Good Mornin

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GUIDED BY :

Dr. AJAY SAXENA Dr. MANOJ CHANDAK (Prof. & Guide) (H.O.D & Prof.)

Dr .Pradnya Nikhade Dr. N. U Manwar (Prof.) (Prof.) Dr. Anant Heda Dr. Anuja Ikhar (Reader) (Reader)

Presented by Dr. Abhilasha dass

ContentsContents

Introduction History 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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Sr.no. Learning objective Domain Level Criteria

1 Steps in casting procedure

Cognitive Must know All

2 Preparation of a die Cognitive Must know All

3Wax Pattern fabrication

Cognitive Must know All

4 Sprue former Cognitive Must know All

5 Casting Ring liner Cognitive Must know All

6 Investing Cognitive Must know All

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7 Burnout Cognitive Must know All

8 Lost wax Technique Cognitive Must know All

9 Crucibles cognitive Must know All

10 Casting machines cognitive Must know All

11 Finishing & polishing cognitive Must know All

Introduction :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, onlays, crowns, bridges and removable partial dentures. 

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.

Successful castings depend on attention to detail and consistency of technique.

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HistoryHistory In approximately 500 BC, the Etruscans produced

bridges made of soldered gold bands.  The most ancient dental prosthesis fabricated from

gold wire was found in Egypt and dated as far back as 2500 BC. 

Copper was cast in Mesopotamia about 3000 B.C. 

The oldest dental castings were gold inlays found in teeth from the natives of Ecuador in about first Century AD. 

11th Century A.D→ Theophilus → Described lost wax technique, which was a common practice in jewelry.

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• The renaissance craftsman and sculptor Benvenuto Cellini (1500-1571) in his autobiography described his method of casting in both gold and bronze by coating his finely detailed wax model with a reinforced refractory shell.    

• 1558 → B. Cellini → attempted use of wax and clay for

preparation of castings.

18981898- Dr. Philbrook demonstrated the 1st cast gold inlay in the Dental Society.

1903 Lentz cast occlusal surfaces to banded gold crowns by the lost wax method of mould formation but apparently did not apply his technique to the fabrication of cast inlays.

19071907- Introduction of lost wax technique by W.H Taggart.

19101910 Van Horn introduced a different method of compensation recommending that the wax pattern be invested at a temperature equal to mouth temperature.  

1928 The low heat casting technique was finally abandoned when Coleman published his research paper No.32 for the United States National Bureau of Standards in which he demonstrated the great shrinkage which occurred when investment is cooled after heating. 

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Weinstein in 19291929 found that adding boric acid to customary investment mixtures prevented the shrinkage.

Sweeney, Paftenbarger  (1930-33) (1930-33) studied use of cristobalite as a refractory in dental casting investment and found that a cristobalite based investment (75% cristobalite & 25% plaster) heated to temperatures between 400ºC and 800ºC produced dimensionally accurate castings.

19421942-Sonder- recognised thermal expansion of investment was greatly inhibited by rigid metal casting ring- advocated lining with soft asbestos.

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19341934- Classification of gold based casting alloys.

Goldberg (1937) (1937) recommended the use of the  wax with 0.1% Contraction for the direct technique and the wax with 0.38% contraction for the a  patterns made on a die at room temperature in the indirect technique.

 

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• 19451945- George D. Estes, introduced the vacuum investing technique to prevent formation of air bubbles on the surface of the wax.

• 19491949- - Moore and Walt developed phosphate bonded investment

• 19501950-development of resin veneers for gold alloys

1959 –Asgar & Peyton stated that flaring should occur at the sprue/ wax pattern junction.

1959 - Strickland et al stated the importance of the type, shape location & direction other than the size of the sprue .

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1959 - Morrison and Warmick reported the

findings of ethyl silicate refractory material

for dental use.

• 19681968-Pd based alloys alternatives for gold

• 19711971- Ni based alloys alternatives for gold• 1980’s1980’s-Introduction of all-ceramic technology

Moore (1993) (1993) discovered by the addition of chlorides to investments he could obtain a thermal expansion of as much as 1.1% from quartz based investment. 

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Definition Definition

Casting is defined as 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 mold.(GPT)

“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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Objectives of casting :Objectives of casting :

1) To heat the alloy as quickly as possible to a completely molten condition.2) To prevent oxidation by heating the metal with a well adjusted torch .3) To produce a casting with sharp details by having adequate pressure to the well melted metal to force into the mold.

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STEPS IN MAKING A CAST STEPS IN MAKING A CAST RESTORATIONRESTORATION

1. TOOTH PREPARATION /CAVITY PREPARATION 2 . IMPRESSION 3. DIE PREPARATION 4. WAX PATTERN FABRICATION 5. SPRUING 6. INVESTING 7.BURNOUT 8.CASTING 9.CLEANING & POLISHING

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Die 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 :PREPRATION OF MASTER DIE :

Commonly used die materials- Type IV gypsum product -

0.1% (setting expansion) Type V gypsum product - 0.3%

(setting expansion)

Disadvantage- Susceptibility to abrasion

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Means to increase abrasion resistance of die : -Silver plating -Coating surface with die hardener - Adding die hardener to gypsum

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Die spacerDie 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 Diestone/investment combinationcombination 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 distortion of wax pattern while removing it from die .

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OTHER DIE MATERIALSOTHER DIE MATERIALS Amalgam Amalgam Acrylic resins (shrinkage 0.6%) Acrylic resins (shrinkage 0.6%) Polyester resin Polyester resin Epoxy resins (less shrinkage then Epoxy resins (less shrinkage then acrylic resins0.1to0.2%) acrylic resins0.1to0.2%)

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ELECTROFORMED DIES :

These dies have high strength, hardness & excellent abrasion resistance.

First step – Treating the surface of impression material so that it conducts electricity. This process is referred as METALLIZING.

PROCESS : A thin layer of metal, such as silver powder, is deposited on the surface of impression material.

Example : Bronzing powder aqueous suspensions of silver powder powdered graphite

ARRANGEMENT IN ELECTROPLATING BATHARRANGEMENT IN ELECTROPLATING BATH

~~Impression- act as cathodeImpression- act as cathode Electroplating bath solution- silver cyanide or Electroplating bath solution- silver cyanide or silver nitrate solution silver nitrate solution (better details) (better details) ~ ~Silver plate- act as anodeSilver plate- act as anode

Electric current of 5-10 mA/cm² square of cathode is Electric current of 5-10 mA/cm² square of cathode is passed for 10 hour. passed for 10 hour.

The impression that contains the electroformed die The impression that contains the electroformed die surface is then filled with dental stone. When the surface is then filled with dental stone. When the stone hardens, it is mechanically locked to the stone hardens, it is mechanically locked to the rough interior of the electroformed metal shell. rough interior of the electroformed metal shell.

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Wax pattern fabricationWax pattern fabrication

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- It is contouring of wax pattern into desired shape and form. - All aspects of final restoration are incorporated into the wax pattern - Shortest time should elapse between the time the pattern is removed from the die & the time it is invested.

Wax Technique Wax Technique

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A) DIRECT

Wax pattern is madedirectly inside themouth, as in inlay

case.

Type-1 medium wax.

Exceptionallydemanding procedure.

B) INDIRECT

Wax pattern is made on the die.

Type -2 soft wax.

More commonly used technique.

Sprue & Sprue formerSprue & Sprue former

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Sprue 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.

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 formerWax pattern attached to the crucible former with a sprue ready for investing. A ring liner is with a sprue ready for investing. A ring liner is in place. in place.

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Spruing of wax pattern :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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SprueSprue

DIAMETERLOCATIONLENGTHATTACHMENTDIRECTION

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SPRUING TECHNIQUE SPRUING TECHNIQUE

DIRECT : sprue former provides direct connection between pattern area & the sprue base/crucible former area.

A basic weakness of direct Spruing is the potential for suck-back porosity at the junction of restoration and the Sprue.

INDIRECT: connector/ reservior bar is positioned between pattern & the crucible former.

Indirect Spruing offers advantages such as greater reliability & predictability in casting plus enhanced control of solidification shrinkage .The Connector bar is often referred to as a “reservoir .

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Sprue formerSprue 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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RESERVOIRRESERVOIR

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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Importance :Importance :

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SURFACE TREATMENT OF SURFACE TREATMENT OF COMPLETED WAX PATTERNCOMPLETED WAX PATTERN

Before the wax pattern is invested, it should be cleaned of any debris, grease, oils and separating medium.

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Preparatory steps for investingPreparatory steps for investing

• Secure wax pattern,• Cleaning the wax pattern of debris,

grease or oil by surfactants-a)Pattern cleanserb)Dil. synthetic detergent sol. c)Debbublizer

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Debubblizer :Debubblizer :Citric acid Sodium 2-phenylpropane-2-

sulfonatePropane-1,2-diol - also known as

propylene glycol

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Casting ring & linerCasting ring & linerThe casting ring serves as a

container for the investment while it sets & restricts setting expansion of the mold.

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Metal casting ring

Sprue base orCrucible former

Ring liner

Casting RingCasting RingThey are available as-1) Shapes - Round

- Oval 2) Complete rings I) - Rigid - Metal (stainless steel)

- Plastic

II) Flexible - Rubber 3) Split rings I) metal II) plastic

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Casting Ring Casting Ring Considerations :Considerations :1) The internal diameter of casting ring should be 5-10mm greater than the widest measurement of the pattern and about 6 mm higher.

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

ASBESTOSLINERS

NON ASBESTOSLINERS

CELLULOSE PAPER AL-SILICATE CERAMIC

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Asbestos:- carcinogenic potential makes it a biohazard.

Functions of a liner Affords greater normal expansion in the investmentThe absorbed water causes a semi hygroscopic expansionThickness – not less than 1mm

Liner techniqueLiner techniqueA)DRY LINER TECHNIQUE - tacked in

position with sticky wax.B)WET LINER TECHNIQUE - lining ring

is immersed in water & excess water is shaken 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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RINGLESS CASTING TECHNIQUE 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 and is designed to allow

unrestricted expansion.

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Crucible formers and cone-shaped plastic rings for a ringless casting system.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.

Investing 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 a mould into which a metal or alloy is cast.

The operation of forming a mold is known as INVESTING

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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 techniquePaint 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 thinlyfrom a height (20-30)cm at a slight angle from bottom to the top of a ring

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Allow the investment to set (45 – 60) min before burnout procedure commences

Place invested pattern in a Place invested pattern in a humidator if burnout process is humidator if burnout process is delayed.delayed.

METHODS OF INVESTING:A) HAND INVESTINGB) VACCUM INVESTING

Advantages of vacuum investing• The amount of Porosity in the

investment is reduced• The texture of cast surface is smoother

with better detail reproduction• The tensile strength is greatly

increased

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Brush technique of investingBrush technique of investing

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Vaccum InvestingVaccum Investing

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•When the investment has set, the "skin" at the top of the ringis trimmed off. •The rubber crucible former is removed, and any loose particlesof investment are blown off. •The ring is then placed in the furnace for the recommended burnout schedule

BURN OUTBURN 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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a) For gypsum investment.500 degree c - hygroscopic

expansion tech.700 degree c - thermal

expansion tech.b) For phosphate investment 700 -

1030 degree cc) Ethyl silicate bonded investment-

1090 degree c

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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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CastingCastingCasting 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 the mold This can be done by use of following different type of force- Vacuum force Air or Gas Pressure Centrifugal force

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Melting temp of pure gold –1063⁰cMelting temp of gold alloy-924-960⁰cMelting temp of base 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. Two basic modes are by using

1) Torch flame– Gas air Gas oxygen Air acetylene Oxygen acetylene. hydrogen oxygen generator2) Electricity

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METHODS OF MELTING METHODS OF MELTING ALLOYALLOY

A)TORCH MELTING-For low

temperature metals

-Mixture of natural/artificial gas, oxygen/tank gas –oxyacetylene

Less faster than electric heating but more faster than resistance heating.

B) ELECTRIC MELTING-For higher

temperature metals.

electric resistance melting, induction melting.

melts alloy faster,& can be easily over heated

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Two type of torch tips:1.Multi-orifice2.Single-orifice

Zones of the blow torch flame:

Zone 1 - colorless zone /Non combustion zone

Zone 2 – Combustion zone

Zone 3 – Reducing zone

Zone 4 - oxidizing zone

Melting methodsMelting methodsGas 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: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 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 oxygenThe 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 due to occluded H2 gas

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CRUCIBLESCRUCIBLES : :

The Melting of alloy requires a crucible to act as a platform on which the heat can be applied to the metal.

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ClayQuartz Carbon

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 a high melting temperature or are sensitive to carbon contamination.(Alloys like High Pd,Pd - Ag ,Ni based or Co based 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 previous

metals

•Sufficient mass of alloy must be present to sustain

adequate casting pressure---

6gm is typically adequate for premolar and anterior casting 10gm is adequate for molar casting

Casting machinesCasting 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.

Air Pressure casting Air Pressure casting machinemachine

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 10-15 psi is usually applied.

Vaccum casting machineVaccum casting machineVaccum 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 MACHINECENTRIFUGAL CASTING MACHINE

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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 itsaxis of symmetry. The mold is kept rotating till the metal has solidified. Mold material like steels, Cast irons, Graphite may be used.

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 arc melting Direct current arc melting machinemachine

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.

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ELECTRICAL RESISTANCE HEATED CASTING ELECTRICAL RESISTANCE HEATED CASTING MACHINEMACHINE

--There is automatic melting of metal in graphite There is automatic melting of metal in graphite crucible .crucible .-This is advantageous for-This is advantageous for metal-ceramicmetal-ceramic restoration in which restoration in which trace amount of basetrace amount of base metals are prevented from metals are prevented from oxidation from torch flameoxidation from torch flame from overheating.from overheating.

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Metal is melted by induction field that develops with in Metal is melted by induction field that develops with in the crucible surrounded by water cooled metal tubing.the crucible surrounded by water cooled metal tubing.Molten metal is forced in to mold by air pressure or Molten metal is forced in to mold by air pressure or both. both.

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INDUCTION CASTING MACHINE

Vacuum Vacuum or pressure- assisted or pressure- assisted casting machinecasting machine Titanium & its alloys

require vacuum arc heating argon pressure casting machines.

The melting point of commercially pure titanium is 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.

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DIVESTINGDIVESTING

“It refers to removal of casting from the investment mold “

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Recovery of a casting from phosphate-bonded Recovery of a casting from phosphate-bonded investment.investment. A, Trimming is done from the bottom end of the A, Trimming is done from the bottom end of the ring. ring. B, Investment is being pushed out of the casting B, Investment is being pushed out of the casting ring.ring.C, The mold is broken open. C, The mold is broken open. D, Investment is removed from the casting. D, Investment is removed from the casting. Care must be taken to avoid damaging the Care must be taken to avoid damaging the marginmargin

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Sandblasting :Sandblasting :The casting is held in a

sandblasting machine to clean the remaining investment from its surface.

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Cleaning the castingCleaning the casting• After the casting has solidified the

ring is 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)

• 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 Pickling Heating a discolored casting in an acid. Mask the dark/tarnished appearance of

adherent oxide. Solution used- 1.(50%) dil HCL, 2. (50%) dil Sulphuric acid, 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 and laboratory metal furniture

Dilute hydrochloric acid should not be used unless necessary neutralizing solutions are immediately at hand

It causes irreversible tissue injury.

Ultrasonic pickling can be carried out while the prostheses is sealed in a Teflon container

Best method of pickling- the casting is placed in a test tube and acid is poured

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After cleaning of casting :After cleaning of casting :

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Trimming & polishing :Trimming & polishing :The casting is trimmed , shaped and

smoothen with suitable burs or stones.The sprue is sectioned off with a

cutting disc.White stone ,rubber wheels, rubber

disks, and fine grit are included in the finishing and polishing agents

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Inspection & finishing of castingInspection & finishing of castingA) 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 Referenes

Craig’s – 13th editionPhillip’s SCIENCE OF DENTAL MATERIALS:

ANUSAVICE – 10th edition &11th editionContemporary fixed prosthodontics –

Rosenstiel & Fujimoto 4th editionMaterials used in dentistry- S.

Mahalaxmi 1st editionVimal sikri- Operative dentistry 3rd

edition

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