CASTING

PROCEDURES

AND

DEFECTS

1

Dr.T.Vivek , I MDS ,Department of Conservative Dentistry And

Endodontics, JSS Dental Hospital, Mysore.

CONTENTS•INTRODUCTION

•HISTORY

•DEFINITIONS

•LOST WAX TECHNIQUE

•FABRICATION OF WAX PATTERN

•WAX PATTERN REMOVAL

•EVALUATION OF WAX PATTERN

•SPRUING THE WAX PATTERN

•VENTING

•RESERVOIR2

•CRUCIBLE FORMER

•CASTING RINGS AND LINERS

•RINGLESS CASTING SYSTEM

•INVESTMENT PROCEDURES

•COMPENSATION FOR SHRINKAGE

•CASTING PROCEDURES- Burn Out

Casting Machines

•RECOVERY AND FINISHING OF CASTING

•CASTING DEFECTS

•CONCLUSION

3

INTRODUCTION

The restoration of lost tooth structure, the replacement of teeth by means of fixed and removable partial denture prosthesis, and full mouth restorations, in fact the practice of dentistry with our present-day concept would virtually be impossible without the dental casting process.

The casting procedure is used to make dental restorations such as Inlays,Onlays,Crowns,post &core and ceramics.Becausecastings must meet stringent dimensional requirements,thecasting process is extremely demanding.

In dentistry the resulting casting must be an accurate reproduction of wax pattern in both surface details and overall dimension.Small variation in investing or casting can significantly effect the quality of the final restoration.

4

HISTORY

3000 B.C Copper was cast by Mesopotomians.

2500 B.C Dental prosthesis fabricated from gold wire was found in egypt.

500 B.C Etruscans made bridges of soldered gold bands.

1571 Benvenuto cellini has done casting of both gold and bronze

1897 Philbrook decsribed a method of casting metal into a mold formed from a wax pattern for restoration of posterior tooth.

5

HISTORY

1907 William Taggart introduced “LOST WAX TECHNIQUE.” But he was unable to overcome the problem of casting shrinkage.

1908 Lane proposed the idea of casting into an enlarged mold which he achieved by using an investment containing high % of silica, heated upto 6500c. He was thus first to introduce mold expansion as a compensation technique.

1910 Van horn discovered the value of wax expansion.

1930 Carl Scheu, discovered the phenomena known as Hygroscopic setting expansion.

6

DEFINITIONS

• The technique is so named because wax pattern of a restoration is invested in a investment material, then the pattern is burned out (lost) to create a space into which molten metal is placed or cast.

• The mold channel through which molten metal or ceramic flows into the mold cavity.

• The base to which sprue former is attached while the wax pattern is invested in refactory investment

7

DEFINITIONS

• Surrounding the wax pattern with a material that can accurately duplicate its shape and anatomic features.

• It is the process of heating an investment mold to eliminate the embedded wax or plastic pattern.

• It is the process in which molten material is thrown into a mold and allowed to harden into the shape of the mold.

8

DEFINITIONS

• Process of rapidly cooling the hot casting by dipping it in water bath.

• Process of removing the investment from a casting.

• Removal of surface film containing oxides,whichinvolves heating the casting in an acid.

9

LOST WAX TECHNIQUE10

Quenching and Pickling

Molten metal is cast into the void created by the wax pattern and the sprue

Invested pattern is heated until all the remnants of wax are burned away

The pattern and sprue are invested

All aspects of final restoration are incorporated into the wax pattern

Wax pattern is first formed on a die or directly on the tooth.

Sprue is removed and the casting is polished and delivered to the patient.

FABRICATION OF WAX PATTERN

The wax pattern is the precursor of the finished cast

restoration that will be placed on the prepared tooth.

A few extra minutes spent on the wax pattern can often

save hours that might be spent correcting the casting.

Requirements of a good inlay wax:

1. It must flow readily when heated, without chipping, flaking

or losing its smoothness.

2. When cooled, it must be rigid.

3. It must be capable of being carved precisely, without

chipping, distorting or smearing.

11

DISTORTION

•Stresses occur in the inlay wax as a

result

of the heating and manipulation of the

wax during fabrication of the pattern.

•Wax “relaxes” as these stresses are

released.

To Minimise the distortion :

•Patterns should never be left off the

die.

•Invested as soon as possible after

fabrication.

12

Two fundamental ways to prepare a wax pattern

for a dental restoration:

DIRECT METHOD:

In this method pattern is prepared on tooth in the

mouth.

Type I wax(medium) is formulated for making

direct wax pattern.

13

INDIRECT METHOD:

In this method a die of tooth is first made,

and the pattern is made on the die.

Type II inlay wax(soft) is used.

The die is lubricated with a wetting agent.

Die spacer is used.

The wax pattern is constructed by:

Dipping method:

To develop a thin, uniform and adapted layer

of thin wax on the die. This is done by dipping the

die into wax that has been thoroughly melted.

14

Addition method:

This is done by melting the wax and dropping it on the die using a heated wax instrument until complete building of

the pattern, and then carved by sharp carver.

Drawback : Stress collected from multiple addition of wax tends to release with time and subsequently distort the wax pattern.

No. 7 Waxing spatula for initial coping

15

WAX INSTRUMENTS

P.K.Thomas No.1 and 2 Wax addition instruments

P.K.Thomas No.3 Burnisher

P.K.Thomas No.4 and 5 Wax carvers

16

Electric Wax Instruments

Advantages: 1) Precise Temperature control of the wax for proper manipulation.

2) Carbon buildup can be kept to a minimum.

17

WAX PATTERN REMOVAL

There is a definite possibility of stressing the wax pattern

while removing it from the die or the tooth, so an effort

should be made to make these stresses even.

Along with the sprue former.

Explorer tip.

Staple like wire( MOD pattern).

Small loop of gold zephyr wire.

Indirect Finger PressureWashed Rubber DamDirect finger pressure

18

EVALUATION OF WAX PATTERN

19

PROXIMAL CONTOUR

20

EMERGENCE PROFILE- Stein and Kuwata

The line between the base of gingival sulcus and height of contour described as emergence profile, which is a straight line which faciliates, access for oral hygiene measures

21

SPRUING THE WAX PATTERN

The purpose of the sprue former, or sprue pin, is to provide a channel

through which molten alloy can reach the mold in an invested ring after the

wax has been eliminated.

Basic requirements of a SPRUE:

It must allow the molten wax to escape from the mold.

It must enable the molten metal to flow into the mold with as little

turbulence as possible.

The metal within it must remain molten slightly longer than the alloy that

has filled the mold. This provides a reservoir to compensate

for the shrinkage that occurs during solidification of

the casting.

22

Materials Used for Sprue

WAX METAL PLASTIC

•Commonly used

•Most preferred

•Easy to manipulate

•Easy to burnout

•Lacks Rigidity

•Stronger than a wax sprue

•Mostly used for small inlays

•They are often Hollow

•It cannot be burned out,

removed along with crucible

•Has the rigidity of the metal

•Minimised Distortion

•May block the escape of the

wax

•Hollow plastic sprues are

available that allow the

escape of the wax.

23

• The gauge selection and design for the sprue former are

often empirical, but the optimal performance during the

casting process is based on the following general principles

:

Sprue Diameter

Sprue Position

Sprue Attachment

Sprue direction

Sprue Length

Venting

Reservoir

24

SPRUE DIAMETER

• It should be of approximately the same size as the thickest

area of the wax pattern

Too large- Distortion

Too small- Premature solidification

Sprue former Diameter

• For small inlay - 1.5mm

• For large inlay - 1.7mm

• For Onlay - 2.1mm

• For large pattern - 2.6mm

10

14

25

SPRUE LOCATION

Site of prefer placement are on the:

• Bulkiest Non-functional cusp tip (Non-critical part)

• Marginal ridge

• Proximal contact

26

SPRUE

ATTACHMENT

• It should be attached to the portion of the pattern with

largest cross sectional area

• The sprue’s point of attachment to the pattern should be

smooth to minimize Turbulence.

• Generally it must be Flared- Facilitating the entry of the

alloy.

27

SPRUE DIRECTION• The sprue former should be directed away from any thin or

delicate parts of the pattern, because the molten metal may

abrade or fracture the investment in this area.

• SHOULD NOT BE ATTACHED AT A RIGHT ANGLE- TURBULENCE

UNEVEN EXPANSION

SUCK BACK

POROSITY

So, the sprue former is attached at an angle (450C)

to allow the molten metal to flow freely to

all the portions of the mold.

Mold Impedance to flow Mold

450900

28

SPRUE LENGTH

• Depends on the length of the casting ring.

Too short- Gases cannot be adequately vented

Too long- Pattern will be too close to the ring,and the

molten alloy may break through the investment during

casting.

Therefore the sprue length should be adjusted so that the

top of the wax pattern is within 6mm (1/8”-1/4”) from the

open end.

29

BACK UP INVESTMENT

VENTING

• In some situations, there is some doubt about the speed

with which the mold gases will escape relative to the speed the melt is entering. This could be due to:

Considerable thickness of Investment walls surrounding

the pattern.

High density of Investment.

• Small auxillary sprues or vents have been recommended

to improve the casting of thin patterns.

• A wax rod is attached to the farthest part of the wax

pattern.

30

• Their action may help gases escape during casting and

ensure that solidification begins in critical areas by acting

as HEAT SINK

• Gases that will not escape fast enough ahead of

ingressing metal will be compressed and trapped in these

vents.

31

RESERVOIR• Patterns can be sprued :

Direct - Connection between the pattern and the crucible

former

Indirect - A reservoir is placed between pattern and crucible.

It should be at a distance of 1-2mm from the wax pattern.

It prevent localized shrinkage porosity in the casting.

The resulting solidification shrinkage occurs in the reservoir bar and not

in the prosthesis.

32

CRUCIBLE FORMER

• The crucible part of the investment is funneled shaped component that is connected to the sprued wax pattern.

• It comes in different Materials, Sizes, inclination, diameter.

• The deeper the crucible is and more the inclined its walls are, the more velocity will be imparted to the melt on its way to the mold.

Crucible for moldable ceramics are metallic and havethe

exact dimensions as the pellet of the raw ceramic whichwill

be injected.

33

CASTING RING AND LINERSCasting ring is use to invest the wax pattern in the investmentmedium.

TypesMetal rings.Rubber rings. Split rings.

With the use of solid metal rings or casting flasks, provisionmust be made to permit investment expansion.

The mold may actually become smaller rather than larger because of the reverse pressure resulting from the confinement of the setting expansion.

This effect can be overcome by using a split ring or flexiblerubber ring that permits the setting expansion of the investment. 34

Setting Expansion

Metal Rings Confinement Reverse Pressure

Mold become small

SOLUTION : Use of Split Ring

Flexible Rubber Ring

Ring liners

35

• A resilient liner is placed inside the ring to provide a buffer of pliable material against which the investment can expand to enlarge the mold.

• NO LINER- Expansion is towards the center of the mold thus distorting the casting

TypesAsbestos Ring Liner- No longer used (Carcinogenic)

Cellulose paper

Fibrous ceramic aluminous silicate

Ceramic cellulose combination. NON-ASBESTOS

GC New Casting Ring liner

Nobil-liner

36

Ring liners are placed to ensure uniform expansion in form

of

• Normal setting expansion.

• Semi hygroscopic expansion- Wet liner.

Minimum Thickness of the

liner should be 1mm.

37

• The liner should fit the inside diameter of the casting ring

with no overlap.

Liner 3.25mm short of Casting ring- provides a “lock”

38

RINGLESS CASTING SYSTEM

• This system provides maximum expansion of the

investment.

• The tapered plastic rings allow easy removal of the

investment after the material has set.

• It is suited for the casting of alloys that require greater

mold expansion than traditional gold-based alloys.

PowerCast Ringless system( Whip-mix Corporation) 39

Preparation of wax pattern

The wax pattern should be cleaned of any debris, grease, or oils.

A commercial pattern cleaner or a diluted synthetic detergent is used.

40

Cleaning the debris is followed

by the application of Debubblizer.

The pattern should be invested

as soon as possible, after it is

removed from the die, and it

should not be subjected to warm

environment during this interval.

In any case it should not stand

more than 20-30 min before

been invested.

41

INVESTING PROCEDURE

42

Investing is the process by which the sprued wax pattern is

embedded in a material called an investment.

An Investment must fulfill these requirements:

The investment must be able to withstand the heat of the

burnout and the actual casting of the molten alloy.

It must confirm to the pattern in a way

such that the size and surface details

are exactly reproduced.

It must expand sufficiently to

compensate for the solidification shrinkage of the alloy.

43

• In general, there are 3 types of investments materials

available:

Gypsum bonded investments:

• For conventional casting of gold alloy inlays,

• Onlays, crowns & FPD’s.

Phosphate bonded investments:

• For metal ceramic restorations

• Pressable ceramics & for base metal alloys.

Ethyl- silicate bonded investments:

• For casting of removable partial dentures with base metal

alloy. 44

• Gypsum bounded is further classified by ADAspecification no 2 (casting investment) into twotypes depending upon the expansion required tocompensate for the contraction of the molten goldalloys during solidification

• TYPE I: THERMAL EXPANSION TYPE

For casting of inlays and crowns

• TYPE II: HYGROSCOPIC EXPANSION TYPE

For casting Inlays , Onlays and Crowns

• TYPE III: Used in construction of partial dentures withgold alloy.

45

GYSPUM BONDED INVESTMENT

BINDER

Calcium Sulphatehemihydrate

• 25-45%

Refractory filler(Silica)

• Quartz

• Cristobalite

• Trydimite

• Fused Quartz

46

REDUCING AGENTS

• Carbon or powdered copper – Provide NON-OXIDISING atmosphere

BORIC ACID/

NaCl

• Regulate the setting expansion and setting time

• Prevent detrimental shrinkage of gypsum

OXALATES• Prevent Sulphur

contamination

MODIFIERS

47

• Mainly used in casting Gold alloys with melting ranges

below 10000c

• When quartz ,tridymite or cristobalite is heated, a change

in crystalline form occurs at a transition temperature

characteristic of the particular form of silica.

Quartz 5750c B-Quartz

Cristobalite 200-2700c B-Cristobalite

The density decreases as the form changes to the B

form with a resulting increase in volume that is

evidenced by a rapid increase in the linear expansion .

48

49

PHOSPHATE BONDED INVESTMENT

BINDER

Magnesium Oxide(base)

Ammonium phosphate(acid)

Refractory filler(Silica)- 80%• Quartz

• Cristobalite

• Trydimite

• Fused Quartz

OTHER AGENTS

Colloidal Silica

50

Basic reaction

Polymerisation

Heated about 300c ammonia is released

Complete Heating 1040c

NH4H2PO4+MgO+5H20 NH4MgPO4+6H2O

Mg2. P2O7 + 2NH3 + 13H20

Forming Colloidal multimolecularaggregate around excess MgO and fillers

Mg3(P2O4)2 crystaline form

51

52

Ethyl silicate bonded investments

• This type of investments loosing popularity because of

the more complicated and time consuming procedures

involved, but it is still used in the construction of the high

fusing base metal partial dentures alloys.

• In this case, the binder is a silica gel that inverts to silica(cristobalite) on heating. Several methods may be used

to produce silica or silicic acid gel binder.

1.

Magnesium Oxide is added to increase strength

Na SiO2 + acid or acid salt bonding silicic acid gel

53

Aqueous solution of colloidal silica + ammonium chloride silica gel

Hydrolysing ethyl silicate + HCl + ethyl alcohol+water colloidal sol of polysilicic acid

A coherent gel of polysilicic acid then forms which is

accompanied by a setting shrinkage.

This soft gel is dried at a temperature below 168 C (334F).

during drying, the gel loses alcohol and H2O to form a

concentrated hard gel---green shrinkage and it occurs in

addition to the setting shrinkage.

54

BRUSH TECHNIQUE

When the investment reaches the level of the pattern, tilt the ring several times to cover and uncover the pattern, thereby minimizing the possible entrapment of air.

Coat the entire pattern with investment, pushing the material ahead of the brush from a single point.

Attach the vacuum hose to the bowl, evacuate the bowl, and mechanically spatulate.

Add investment powder to the liquid in the mixing bowl

In this technique, pattern is first painted with surface tension reducer; the surface must be wet completely.

55

56

VACUUM TECHNIQUE

57

GRINDING THE INVESTMENT

• Carefully Grinding(on a model trimmer) or scraping the

shiny skin off the end of the investment just prior to burn

out.

• Removes the impervious layer, opening the pores of the

investment and facilitating gas release as the alloy is cast

into the mold.

58

COMPENSATION FOR SHRINKAGE

Wax Pattern Expansion-Negligible

Setting Expansion

Thermal Expansion

1-2%

Casting Ring Liners

Ringless Casting System

Hygroscopic1.2-2.2%

Normal0.4%

WAX SHRINKAGE= 0.35% GOLD ALLOY SHRINKAGE= 1.25% BASE-METAL ALLOY= 2.4%

•Wet Liner•2 Liners

Controlled water added technique

59

WAX ELIMINATION &

HEATING

60

• For gypsum bonded investment

– 500°C for hygroscopic technique.

– 700°C for thermal expansion technique.

• Phosphate bonded

– 700°C to 1030°C.

61

HYGROSCOPIC LOW HEAT TECHNIQUE

• Obtains compensation expansion from 3 sources.

37°C water bath expands wax pattern.

Warm water entering the investment mold from top adds some hygroscopic expansion.

Thermal expansion at 500°C provides needed thermal expansion.

62

ADVANTAGES

• Less investment degradation.

• Cooler surface for smoother castings.

• Convenience of placing molds directly in 500°C

furnace.

• Care taken for sufficient burnout time.

• The molds should remain in furnace for atleast 60

min.

DISADVANATAGE:

– Back pressure porosity great hazard in low heat technique.

63

Standardized hygroscopic technique was developed

for alloys with high gold content; the newer noble

alloy may require slightly more expansion. This

added expansion may be obtained by making 1 or

more of following changes.

1. Increasing water bath temperature to 40°C.

2. Using two layers of liners.

3. Increasing burnout temperature to a range of

600°C to 650°C.

64

HIGH-HEAT EXPANSION TECHNIQUE

• Depend almost entirely on high-heat burnout to obtain the required expansion, while at the same time eliminating the wax pattern.

• Additional expansion:

– Slight heating of gypsum investments on setting. Thus expanding the wax pattern.

– Water entering from wet liner adds a small amount of hygroscopic expansion to the normal setting expansion.

65

GYPSUM INVESTMENTS

• Fragile and require use of metal ring.

• Slowly heated to 600°C to 700°C in 60 minutes and held for 15 to 30 minutes at the upper temperature.

• Slow Rate of heating:– Smoothness.– Good Overall dimension of investment.

• Too rapid heating:

• Cracking of investment (outside layer expands >center section).

• Radial cracks interior to outward.

• Casting with fins or spines.66

• Investment decomposition and alloy contamination is

related to the chemical reaction between the residual

carbon and sulfate binder.

• Reduction of calcium sulfate by carbon takes place

rapidly above 700°C.

CaSO4 + 4C CaS + 4CO

3CaSO4 + CaS 4CaO + 4SO2

• This reaction occurs when gypsum investment are

heated above 700°C in presence of carbon. SO2 as a

product of this reaction contaminates gold castings and

makes them extremely brittle.

67

Burn Out of Phosphate Bonded• Obtain their expansion from following sources.

– Expansion of the wax pattern.

– Setting expansion (because of liquid used).

– Thermal expansion

• Phosphate investments much harder and stronger than gypsum investments.

• Burnout temperature range from 750°C to 1030°C.

• It is necessary to use a slow heating rate to prevent cracking.

Two stage burnout

-Placed directly in furnace at 200-3000C

-Held for 20-30 min. then cast68

CASTING MACHINES

69

• Alloys are melted in one of the 4 following ways:

In a separate crucible by a torch flame, and cast centrifugally.

Electrically by a resistance heating or induction furnace. Centrifugally by motor or spring action.

By induction heating. Centrifugally by motor or spring action.

Vacuum arc melted. Cast by pressure in argon atmosphere.

70

Parts of Torch Flame

Melted by placing on inner sidewall of crucible.

Fuel used -Natural gas(Propane)+ Oxygen Gold Alloys

-Artificial gas and air.

- Acetylene + Oxygen air

Non-luminous brush flame with different

combustion zones should be obtained. Mixing zone Combustion zone.

Reducing zone.

Oxidizing zone.

71

• Gas-air torch is used to melt conventional noble metal alloys (used for inlays, crown and bridge) whose melting points less than 1000⁰c

• Alloy should be approx. 38°C to 66°C above

liquidus temperature

It will becomeRed

Orange

White (dull)

White (Mirror-like)

•When the gold is orange, transfer the ring from the furnace to the cradle of the casting machine

•When the molten alloy surface is shiny and shimmering-It indicates the alloy is not contaminated with oxides

AIR GAS

72

USE OF FLUX FOR GOLD ALLOYS

• Minimize porosity

• To increase the fluidity of the metal i.e so as to increase the flow of the molten alloy.

• Film of flux formed over surface of

the alloy prevents oxidation

Example:

1. powdered charcoal

2. Fused borax powder with boric acid powder

73

ELECTRICALThere are two methods by which electricity can be employed

to melt the alloy.

Induction.Electric Arc (Resistance).

Both the methods work on same principle i.e. heat energy isproduced when electric current is passed through a

conductordepending upon the voltage applied across it.

74

The apparatus is simple in design consisting of high frequencyinduction coil across which high voltage is applied. This highfrequency coil surrounds the crucible in which the alloy / metalpellets are melted.

Graphite is a good conductor of heat and electricity.

It transmits the heat produced by the high frequency induction coil, to the metal / alloy pellets in the crucible.

This process is the indirect heating of the pellets in the graphite crucible.

Used for Noble metal alloys.

Ceramic is a bad conductor of heat and electricity.

The surrounding high frequency coil induces intrinsic Currents in the alloy / metal pellets, which

produce heat required to melt the alloys / metal pellets.

used for base metal alloys

Graphite crucible Ceramic crucible

75

Electric Arc / Resistance

This is a crucible free technique where by the metal / alloy

pellet is directly melted by the heat produced as a result of

resistance offered by it to high voltage current.

The apparatus consist of an electrode and a base plate on

which the metal / alloy pellet is placed.

The contact between the electrode terminal and the metal /alloy pellet is in form of an ‘electric arc’ thus the term ‘electric arc furnace’.

76

CENTRIFUGAL CASTING MACHINE

Casting machine spring wound from 2 to 5 turns

Alloy is melted by a torch flame in a glazed ceramic crucible attached to the “broken-arm” of casting machine.

Torch flame is generated from a gas mixture of propane and air.

Machine is released and spring triggers the rotation motion.

As the metal fills the mold, a hydrostatic pressure gradient develops along the length of the casting.

Broken arm feature accelerates the initial rotational speed of thecrucible and casting ring.

Casting pressure = 30-40psi

77

The metal / alloy pellets are placedin the crucible.

Flame/blowpipe is use to heatthe alloy/metal in the crucible.

Once the alloy / metal is in a molten form.

The flame is removed and the broken-arm is released. 78

ELECTRICAL RESISTANCE-HEATED CASTING MACHINE

• Current is passed through a resistance heating conductor,

and automatic melting of the alloy occurs in a graphite or

ceramic crucible.

• Advantages:

– For metal ceramic prosthesis.

– Base metals in trace amounts that tend to oxidize on overheating.

– Crucible located flush against casting ring.

79

80

INDUCTION MELTING MACHINE

• Alloy is melted by induction field that develops within a crucible surrounded by water cooled metal tubing.

• The electrical induction furnace is a transformer in which an alternating current flows through the primary winding coil and generates a variable magnetic field in the location of the alloy to be melted in crucible.

• Once the alloy reaches casting temperature in air/vacuum it is forced into mold by centrifugal force by air pressure, or by vacuum.

• More commonly used for melting base metal alloys.

81

82

DIRECT CURRENT ARC MELTING MACHINE

• Direct current arc is produced between two electrodes-

the alloy and water cooled tungsten electrode.

• The temperature between the arc exceeds 4000°C.

• Has high risk of overheating.

83

GAS PRESSURE

• This is introduced over the melt, in a closed

compartment

• Gas used- Carbon dioxide

Carbon monoxide Inert gases

Nitrogen

• The gas should be chosen so that the cast material has

no affinity towards it.

• The Casting pressure … 15-20psi

84

VACUUM OR PRESSURE ASSISTED CASTING

MACHINE

• Molten alloy is heated to casting temperature drawn into

the evacuated mold by gravity or vacuum and subjected to

additional pressure to force the alloy into the mold.

• Used for titanium and titanium alloys. Under vacuum arc

heated-argon pressure casting machine.

• Casting pressure used here is about

0.1 Mpa(15psi) which is very less.

• So vacuum pressure is always used in

combination with other methods.

85

•STEAM PRESSURE•Moist Asbestos pad

86

RECOVERY AND FINISHING OF CASTING

87

This is the process of rapidly cooling the hot casting by dipping it in water.

After the casting has solidified the ring is removed and quenched in water as soon as the button has lost its glow.

After the casting has solidified the ring is removed and

quenched in water as soon as the button exhibits a dull

yellow glow. The ring in quenched in

cold water.

QUENCHING

88

Advantages of Quenching

When the water contact hot investment, a violent

reaction

ensues(Thermal shock), resulting in as soft, granular

investment that is easily removed.

The noble metal alloy is left in an annealed condition for

burnishing and polishing.

89

PICKLING

The surface of casting appears dark with oxide and tarnish. Sucha surface film can be removed by the process called Pickling.which consist of heating the discolored casting in a acid.

Used for cleaning gold castings.

Pickling solutions used :

50% Hydrochloric acid.

50% Sulfuric acid + Pottasium dichromate

Hydrofluoric acid- For phospahte bonded invested

AQUA REGIA : Hcl + Nitric acid (1:3)

90

Precautions to be taken

Ultrasonics –prosthesis is sealed in

teflon container.

Casting should not be held with steel

tongs

Always use plastic covered pliers to

introduce and remove the casting from

the pickling solution

When steel tongs come into contact

with gold in strong solution,

electrodeposition may occur on the

surface of the casting

91

SANDBLASTING

Gold based and palladium based metal ceramic and

base metal alloy are bench cooled to room temperature

before casting is removed from investment and then

subjected to sandblasting.

Recycled Aluminium oxide(50µ)with pressure of 100psi

Acid is NOT used for BASE METAL

ALLOYS.

92

REMOVAL OF SPRUE

Remove the sprue with double sided

Carborundum disc/diamond disc.

93

FINISHING

This is done to remove small irregularities on the surface

Fitting surface- Blebs(Extrusions) removed using sharp chisel or finishing bur

External surface- Carbide trimming burs

Try Casting on the die- Look for the Rocking

movement

Marginal Burnishability- Ball/Beaver

To prevent cement line exposure

To protect friable enamel at the margins

Sliding lap joint at gingival cavosurface margin

94

POLISHING

Remove surface scratches

Obtain smooth shiny surface

Seat casting

Polishing of Gold casting is done with

carbide burs , green stones, pink stone,

tripoli or rouge.

Polishing of Base metal alloy casting is

done with aluminum oxide, tin oxide.

Note:

•Use Slow speed

•Coolant

•Used in slurry

•Decreasing order of abrasive

95

CASTING DEFECTS

96

A mole on the chin, a dimple on the cheek, imperfections have

long been considered signs of beauty and individuality. Not so in

the field of dental sciences and specifically casting technology.

Any imperfections or irregularities that result in unsuccessful casting which interferes with the fit of final restoration or its esthetic and mechanical properties.

Defects in casting can be classified under four headings.

(1) Distortion

(2) Surface roughness and irregularities

(3) Porosity

(4) Incomplete or missing detail

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DISTORTION

Distortion

Impression making- The impression(Alginate) must be poured

immediately within 30 min.

Rubber base impressions can be poured even after a day ..!!

Warm wax thoroughly

before creating pattern.

Invest it quickly

•Change in the shape of the casting

•Mainly incorporated in the wax pattern stage

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DISCOLORATION:

Over-heating: Attracts oxides

Under-heating: Incomplete wax elimination

Carbon Inclusions: Crucible/Investment

Mixture of Alloys: Re-use of Sprue and button

Mercury Contamination

Solder

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SURFACE ROUGHNESS AND

IRREGULARITIES

Surface roughness is defined as relatively

finely spaced surface imperfections that

predominant surface pattern.

Surface irregularities are isolated

imperfections, such as nodules, that are not

characteristic of the entire surface area

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•Air Bubbles

•Water film-Excess

Vibration

•Foreign body- Broken Investment Pieces,Bits of the carbon from the flux

•Higher L:P ratio- Weak

Investment

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Rapid Heating of the mold- FINS

High temperature of molten alloy

Casting Pressure : Too high a pressure during casting produces a rough surface on the casting. A gauge pressure of 0.10 to 0.14 Mpa (15 to 20 psi) in an air pressure casting machine or three to four turns of the spring in an average type of centrifugal casting machine(30-40psi) is sufficient

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Composition of the Investment

The ratio of the binder to the quartz influence, thesurface texture of the casting. In addition, a coarse silicacauses more surface roughness

Multiple Patterns:Should not be placed too close togetherShould not place many patterns in same planeSpace between the pattern is atleast 3mm

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POROSITIES

I. Solidification defects :

A. Localized shrinkage porosity

B. Suck-back porosity

C. Microporosity

II. Trapped gases :

A.Pinhole porosity

B.Gas inclusion porosity

C.Subsurface porosity

III. Residual air : Back pressure porosity

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PROBLEM

SOLIDIFICATION DEFECTS

LIKELY CAUSES REMEDIES

Localized shrinkage porosity•Premature termination of

flow of molten metal.

•It mainly occurs at sprue-casting

junction.

Use of reservoir.

Increase the sprue diameter

Suck-back porosity•Hot spot created by hot

metal impinging on point

on mold surface.

•It often occurs at OCCLUSOAXIAL

OR INCISOAXIAL LINE ANGLE

•Flare the point of

attachment of sprue.

•Lowering the casting

temperature by about 30°C.

Micro PorositySmall Irregular

•Rapid solidification of the mold

•Casting temperature is too low.

Increase the mold

or casting temperature

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TRAPPED

GASESPROBLEM LIKELY CAUSES REMEDIES

•poorly adjusted torch flame, or

the use of the mixing or oxidizing

zones

• Gas mechanically trapped

by molten metal in mold.

•Gas incorporated during

casting procedures.

•Absorbed gases are expelled

on solidification.

Pin hole PorositySmall Spherical

Gas inclusion

PorosityLarge Spherical

Sub Surface

Porosity

•Simultaneous nucleation

of solid grains and gas

bubbles.

•Rate of entering of

molten metal in mold.

•Correctly adjusting

and positioning the

torch during melting.

•Prevent oxidation

of alloys-Flux

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RESIDUAL AIR

PROBLEM LIKELY CAUSES REMEDIES

Back Pressure

Porosity

•Inability of gases in mold

to escape.

•Pressure gradient that displace

air towards the end of

investment.

•Tendency for mold to clog with

residual carbon.

•Proper venting.

•Place pattern no more than

6-8mm from the end

of the casting ring

•Sufficient casting

pressure.

•Proper burn out

temperature.

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INCOMPLETE CASTING

Rounded and incomplete

margins- Insuffecient casting

pressure

Rounded and shiny- Incomplete wax elimination

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Insuffecient alloy

Review Of Various Studies…..

Surface Porosity of Different Investment Materials with

Different Mixing Techniques

Ahmed A et al. Al–Rafidain Dent J. 2009; 9(2): 307–314

Conclusion: Manual technique showing higher value of

surface area and number of porosities than the Vacuum

technique.

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The effect of sprue attachment design on castability and

porosity.

Verrett RG, Duke ES. J Prosthet Dent. 1989 Apr;61(4):418-

24.

Conclusion: Flared and straight sprue attachments

optimized castability and minimized porosity than compared

to abrupt constriction, and gradual constriction

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The effect of casting ring liners on the potential

expansion of a gypsum-bonded investment.

Earnshaw R J Dent Res. 1988 Nov;67(11):1366-70

Conclusion: Wet lining materials showed an increased

total expansion (in the range 2.2 to 2.3%), where as dry

surface showed a total expansion of 1.7%

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Effect of wax melting range and investment liquid

concentration on the accuracy of a three-quarter

crown casting

Michio Ito et al,The Journal of Prosthetic Dentistry Volume

87, Issue 1, January 2002, Pages 57–61

Conclusion:

1) The higher the softening temperature, the larger the

casting shrinkage.

2) Casting shrinkage was smaller with the use of 100%

special liquid.

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Effects of rapid burnout type gypsum-bonded investment on performance of

castings.

Murakami S et al, Dent Mater J.1994 Dec;13(2):240-50.

Rapid burnout type gypsum-bonded investment materials were developed to shorten

the time required for dental casting procedures

Conclusion:

1)When the investment block was rapidly heated at 7000 C, no fractures were

observed in the rapid burnout type investments with one exception, while a

conventional cristobalite investment broke into pieces shortly after being placed in

the furnace

2) Casting fins were sometimes induced only for the material which showed

fracturing on rapid heating

3) The 30 min-setting expansion was significantly different among the materials

although there were no differences in thermal expansion, and the material showing

greater 30 min-setting expansion was efficient to obtain better fit of the crown as in

the conventional casting procedures.

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Dimensional accuracy of castings produced with

ringless and metal ring investment systems

Pelopidas Lombardas The Journal of Prosthetic Dentistry

Volume 84, Issue 1,July 2000, Pages 27–31

Conclusion: The ringless technique may produce

accurate castings when compared to metal rings

114

Effect of internal microblasting on retention of cemented

cast crowns.

Randolph P. O'Connor. The Journal of Prosthetic Dentistry

Volume 64, Issue 5, November 1990, Pages 557–562.

This investigation measured the retention of type III cast gold

crowns after cleansing with pickling solution (group I) or

microblasting with 50-micron aluminum oxide (group II) or 50-

micron glass beads (group III)

Conclusion: Microblasting the internal surface of cast crowns

with 50-micron aluminum oxide significantly improved the

retention of castings luted with zinc phosphate cement.

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Measurement of paint-on die spacers used for casting

relief

W.V. Campagni The Journal of Prosthetic Dentistry

Volume 47, Issue 6, June 1982, Pages 606–611

Three paints used for die spacing were measured for film

thickness. Two of the materials are marketed as die

spacers, and the third is sold as a model airplane paint.

Conclusion:

The three die spacing materials were capable of achieving

clinically acceptable relief in the range of 20 to 40 μ by

using a variable number of coats.

116

CONCLUSION117

Investing and casting, a series of highly technique sensitive

steps,coverts the wax pattern into a metal casting.

Accurate and smooth restorations can be obtained if the

operator pays special attention to each step in the technique

When initial attempts at casting produce errors or defects,

appropriate corrective measures must be taken so that they

do not recur.

Thus a thorough and in-depth knowledge of defects,their

etiology and prevention will go a long way in avoiding these

undesirable imperfections allowing the fulfillment of dreamOf

every dentist and desire of every individual- The ideal

restoration, and perfect rehabilitation yielding Patient function

and healing.

118