Casting in dentistry

Cotton Candy Wind

GOOD MORNING!!!

Dr. V. VasundharaFirst year PGCASTING

INTRODUCTIONHISTORYBASIC STEPS OF CASTING PROCEDURESPRUE FORMER AND ITS ATTACHMENTCRUCIBLE FORMERSCASTING RINGS AND LINERS

INVESTING PROCEDUREWAX BURNOUT CASTING OF ALLOY INTO MOLDHEAT SOURCEMACHINES TO INDUCE CASTING FORCERECOVERY AND CLEANING OF CASTINGCASTING OF TITANIUM ALLOYS CASTING OF CASTABLE CERAMICSCONCLUSIONREFERENCES

INTRODUCTIONCasting is an object, formed by the solidification of a fluid that has been poured or injected into the mold.(GPT)

Objective of casting procedure is to provide a metallic duplication of missing tooth structure with as much accuracy as possible

HISTORY

The meticulous procedure of casting was used by various craftsmen to produce jewelry and ornaments. Its history can be traced back around 3000 B.C. But origin of lost wax technique when viewed, history makes its presence in the writings of Theophilus (11th Century).

1907 Taggart devised a practically useful casting machine.

5/12/2014Free Template from www.brainybetty.com101907-LOST WAX PROCESS1933-Replacement of CO-Cr for gold in RPD1950-Development of resin veneers for gold alloys1959-Porcelain fused to metal technique1971-The Gold standard1980-ALL-CERAMIC TECHNOLOGIES1999-Gold alloy as alternative to Pd-Based alloys

Taggart ( end of nineteenth century)

CASTING ALLOYA thorough knowledge about alloy system used for casting is helpful in selecting the appropriate alloy. There are various classifications of alloy systems. Commonly used is A) Based on physical properties yield strength and minimum elongation proportion ADA SP 5 Type I Soft Type II Medium Type III HardType IV Extra hard (Metal-Ceramic alloys)

B) Based on alloy nobility based on percentage of noble metal present. High noble metal 40 wt% AuNoble metal 25 wt% Au Base metal < 25 wt% Au

Noble metal alloy a commonly invested in gypsum bonded investments ,they can be pickled, but steel instruments should not be used to of place and remove casting from pickling solution. They should be allowed to cool for 5 min after casting, for age hardening (allow adequate grain growth).

Base metal alloys They need phosphate bonded investment, they should never be pickled, and need large amount of mold expansion.

Palladium alloys :- carbon crucibles should not be used for melting alloy, Type II alloys cannot be age hardened

Titanium and its alloy The main problem in casting titanium is its high reactivity to oxygen above 6000c. So a specially designed investment material and casting environment to be prepared

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QUANTITY OF ALLOY=

weight of the wax pattern x 8.2+5 g.

Solidification shrinkage: - seen with various alloy. Gold alloy 1.5% Base metal alloy 2.4%

Procedure of casting

CHANGE COLORS17

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SPRUINGSPRUE FORMERSPRUE

SPRUINGThe process of attaching a sprue former to the wax pattern is called as spruingThe sprue former is attached to provide a channel through which molten alloy reach the mold

The process of attaching a sprue former/sprue pin to the wax pattern is called as spruingSprue - The channel left by sprue former following burnout through which molten alloy can reach the mold.

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Types of sprue former

CHANGE PREST20

Wax sprue former They are preferred for most of the castings .

2mm 2.5mm 4mm

They are preferred for most of the castings because they melt at the same rate as the pattern and thus allow easy escape of the molten wax21

SOLID PLASTIC SPRUE FORMER used for castings of alloys which use 2 stage burn out with Po4 bonded investment Their main disadvantage is its softening temperature, which is higher than wax pattern. And may block escape of wax.

SOLID PLASTIC SPRUE FORMER It softens at higher temperature than wax pattern so it may block the escape of wax and will result in casting roughness.

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Hollow plastic sprues are available that permit easy escape of gasses.

It can be used in casting FPD in one piece due to their added rigidity.

Metal sprue formerIt should be made of non rusting metal to avoid possible contamination of wax.

They are usually removed from the investment at the same time as crucible former is removed.

Basic requirements of sprue former :

FLOWFlow is the quantity of material passing a given area in a unit time period.

Turbulent flow needs more pressure to drive it. For a given pressure difference, you will have a lesser flow with turbulent flow than with laminar flow.

He found with his experiments, that when the Reynolds number was below 2000 that the flow was laminar and that when the Reynolds number was above 2000, the flow was turbulent.Thus factors that increase Reynolds number promote turbulence. From the equation, you can see that an higher fluid velocity (forward speed), higher density of the fluid, and higher diameter of the pipe favour the flow to change from laminar to turbulent.

Types of spruing

Direct Spruing

Indirect Spruing

DIRECT SPRUING :The sprue former provides a direct connection between the pattern area & sprue base or crucible former area.

INDIRECT SPRUING :A connector or reservoir bar is positioned between the pattern & crucible former. It is common to use indirect spruing for multiple single units& FPDs.

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Diameter of sprue : - Larger diameter is recommended for improved flow of metal.

- 10 gauge (2.5mm) Molar, metal ceramic pattern

- 12 gauge (2mm) Premolar, partial coverage

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2) Location of sprue :

Should be attached to bulkiest part of pattern away from margin and occlusal contacts.

Normally non centric cusp is used.

Molten metal should flow from thick section towards margin, not reverse

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3) Sprue former direction :

-Should be directed away from any thin or delicate parts of pattern because molten metal can fracture investment in this part.

Should not be attached at 90 degree, it may lead to turbulence it should be attached 45 degree to bulkiest part of pattern

Should not be attached at 90 degree, it may lead to turbulence.

-So it should be attached to 45 degree to bulkiest part of pattern.

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4) Attachment of sprue : - Point of attachment should be smooth to minimize turbulence.

- It is usually flared for high density gold alloy which acts as same way as reservoir.

Internal sprue: If there is broad expense of paper thin wax between sprue and margin, bridge it with a narrow (.5mm) strip of wax that will serve as an internal sprue.

5) SPRUE FORMER LENGTH

It should be adjusted such that top of wax pattern is with in 6 mm of open end of ring for gypsum investments and 3-4 mm for phosphate bonded investments.

5) Sprue former length : - It depends on length of casting ring. - For accuracy it should be as close to centre of ring. - It should be adjusted such that top of wax pattern is with in 6 mm of open end of ring for gypsum investments and 3-4 mm for phosphate bonded investments.

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Wax pattern removal : - Sprue former should be attached such that pattern can be removed directly in the line with its path of withdrawal from die. - Any motion that may distort the wax pattern should be avoided.

VENTING

Small auxiliary sprues or vents are recommended to improve casting of thin patterns.

- Their action may help gases to escape during casting or ensure that solidification begins in critical areas by acting as heat sink.

7) Venting : - Small auxiliary sprues or vents are recommended to improve casting of thin patterns. - Their action may help gasses to escape during casting or ensure that solidification begins in critical areas by acting as heat sink.

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Reservoir

It is added to sprue network to prevent localize shrinkage porosity. Pattern should always solidify first and reservoir last

ReservoirIt is added to sprue network to prevent localize shrinkage porosity. Pattern should always solidify first and reservoir last

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Because of large mass of alloy and heat at centre part of ring it remains molten to furnish liquid alloy into mold as it solidifies

Crucible former

The sprue is attached to crucible former which constitutes the base of casting relation with casting ring during investing

They form a conical depression in investment, which guides flow of molten metal..

Crucible formerThe sprue is attached to crucible former which constitutes the base of casting relation with casting ring during investing

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They are available as

Rubber crucible former Metallic Crucible formerPlastic crucible former

Casting ring

They are available as: 1) Shapes - Round - Oval

2) I) Complete rings - Rigid - Metal (stainless steel) - Plastic - Flexible - Rubber II) Split rings - Metal - Plastic3) I) Cylindrical II) Conical

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Considerations in selection of castings rings

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 as used. Diameter - 32 mm

For large fixed partial denture 63mm round/oval shaped casting ring are used.

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Ringless casting system:

Plastic ring with rubber crucible formers are used. The ring is conical in shape with tapering walls. As the investment sets the investment is tapped out of ring.Then burnout is done with out casting ring, this causes greater expansion. It is used for traditional gold- based alloys

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Usually casting rings are rigid in nature. Because of this the mold may become smaller rather than larger due to the reverse pressure resulting from confinement of the setting expansion.

To overcome this, flexible rings/ split rings are used. But the most commonly used technique is to provide expansion by lining the ring with a ring liner.

Casting ring linersCasting ring holds the investment in place during setting and restricts the expansion of mold.

Provision must be made to permit investment expansion so a ring liner is used inside the ring for more expansion.

Materials used as liner :

Traditionally asbestos ( carcinogenic )

Aluminum silicate ceramic liner

Cellulose ( paper ) liner

Functions of liner

To ensure uniform expansion, a liner is cut to fit the inside diameter of the casting ring with no overlap.

The dry liner is tacked in position with sticky wax, & then is used either dry or wet.

Wet liner technique- The lined ring is immersed in water for a time & excess water is shaken away.

Squeezing the liner should be avoided, because this leads to variable amounts of water removal & no uniform expansion.

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Cellulose linersBeing paper products, are burned away during the burnout procedure, so a technique must be found to secure the investment in the ring

It should be 3.25 mm short of ends of the ring to keep the investment in contact with ring after burnout

This also restricts the longitudinal setting and hygroscopic expansion

Ceramic ring liner: They do not absorb water, but its network of fibers can retain small amount of water on its surface/wetting agents can be used to increase the water sorption on surface .

The binders used in ceramic liner (Ex neoprene-latex) can contribute to toxicity (stimulate fibrosis. act as adsorbent surface for carcinogenesis).

Investing There are 3 types of investing materials for dental castings :

1) Gypsum bonded : type 2, 3 & 4 gold alloys

2) Phosphate bonded : metal ceramic framework

3) Silica bonded : high melting base metal alloy in CPD

Wax pattern cleaning5/12/2014Free Template from www.brainybetty.com54

There are 2 techniques :

1) Hand investing or Brush technique

2) Vaccum technique

5/12/2014Free Template from www.brainybetty.com56Brush techniquePaint the pattern with surface tension reducer

Add powder & liquid in bowl, & quickly mix by hand

Attach vaccum hose to bowl, evacuate the bowl & mechanically spatulate

Coat the entire pattern with investment with brush

Fill the ring slowly from bottom to upward

5/12/2014Free Template from www.brainybetty.com57Vaccum technique Hand spatulate the mix

With crucible former & pattern, attach ring to mixing bowl

Attach vacuum hose & mix

Invert the bowl & fill the ring under vibration

Remove the vacuum hose before shutting off the mixer

Remove filled ring & crucible former from bowl

Burnout/wax elimination

Purpose of wax elimination:-To create a mold space By wax elimination To provide thermal expansion of investment.To remove residual water in investmenteliminates the temperature difference between the molten alloy and investment

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5/12/2014Free Template from www.brainybetty.com59 Remove crucible former

Remove metal sprue if used

Place ring with sprue facing down in furnace on ceramic tray

Bring furnace to 400 degree C & hold for 30 min

Increase the heat to final burn out temp 650 degree C for gypsum & 700-870 C for phosphate bonded

During burn out, residual carbon may get trapped in porous investment so it is advised to begin burn out when mold is still wet.

When transferring the mold to casting machine, visually check, it is cherry red color.


Hygroscopic low-heat technique:Used with gypsum bonded investment which are allowed to set under water. They are used in casting gold alloys. The temperature used in this technique is 4800c for 60-90 mins. This technique causes 0.55% of expansionGreater chances of back pressure porosity

This technique can only be used in hygroscopic low heat technique because in high heat technique, wax pattern will be melted before most of this expansion takes place.

This technique results in casting fits that are more accurate and reproducible than those produced by either of the commonly used technique.

High heat thermal expansion technique :Gypsum bonded investment is slowly heated to 6500c - 7000c in 60mins. Then maintained for 15-30 mins.

Phosphate bonded investment is heated after initial slow raise of temp to 3150c, the temperature is rapidly raised to 750-9000c and maintained for 30 minsThe technique cause 1.33-1.58 % of Thermal expansion

Once the 2nd stage of heat application is over, casting of alloy into mold should be done immediately. This prevents cracking of investment and reheating, which cause change in dimension of mold (with loss of some expansion previously obtained) ). The maximum allowable time between the completion of burnout and casting of alloy is 1 minute

Accelerated casting method :

Accelerated casting method : (J Prosthet dent. 66: 155,1991) This technique uses phosphate bonded investment which sets in 15 mins and then 15 min burn out is done at 8150c. This method is used for preparing post and core restorations. The casting produced show similar accuracy and surface roughness as seen with fractional casting method

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Casting crucibleClay for crown and bridge, high noble metalCarbon- for noble metal & high fusing gold alloysQuartz high fusing alloys of all types, Pd, Ni, Co

CastingA) Heat source to melt the alloy

B) Casting force to force molten alloy into mold Casting force > surface tension of alloy + resistance offered by gas in the mold.

Heat Source

A)Torch flame source of flame Gas air Gas oxygen Air acetylene Oxygen acetylene. Hydrogen oxygen GENERATOR B)Electricity electrical resistance melting Arc melting Electrical\ induction melting

Zones of blow-torch flame: It should be like non luminous brush flame.

Zone 1 Colorless Zone

Zone 2 - Combustion zone

Zone 3 - Reducing zone

Zone 4 - oxidizing zone

Zone 1 Colorless Zone This zone indicates the mixture of gas and air. No heat is present in this zoneZone 2 - Combustion zone green in color - This is an oxidizing zone. Here gases are partially burnt. This zone should be kept away from metal during fusing.

Zone 3 - Reducing zone dimly blue. It is the hottest part of flame. This area should be in contact with the metal.

Zone 4 - oxidizing zone outer cone the combustion occurs with oxygen in air. This part should be never in contact with metal.

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Mixing zone dark in color, no heat present.

Combustion zone green in color, zone of partial combustion & has oxidizing nature so should be kept away from molten metal5/12/2014Free Template from www.brainybetty.com713. Reducing zone dim blue in color, hottest part of flame, it is used for fusion of casting alloy.

4. Oxidizing zone has lower temp. than reducing zone, it oxidizes the metal so should never be used for melting alloys

The changes seen in metal during fusing are : Initially appear spongy later small globules of fused metal appear later molten metal flows assuming a spherical shape at proper casting temperature the molten alloy is light orange and tend to spin/follow the flame when the latter is moved.

At this stage the temperature of molten alloy is 380c above its liquidus temperature.

Flux: During melting of gold alloys flux may be added to fusing of metal prevent oxidation porosity

Commonly used fluxes are fused borax powder ground with boric acid powder.

Electrical resistance heated casting machine :- An electric heater is an electrical appliance that converts electrical energy into heat. The heating element inside every electric heater is simply an electrical resistor, and works on the principle of Joule heating: an electric current through a resistor converts electrical energy into heat energy.

It is used to melt ceramic alloys. Here the alloy is automatically melted in graphite crucible.It provides best means of temperature control. It is quite convenient as compared to blow torch

Advantage : - Prevents over heating of the alloy The crucible is always against the casting ring. So the metal button remain molten slightly longer and ensures complete solidification.

Precautions :- Carbon crucible should not be used with palladium alloys whose melting temperature 15040C

Electrical arc melting: - It is used to melt higher fusing alloys. Electrodes are placed about 5cm apart.

Safety measures need use of Black apron -prevents glare Facial viser protect eyes Foot switch control the current in electrodes

High fusing alloys show rounding off of corners and signs of collapsing, at which point they are thrust into the mould.

The electrodes must be raised from the crucible before a centrifugal casting machine is set in .

Induction casting :It is used to melt base metal alloys of high melting temperature.The centrifugal casting machine is controlled by electricity.

Principle :- Induction casting is based on electric currents in a metal core caused by induction from a magnetic field. This apparatus consists of specially designed crucible around which copper coil is wound circularly

When high density alternating current (of high frequency 1500 KHz) is passed through copper coil a magnetic field (I2P) is set up around the coil.

This AC current also sets up eddy currents in crucible.

Rapid change of polarity produced causes the molecules of the alloy to oscillate and their molecular bonds to break down

DIFFERENT TYPES OF CASTING FORCE

Parts of spring wound centrifugal casting machine :It consists two arms one contains the casting assembly.

This arm may be rigid / broken. The broken arm is commonly used. the incorporated broken arm principle accelerates the effective initial rotation speed of crucible and casting ring this increases the linear speed of metal into the mold. The other arm consists of appropriate counter weight for proper rotation.

Cradle to seat the casting ring. Bracket to place the crucible against the ring. Head plate to prevent displacement of casting ring.

Basic principle : In broken arm casting machine the metal entry into the mold is controlled by 2 forces. Rotational force which outs initially to force the metal to the right. Centrifugal force attempts to push the alloy laterally in a direction parallel to the long axis of rotating arm. This force maintains the constant force on the molten alloy during rotation.

To allow proper orientation of casting ring (consequently the wax pattern) in the arm of casting machine, a method of marking the sprue former on the casting ring should be used.Locate the pattern in the trailing half of casting ring, 90 to 135 away from the sprue position.Once the pattern is in position, locate it downward in the lower outer quarter of the trailing half of the casting ring and secure it in that position. J Prosthet Dent 1979;41:531-34

A variety of forces act upon a liquid metal subjected to a centrifugal force during the casting process:Acceleration force (opposite the direction of rotation)Centrifugal force (outward)Gravitational force (downward)The resultant force will be in the direction of the lower outer quarter of the trailing half.

J Prosthet Dent 1979;41:531-34

The pressure gradient from the tip of the casting to the bottom surface is quite sharp and parabolic in form

Maximum at tip of casting and reaching zero at the button surface. More the pressure gradient

Greater the heat transfer

Argon gas pressure is used in Ti casting.

Piston plunger force is used in compressing ceramics

CLEANING THE CASTINGAfter casting has been completed, the ring is removed & quenched in water as soon as the button exhibits a dull red glow.ADVANTAGES OF QUENCHING

PICKLING50% HCl solution It aids in removal of any residual investment as well as of the oxide coating.

The disadvantage of use of HCl is:- 1) The fumes from the acid are likely to corrode lab. metal furnishings. 2) Health hazard & should be vented via a fume hood.

Surface of casting appears dark with oxides & tarnish. Such surface film can be removed by a process known as pickling, which consists of heating the discoloured casting in an acid.For gypsum 50% HCl solution It aids in removal of any residual investment as well as of the oxide coating.

The disadvantage of use of HCl is:- 1) The fumes from the acid are likely to corrode lab. metal furnishings. 2) Health hazard & should be vented via a fume hood.

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Sulfuric acid is more advantageous.

Ultrasonic devices are available for cleaning the casting.

Best method for Pickling is to place the casting in a test tube or dish & pour the acid over it.

Pickling solution should be renewed frequently, since it is likely to become contaminated.

In no case the casting be held with steel tongs because, when both the casting & tongs come into contact with pickling solution, it may contaminate the casting. The pickling solution usually contains small amounts of copper dissolved from previous castings.

When steel tongs contact with this electrolyte, a small galvanic cell is created & copper is deposited on the casting at the point where tongs grip it.

Electrolytic polishing

Electrolytic preparation uses an electrolytic reaction cell containing a liquid electrolyte with two electrodes: an anode and cathode. The sample to be polished/etched forms the anode.

Anodic dissolution results in a leveling and brightening of the specimen surface.

TITANIUM CASTING

Spruing should be done with 6 gauge multiple (3) sprues for three unit FPD to minimize porosity(90% porosity free castings) J Prosthet Dent,1995;73:534-41

Burnout cycle

Casting machine

Titanium ingot

CASTABLE GLASS CERAMICS

A cast glass ceramic offers unique advantages to dentist dental technicians and patients. They can be used to restore accurate form and esthetics in complete and partial coverage restorations. Spruing :- Casting ring used 1 .81 inches long Sprue size used 8 gauge

Single sprue for anterior teeth, 2 sprues for posterior teeth the recommended sprue length 3-4mm.

With pattern position 1/8 inches from the top of ring.

A generous reservoir is needed to ensure that the molten glass is amplified into the mold during casting.

Investing :Ceramic ring liner is used inside the ring and invested with phosphate bonded investment which shows 1.6% thermal expansion. Investing done with brush technique

Setting time 1 hr (in open air)

Burn out 2 stage burnout is usual.

Initial temperature - 4800F for 30min.

Final temperature 17500F (9500c) for 2 hrs.

Casting: - The crucible is loaded with 4 gram ingot. The casting temperature is 13580C(24760F).

Molten glass is forced into the mold cavity using piston-plunger force and compressed.

Devesting Cut approximately inch from the end of the ring to remove the casting from the investment .

The remaining investment is removed using air abrasive tool and 25m aluminum oxide at a pressure of 40Psi while a finger is placed over the margin areas to prevent chipping.

Sprue removal : - A double sided diamond disk is used to slowly separate the crown at the junction of the sprue and crown. The excess sprue is removed and finding completed with a white point by using slow revolutions to avoid chipping. At this stage the casting is in a non-crystalline glass form.

EmbeddingThe bottom of each pocket in the ceramming tray is covered with the embedment material and vibrated while the inside of each crown is filled. One crown is placed in each pocket of tray, covered completely with additional embedment material and allowed to set for 15-30 minutes

Ceramming During this procedure the casting converts into a form that is 55% crystalline. Ceramming temperature of 10750C (19600F) a for 6 hours. Furnace is cooled to 3920F

Post-ceram divesting :- After the tray has cooled to room temperature, the embedment is broken apart by using finger pressure.

The crown is blasted with 25m aluminum oxide at a pressure of 40Psi without abrading the margins.

Crown finishing is done with extra fine diamond point and placed in distilled water for ultrasonic cleaning.

Cooling done to imitate the natural hue and the light entering the cast glass ceramic is selectively filtered by colorcuts incorporated into the exterior shading porcelains

Casting defectsClassification :

Distortion

Surface roughness & irregularities

Porosity

Incomplete casting


DistortionCauses :

Distortion of wax pattern

Proper handling of wax pattern should be done.

Thermal changes in different thickness of pattern can produce distortion

If wax pattern has to be invested after more than 30 min then store in refrigeration.


2) Due to hygroscopic or setting expansionThese expansion movements of investment produce uneven movement of proximal walls of patterns, when it hardens around it.

Gingival margins are forced apart by expansion while solid occlusal bar of wax resists expansion.


Thinner the pattern, greater is distortion.

Lesser the setting expansion, lesser is distortion.


Surface roughness, irregularities, discolorationSurface roughness is defined as relatively finely spaced surface imperfections whose height, width & direction establish the predominant surface pattern.

Surface roughness is related to particle size of investment & its ability to reproduce wax pattern in microscopic detail.


2 most common cause of surface roughness are presence of water droplets & air bubbles on surface of pattern.

Surface irregularities refers to surface imperfections such as nodules that do not characterize the total of area.


Factors that contribute to it :Air bubbles It may be entrapped during investment procedure & results in a nodule in casting.

Best method to avoid it by vacuum mixing, & applying surface tension reducing agent.


2) Water filmsappears as minute ridge or veins on surface

Causes Pattern is vibrated after investing or moved.

-Too high L/P ratio

-painting procedure is not correct

Solution Apply wetting agent


3) Rapid heating rates :It results in fins or spines or characteristic surface roughness can be seen due to flaking of investment.Mold should be heated gradually, at least 60min should elapse from room temp to 700 degree Celsius.Greater the bulk of investment, more slowly it is heated.


4) Under heating :Incomplete elimination of wax residue occur.

Voids or porosities occur when hot alloy comes in contact with carbonaceous residue.

Tenacious carbon coating is seen that is impossible to clean even by pickling.


5) Liquid powder ratio :

Higher the L/P ratio, rougher is surface, so it should be accurate.

If L/P ratio is very low then investment will be thick & cause air entrapment.


6) Prolonged heating :

It causes disintegration of investment & walls of investment are roughened as result.

Also product of decomposition, sulphur may contaminate the alloy.


7) Temperature of alloy :

If alloy is heated to too high temp. before casting, surface of investment is likely to be attached & results in surface irregularities.

Prolonged heating of gold alloy causes loss of its constituents& become less fluid.


Alloy should be heated as quickly as possible until they are fully molten, should not be overheated.

Time elapsed between heating & applying casting force should be as short as possible to avoid any cooling at this critical stage.


8) Casting procedure :

Too high pressure during casting can produce rough surface.

0.10 to 0.14 Mpa of air pressure & 3-4 turns of spring in centrifugal machine is sufficient.


9) Foreign body :

Any casting with sharp, well defined deficiencies indicates presence of foreign body in mold.

It may occur due to rough crucible former or careless removal of sprue former.


10) Impact of molten alloy :

Direction of sprue should be such that molten alloy does not strike a wear portion of mold surface.

Molten alloy may fracture the mold surface & causes raised area in casting and goes undetected until casting does not seat.


11) Pattern position :

When many patterns are invested together, they should not be placed too close to each other.

Many patterns at same plane should be avoided.

Expansion of wax is much greater than investment so spaces between them should not be less than 3 mm.


PorosityClassification :

1.Solidification defects a) Localize shrinkage porosity b) Micro porosity c) suck-back porosity

2. Trapped gasses- a) Pin holes b) Gas inclusions c) Subsurface porosity

3. Residual air a) back pressure porosity


1) Localise shrinkage porosity :

Occur due to incomplete feeding of metal during solidification.

1.25% of linear contraction occur in noble metal alloys when changes from liquid to solid state.


Porosity in pontic area is due to ability of pontic to retain heat because of its bulk & is located in heat centre of ring.

It can be solved by attaching 18 gauge sprue at surface most distant from main sprue, extending laterally within 5 mm of ring. It carries heat away from pontic.


Suck back porosity :

-It occur externally, usually in interior of crown near area of sprue when a hot spot is created by hot metal impinging from sprue channel on point of mold wall.

-It causes this local region to freeze last & causes suck back porosity.


It occur at occlusoaxial line angle or inciso axial line angle that is not well rounded.

It is solved by flaring the point of attachment of sprue & by reducing the mold melt temp. or by placing a Y shaped sprue with 2 arms wide apart.


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2) Microporosity :

It occur from rapid solidification if mold or casting temperature is too low.

It is not a serious defect because it can not be detected until casting is sectioned.


It also occur from solidification shrinkage, when solidification is very rapid for the micro voids to segregate to liquid pool. Solubility of gasses in liquid is high so gas is ejected during cooling.

Porosity due to shrinkage- rough voids Porosity due to gasses- smooth bubbles


3) Pin holes and gas inclusion porosity :

These are entrapment of gas during solidification.

They are same & characterize by spherical contour but vary in size.

Gas inclusion porosities are larger than pin hole porosity.


Metals dissolve & liberate gas when they are melted. On solidification absorbed gasses are expelled & pin hole porosity results.


4) Subsurface porosity :

It occur due to simultaneous nucleation of sold grains & gases bubbles at first moment that the metal freezes at the mold walls.

Controlling the rate at which molten metal enter the mold can minimize this.


5) Entrapped air or back pressure porosity:

Occur on inner surface of casting & can produce large concave depressions.

It is caused by inability of air in the mold to escape through the pores in investment or by pressure gradient that displaces the air pocket towards the end of investment via molten sprue or button.


It occur because of modern dense investment, increase in mold density by vacuum mixing & tendency of mold to clog with residual carbon.

It is solved by proper burn out, adequate mold & casting temperature, sufficient high casting pressure & proper L/P ratio.

Thickness of investment from pattern should not be more than 6 mm.


6) Incomplete casting :

It is because molten metal alloy is prevented from completely filling the mold.

Factors that lead to this

Venting of the mold

High viscosity of fused metal


Insufficient venting is directly related to back pressure exerted by air in the mold.

So casting pressure should overcome back pressure, pressure should be applied for 4 seconds.

Mold is filled & metal is solidified in 1 sec or less, yet it is soft in early stage. So pressure should be maintained for slightly longer.

It can also occur by incomplete elimination of wax residue.


7) Small casting :

If compensation for shrinkage of alloy is not done, by adequate expansion of mold cavity, then small casting is resulted.

Too bright & shiny castings withy short & rounded margins are formed.


CONCLUSION Investing & casting, a series of highly technique sensitive steps, converts the wax pattern into metal casting. Accurate & smooth restoration can be obtained by paying special attention to each step in the technique.

When initial attempts in casting produce errors or defects, appropriate correction must be done at that stage only.

SummaryInvesting and casting , a series of highly technique sensitive steps,converts the wax pattern into a metal casting. Accurate and smooth restorations can be obtained if the operator pays special attention to each step in the technique.when initial attempts at casting produce errors or defects,appropriate corrective measures must be taken so they do not recur.

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REFERENCES Anusavice K.J. Phillips science of dental materials 11th edition, 2003

Craig R.G. & Powers J.M.- Restorative dental materials 11th edition 2001

Rosenstiel Contemporary fixed prosthodontics

Rudd K.D., Morrow R.M. Dental laboratory procedures FPD ans CPD 2nd edition, 1986

Stananought D.- Laboratory procedures for inlays, crowns & bridges, 1975

Bert T. Cecconi, D.D.S., M.S.and Kamal Asgar, Ph.D Modified hygroscopic gold casting technique J Prosthet Dent 1975;33(2):216-220

Ernest Dewald, The relationship of pattern position to the flow of gold and casting completeness J Prosthet Dent 1979;41(5):531-534

Asgar K., Arfaei A.H.- Castability of crowns & bridge alloys J Prosthet Dent 1985;54:60-63.

Wayne V Campagni, Michael Majchrowicz, An accelerated technique for casting post and core restorations J Prosthet Dent 1991;66:155-156.

Health & Medicine

Casting in dentistry