casting defects/ dentist laboratory

Casting defectsCasting defectsINDIAN DENTAL ACADEMY

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ContentsContents• INTRODUCTIONINTRODUCTION• 2 CLASSIFICATION OF DEFECTS-2 CLASSIFICATION OF DEFECTS-• A) Defects in the wax patternA) Defects in the wax pattern• B) Defects in the investing procedureB) Defects in the investing procedure• C) Problems in burnout procedureC) Problems in burnout procedure• D) Problems during casting D) Problems during casting

procedureprocedure


• CLASSIFICATION OF DEFECTS CLASSIFICATION OF DEFECTS (ANUSAVICE)(ANUSAVICE)

• CLASSIFICATION OF DEFECT ( RUDD CLASSIFICATION OF DEFECT ( RUDD AND MORROW)AND MORROW)

• CONCLUSIONCONCLUSION• REFERENCESREFERENCES


IntroductionIntroduction• In dentistry the lost-wax casting technic was In dentistry the lost-wax casting technic was

not popular until 1907 when W.H. Taggart not popular until 1907 when W.H. Taggart intro duced his technic and casting machine. intro duced his technic and casting machine. Today the lost-wax technic is common Today the lost-wax technic is common practice and is used for a variety of casting practice and is used for a variety of casting operationsoperations

• The casting method consists of forming a The casting method consists of forming a wax pattern, surrounding it with investment wax pattern, surrounding it with investment material, and later heating the investment material, and later heating the investment mold to remove the wax before casting the mold to remove the wax before casting the molten metal into the mold molten metal into the mold


• Defects in Wax PatternDefects in Wax Pattern• 1. Distortion1. Distortion • 2. Surface roughness and 2. Surface roughness and

irregularities irregularities • 3. Air bubbles3. Air bubbles• 4. Water film4. Water film


Defects in Burn out Defects in Burn out ProcedureProcedure• 1. Rapid Heating Rates1. Rapid Heating Rates• 2. Under heating 2. Under heating • 3. Prolonged heating3. Prolonged heating


• Rough surface on casting Rough surface on casting • Due to prolonged overheating of Due to prolonged overheating of

gypsum bonded investment. gypsum bonded investment. • Prevention- use correct heating cycle Prevention- use correct heating cycle

for burnout procedure. Do not heat for burnout procedure. Do not heat above 1290F (700C)above 1290F (700C)


• Problems DURING CASTING PROCEDUREProblems DURING CASTING PROCEDURE1. 1. Incomplete castingsIncomplete castings Alloy not hot enoughAlloy not hot enough• Have casting temperature above indicated Have casting temperature above indicated

fusion temperature of alloyfusion temperature of alloy• Mold too coldMold too cold• Oven too cold- check pyrometer; mould should Oven too cold- check pyrometer; mould should

heat soak 1 hour at burnout temperature.heat soak 1 hour at burnout temperature.


--Insufficient casting forceInsufficient casting force• Force alloy into the mold rapidly;use Force alloy into the mold rapidly;use

centrifugal machine with good springs centrifugal machine with good springs and several winds.Use air pressure and and several winds.Use air pressure and vacuum machines with proper force.vacuum machines with proper force.

• --Insufficient goldInsufficient gold• Cast enough gold to allow for good Cast enough gold to allow for good


• 2. Rough Surface on casting2. Rough Surface on casting• Due to -Too much casting pressure causing Due to -Too much casting pressure causing

investment breakdown from force of alloy.investment breakdown from force of alloy.• Elimination-Elimination-• Do not wind machine too tightly.Do not wind machine too tightly.• A gauge pressure of .10 to.14MPa in an air A gauge pressure of .10 to.14MPa in an air

pressure casting machine pressure casting machine • Three to four turns of the spring in an Three to four turns of the spring in an

average type of centrifugal casting average type of centrifugal casting machines is sufficient for small castings machines is sufficient for small castings


Classification of casting defects Classification of casting defects (ANUSAVICE) (ANUSAVICE)

• Defects in castings can be classified Defects in castings can be classified under four headings: under four headings:

• (1) Distortion;(1) Distortion;• (2) Sur face roughness and (2) Sur face roughness and

irregularities; irregularities; • (3) Porosity; (3) Porosity; • (4) Incomplete or missing detail (4) Incomplete or missing detail


• DistortionDistortion• Any marked distortion of the casting Any marked distortion of the casting

is probably related to a distortion of is probably related to a distortion of the wax pattern.the wax pattern.

• This type of distortion can be This type of distortion can be minimized or prevented by proper minimized or prevented by proper manipulation of the wax and handling manipulation of the wax and handling of the pattern.of the pattern.


• Some distortion of the wax pattern Some distortion of the wax pattern occurs as the investment hardens occurs as the investment hardens around it.around it.

• The setting and hygroscopic The setting and hygroscopic expansions of the investment may expansions of the investment may produce a nonuniform expansion of produce a nonuniform expansion of the walls of the pattern.the walls of the pattern.


• This type of distortion occurs in part This type of distortion occurs in part from the nonuniform outward from the nonuniform outward movement of the proximal walls, the movement of the proximal walls, the gingival margins are forced apart by gingival margins are forced apart by the mold expansion, whereas the the mold expansion, whereas the solid occlusal bar of wax resists solid occlusal bar of wax resists expansion during the early stages of expansion during the early stages of setting setting


• The configuration of the pattern, the The configuration of the pattern, the type of wax, and the thickness type of wax, and the thickness influence the distortion that occurs. influence the distortion that occurs.

• Distortion increases as the thickness Distortion increases as the thickness of the pattern decreases. of the pattern decreases.

• The less the setting expansion of the The less the setting expansion of the investment, the less the distortion. investment, the less the distortion.


Surface Roughness, Irregularities, Surface Roughness, Irregularities, and Discolorationand Discoloration

• The surface of a dental casting should The surface of a dental casting should be an accurate reproduction of the wax be an accurate reproduction of the wax pattern from which it is made. pattern from which it is made.

• Excessive roughness or irregularities Excessive roughness or irregularities on outer surface of the casting on outer surface of the casting necessitate additional finishing and necessitate additional finishing and polishing, whereas irregularities on the polishing, whereas irregularities on the cavity surface prevent a proper seating cavity surface prevent a proper seating of an otherwise accurate casting.of an otherwise accurate casting.


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

• Surface irregularities are isolated Surface irregularities are isolated imperfections, such as nodules, that imperfections, such as nodules, that are not characteristic of the entire are not characteristic of the entire surface area. surface area.


• The surface roughness of the dental The surface roughness of the dental casting is invariably somewhat casting is invariably somewhat greater than that of the wax pattern greater than that of the wax pattern from which it is made. from which it is made.

• The difference is probably related to The difference is probably related to the particle size of the investment the particle size of the investment and its ability to reproduce the wax and its ability to reproduce the wax pattern in microscopic detail.pattern in microscopic detail.


• Surface irregularities on an experimental casting caused by air bubbles (A),water film (B), and inclusion of foreign bodies(C).


Air BubblesAir Bubbles• Small nodules on a casting are caused Small nodules on a casting are caused

by air bubbles that become attached by air bubbles that become attached to the pattern during or subsequent to to the pattern during or subsequent to the investing procedure. the investing procedure.

• The best method to avoid air bubbles The best method to avoid air bubbles is to use the vacuum investing is to use the vacuum investing technique.technique.


• If a manual method is used, various precautions If a manual method is used, various precautions can be observed from the investment mix before can be observed from the investment mix before the investing. the investing.

• The use of a mechanical mixer with vibration both The use of a mechanical mixer with vibration both before and after mixing should be practiced before and after mixing should be practiced routinely.routinely.

• Wetting agentWetting agent;;• A wetting agent may be helpful in preventing the A wetting agent may be helpful in preventing the

collection of air bubbles on the surface of the collection of air bubbles on the surface of the pattern.pattern.

• The wetting agent be applied in a thin layer.The wetting agent be applied in a thin layer.• Air-dry the wetting agent, because any excess Air-dry the wetting agent, because any excess

liquid dilutes the investment, possi bly producing liquid dilutes the investment, possi bly producing surface irregularities on the casting. surface irregularities on the casting.


• Water FilmsWater Films• Wax is repellent to water, and if the Wax is repellent to water, and if the

investment becomes separated from investment becomes separated from the wax pat tern in some manner, a the wax pat tern in some manner, a water film may form irregularly over water film may form irregularly over the surface. the surface.

• Occasionally, this type of surface Occasionally, this type of surface irregularity appears as minute ridges irregularity appears as minute ridges or veins on the surface or veins on the surface


• If the pattern is slightly moved, jarred, or If the pattern is slightly moved, jarred, or vibrated after investing, or if the painting vibrated after investing, or if the painting procedure does not result in an intimate procedure does not result in an intimate contact of the investment with the pattern, contact of the investment with the pattern, such a condition may result.such a condition may result.

• A wetting agent is of aid in the prevention of A wetting agent is of aid in the prevention of such irregularities. Too high a L/P ratio may such irregularities. Too high a L/P ratio may also produce these surface irregularities. also produce these surface irregularities.


• Rapid Heating Rapid Heating • May result in fins or spines on the May result in fins or spines on the

casting.casting.• Also, a characteristic surface roughness Also, a characteristic surface roughness

may be evident because of flaking of the may be evident because of flaking of the investment when the water or steam investment when the water or steam pours into the mold. Furthermore, such a pours into the mold. Furthermore, such a surge of steam or water may carry some surge of steam or water may carry some of the salts used as modifiers into the of the salts used as modifiers into the mold, and these salts are left as deposits mold, and these salts are left as deposits on the walls after the water evaporates. on the walls after the water evaporates.


• . The mold should be heated . The mold should be heated gradually; at least 60 min should gradually; at least 60 min should elapse during the heating of the elapse during the heating of the investment-filled ring from room investment-filled ring from room temperature to 700° C. temperature to 700° C.

• The greater the bulk of the The greater the bulk of the investment, the more slowly it should investment, the more slowly it should be heated.be heated.


• UnderheatingUnderheating• Incomplete elimination of wax Incomplete elimination of wax

residues may occur if the heating time is residues may occur if the heating time is too short or if insufficient air is available in too short or if insufficient air is available in the furnace. These factors are particularly the furnace. These factors are particularly important with the low-temperature important with the low-temperature investment techniques.investment techniques.

• Voids or porosity may occur in the casting Voids or porosity may occur in the casting from the gases formed when the hot alloy from the gases formed when the hot alloy comes in contact with carbon residues. comes in contact with carbon residues. Occasionally, the casting may be covered Occasionally, the casting may be covered with a tenacious carbon, coating that is with a tenacious carbon, coating that is virtually impossible to remove by pickling.virtually impossible to remove by pickling.


• Liquid/Powder RatioLiquid/Powder Ratio• The amount of water and investment The amount of water and investment

should be measured accurately.should be measured accurately.• The higher the L/P ratio, the rougher the The higher the L/P ratio, the rougher the

casting. However, if too little water is used, casting. However, if too little water is used, the investment may be unmanageably thick the investment may be unmanageably thick and cannot be properly applied to the and cannot be properly applied to the pattern. pattern.

• In vacuum investing, the air may not be In vacuum investing, the air may not be sufficiently removed. In either instance, a sufficiently removed. In either instance, a rough surface on the casting may result.rough surface on the casting may result.


• Prolonged HeatingProlonged Heating• When the high-heat casting technique is When the high-heat casting technique is

used, a prolonged heating of the mold at used, a prolonged heating of the mold at the casting temperature is likely to cause a the casting temperature is likely to cause a disintegration of the gypsum-bonded disintegration of the gypsum-bonded investment, and the walls of the mold are investment, and the walls of the mold are roughened as a result. Furthermore, roughened as a result. Furthermore, products of decomposition are sulfur products of decomposition are sulfur compounds that may contaminate the alloy compounds that may contaminate the alloy to the extent that the surface texture is to the extent that the surface texture is affected affected


• . Such contamination may be the reason . Such contamination may be the reason that the surface of the casting that the surface of the casting sometimes does not respond to pickling.sometimes does not respond to pickling.

• When the thermal expansion technique When the thermal expansion technique is employed, the mold should be heated is employed, the mold should be heated to the casting temperature—never to the casting temperature—never higher—and the casting should be made higher—and the casting should be made immediately.immediately.


• Temperature of the AlloyTemperature of the Alloy• If an alloy is heated to too high a If an alloy is heated to too high a

temperature before casting, the surface temperature before casting, the surface investment is likely to be attacked, and a investment is likely to be attacked, and a surface roughness may result. surface roughness may result.

• In all probability, the alloy will not be In all probability, the alloy will not be overheated with a gas-air torch when used overheated with a gas-air torch when used with the gas supplied in most localities. with the gas supplied in most localities.

• If oil is used, special care should be If oil is used, special care should be observed that the color emitted by the observed that the color emitted by the molten alloy, for example, is no lighter than molten alloy, for example, is no lighter than a light orange.a light orange.


• Casting PressureCasting Pressure• Too high a pressure during casting Too high a pressure during casting

can produce a rough surface on the can produce a rough surface on the casting. casting.

• A gauge pressure of 0.10 to 0.14 MPa A gauge pressure of 0.10 to 0.14 MPa in an air pressure casting machine or in an air pressure casting machine or three to four turns of the spring in an three to four turns of the spring in an average type of centrifugal casting average type of centrifugal casting machine is sufficient for small machine is sufficient for small castings.castings.


• Composition of the InvestmentComposition of the Investment• The ratio of the binder to the quartz The ratio of the binder to the quartz

influences the surface texture of the influences the surface texture of the casting. casting.

• A coarse silica causes a surface A coarse silica causes a surface roughness. roughness.

• If the investment meets ANSI/ ADA If the investment meets ANSI/ ADA Specification No. 2, the composition Specification No. 2, the composition is not a factor in surface roughness is not a factor in surface roughness


• Foreign BodiesForeign Bodies• When foreign substances get into the mold, When foreign substances get into the mold,

a surface roughness may be produced. a surface roughness may be produced. • For example, a rough crucible former with For example, a rough crucible former with

investment clinging to it may roughen the investment clinging to it may roughen the investment on its removal so that bits of investment on its removal so that bits of investment are carried into the mold with investment are carried into the mold with the molten alloy. the molten alloy.

• Carelessness in the removal of the sprue Carelessness in the removal of the sprue former may also be a cause.former may also be a cause.


• Usually, contamination results not only in Usually, contamination results not only in surface roughness but also in incom plete surface roughness but also in incom plete areas or surface voids. areas or surface voids.

• Any casting that shows sharp, well-defined Any casting that shows sharp, well-defined deficiencies indicates the presence of some deficiencies indicates the presence of some foreign particles in the mold, such as pieces foreign particles in the mold, such as pieces of investment and bits of carbon from a flux. of investment and bits of carbon from a flux.

• Bright-appearing concavities may be the Bright-appearing concavities may be the result of flux being carried into the mold with result of flux being carried into the mold with the metal.the metal.


• Surface discoloration and roughness Surface discoloration and roughness can result from sulfur contamination, can result from sulfur contamination, either from investment breakdown at either from investment breakdown at elevated temperatures or from a high elevated temperatures or from a high sulfur content of the torch flame. The sulfur content of the torch flame. The interaction of the molten alloy with interaction of the molten alloy with sulfur produces a black or grey layer on sulfur produces a black or grey layer on the surface of gold alloys that is brittle the surface of gold alloys that is brittle and does not clean readily during and does not clean readily during pickling.pickling.


• Impact of Molten AlloyImpact of Molten Alloy• The direction of the sprue former should be The direction of the sprue former should be

such that the molten gold alloy does not such that the molten gold alloy does not strike a weak portion of the mold surface.strike a weak portion of the mold surface.

• Occasionally, the molten alloy may fracture Occasionally, the molten alloy may fracture or abrade the mold surface on impact, or abrade the mold surface on impact, regardless of its bulk. regardless of its bulk.

• It is unfortu nate that sometimes the It is unfortu nate that sometimes the abraded area is smooth so that it cannot be abraded area is smooth so that it cannot be detected on the surface of the casting detected on the surface of the casting


• Such a depression in the mold is Such a depression in the mold is reflected as a raised area on the reflected as a raised area on the casting, often too slight to be noticed casting, often too slight to be noticed yet sufficiently large to prevent yet sufficiently large to prevent complete seating of the casting. This complete seating of the casting. This type of surface roughness or irregularity type of surface roughness or irregularity can be avoided by proper spruing so as can be avoided by proper spruing so as to prevent the direct impact of the to prevent the direct impact of the molten metal at an angle of 90 degrees molten metal at an angle of 90 degrees to the investment surface.to the investment surface.


• Pattern PositionPattern Position• If several patterns are invested in the same If several patterns are invested in the same

ring, they should not be placed too close ring, they should not be placed too close together. Likewise, positioning too many together. Likewise, positioning too many patterns in the same plane in the mold should patterns in the same plane in the mold should be avoided. The expansion of wax is much be avoided. The expansion of wax is much greater than that of the invest ment, causing greater than that of the invest ment, causing breakdown or cracking of the investment if the breakdown or cracking of the investment if the spacing between pat terns is less than 3 mm.spacing between pat terns is less than 3 mm.


• Carbon InclusionsCarbon Inclusions• Carbon, as from a crucible, an Carbon, as from a crucible, an

improperly adjusted torch, or a improperly adjusted torch, or a carbon-containing investment, can carbon-containing investment, can be absorbed by the alloy during be absorbed by the alloy during casting. These particles may lead to casting. These particles may lead to the formation of carbides or even the formation of carbides or even create visible carbon inclusions.create visible carbon inclusions.


• Other CausesOther Causes• Certain surface discolorations and Certain surface discolorations and

roughness may not be evident when the roughness may not be evident when the casting is completed but may appear casting is completed but may appear during service. For example, various gold during service. For example, various gold alloys, such as solders, bits of wire, and alloys, such as solders, bits of wire, and mixtures of different casting alloys should mixtures of different casting alloys should never be melted together and reused. never be melted together and reused. Discoloration and corrosion may also occur.Discoloration and corrosion may also occur.


• A source of discoloration often overlooked is A source of discoloration often overlooked is the surface contamination of a gold alloy the surface contamination of a gold alloy restoration by mercury. Mercury penetrates restoration by mercury. Mercury penetrates rapidly into the alloy and causes a marked rapidly into the alloy and causes a marked loss in ductility and a greater susceptibility to loss in ductility and a greater susceptibility to corrosion.corrosion.

• Thus it is not a good practice to place a new Thus it is not a good practice to place a new amalgam restoration adjacent to a high noble amalgam restoration adjacent to a high noble alloy restoration. In addition, these dissimilar alloy restoration. In addition, these dissimilar metals form a galvanic cell that can lead to metals form a galvanic cell that can lead to breakdown of the anode (amalgam) relative breakdown of the anode (amalgam) relative to the cathode (noble alloy). to the cathode (noble alloy).


• PorositiesPorosities in noble metal alloy castings may be in noble metal alloy castings may be classified as follows:classified as follows:

• I. Solidification defectsI. Solidification defects• A. Localized shrinkage porosity A. Localized shrinkage porosity • B. MicroporosityB. Microporosity• II. Trapped gasesII. Trapped gases• A. Pinhole porosityA. Pinhole porosity• H. Gas inclusionsH. Gas inclusions• C. Subsurface porosityC. Subsurface porosity• III. Residual airIII. Residual air


• Localized shrinkage Localized shrinkage • generally caused by premature generally caused by premature

termination of the flow of molten termination of the flow of molten metal during solidification. metal during solidification.

• The linear contraction of noble metal The linear contraction of noble metal alloys in changing from a liquid to a alloys in changing from a liquid to a solid is at least 1.25%. Therefore solid is at least 1.25%. Therefore continual feeding of molten metal continual feeding of molten metal through the sprue must occur to make through the sprue must occur to make up for the shrinkage of metal volume up for the shrinkage of metal volume during solidification during solidification


• . If the sprue freezes in its cross-. If the sprue freezes in its cross-section before this feeding is section before this feeding is completed to the casting proper, a completed to the casting proper, a localized shrinkage void will occur in localized shrinkage void will occur in the last portion of the casting that the last portion of the casting that solidifies.solidifies.


Localized shrinkage porosity due to Localized shrinkage porosity due to delayed solidificationdelayed solidification


• The porosity in the pontic area The porosity in the pontic area • caused by the ability of the pontic to caused by the ability of the pontic to

retain heat because of its bulk retain heat because of its bulk • It was located in the heat center of It was located in the heat center of

the ring. the ring.


• This problem can be solved by This problem can be solved by attaching one or more small-gauge attaching one or more small-gauge sprues sprues

• These small chill set sprues ensure These small chill set sprues ensure that solidification begins within the that solidification begins within the sprues, and they act as cooling pins sprues, and they act as cooling pins to carry heat away from the pontic.to carry heat away from the pontic.


• Suck-back porosity often occurs at Suck-back porosity often occurs at an occlusoaxial line angle or incisoaxial an occlusoaxial line angle or incisoaxial line angle that is not well rounded. line angle that is not well rounded.

• The entering metal impinges onto the The entering metal impinges onto the mold sur face at this point and creates a mold sur face at this point and creates a higher localized mold temperature in higher localized mold temperature in this region, known as a this region, known as a hot spothot spot


Suckback - porositySuckback - porosity


• A A hot spot may retain a localized pool of hot spot may retain a localized pool of molten metal after other areas of the molten metal after other areas of the casting have solidified.casting have solidified.

• This in turn creates a shrinkage void or This in turn creates a shrinkage void or suck-back porosity. suck-back porosity.

• Elimination by flaring the point of sprue Elimination by flaring the point of sprue attachment and reducing the mold-melt attachment and reducing the mold-melt temperature differential, that is ,lowering temperature differential, that is ,lowering the casting temperature by about 30° C.the casting temperature by about 30° C.


MICROPOROSITY AND PINHOLE MICROPOROSITY AND PINHOLE POROSITYPOROSITY


• Microporosity Microporosity • occurs from solidification shrinkage but is occurs from solidification shrinkage but is

generally present in fine-grain alloy castings generally present in fine-grain alloy castings when the solidification is too rapid for the when the solidification is too rapid for the microvoids to segregate to the liquid pool. microvoids to segregate to the liquid pool.

• Such phenomena can occur from rapid Such phenomena can occur from rapid solidification if the mold or casting tem solidification if the mold or casting tem perature is too low. perature is too low.

• This type of defect is not detectable unless This type of defect is not detectable unless the casting is sectioned. the casting is sectioned.


• pinhole and the gas inclusion pinhole and the gas inclusion porositiesporosities

• are related to the entrapment of gas are related to the entrapment of gas during solidification. during solidification.

• both are characterized by a spherical both are characterized by a spherical contour, but they are decidedly contour, but they are decidedly different in size. different in size.

• The gas inclusion porosities are usually The gas inclusion porosities are usually much larger than the pinhole porosity much larger than the pinhole porosity


• Many metals dissolve or occlude gases Many metals dissolve or occlude gases while they are molten.while they are molten.

• For example, both copper and silver For example, both copper and silver dissolve oxygen in large amounts in dissolve oxygen in large amounts in the liquid state.the liquid state.

• platinum and palladium have a strong platinum and palladium have a strong affinity for hydrogen as well as oxygen. affinity for hydrogen as well as oxygen.

• On solidification, the absorbed gases On solidification, the absorbed gases are expelled and pinhole porosity are expelled and pinhole porosity results results


• The larger voids may also result The larger voids may also result from the same cause, but it is more from the same cause, but it is more logical to assume that such voids are logical to assume that such voids are caused by gas that is mechanically caused by gas that is mechanically trapped by the molten metal in the trapped by the molten metal in the mold or by gas that is incorporated mold or by gas that is incorporated during the casting procedure. during the casting procedure.


• All castings probably contain a All castings probably contain a certain amount of porosity. However, certain amount of porosity. However, the porosity should be kept to a the porosity should be kept to a minimum because it may adversely minimum because it may adversely affect the physical properties of the affect the physical properties of the casting. casting.


• . Castings that are severely . Castings that are severely contaminated with gases are usually contaminated with gases are usually black when they are removed from black when they are removed from the investment and do not clean the investment and do not clean easily on pickling.easily on pickling.


A BLACK-COATED NOBLE A BLACK-COATED NOBLE METAL ALLOY CASTING FROM METAL ALLOY CASTING FROM SULFUR CONTAMINATION OR SULFUR CONTAMINATION OR OXIDATIONOXIDATION


• The porosity that extends to the The porosity that extends to the surface is usually in the form of small surface is usually in the form of small pinpoint holes. When the surface is pinpoint holes. When the surface is polished, other pinholes appear.polished, other pinholes appear.


• Larger spherical porosities can be caused by Larger spherical porosities can be caused by gas occluded from a poorly adjusted torch gas occluded from a poorly adjusted torch flame, or by use of the mixing or oxidizing flame, or by use of the mixing or oxidizing zones of the flame rather than the reducing zones of the flame rather than the reducing zone.zone.

• These types of porosity can be minimized by These types of porosity can be minimized by pre-melting the gold alloy on a graphite pre-melting the gold alloy on a graphite crucible or a graphite block, crucible or a graphite block,

• by correctly adjusting and positioning the torch by correctly adjusting and positioning the torch flame during melting.flame during melting.


• Subsurface porosity Subsurface porosity may be caused may be caused by the simultaneous nucleation of by the simultaneous nucleation of solid grains and gas bubbles at the solid grains and gas bubbles at the first moment that the alloy freezes at first moment that the alloy freezes at the mold walls. the mold walls.

• can be diminished by controlling the can be diminished by controlling the rate at which the molten metal rate at which the molten metal enters the mold.enters the mold.


• Entrapped-air porosity or Back Entrapped-air porosity or Back pressure porosity pressure porosity

• occurs occurs on the inner surface of the on the inner surface of the casting,casting,

• produce large concave depressions .produce large concave depressions .


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

• The entrapment is frequently found in a The entrapment is frequently found in a "pocket" at the cavity surface of a crown "pocket" at the cavity surface of a crown or mesio-occlusal-distal casting. or mesio-occlusal-distal casting.


• Occasionally, it is found even on the outside Occasionally, it is found even on the outside surface of the casting when the casting surface of the casting when the casting temperature or mold temperature is so low temperature or mold temperature is so low that solidification occurs before the that solidification occurs before the entrapped air can escape. entrapped air can escape.

• The incidence of entrapped air can be The incidence of entrapped air can be increased by use of the dense modern increased by use of the dense modern investments, by an increase in mold density investments, by an increase in mold density produced by vacuum investing, and by the produced by vacuum investing, and by the tendency for the mold to clog with residual tendency for the mold to clog with residual carbon when the low-heat technique is used. carbon when the low-heat technique is used.

• Each of these factors tends to slow the Each of these factors tends to slow the venting of gases from the mold during venting of gases from the mold during casting.casting.


• Proper burnout, an adequate mold and Proper burnout, an adequate mold and casting temperature, a sufficiently high casting temperature, a sufficiently high casting pressure, and proper L/P ratio casting pressure, and proper L/P ratio can help to eliminate entrapped-air can help to eliminate entrapped-air porosity. It is good practice to make sure porosity. It is good practice to make sure that the thickness of investment that the thickness of investment between the tip of the pattern and the between the tip of the pattern and the end of the ring not be greater than 6 end of the ring not be greater than 6 mm.mm.


CASTING DEFECTS BY RUDD CASTING DEFECTS BY RUDD AND MORROWAND MORROW•Incomplete Incomplete

castingcasting -Inadequate spruing-Inadequate spruing -Alloy not hot -Alloy not hot

enoughenough -Mold too cold-Mold too cold -Ingate obstructed-Ingate obstructed -Insufficient casting -Insufficient casting

forceforce


Rounded marginsRounded margins• Incomplete burnout Incomplete burnout

of wax patternof wax pattern


• Insufficient heating of Insufficient heating of alloybefore castingalloybefore casting

• Margins melted while Margins melted while attaching pattern to attaching pattern to sprue or formersprue or former

• Improper length/diam Improper length/diam of sprue restricts flow of sprue restricts flow of alloy into mold of alloy into mold metal freezes before metal freezes before margins are margins are completecomplete


PorosityPorosity

• Improper sprue Improper sprue former,size,length former,size,length or placementor placement


Rough surface on castingRough surface on casting• Excess moisture on Excess moisture on

patternpattern• Water powder ratio Water powder ratio

too high in mixing too high in mixing investment investment

• Too much casting Too much casting pressure causing pressure causing investment investment breakdown from breakdown from force of alloyforce of alloy


• Prolonged Prolonged overheating of overheating of gypsum bonded gypsum bonded investmentinvestment


Pits in castingPits in casting• Debris in mouldDebris in mould• Dirty waxDirty wax• Loose debris in Loose debris in

cruciblecrucible• Mold temperature Mold temperature

too hottoo hot


Fins on castingFins on casting• Overvibration Overvibration

during investment during investment or disturbing ring or disturbing ring during setduring set

• Heating ring too Heating ring too rapidly causes rapidly causes moisture in mold to moisture in mold to form steam and form steam and rupture out moldrupture out mold


Bubbles or nodules on Bubbles or nodules on castingcasting• Investment not Investment not

mixed under mixed under enough vacuum to enough vacuum to remove entrapped remove entrapped air bubblesair bubbles


• Air leak between Air leak between crucibles,sprue crucibles,sprue former,and casting former,and casting allowed bubble to allowed bubble to form and become form and become trapped under trapped under patternpattern


• Incomplete CastingIncomplete Casting• Occasionally, only a partially Occasionally, only a partially

complete casting, or perhaps no complete casting, or perhaps no casting at all, is found. The obvious casting at all, is found. The obvious cause is that the molten alloy has cause is that the molten alloy has been prevented, in some man ner, been prevented, in some man ner, from completely filling the mold.from completely filling the mold.

• At least two factors that may inhibit At least two factors that may inhibit the ingress of the liquefied alloy are the ingress of the liquefied alloy are

• insufficient venting of the mold insufficient venting of the mold • high viscosity of the fused metal.high viscosity of the fused metal.


• The first consideration, insufficient The first consideration, insufficient venting, is directly related to the venting, is directly related to the back pres sure exerted by the air in back pres sure exerted by the air in the mold.the mold.

• If the air cannot be vented quickly, If the air cannot be vented quickly, the molten alloy does not fill the the molten alloy does not fill the mold before it solidifies. In such a mold before it solidifies. In such a case, the magnitude of the casting case, the magnitude of the casting pressure should be suspected pressure should be suspected


• . If insufficient casting pressure is used, the . If insufficient casting pressure is used, the back pressure cannot be overcome. back pressure cannot be overcome. Furthermore, the pressure should be Furthermore, the pressure should be applied for at least 4 sec. applied for at least 4 sec.

• The mold is filled and the alloy is solidified The mold is filled and the alloy is solidified in 1 sec or less; yet it is quite soft during in 1 sec or less; yet it is quite soft during the early stages. Therefore the pressure the early stages. Therefore the pressure should be maintained for a few seconds should be maintained for a few seconds beyond this point. These failures are beyond this point. These failures are usually exemplified in rounded, incomplete usually exemplified in rounded, incomplete margins. margins.


Rounded,incomplete margins Rounded,incomplete margins are evidence of insufficient are evidence of insufficient casting pressurecasting pressure


• A second common cause for an A second common cause for an incomplete casting is incomplete incomplete casting is incomplete elimination of wax residues from elimination of wax residues from the mold. If too many products of the mold. If too many products of combustion remain in the mold, combustion remain in the mold, the pores in the investment may the pores in the investment may become filled so that the air become filled so that the air cannot be vented completelycannot be vented completely


• If moisture or particles of wax remain, If moisture or particles of wax remain, the contact of the molten alloy with the contact of the molten alloy with these foreign substances produces an these foreign substances produces an explosion that may produce suffi cient explosion that may produce suffi cient back pressure to prevent the mold from back pressure to prevent the mold from being filled. This shiny condition of the being filled. This shiny condition of the metal is caused by the strong reducing metal is caused by the strong reducing atmosphere created by carbon atmosphere created by carbon monoxide left by the residual wax. monoxide left by the residual wax.


Incomplete casting results Incomplete casting results from incomplete wax removalfrom incomplete wax removal


• Different alloys exibit varying viscosities in Different alloys exibit varying viscosities in molten state depending on composition and molten state depending on composition and temperature.temperature.

• However, both the surface ten sion and the However, both the surface ten sion and the viscosity of a molten alloy are decreased viscosity of a molten alloy are decreased with an increase in tempera ture. with an increase in tempera ture.

• An incomplete casting resulting from too An incomplete casting resulting from too great a viscosity can be attributed to great a viscosity can be attributed to insufficient heating. The temperature of the insufficient heating. The temperature of the alloy should be raised higher than its alloy should be raised higher than its liquidus liquidus


• temperature so that its viscosity and temperature so that its viscosity and surface tension are lowered and so that surface tension are lowered and so that it does not solidify prematurely as it it does not solidify prematurely as it enters the mold. Such premature enters the mold. Such premature solidification may account for the greater solidification may account for the greater susceptibility of the white gold alloys to susceptibility of the white gold alloys to porosity because their liquidus porosity because their liquidus temperature are higher. Thus they are temperature are higher. Thus they are more difficult to melt with a gas- air more difficult to melt with a gas- air torch flame. torch flame.


• Sprue former selection is based on the Sprue former selection is based on the following five principles;following five principles;

• . For a small pattern a small size sprue is . For a small pattern a small size sprue is selected because a large sprue attached to selected because a large sprue attached to a thin delicate pattern could cause a thin delicate pattern could cause distortiondistortion

• A sprue former diameter if too thin will A sprue former diameter if too thin will lead to a shrinkage porosity (“suck- back) lead to a shrinkage porosity (“suck- back) porosity.porosity.

• Reservoir sprues are used to overcome Reservoir sprues are used to overcome this problem.this problem.


• 2-The sprue former should be 2-The sprue former should be attached to the portion of the pattern attached to the portion of the pattern with the largest cross sectional area. with the largest cross sectional area. It is best for the molten alloy to flow It is best for the molten alloy to flow from a thick section to a surrounding from a thick section to a surrounding thin area (e.g margins) and not the thin area (e.g margins) and not the reverse. This design minimizes the reverse. This design minimizes the risk for turbulence.risk for turbulence.


• 3-The length of the sprue former 3-The length of the sprue former should be long enough to properly should be long enough to properly position the pattern in the casting position the pattern in the casting ring within 6 mm of the trailing end ring within 6 mm of the trailing end and yet short enough so that molten and yet short enough so that molten alloy does not solidify before it fills alloy does not solidify before it fills the mold.the mold.


• A reservoir should be added to a spruing A reservoir should be added to a spruing network to prevent localized shrinkage network to prevent localized shrinkage porosity.porosity.

• When the molten metal alloy fills the heated When the molten metal alloy fills the heated casting ring, the pattern area should solidify casting ring, the pattern area should solidify first and the reservoir last. Because of its large first and the reservoir last. Because of its large mass of alloy and position in the heat centre of mass of alloy and position in the heat centre of the ring, the reservoir remains molten to the ring, the reservoir remains molten to furnish liquid alloy into the mold as it solidifies. furnish liquid alloy into the mold as it solidifies. The resulting solidification shrinkage occurs in The resulting solidification shrinkage occurs in the reservoir bar and not in the restoration.the reservoir bar and not in the restoration.


• . Flaring facilitates the entry of fluid . Flaring facilitates the entry of fluid alloy into the pattern area.alloy into the pattern area.


• Sprue Former Direction-Sprue Former Direction-The sprue The sprue former should be directed away from former should be directed away from any thin or delicate parts of the pattern, any thin or delicate parts of the pattern, because the molten metal may abrade because the molten metal may abrade or fracture investment in this area and or fracture investment in this area and result in a casting failure. It should not result in a casting failure. It should not be attached at a right angle to a broad be attached at a right angle to a broad flat surface as this leads to turbulence flat surface as this leads to turbulence within the mold cavity and severe within the mold cavity and severe porosity in the region. The sprue former porosity in the region. The sprue former should always be attached to pattern at should always be attached to pattern at 45 degree angle.45 degree angle.


• Sprue Former Length-Sprue Former Length-The length of the The length of the sprue former depends on the length of the sprue former depends on the length of the casting ring. The sprue length should be casting ring. The sprue length should be adjusted so that the top of the wax pattern adjusted so that the top of the wax pattern is within 6 mm of the open end of the ring is within 6 mm of the open end of the ring for gypsum bonded investments. If the for gypsum bonded investments. If the sprue is too short, the wax pattern may be sprue is too short, the wax pattern may be so far removed from the end of the casting so far removed from the end of the casting ring that gases cannot be vented to permit ring that gases cannot be vented to permit the molten alloy to fill the ring completely the molten alloy to fill the ring completely


• When these gases are not When these gases are not completely eliminated porosity completely eliminated porosity results. With the higher strength results. With the higher strength phosphate bonded investments the phosphate bonded investments the sprue former should be within 3 to 4 sprue former should be within 3 to 4 mm of the top of the investment. The mm of the top of the investment. The pattern should always be placed as pattern should always be placed as close to the centre of the ring as close to the centre of the ring as possible.possible.


• The type, size, location, and direction The type, size, location, and direction of the sprue are factors that contribute of the sprue are factors that contribute to the quality of the casting. A hollow to the quality of the casting. A hollow sprue pin provides a stronger sprue pin provides a stronger attachment to the pattern and is rec attachment to the pattern and is rec ommended over the solid pin. Where ommended over the solid pin. Where die solid sprue pin is necessary, as in die solid sprue pin is necessary, as in some types of double sprues having a some types of double sprues having a Y shape, a minute amount of sticky Y shape, a minute amount of sticky wax should be used to enhance the wax should be used to enhance the union of the pin and the wax.union of the pin and the wax.


• The diameter of the sprue, in The diameter of the sprue, in conjunction with the pressure of the conjunction with the pressure of the casting machine and density of the casting machine and density of the molten metal, controls the rate of flow molten metal, controls the rate of flow of the molten metal into the mold of the molten metal into the mold cavity. The larger the diameter of the cavity. The larger the diameter of the sprue or the higher the pressure of the sprue or the higher the pressure of the casting machine and density of the casting machine and density of the molten metal, the faster the molten molten metal, the faster the molten metal should enter the mold cavity metal should enter the mold cavity


Localized shrinkage caused by Localized shrinkage caused by using a sprue of improper using a sprue of improper diameterdiameter


Detached sprue bcoz of Detached sprue bcoz of turbulence caused by improper turbulence caused by improper sprue anglesprue angle


• Surface treatment Surface treatment • Pickling consists of heating the discolored Pickling consists of heating the discolored

casting in an acid. For gypsum bonded casting in an acid. For gypsum bonded investments the best pickling solution is investments the best pickling solution is 50% hydrochloric acid solution. It aids in 50% hydrochloric acid solution. It aids in the removal of any residual investment as the removal of any residual investment as well as of the oxide coating. well as of the oxide coating.

• Disadvantage is that acid fumes are likely Disadvantage is that acid fumes are likely to corrode laboratory metal furnishings. to corrode laboratory metal furnishings. Never should the casting be held with steel Never should the casting be held with steel tongs so that both the casting and the tongs so that both the casting and the tongs come in contact with the pickling tongs come in contact with the pickling solution, as this may contaminate the solution, as this may contaminate the casting.casting.


• Gold based and palladium based metal Gold based and palladium based metal ceramic alloys and base metal alloys ceramic alloys and base metal alloys are bench cooled to room temperature are bench cooled to room temperature before the casting is removed from the before the casting is removed from the investment. Castings from these alloys investment. Castings from these alloys are generally not pickled, and when it are generally not pickled, and when it is recommended for certain metal –is recommended for certain metal –ceramic alloys, it is only to selectively ceramic alloys, it is only to selectively remove specific surface oxides.remove specific surface oxides.


CONCLUSIONCONCLUSION• An unsucessfull casting results in An unsucessfull casting results in

considerable trouble and loss of time,in considerable trouble and loss of time,in almost all instances,defects in casting can be almost all instances,defects in casting can be avoided by strict observance of procedures avoided by strict observance of procedures governed by certain fundamental rules and governed by certain fundamental rules and principles. Seldom is a defect in a casting principles. Seldom is a defect in a casting attributable to factors others than the attributable to factors others than the carelessness or ignorance of the operator. carelessness or ignorance of the operator. With present techniques,casting failures With present techniques,casting failures should be the exception,not the rule..should be the exception,not the rule..


REFERENCESREFERENCESSCIENCE OF DENTAL MATERIALS--- SCIENCE OF DENTAL MATERIALS---

ANUSAVICE—11ANUSAVICE—11THTH EDITION EDITIONCONTEMPORARY FIXED CONTEMPORARY FIXED

PROSTHODONTICS ---STEPHEN F. PROSTHODONTICS ---STEPHEN F. ROSENSTIEL---2ROSENSTIEL---2NDND EDITION EDITION

DENTAL LABORATORY PROCEDURES---DENTAL LABORATORY PROCEDURES--- RHOADS.RUDD.MORROWRHOADS.RUDD.MORROW SCHILLINBURG . FUNDEMENTALS OF SCHILLINBURG . FUNDEMENTALS OF

FIXED PROTHODONTICS FIXED PROTHODONTICS


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