casting process · 2020-03-17 · introduction casting is the process by which a wax pattern of a...

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.

7

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)

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

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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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.

22

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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27

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.

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.

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.

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 ” .

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.

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 .

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

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 .

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

1-Refractory materials : 65%

• A material that will not decompose or disintegrate

on heating such as : silica

• Quartz and cristobalite

• ROLE OF REFRACTORY MATERIAL

• -Resist high temperature .

• -Produce thermal expansion.

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 .

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 .

Types of investment materials according to the

type of the binder:

• Gypsum bonded investment material

• Phosphate bonded investment material

• Silicate bonded investment material

Gypsum bonded material :

• Refractory material : Silica is present in one of

its allotropic forms quartz or cristobalite.

• The binder : Is α-calcium sulphate hemihydrate

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

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.

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

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 .

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 .

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 .

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

• 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

Properties of phosphate investment

• 1) Dimensional changes

• Setting expansion

• Hygroscopic expansion

• Thermal expansion

• Thermal shrinkage

• 2) strength

• 3) porosity

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)

Propoperties

• Dimensional changes

• Setting shrinkage

• Green shrinkage

• Thermal expansion

• Porosity

• strength

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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60

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

75

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.

77

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.

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 .

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.

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.

86

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.

88

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.

89

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

90

DIVESTING

“It refers to removal of casting from

the investment mold “

91

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

92

•

93

Sandblasting : The casting is held in a sandblasting

machine to

investment

clean the remaining

from its surface.

94

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.

95

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.

96

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

97

After cleaning of casting :

98

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

99

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.

100

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.

101

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

102

103

THANK

YOU