Casting defects/ dentist laboratory

Casting defectsCasting defects
INDIAN 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 procedureD) Problems during casting procedure

• CLASSIFICATION OF DEFECTSCLASSIFICATION OF DEFECTS
(ANUSAVICE)(ANUSAVICE)
• CLASSIFICATION OF DEFECT ( RUDDCLASSIFICATION OF DEFECT ( RUDD
AND MORROW)AND MORROW)
• CONCLUSIONCONCLUSION
• REFERENCESREFERENCES

IntroductionIntroduction
• In dentistry the lost-wax casting technic was notIn dentistry the lost-wax casting technic was not
popular until 1907 when W.H. Taggart intropopular until 1907 when W.H. Taggart intro
duced his technic and casting machine. Todayduced his technic and casting machine. Today
the lost-wax technic is common practice and isthe lost-wax technic is common practice and is
used for a variety of casting operationsused for a variety of casting operations
• The casting method consists of forming a waxThe casting method consists of forming a wax
pattern, surrounding it with investment material,pattern, surrounding it with investment material,
and later heating the investment mold to removeand later heating the investment mold to remove
the wax before casting the molten metal into thethe wax before casting the molten metal into the
moldmold

• Defects in Wax PatternDefects in Wax Pattern
• 1. Distortion1. Distortion
• 2. Surface roughness and irregularities2. Surface roughness and irregularities
• 3. Air bubbles3. Air bubbles
• 4. Water film4. Water film

Defects in Burn out ProcedureDefects in Burn out Procedure
• 1. Rapid Heating Rates1. Rapid Heating Rates
• 2. Under heating2. Under heating
• 3. Prolonged heating3. Prolonged heating

• Rough surface on castingRough surface on casting
• Due to prolonged overheating of gypsumDue to prolonged overheating of gypsum
bonded investment.bonded investment.
• Prevention- use correct heating cycle forPrevention- use correct heating cycle for
burnout procedure. Do not heat aboveburnout procedure. Do not heat above
1290F (700C)1290F (700C)

• Problems DURING CASTINGProblems DURING CASTING
PROCEDUREPROCEDURE
1.1. Incomplete castingsIncomplete castings
Alloy not hot enoughAlloy not hot enough
• Have casting temperature above indicatedHave casting temperature above indicated
fusion temperature of alloyfusion temperature of alloy
• Mold too coldMold too cold
• Oven too cold- check pyrometer; mould shouldOven 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;useForce alloy into the mold rapidly;use
centrifugal machine with good springs andcentrifugal machine with good springs and
several winds.Use air pressure andseveral winds.Use air pressure and
vacuum machines with proper force.vacuum machines with proper force.
• --Insufficient goldInsufficient gold
• Cast enough gold to allow for goodCast enough gold to allow for good

• 2. Rough Surface on casting2. Rough Surface on casting
• Due to -Too much casting pressure causingDue 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 airA gauge pressure of .10 to.14MPa in an air
pressure casting machinepressure casting machine
• Three to four turns of the spring in an averageThree to four turns of the spring in an average
type of centrifugal casting machines is sufficienttype of centrifugal casting machines is sufficient
for small castingsfor small castings

Classification of casting defectsClassification of casting defects
(ANUSAVICE)(ANUSAVICE)
• Defects in castings can be classifiedDefects in castings can be classified
under four headings:under four headings:
• (1) Distortion;(1) Distortion;
• (2) Surface roughness and irregularities;(2) Surface roughness and irregularities;
• (3) Porosity;(3) Porosity;
• (4) Incomplete or missing detail(4) Incomplete or missing detail

• DistortionDistortion
• Any marked distortion of the casting isAny marked distortion of the casting is
probably related to a distortion of the waxprobably related to a distortion of the wax
pattern.pattern.
• This type of distortion can be minimized orThis type of distortion can be minimized or
prevented by proper manipulation of theprevented by proper manipulation of the
wax and handling of the pattern.wax and handling of the pattern.

• Some distortion of the wax pattern occursSome distortion of the wax pattern occurs
as the investment hardens around it.as the investment hardens around it.
• The setting and hygroscopic expansionsThe setting and hygroscopic expansions
of the investment may produce aof the investment may produce a
nonuniform expansion of the walls of thenonuniform expansion of the walls of the
pattern.pattern.

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

• The configuration of the pattern, the typeThe configuration of the pattern, the type
of wax, and the thickness influence theof wax, and the thickness influence the
distortion that occurs.distortion that occurs.
• Distortion increases as the thickness ofDistortion increases as the thickness of
the pattern decreases.the pattern decreases.
• The less the setting expansion of theThe 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 beThe surface of a dental casting should be
an accurate reproduction of the waxan accurate reproduction of the wax
pattern from which it is made.pattern from which it is made.
• Excessive roughness or irregularities onExcessive roughness or irregularities on
outer surface of the casting necessitateouter surface of the casting necessitate
additional finishing and polishing, whereasadditional finishing and polishing, whereas
irregularities on the cavity surface preventirregularities on the cavity surface prevent
a proper seating of an otherwise accuratea proper seating of an otherwise accurate
casting.casting.

• Surface roughness is defined as relativelySurface roughness is defined as relatively
finely spaced surface imperfections whosefinely spaced surface imperfections whose
height, width, and direction establish theheight, width, and direction establish the
predominant surface pattern.predominant surface pattern.
• Surface irregularities are isolatedSurface irregularities are isolated
imperfections, such as nodules, that areimperfections, such as nodules, that are
not characteristic of the entire surfacenot characteristic of the entire surface
area.area.

• The surface roughness of the dentalThe surface roughness of the dental
casting is invariably somewhat greatercasting is invariably somewhat greater
than that of the wax pattern from which itthan that of the wax pattern from which it
is made.is made.
• The difference is probably related to theThe difference is probably related to the
particle size of the investment and itsparticle size of the investment and its
ability to reproduce the wax pattern inability to reproduce the wax pattern in
microscopic detail.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 bySmall nodules on a casting are caused by
air bubbles that become attached to theair bubbles that become attached to the
pattern during or subsequent to thepattern during or subsequent to the
investing procedure.investing procedure.
• The best method to avoid air bubbles isThe best method to avoid air bubbles is
to use the vacuum investing technique.to use the vacuum investing technique.

• If a manual method is used, various precautions can beIf a manual method is used, various precautions can be
observed from the investment mix before the investing.observed from the investment mix before the investing.
• The use of a mechanical mixer with vibration both beforeThe use of a mechanical mixer with vibration both before
and after mixing should be practiced routinely.and after mixing should be practiced routinely.
• Wetting agentWetting agent ;;
• A wetting agent may be helpful in preventing theA wetting agent may be helpful in preventing the
collection of air bubbles on the surface of the pattern.collection of air bubbles on the surface of the 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 liquidAir-dry the wetting agent, because any excess liquid
dilutes the investment, possibly producing surfacedilutes the investment, possibly producing surface
irregularities on the casting.irregularities on the casting.

• Water FilmsWater Films
• Wax is repellent to water, and if theWax is repellent to water, and if the
investment becomes separated from theinvestment becomes separated from the
wax pattern in some manner, a water filmwax pattern in some manner, a water film
may form irregularly over the surface.may form irregularly over the surface.
• Occasionally, this type of surfaceOccasionally, this type of surface
irregularity appears as minute ridges orirregularity appears as minute ridges or
veins on the surfaceveins on the surface

• If the pattern is slightly moved, jarred, orIf the pattern is slightly moved, jarred, or
vibrated after investing, or if the paintingvibrated after investing, or if the painting
procedure does not result in an intimate contactprocedure does not result in an intimate contact
of the investment with the pattern, such aof the investment with the pattern, such a
condition may result.condition may result.
• A wetting agent is of aid in the prevention ofA wetting agent is of aid in the prevention of
such irregularities. Too high a L/P ratio may alsosuch irregularities. Too high a L/P ratio may also
produce these surface irregularities.produce these surface irregularities.

• Rapid HeatingRapid Heating
• May result in fins or spines on the casting.May result in fins or spines on the casting.
• Also, a characteristic surface roughnessAlso, a characteristic surface roughness
may be evident because of flaking of themay be evident because of flaking of the
investment when the water or steam poursinvestment when the water or steam pours
into the mold. Furthermore, such a surgeinto the mold. Furthermore, such a surge
of steam or water may carry some of theof steam or water may carry some of the
salts used as modifiers into the mold, andsalts used as modifiers into the mold, and
these salts are left as deposits on thethese salts are left as deposits on the
walls after the water evaporates.walls after the water evaporates.

• . The mold should be heated gradually; at. The mold should be heated gradually; at
least 60 min should elapse during theleast 60 min should elapse during the
heating of the investment-filled ring fromheating of the investment-filled ring from
room temperature to 700° C.room temperature to 700° C.
• The greater the bulk of the investment, theThe greater the bulk of the investment, the
more slowly it should be heated.more slowly it should be heated.

• UnderheatingUnderheating
• Incomplete elimination of wax residues mayIncomplete elimination of wax residues may
occur if the heating time is too short or ifoccur if the heating time is too short or if
insufficient air is available in the furnace. Theseinsufficient air is available in the furnace. These
factors are particularly important with the low-factors are particularly important with the low-
temperature investment techniques.temperature investment techniques.
• Voids or porosity may occur in the casting fromVoids or porosity may occur in the casting from
the gases formed when the hot alloy comes inthe gases formed when the hot alloy comes in
contact with carbon residues. Occasionally, thecontact with carbon residues. Occasionally, the
casting may be covered with a tenacious carbon,casting may be covered with a tenacious carbon,
coating that is virtually impossible to remove bycoating that is virtually impossible to remove by
pickling.pickling.

• Liquid/Powder RatioLiquid/Powder Ratio
• The amount of water and investment shouldThe amount of water and investment should
be measured accurately.be measured accurately.
• The higher the L/P ratio, the rougher theThe higher the L/P ratio, the rougher the
casting. However, if too little water is used, thecasting. However, if too little water is used, the
investment may be unmanageably thick andinvestment may be unmanageably thick and
cannot be properly applied to the pattern.cannot be properly applied to the pattern.
• In vacuum investing, the air may not beIn vacuum investing, the air may not be
sufficiently removed. In either instance, a rough sufficiently removed. In either instance, a rough
surface on the casting may result.surface on the casting may result.

• Prolonged HeatingProlonged Heating
• When the high-heat casting technique is used, aWhen the high-heat casting technique is used, a
prolonged heating of the mold at the castingprolonged heating of the mold at the casting
temperature is likely to cause a disintegration oftemperature is likely to cause a disintegration of
the gypsum-bonded investment, and the walls ofthe gypsum-bonded investment, and the walls of
the mold are roughened as a result.the mold are roughened as a result.
Furthermore, products of decomposition areFurthermore, products of decomposition are
sulfur compounds that may contaminate thesulfur compounds that may contaminate the
alloy to the extent that the surface texture isalloy to the extent that the surface texture is
affectedaffected

• . Such contamination may be the reason. Such contamination may be the reason
that the surface of the casting sometimesthat the surface of the casting sometimes
does not respond to pickling.does not respond to pickling.
• When the thermal expansion technique isWhen the thermal expansion technique is
employed, the mold should be heated toemployed, the mold should be heated to
the casting temperature—never higher—the casting temperature—never higher—
and the casting should be madeand the casting should be made
immediately.immediately.

• Temperature of the AlloyTemperature of the Alloy
• If an alloy is heated to too high a temperatureIf an alloy is heated to too high a temperature
before casting, the surface investment is likely tobefore casting, the surface investment is likely to
be attacked, and a surface roughness maybe attacked, and a surface roughness may
result.result.
• In all probability, the alloy will not be overheatedIn all probability, the alloy will not be overheated
with a gas-air torch when used with the gaswith a gas-air torch when used with the gas
supplied in most localities.supplied in most localities.
• If oil is used, special care should be observedIf oil is used, special care should be observed
that the color emitted by the molten alloy, forthat the color emitted by the molten alloy, for
example, is no lighter than a light orange.example, is no lighter than a light orange.

• Casting PressureCasting Pressure
• Too high a pressure during casting canToo high a pressure during casting can
produce a rough surface on the casting.produce a rough surface on the casting.
• A gauge pressure of 0.10 to 0.14 MPa inA gauge pressure of 0.10 to 0.14 MPa in
an air pressure casting machine or threean air pressure casting machine or three
to four turns of the spring in an averageto four turns of the spring in an average
type of centrifugal casting machine istype of centrifugal casting machine is
sufficient for small castings.sufficient for small castings.

• Composition of the InvestmentComposition of the Investment
• The ratio of the binder to the quartzThe ratio of the binder to the quartz
influences the surface texture of theinfluences the surface texture of the
casting.casting.
• A coarse silica causes a surfaceA coarse silica causes a surface
roughness.roughness.
• If the investment meets ANSI/ ADAIf the investment meets ANSI/ ADA
Specification No. 2, the composition is notSpecification No. 2, the composition is not
a factor in surface roughnessa factor in surface roughness

• Foreign BodiesForeign Bodies
• When foreign substances get into the mold, aWhen foreign substances get into the mold, a
surface roughness may be produced.surface roughness may be produced.
• For example, a rough crucible former withFor example, a rough crucible former with
investment clinging to it may roughen theinvestment clinging to it may roughen the
investment on its removal so that bits ofinvestment on its removal so that bits of
investment are carried into the mold with theinvestment are carried into the mold with the
molten alloy.molten alloy.
• Carelessness in the removal of the sprue formerCarelessness in the removal of the sprue former
may also be a cause.may also be a cause.

• Usually, contamination results not only inUsually, contamination results not only in
surface roughness but also in incomplete areassurface roughness but also in incomplete areas
or surface voids.or surface voids.
• Any casting that shows sharp, well-definedAny casting that shows sharp, well-defined
deficiencies indicates the presence of somedeficiencies indicates the presence of some
foreign particles in the mold, such as pieces offoreign particles in the mold, such as pieces of
investment and bits of carbon from a flux.investment and bits of carbon from a flux.
• Bright-appearing concavities may be the resultBright-appearing concavities may be the result
of flux being carried into the mold with the metal.of flux being carried into the mold with the metal.

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

• Impact of Molten AlloyImpact of Molten Alloy
• The direction of the sprue former should be suchThe direction of the sprue former should be such
that the molten gold alloy does not strike a weakthat the molten gold alloy does not strike a weak
portion of the mold surface.portion of the mold surface.
• Occasionally, the molten alloy may fracture orOccasionally, the molten alloy may fracture or
abrade the mold surface on impact, regardlessabrade the mold surface on impact, regardless
of its bulk.of its bulk.
• It is unfortunate that sometimes the abradedIt is unfortunate that sometimes the abraded
area is smooth so that it cannot be detected onarea is smooth so that it cannot be detected on
the surface of the castingthe surface of the casting

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

• Pattern PositionPattern Position
• If several patterns are invested in the same ring,If several patterns are invested in the same ring,
they should not be placed too close together.they should not be placed too close together.
Likewise, positioning too many patterns in theLikewise, positioning too many patterns in the
same plane in the mold should be avoided. Thesame plane in the mold should be avoided. The
expansion of wax is much greater than that ofexpansion of wax is much greater than that of
the investment, causing breakdown or crackingthe investment, causing breakdown or cracking
of the investment if the spacing between patof the investment if the spacing between pat
terns is less than 3 mm.terns is less than 3 mm.

• Carbon InclusionsCarbon Inclusions
• Carbon, as from a crucible, an improperlyCarbon, as from a crucible, an improperly
adjusted torch, or a carbon-containingadjusted torch, or a carbon-containing
investment, can be absorbed by the alloyinvestment, can be absorbed by the alloy
during casting. These particles may leadduring casting. These particles may lead
to the formation of carbides or even createto the formation of carbides or even create
visible carbon inclusions.visible carbon inclusions.

• Other CausesOther Causes
• Certain surface discolorations and roughnessCertain surface discolorations and roughness
may not be evident when the casting ismay not be evident when the casting is
completed but may appear during service. Forcompleted but may appear during service. For
example, various gold alloys, such as solders,example, various gold alloys, such as solders,
bits of wire, and mixtures of different castingbits of wire, and mixtures of different casting
alloys should never be melted together andalloys should never be melted together and
reused. Discoloration and corrosion may alsoreused. Discoloration and corrosion may also
occur.occur.

• A source of discoloration often overlooked is theA source of discoloration often overlooked is the
surface contamination of a gold alloy restorationsurface contamination of a gold alloy restoration
by mercury. Mercury penetrates rapidly into theby mercury. Mercury penetrates rapidly into the
alloy and causes a marked loss in ductility and aalloy and causes a marked loss in ductility and a
greater susceptibility to corrosion.greater susceptibility to corrosion.
• Thus it is not a good practice to place a newThus it is not a good practice to place a new
amalgam restoration adjacent to a high nobleamalgam restoration adjacent to a high noble
alloy restoration. In addition, these dissimilaralloy restoration. In addition, these dissimilar
metals form a galvanic cell that can lead tometals form a galvanic cell that can lead to
breakdown of the anode (amalgam) relative tobreakdown of the anode (amalgam) relative to
the cathode (noble alloy).the cathode (noble alloy).

• PorositiesPorosities in noble metal alloy castings may bein noble metal alloy castings may be
classified as follows:classified as follows:
• I. Solidification defectsI. Solidification defects
• A. Localized shrinkage porosityA. 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 shrinkageLocalized shrinkage
• generally caused by prematuregenerally caused by premature
termination of the flow of molten metaltermination of the flow of molten metal
during solidification.during solidification.
• The linear contraction of noble metalThe linear contraction of noble metal
alloys in changing from a liquid to a solidalloys in changing from a liquid to a solid
is at least 1.25%. Therefore continualis at least 1.25%. Therefore continual
feeding of molten metal through the spruefeeding of molten metal through the sprue
must occur to make up for the shrinkagemust occur to make up for the shrinkage
of metal volume during solidificationof metal volume during solidification

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

Localized shrinkage porosity due to delayedLocalized shrinkage porosity due to delayed
solidificationsolidification

• The porosity in the pontic areaThe porosity in the pontic area
• caused by the ability of the pontic tocaused by the ability of the pontic to
retain heat because of its bulkretain heat because of its bulk
• It was located in the heat center of theIt was located in the heat center of the
ring.ring.

• This problem can be solved by attachingThis problem can be solved by attaching
one or more small-gauge spruesone or more small-gauge sprues
• These small chill set sprues ensure thatThese small chill set sprues ensure that
solidification begins within the sprues, andsolidification begins within the sprues, and
they act as cooling pins to carry heat awaythey act as cooling pins to carry heat away
from the pontic.from the pontic.

• Suck-back porosity often occurs at anSuck-back porosity often occurs at an
occlusoaxial line angle or incisoaxial lineocclusoaxial line angle or incisoaxial line
angle that is not well rounded.angle that is not well rounded.
• The entering metal impinges onto theThe entering metal impinges onto the
mold surface at this point and creates amold surface at this point and creates a
higher localized mold temperature in thishigher localized mold temperature in this
region, known as aregion, known as a hot spothot spot

Suckback - porositySuckback - porosity

• AA hot spot may retain a localized pool of moltenhot spot may retain a localized pool of molten
metal after other areas of the casting havemetal after other areas of the casting have
solidified.solidified.
• This in turn creates a shrinkage void or suck-This in turn creates a shrinkage void or suck-
back porosity.back porosity.
• Elimination by flaring the point of sprueElimination by flaring the point of sprue
attachment and reducing the mold-meltattachment and reducing the mold-melt
temperature differential, that is ,lowering thetemperature differential, that is ,lowering the
casting temperature by about 30° C.casting temperature by about 30° C.

MICROPOROSITY AND PINHOLEMICROPOROSITY AND PINHOLE
POROSITYPOROSITY

• MicroporosityMicroporosity
• occurs from solidification shrinkage but isoccurs from solidification shrinkage but is
generally present in fine-grain alloy castingsgenerally present in fine-grain alloy castings
when the solidification is too rapid for thewhen 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 rapidSuch phenomena can occur from rapid
solidification if the mold or casting temperaturesolidification if the mold or casting temperature
is too low.is too low.
• This type of defect is not detectable unless theThis type of defect is not detectable unless the
casting is sectioned.casting is sectioned.

• pinhole and the gas inclusionpinhole and the gas inclusion
porositiesporosities
• are related to the entrapment of gasare related to the entrapment of gas
during solidification.during solidification.
• both are characterized by a sphericalboth are characterized by a spherical
contour, but they are decidedly different incontour, but they are decidedly different in
size.size.
• The gas inclusion porosities are usuallyThe gas inclusion porosities are usually
much larger than the pinhole porositymuch larger than the pinhole porosity

• Many metals dissolve or occlude gasesMany metals dissolve or occlude gases
while they are molten.while they are molten.
• For example, both copper and silverFor example, both copper and silver
dissolve oxygen in large amounts in thedissolve oxygen in large amounts in the
liquid state.liquid state.
• platinum and palladium have a strongplatinum and palladium have a strong
affinity for hydrogen as well as oxygen.affinity for hydrogen as well as oxygen.
• On solidification, the absorbed gases areOn solidification, the absorbed gases are
expelled and pinhole porosity resultsexpelled and pinhole porosity results

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

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

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

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

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

• Larger spherical porosities can be caused byLarger spherical porosities can be caused by
gas occluded from a poorly adjusted torch flame,gas occluded from a poorly adjusted torch flame,
or by use of the mixing or oxidizing zones of theor by use of the mixing or oxidizing zones of the
flame rather than the reducing zone.flame rather than the reducing zone.
• These types of porosity can be minimized byThese types of porosity can be minimized by
pre-melting the gold alloy on a graphite cruciblepre-melting the gold alloy on a graphite crucible
or a graphite block,or a graphite block,
• by correctly adjusting and positioning the torchby correctly adjusting and positioning the torch
flame during melting.flame during melting.

• Subsurface porositySubsurface porosity may be caused bymay be caused by
the simultaneous nucleation of solid grainsthe simultaneous nucleation of solid grains
and gas bubbles at the first moment thatand gas bubbles at the first moment that
the alloy freezes at the mold walls.the alloy freezes at the mold walls.
• can be diminished by controlling the ratecan be diminished by controlling the rate
at which the molten metal enters the mold.at which the molten metal enters the mold.

• Entrapped-air porosity or Back pressureEntrapped-air porosity or Back pressure
porosityporosity
• occursoccurs on the inner surface of the casting,on the inner surface of the casting,
• produce large concave depressions .produce large concave depressions .

• This is caused by the inability of the air in theThis is caused by the inability of the air in the
mold to escape through the pores in themold to escape through the pores in the
investment or by the pressure gradient thatinvestment or by the pressure gradient that
displaces the air pocket toward the end of thedisplaces the air pocket toward the end of the
investment via the molten sprue and button.investment via the molten sprue and button.
• The entrapment is frequently found in a "pocket"The entrapment is frequently found in a "pocket"
at the cavity surface of a crown or mesio-at the cavity surface of a crown or mesio-
occlusal-distal casting.occlusal-distal casting.

• Occasionally, it is found even on the outsideOccasionally, it is found even on the outside
surface of the casting when the castingsurface of the casting when the casting
temperature or mold temperature is so low thattemperature or mold temperature is so low that
solidification occurs before the entrapped air cansolidification occurs before the entrapped air can
escape.escape.
• The incidence of entrapped air can be increasedThe incidence of entrapped air can be increased
by use of the dense modern investments, by anby use of the dense modern investments, by an
increase in mold density produced by vacuumincrease in mold density produced by vacuum
investing, and by the tendency for the mold toinvesting, and by the tendency for the mold to
clog with residual carbon when the low-heatclog with residual carbon when the low-heat
technique is used.technique is used.
• Each of these factors tends to slow the ventingEach of these factors tends to slow the venting
of gases from the mold during casting.of gases from the mold during casting.

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

CASTING DEFECTS BY RUDDCASTING DEFECTS BY RUDD
AND MORROWAND MORROW
• IncompleteIncomplete
castingcasting
 -Inadequate spruing-Inadequate spruing
 -Alloy not hot enough-Alloy not hot enough
 -Mold too cold-Mold too cold
 -Ingate obstructed-Ingate obstructed
 -Insufficient casting-Insufficient casting
forceforce

Rounded marginsRounded margins
• Incomplete burnout ofIncomplete burnout of
wax patternwax pattern

• Insufficient heating ofInsufficient heating of
alloybefore castingalloybefore casting
• Margins melted whileMargins melted while
attaching pattern toattaching pattern to
sprue or formersprue or former
• Improper length/diamImproper length/diam
of sprue restricts flowof sprue restricts flow
of alloy into moldof alloy into mold
metal freezes beforemetal freezes before
margins are completemargins are complete

PorosityPorosity
• Improper sprueImproper sprue
former,size,length orformer,size,length or
placementplacement

Rough surface on castingRough surface on casting
• Excess moisture onExcess moisture on
patternpattern
• Water powder ratioWater powder ratio
too high in mixingtoo high in mixing
investmentinvestment
• Too much castingToo much casting
pressure causingpressure causing
breakdown from forcebreakdown from force
of alloyof alloy

• ProlongedProlonged
overheating ofoverheating of
gypsum bondedgypsum bonded

Pits in castingPits in casting
• Debris in mouldDebris in mould
• Dirty waxDirty wax
• Loose debris inLoose debris in
cruciblecrucible
• Mold temperature tooMold temperature too
hothot

Fins on castingFins on casting
• Overvibration duringOvervibration during
investment orinvestment or
disturbing ring duringdisturbing ring during
setset
• Heating ring tooHeating ring too
rapidly causesrapidly causes
moisture in mold tomoisture in mold to
form steam andform steam and
rupture out moldrupture out mold

Bubbles or nodules on castingBubbles or nodules on casting
• Investment not mixedInvestment not mixed
under enoughunder enough
vacuum to removevacuum to remove
entrapped air bubblesentrapped air bubbles

• Air leak betweenAir leak between
crucibles,spruecrucibles,sprue
former,and castingformer,and casting
allowed bubble toallowed bubble to
form and becomeform and become
trapped under patterntrapped under pattern

• Incomplete CastingIncomplete Casting
• Occasionally, only a partially completeOccasionally, only a partially complete
casting, or perhaps no casting at all, iscasting, or perhaps no casting at all, is
found. The obvious cause is that thefound. The obvious cause is that the
molten alloy has been prevented, in somemolten alloy has been prevented, in some
manner, from completely filling the mold.manner, from completely filling the mold.
• At least two factors that may inhibit theAt least two factors that may inhibit the
ingress of the liquefied alloy areingress of the liquefied alloy are
• insufficient venting of the moldinsufficient venting of the mold
• high viscosity of the fused metal.high viscosity of the fused metal.

• The first consideration, insufficient venting,The first consideration, insufficient venting,
is directly related to the back pressureis directly related to the back pressure
exerted by the air in the mold.exerted by the air in the mold.
• If the air cannot be vented quickly, theIf the air cannot be vented quickly, the
molten alloy does not fill the mold before itmolten alloy does not fill the mold before it
solidifies. In such a case, the magnitude ofsolidifies. In such a case, the magnitude of
the casting pressure should be suspectedthe casting pressure should be suspected

• . If insufficient casting pressure is used, the back. If insufficient casting pressure is used, the back
pressure cannot be overcome. Furthermore, thepressure cannot be overcome. Furthermore, the
pressure should be applied for at least 4 sec.pressure should be applied for at least 4 sec.
• The mold is filled and the alloy is solidified in 1The mold is filled and the alloy is solidified in 1
sec or less; yet it is quite soft during the earlysec or less; yet it is quite soft during the early
stages. Therefore the pressure should bestages. Therefore the pressure should be
maintained for a few seconds beyond this point.maintained for a few seconds beyond this point.
These failures are usually exemplified inThese failures are usually exemplified in
rounded, incomplete margins.rounded, incomplete margins.

Rounded,incomplete margins areRounded,incomplete margins are
evidence of insufficient castingevidence of insufficient casting
pressurepressure

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

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

Incomplete casting results fromIncomplete casting results from
incomplete wax removalincomplete wax removal

• Different alloys exibit varying viscosities inDifferent alloys exibit varying viscosities in
molten state depending on composition andmolten state depending on composition and
temperature.temperature.
• However, both the surface tension and theHowever, both the surface tension and the
viscosity of a molten alloy are decreased with anviscosity of a molten alloy are decreased with an
increase in temperature.increase in temperature.
• An incomplete casting resulting from too great aAn incomplete casting resulting from too great a
viscosity can be attributed to insufficient heating.viscosity can be attributed to insufficient heating.
The temperature of the alloy should be raisedThe temperature of the alloy should be raised
higher than its liquidushigher than its liquidus

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

• Sprue former selection is based on the followingSprue former selection is based on the following
five principles;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 aselected because a large sprue attached to a
thin delicate pattern could cause distortionthin delicate pattern could cause distortion
• A sprue former diameter if too thin will lead to aA sprue former diameter if too thin will lead to a
shrinkage porosity (“suckback) porosity.shrinkage porosity (“suckback) porosity.
• Reservoir sprues are used to overcome thisReservoir sprues are used to overcome this
problem.problem.

• 2-The sprue former should be attached to2-The sprue former should be attached to
the portion of the pattern with the largestthe portion of the pattern with the largest
cross sectional area. It is best for thecross sectional area. It is best for the
molten alloy to flow from a thick section tomolten alloy to flow from a thick section to
a surrounding thin area (e.g margins) anda surrounding thin area (e.g margins) and
not the reverse. This design minimizes thenot the reverse. This design minimizes the
risk for turbulence.risk for turbulence.

• 3-The length of the sprue former should3-The length of the sprue former should
be long enough to properly position thebe long enough to properly position the
pattern in the casting ring within 6 mm ofpattern in the casting ring within 6 mm of
the trailing end and yet short enough sothe trailing end and yet short enough so
that molten alloy does not solidify before itthat molten alloy does not solidify before it
fills the mold.fills the mold.

• A reservoir should be added to a spruingA reservoir should be added to a spruing
network to prevent localized shrinkage porosity.network to prevent localized shrinkage porosity.
• When the molten metal alloy fills the heatedWhen the molten metal alloy fills the heated
casting ring, the pattern area should solidify firstcasting ring, the pattern area should solidify first
and the reservoir last. Because of its large massand the reservoir last. Because of its large mass
of alloy and position in the heat centre of theof alloy and position in the heat centre of the
ring, the reservoir remains molten to furnishring, the reservoir remains molten to furnish
liquid alloy into the mold as it solidifies. Theliquid alloy into the mold as it solidifies. The
resulting solidification shrinkage occurs in theresulting solidification shrinkage occurs in the
reservoir bar and not in the restoration.reservoir bar and not in the restoration.

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

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

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

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

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

• The diameter of the sprue, in conjunctionThe diameter of the sprue, in conjunction
with the pressure of the casting machinewith the pressure of the casting machine
and density of the molten metal, controlsand density of the molten metal, controls
the rate of flow of the molten metal into thethe rate of flow of the molten metal into the
mold cavity. The larger the diameter of themold cavity. The larger the diameter of the
sprue or the higher the pressure of thesprue or the higher the pressure of the
casting machine and density of the moltencasting machine and density of the molten
metal, the faster the molten metal shouldmetal, the faster the molten metal should
enter the mold cavityenter the mold cavity

Localized shrinkage caused byLocalized shrinkage caused by
using a sprue of improper diameterusing a sprue of improper diameter

Detached sprue bcoz of turbulenceDetached sprue bcoz of turbulence
caused by improper sprue anglecaused by improper sprue angle

• Surface treatmentSurface treatment
• Pickling consists of heating the discoloredPickling consists of heating the discolored
casting in an acid. For gypsum bondedcasting in an acid. For gypsum bonded
investments the best pickling solution is 50%investments the best pickling solution is 50%
hydrochloric acid solution. It aids in the removalhydrochloric acid solution. It aids in the removal
of any residual investment as well as of theof any residual investment as well as of the
oxide coating.oxide coating.
• Disadvantage is that acid fumes are likely toDisadvantage is that acid fumes are likely to
corrode laboratory metal furnishings. Nevercorrode laboratory metal furnishings. Never
should the casting be held with steel tongs soshould the casting be held with steel tongs so
that both the casting and the tongs come inthat both the casting and the tongs come in
contact with the pickling solution, as this maycontact with the pickling solution, as this may
contaminate the casting.contaminate the casting.

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

CONCLUSIONCONCLUSION
• An unsucessfull casting results in considerableAn unsucessfull casting results in considerable
trouble and loss of time,in almost alltrouble and loss of time,in almost all
instances,defects in casting can be avoided byinstances,defects in casting can be avoided by
strict observance of procedures governed bystrict observance of procedures governed by
certain fundamental rules and principles. Seldomcertain fundamental rules and principles. Seldom
is a defect in a casting attributable to factorsis a defect in a casting attributable to factors
others than the carelessness or ignorance of theothers than the carelessness or ignorance of the
operator. With present techniques,castingoperator. With present techniques,casting
failures should be the exception,not the rule..failures should be the exception,not the rule..

REFERENCESREFERENCES
SCIENCE OF DENTAL MATERIALS---SCIENCE OF DENTAL MATERIALS---
ANUSAVICE—11ANUSAVICE—11THTH
EDITIONEDITION
CONTEMPORARY FIXEDCONTEMPORARY FIXED
PROSTHODONTICS ---STEPHEN F.PROSTHODONTICS ---STEPHEN F.
ROSENSTIEL---2ROSENSTIEL---2NDND
EDITIONEDITION
DENTAL LABORATORYDENTAL LABORATORY
PROCEDURES---PROCEDURES---
RHOADS.RUDD.MORROWRHOADS.RUDD.MORROW
 SCHILLINBURG . FUNDEMENTALSSCHILLINBURG . FUNDEMENTALS
OF FIXED PROTHODONTICSOF FIXED PROTHODONTICS

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