DefinitioDefinitionn

The process of pouring the molten The process of pouring the molten alloy in the mold cavity.alloy in the mold cavity.

When to start the When to start the castingcasting??

Within 30 seconds from the time the ring Within 30 seconds from the time the ring is removed from the burn out furnace to is removed from the burn out furnace to avoid heat loss and contraction of the mold avoid heat loss and contraction of the mold cavity. cavity.

To complete the casting process To complete the casting process successfully the following items should successfully the following items should be considered carefully;be considered carefully;

-- The metal to be casted.The metal to be casted.

-- Heat source to melt the metal.Heat source to melt the metal.

-- The casting force to force the molten The casting force to force the molten metal in the mold cavity.metal in the mold cavity.

I - MetalI - Metal

There are various types of There are various types of precious ,semiprecious and no precious ,semiprecious and no precious alloys. . It is important to precious alloys. . It is important to use enough weight of metal that use enough weight of metal that suits every case. These were suits every case. These were estimated to be 9grams for molars, 6 estimated to be 9grams for molars, 6 grams for premolars and anteriors grams for premolars and anteriors and12 grams for a pontic.and12 grams for a pontic.

II - Heat SourceII - Heat Source

It is an essential part of the casting It is an essential part of the casting process to melt the metal with either process to melt the metal with either

a)a) Blowpipe torch.Blowpipe torch. b)b) Electric melting.Electric melting.

Blowpipe torchBlowpipe torch

When a torch is used, the major objective in the When a torch is used, the major objective in the melting procedure is to develop the most melting procedure is to develop the most efficient gas-air flame that quickly yet cleanly efficient gas-air flame that quickly yet cleanly melts the metal.melts the metal.

Different torch types are used in the Different torch types are used in the casting procedures.casting procedures.

1)1) Town gas and compressed air which is Town gas and compressed air which is used for melting alloys at a range below used for melting alloys at a range below 1000ºC.1000ºC.

2)2) Gas and oxygen torch which is used Gas and oxygen torch which is used mainly for melting alloys of high fusing mainly for melting alloys of high fusing temperature (about 1300ºC).temperature (about 1300ºC).

3) Oxyacetylene torch which is a multiorifice tip 3) Oxyacetylene torch which is a multiorifice tip used for melting base metal alloys.used for melting base metal alloys.

Zones of the flameZones of the flame

1) Mixing zone1) Mixing zone The inner most blank- appearing cone, directly emanating The inner most blank- appearing cone, directly emanating

from the nozel in which the air and gas combine prior to from the nozel in which the air and gas combine prior to combustion (it is cool and colorless).combustion (it is cool and colorless).

2) Combustion zone2) Combustion zoneIt is slightly greenish and immediately surrounds the first ,it is It is slightly greenish and immediately surrounds the first ,it is

an oxidizing zone and should be kept away from the metal an oxidizing zone and should be kept away from the metal during the fusion.during the fusion.

3) Reducing zone3) Reducing zoneThe hottest part of the flame and has a bluish color , the area The hottest part of the flame and has a bluish color , the area

near the tip of the cone is the part of the flame that must near the tip of the cone is the part of the flame that must stay on the metal during fusion.stay on the metal during fusion.

4) Oxidizing zone4) Oxidizing zoneThe outer zone in which combustion occurs with the oxygen of The outer zone in which combustion occurs with the oxygen of

the air. This part of the flame should never be used to melt the air. This part of the flame should never be used to melt the alloy ,since its temperature is too low and it causes the alloy ,since its temperature is too low and it causes oxidation of the metaloxidation of the metal

Electric melting of the Electric melting of the alloyalloy

In this method the alloy is melted by an In this method the alloy is melted by an electric power, this either performed by:electric power, this either performed by:

a-a-Convection from a heating muffle.Convection from a heating muffle.

b-b- Generation of an induction current in Generation of an induction current in the alloy when the electrical resistance the alloy when the electrical resistance melting furnace casting machine is used.melting furnace casting machine is used.

This method provides a more easily This method provides a more easily controlled heating ,however ,these casting controlled heating ,however ,these casting machines are expensive.machines are expensive.

III- Casting forceIII- Casting force

The force required to push the melted metal The force required to push the melted metal to the mold cavity. It should be high enough to the mold cavity. It should be high enough to overcome the high surface tension of the to overcome the high surface tension of the molten alloy As well as the resistance of the molten alloy As well as the resistance of the gases within the mold cavity.gases within the mold cavity.

The casting machines are either;The casting machines are either;

1.1. Pneumatic (air pressure and steam Pneumatic (air pressure and steam pressure).pressure).

2.2. Centrifugal.Centrifugal.

11 ) )Air pressure casting Air pressure casting machinemachine

-- The alloy is melted (with torch ) directly The alloy is melted (with torch ) directly in the crucible formed in the investment.in the crucible formed in the investment.

-- Above the ring there is a piston through Above the ring there is a piston through which the air pressure is applied to force which the air pressure is applied to force the molten alloy into the mold.the molten alloy into the mold.

-- The table below the ring is connect to a The table below the ring is connect to a source of vacuum ,which is activated just source of vacuum ,which is activated just before the pressure starts.before the pressure starts.

N.B.N.B. Such machines evacuate the mold Such machines evacuate the mold cavity which has been shown to improve cavity which has been shown to improve mold filling.mold filling.

2) Steam pressure casting 2) Steam pressure casting machinemachine

-- A typical steam casting machine is A typical steam casting machine is the solbrigthe solbrig

-- Such machine uses steam to force Such machine uses steam to force the metal into the mold.the metal into the mold.

3) Centrifugal casting 3) Centrifugal casting machinemachine

The molten metal is centrifugally forced into The molten metal is centrifugally forced into the mold.the mold.

There are two centrifugal machines.There are two centrifugal machines.

a)a) Broken-arm. centrifugal casting Broken-arm. centrifugal casting machine.machine.

b)b) Spring wound centrifugal casting Spring wound centrifugal casting machine.machine.

Broken arm centrifugal Broken arm centrifugal casting machinecasting machine

-- It consists of two arms which meet at a It consists of two arms which meet at a central spindle. One arm contains a central spindle. One arm contains a cradle for holding the ring and a crucible cradle for holding the ring and a crucible in which the metal is melted with a torch in which the metal is melted with a torch ,while the other arm carries a movable ,while the other arm carries a movable weight used to balance the machine.weight used to balance the machine.

-- It is powered by the spring at the base It is powered by the spring at the base

B) Spring-wound centrifugal B) Spring-wound centrifugal casting machinecasting machine

It is a spring-wound with an attached It is a spring-wound with an attached electrical resistance melting furnace. electrical resistance melting furnace. The alloy is electrically melted in the The alloy is electrically melted in the furnace and then centrifugally cast into furnace and then centrifugally cast into the mold so there is no need for a the mold so there is no need for a torch .torch .

Spring-wound centrifugal Spring-wound centrifugal casting machinecasting machine

Induction casting Induction casting machinemachine

-- Metal is melted by an induction field Metal is melted by an induction field that develops within a crucible that develops within a crucible surround by water cooled tubing .The surround by water cooled tubing .The molten metal is forced into the mold by molten metal is forced into the mold by air pressure, vacuum or both at the air pressure, vacuum or both at the other end of the ring.other end of the ring.

-- It is used for casting base metal alloys.It is used for casting base metal alloys.

Induction casting machine

Steps of Casting Steps of Casting techniquetechnique

Steps of Casting Steps of Castingtechniquetechnique

Casting armamentariumCasting armamentarium

Centrifugal casting Centrifugal casting machinemachine

CrucibleCrucible BlowtorchBlowtorch Protective colored Protective colored

gogglesgoggles TongsTongs Casting alloyCasting alloy

1) Line the crucible with wet asbestos. Take 1) Line the crucible with wet asbestos. Take care not to obstruct the aperture at the end care not to obstruct the aperture at the end of the crucible. Heat the liner with gas air of the crucible. Heat the liner with gas air blow pipe to remove the impurities from the blow pipe to remove the impurities from the asbestos thus prolongs the life of the asbestos thus prolongs the life of the crucible and prevents the metal crucible and prevents the metal contamination.contamination.

2) Place the crucible in its place on the casting 2) Place the crucible in its place on the casting machine.machine.

3) Place the casting metal on the asbestos line 3) Place the casting metal on the asbestos line in the crucible in the crucible

4) Light the gas-air blowpipe and adjust the 4) Light the gas-air blowpipe and adjust the air and gas knobs to produce conical brush air and gas knobs to produce conical brush flame.flame.

5) Melt the gold using the reducing zone until 5) Melt the gold using the reducing zone until it is shinny and mirror like.it is shinny and mirror like.

When the metal reaches the casting When the metal reaches the casting temperature:-temperature:-

-- It has straw-yellow in color. It has straw-yellow in color.

-- It wiggles easily in the crucible if it is It wiggles easily in the crucible if it is tapped.tapped.

-- It will follow the flame if it is moved slightly.It will follow the flame if it is moved slightly.

6) Shake small amount of flux 6) Shake small amount of flux (powered boric acid and borax) on (powered boric acid and borax) on the metal.the metal.

-- It acts as a reducing agent to It acts as a reducing agent to prevent oxidation of the metal alloy. prevent oxidation of the metal alloy.

- It enhances alloy melting.It enhances alloy melting.

Flux added

7) Keep the flame on the melted 7) Keep the flame on the melted metal ,remove the ring from the burn out metal ,remove the ring from the burn out furnace and place it in the cradle with furnace and place it in the cradle with the sprue hole toward the crucible, it is the sprue hole toward the crucible, it is then slide along the casting arm until it then slide along the casting arm until it lies firmly against the casting ring lies firmly against the casting ring

8) Hold the blow pipe in one hand and 8) Hold the blow pipe in one hand and put gentle pressure on the counter put gentle pressure on the counter weight with other hand until the pin weight with other hand until the pin drops. drops.

Don’t lift the pipe out of the position Don’t lift the pipe out of the position until the arm of the machine has until the arm of the machine has been released. Allow the machine to been released. Allow the machine to spin itself to stop.spin itself to stop.

Cleaning the Cleaning the castcast

1.1. Remove the casting ring with the casting togs.Remove the casting ring with the casting togs.2.2. Wait 3-5 min. until the red glow is disappeared Wait 3-5 min. until the red glow is disappeared

from the button.from the button.3.3. Quench the ring in cold water to;Quench the ring in cold water to;-- Disintegrate the hot investment.Disintegrate the hot investment.-- Anneal the gold thus providing better working Anneal the gold thus providing better working

qualities during finishing.qualities during finishing.N.B:N.B: Quenching the ring in water while it is still Quenching the ring in water while it is still

red will result in sudden drop in temperature red will result in sudden drop in temperature leading to, distortion of the margin and thin areas.leading to, distortion of the margin and thin areas.

4.4. Push the investment out of the ring, clean the cast Push the investment out of the ring, clean the cast as possible with a tooth brush.as possible with a tooth brush.

a) Devestment

B) Pickling

- - Put the casting in a porcelain container and Put the casting in a porcelain container and cover it with cover it with 50% dilute HCL50% dilute HCL or sulphoric acid or sulphoric acid or Jel-Pac liquid warmed gently below the or Jel-Pac liquid warmed gently below the boiling temperature for few moments until the boiling temperature for few moments until the surface become bright.surface become bright.

- Dilute solution of nitric acid can be used for - Dilute solution of nitric acid can be used for badly stained casting.badly stained casting.

Precautions during picklingPrecautions during pickling

1-1- The cast is placed in porcelain or Pyrex container.The cast is placed in porcelain or Pyrex container.2-2- Heat over a Bunsen .Don’t allow boil to it .Heat over a Bunsen .Don’t allow boil to it .3-3- Don’t heat the cast and put it in cold acid, this will Don’t heat the cast and put it in cold acid, this will

result in :result in : -- Distortion of the thin margins.Distortion of the thin margins. -- Explosion that may damage the operator.Explosion that may damage the operator.4-4- The cast should never be healed with a steel tweezers The cast should never be healed with a steel tweezers

or tongs and heated over a Bunsen burner as the or tongs and heated over a Bunsen burner as the insertion of the steel tongs in the acid will result in insertion of the steel tongs in the acid will result in galvanic current that will deposit a deep layer of galvanic current that will deposit a deep layer of Cupper on the casting causing its distortion. So you Cupper on the casting causing its distortion. So you have to use a plastic coated tweezers to remove the have to use a plastic coated tweezers to remove the clean casting from the pickling solution and rinse it clean casting from the pickling solution and rinse it under cold running water.under cold running water.

- Air abrasion with small particles size abrasives is - Air abrasion with small particles size abrasives is another preferable method for cleaning castings.another preferable method for cleaning castings.

Failure to obtain a perfect casting is certainly the result of Failure to obtain a perfect casting is certainly the result of deviation from the correct technique.deviation from the correct technique.Manifestation and possible causes of casting failure could Manifestation and possible causes of casting failure could be summarized as:be summarized as:

I) Surface roughnessI) Surface roughnessII) Black, Rough castingII) Black, Rough casting III) NodulesIII) NodulesIV) FinsIV) FinsV) Incomplete CastingV) Incomplete Casting VI) PorosityVI) PorosityVII) Over or under sized castingVII) Over or under sized castingVIII) Wrapped or distorted restorationVIII) Wrapped or distorted restorationIX) Dark discolored casting resisting IX) Dark discolored casting resisting

picklingpickling

I) Surface roughnessI) Surface roughness

The surface of the casting should be smooth, The surface of the casting should be smooth, although finishing and polishing are although finishing and polishing are required .These surface roughness maybe the required .These surface roughness maybe the result of :result of :

1) Rough surface of the wax pattern. 1) Rough surface of the wax pattern. 2) Excessive wetting agent on the wax pattern.2) Excessive wetting agent on the wax pattern.

3) Investment with large grain size.3) Investment with large grain size.4) Foreign body inclusion of investment 4) Foreign body inclusion of investment

particles or flux powder.particles or flux powder.5) Over heating of the investment leads to 5) Over heating of the investment leads to

rough ,brittle black surface due to rough ,brittle black surface due to decomposing and sulfur compounds.decomposing and sulfur compounds.

II) Black, rough castingII) Black, rough casting

Oxidation of the metal during Oxidation of the metal during melting due to:melting due to:

-Flame not properly adjusted.-Flame not properly adjusted.

-Flux not powdered on the melted -Flux not powdered on the melted metal.metal.

III) NodulesIII) Nodules

Common form of surface roughness usually Common form of surface roughness usually occur due to the presence of bubbles on the occur due to the presence of bubbles on the surface of the wax pattern , these air surface of the wax pattern , these air bubbles may be due to ; bubbles may be due to ;

a-a-Lack of surfactant on the wax pattern.Lack of surfactant on the wax pattern.

b-b- Inadequate vacuum during investing Inadequate vacuum during investing (mixing and pouring).(mixing and pouring).

C-C- Thick mix of the investment.Thick mix of the investment.

d-d- Movement of the ring during setting of Movement of the ring during setting of the investment.the investment.

IV) FinsIV) Fins

This failure is due toThis failure is due to ; ;

1.1.Dropped ring.Dropped ring.

2.2.Sudden increase in the temperature Sudden increase in the temperature during burn out leads to cracks in the during burn out leads to cracks in the investment into which the metal will pass investment into which the metal will pass through.through.

3.3.Excessive casting force.Excessive casting force.

4.4.increase W/P ratioincrease W/P ratio

5.5. improper pattern positioningimproper pattern positioning

V) Incomplete CastingV) Incomplete Casting

1.1. Too thin areas in wax pattern.Too thin areas in wax pattern.2.2. Presence of investment fragments in the sprue Presence of investment fragments in the sprue

channel.channel.3.3. Incomplete wax elimination.Incomplete wax elimination.4.4. The molten alloy not properly directed to the mold The molten alloy not properly directed to the mold

cavity.cavity.5.5. Inadequate metal.Inadequate metal.6.6. Cold ring (the casting not completed within 30 sec.)Cold ring (the casting not completed within 30 sec.)7.7. Inadequate melting of the alloy.Inadequate melting of the alloy.8.8. Low casting force.Low casting force.9.9. Back pressure.Back pressure.10.10. Fracture of the investment in the base of the ring Fracture of the investment in the base of the ring

VI) PorosityVI) Porosity

Reduction in the density of casting by the presence of Reduction in the density of casting by the presence of voids due to the absorption of mold gases and lack of voids due to the absorption of mold gases and lack of precautions to compensate for alloy contraction. It precautions to compensate for alloy contraction. It may be :-may be :-

a- a- Externally, lead to surface roughness of the Externally, lead to surface roughness of the cast.cast.

b-b- Internally, leads to;Internally, leads to;- Weakening the casting- Weakening the casting- Discoloration . - Discoloration . - If sever leads to leakage at tooth restoration interface. - If sever leads to leakage at tooth restoration interface. N.B:N.B: Voids in the marginal area may be caused by Voids in the marginal area may be caused by

debris trapped in the mold, a well waxed smooth debris trapped in the mold, a well waxed smooth sprue will help prevent them. sprue will help prevent them.

Types of porosityTypes of porosity

a-a- Localized porosityLocalized porosity (contraction porosity, (contraction porosity, suck back pressure). resulting from suck back pressure). resulting from solidification shrinkage which occurs if the solidification shrinkage which occurs if the metal in the sprue channel solidify before that metal in the sprue channel solidify before that in the mold, when the sprue is too narrow ,too in the mold, when the sprue is too narrow ,too long ,incorrectly located in a thin area or when long ,incorrectly located in a thin area or when large casting is made in the absence of a vent.large casting is made in the absence of a vent.

b- Subsurface porosity:b- Subsurface porosity: It is caused by It is caused by entrapment of solid particles or gas bubbles entrapment of solid particles or gas bubbles during casting. It appears during finishing of during casting. It appears during finishing of the castingthe casting

contraction porosity at the attachment of the contraction porosity at the attachment of the

thin spruethin sprue

c- Gaseous porosityc- Gaseous porosity(gas inclusion porosity or occluded (gas inclusion porosity or occluded

gases) gases)-Related to the entrapment of gases during -Related to the entrapment of gases during

solidification.solidification.

-Characterized by spherical contour, small in -Characterized by spherical contour, small in size.size.

-Gases may dissolve in molten alloy during -Gases may dissolve in molten alloy during melting and leave porosity defects.melting and leave porosity defects.

-Gases trapped mechanically in the mold by the -Gases trapped mechanically in the mold by the molten metal during casting.molten metal during casting.

-Spherical larger porosity can be caused by gas -Spherical larger porosity can be caused by gas occluded from poorly adjusted blowpipe flame occluded from poorly adjusted blowpipe flame or if the reducing zone is not used or if the reducing zone is not used

Gaseous porosityGaseous porosity

d- Back pressure porosityd- Back pressure porosity Entrapped air on the inner surface of the casting, Entrapped air on the inner surface of the casting,

result from inability of the air in the mold to escape result from inability of the air in the mold to escape through the investment (Fig 23). The entrapment is through the investment (Fig 23). The entrapment is frequently found in a pocket at the margin.frequently found in a pocket at the margin.

N.BN.B-As the molten metal enter the mold space, the -As the molten metal enter the mold space, the

gases contained in there must escape, gases contained in there must escape, preferably through the bottom of the mold.preferably through the bottom of the mold.

-To assist the escape of gases, the investing -To assist the escape of gases, the investing material between the casting and the end of the material between the casting and the end of the ring should be as thin as consistent with ring should be as thin as consistent with strength. strength.

-Also the ring should not be covered completely by -Also the ring should not be covered completely by any part of the casting machine. The plate of any part of the casting machine. The plate of metal which supports the ring must be metal which supports the ring must be perforated.perforated.

VII) Over or under sized VII) Over or under sized castingcasting

Possible CausesPossible Causes

1. Improper investment mixing ratio

2. Stored beyond the date of expiration

3. Improper ratio of investment liquid to water

11 - -Over sized castingOver sized casting

Possible CausesPossible Causes

1. Improper investment mixing ratio

2. Stored beyond the date of expiration

3. Improper ratio of investment liquid to water

4. Missing ring liner in the casting ring

5. Burn-out temperature too low

22 - -Under sized castingUnder sized casting

VIII) Wrapped or distorted VIII) Wrapped or distorted restorationrestoration

It is due improper following to the It is due improper following to the precautionary measures during precautionary measures during every step of the laboratory every step of the laboratory procedures.procedures.

IX) Dark discolored casting IX) Dark discolored casting resisting picklingresisting pickling

It is due to overheating of the investment It is due to overheating of the investment leading to sulfur compounds that leading to sulfur compounds that contaminated the castingcontaminated the casting..