with investment material may result in
entrapment of large volume of air.
This will result in large nodule over the metal blocking the whole inner surface.
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Referenes
Craig’s – 13t
h
edition
Phillip’s SCIENCE OF DENTAL MATERIALS: ANUSAVICE – 10th edition &11th edition
Contemporary fixed prosthodontics –
Rosenstiel & Fujimoto 4t
h
edition
Materials used in dentistry- S. Mahalaxmi 1st
edition
Vimal sikri- Operative dentistry 3r
d
edition
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THANK
YOU