INSTITUTE OF DENTAL SCIENCE

DEPARTMENT OF CONSERVATIVE DENTISTRY

Presented by:-DR. TANMAY SINGH PATHANI {56}BDS, MPH

SEMINAR ON INVESTMENT & CASTING

•Definition

•Introduction

•Layout of steps involved casting procedure.

•Desired accuracy in casting procedure.

•Spruing

•Casting ring liners

•Investing

•Wax burnout procedure in

•Gypsum bonded investment

•Phosphate bonded investment

•Casting machines and procedure.

•Defects in casting and their prevention.

Contents

DEFINITIONS

Casting:

Something that has been cast in a mold.Or

An object formed by the solidification of a fluid that has been poured or injected into a mold.

OrThe act of forming an object in a mold.

GPT 8th edition.

Sprue former- A wax, plastic, or metal pattern used to form the channel allowing molten metal to flow in the mold to make a casting.

Investing –

The process of covering or enveloping wholly or in part , an object such as denture , tooth, wax form, crown etc. with a suitable investment material before processing or casting.

INTRODUCTIONThe casting process is used to make dental restorations such as inlays, onlays, crowns, bridges, and removable partial dentures

• W.H.Taggart introduced lost-wax casting technique in 1907.

• This was followed by Jameson who introduced the centrifugal-casting machine and Solbrig who introduced the steam-pressure casting machine.

centrifugal-casting machine

The Layout Of Steps In Making Casting

1. Preparation of the tooth or teeth

2. Making of an impression.

3. Pour gypsum slurry into the impression to make a positive cast,

4. Make a wax pattern

5. Sprue the wax pattern (fix it in space)

1. Invest the wax pattern.

2. Eliminate the wax pattern by burning the wax out of the investment in a furnace, thus making the mold.

3. Force molten metal into the mold using one of a variety of means.

4. Clean the cast.

5. Remove the sprue from the casting.

6. Finish and polish the casting on the die.

7. Cement the finished cast restoration on the prepared tooth

The direct technique (in which the pattern is made on the tooth rather than on the die) if used, steps 2 and 3 are omitted

DESIRED ACCURACY OF CASTING PROCEDURE

The desired accuracy of the casting is about 0.1 % Therefore, the lost wax procedure requires specially developed materials that compensate for the dimensional changes indicated by thefollowing equation.

SHRIKANGE EXPANSION

(wax+alloy) (wax+setting+hygroscopic+thermal)

ShrinkageWax Direct 0.4%

O'Brien 3rd editionIndirect 0.2%

Alloys 1.25%-2.5% Craig 11th edition.

Setting expansion(%)

Hygroscopic expansion(%)

Thermal expansion(%)

Total expansion.(%)

•High heat (cristobalite)

0.35 .70 1.25(700°C) 1.95

•Hygroscopic immersion (Beauty Cast*)

0.30 1.50 .55(480°C) 2.05

•Hygroscopic water added (Hygrotrol*)

0.75 2 .55(480°C) 2.55

•Phosphate-high heat (Ceromigoid *)

0.23-0.50 .35-1.20 1.33-1.58(700°C)

1.88-2.88

Expansion in Gypsum Bonded Investment.

Hi-Temp Carbon-free investment for high fusing alloys

1.2%TE at1300-1800°F(700-900°C

.7% SettingExpansion

Cera-Fina Carbon-free fine grained investment for high fusing alloys

1.3% TE at1300-1800°F(700-900°C)

.3% SettingExpansion

WiroFine partial denture investment material

Linear thermal expansion [%] 0,8

Bellavest® SHGraphite-free, phosphate-bondedprecision casting investment material

rapidly or conventionally heatable casting investment material for crowns and bridges made of precious metal, non-precious metal alloys and press ceramic

50% BegoSol® 1.7% (total expansion)

80% BegoSol® 2.2%(total expansion)

Expansion in Phosphate Bonded Investment Material

Die hardener

Oil

Spacer applied

Waxing instrument

(PKT set)

Wax pattern

SPRUINGPURPOSE-1. Fix the pattern in space so a mold can be made.2. To create a channel for elimination of wax during

burnout.3. To form a channel for the ingress of molten alloy

during casting.4. To compensate for alloy shrinkage during

solidification.

O'Brien 3rd edition

Sprue former

1. Wax sprues are preferred and commonly used as they allow easy escape of molten wax.

2. Plastic sprues : • Solid plastic sprues soften at higher

temperature than the wax pattern • May block the escape of wax because the wax

melts at lower temperature than plastic and • Thermal expansion of wax is 5 times greater

than plastic which will result in excessive pressure build up inside the mold during the burnout before plastic sprue softens resulting in casting defects

O'Brien 3rd edition

3.Metal sprues can be solid or hollow tubes. Later one is preferable.

• If metal sprue is used, it should be made of non-rusting to avoid possible contamination of wax pattern.

• They removed from the investment at the same time when the crucible former is removed.

care should be taken to examine the orifice while removing because small amount of investment chip-off which result in incomplete casting or inclusion of foreign object in the casting

O'Brien 3rd edition

SELECTION OF SPRUE FORMER

Based on following criteria DIAMETER- Should be = the thickest portion of wax

pattern If large- causes distortion If small- will solidify before casting itself and cause localized shrinkage porosity (suck back porosity)

The sprue former should be at least 1. 7 mm(14 guage) in diameter unless the pattern is extremely small,and sprue formers up to 2.5 mm(10 guage) in diameter can be used for very large patterns (bulky) full crown

Murrow and Rudd

When small patterns need long sprues, • wax reservoir placed approximately 1/6 inch (1.5 mm) from

pattern. • These reservoirs should have diameter greater than thickest

portion of pattern

Murrow and Rudd

ATTACHMENT AND ORIENTATION Should be attached at greatest cross-section

of the pattern As It is best for molten metal to flow from

thick to thin area i.e. from the marginal ridges

to the gingival margins and not the reverse.

This also minimizes the risk of turbulence

LENGTH- Sprue former should be long enough so as to properly

position the pattern in casting ring within 6 mm (1/4 inch) of trail end in gypsum bonded

investment and 3 to 4mm (1/8 inch) for phosphate bonded

investment

Direction should be 45º to the proximal area It should be directed away from any thin or delicate

part of investment. Molten alloy may abrade or fracture investment in this area

DIRECTION

CASTING RING LINER

Provides investment expansion. If not used the mold may become smaller because of reverse pressure resulting

from the confinement of the setting expansion. Types- Asbestos ( Was used earlier, but is not used now because of its carcinogenic potential) Aluminum silicate /ceramic liner Cellulose liner (it has an advantage that that it can be made

wet with water and allow semi hygroscopic expansion) Use of two layers of liner allow more expansion.

A B C

(A)Casting ring

(B) Ring liner

(C) Base former

PLACEMENTLINER If PLACED 3.25 mm short of ends of the

ring, produces more uniform expansion

Placement of the liner within the ring influences expansion of the mold

√ when maximum expansion is required, a liner flush with the open end of the should be used.

√ A liner that is 3 or 6 mm short of the open end of the ring will produce a smaller casting. The greater the unlined portion of the ring. the greater the restrictive effect; thus the distance by which the liner is short should be carefully controlled

INVESTING Wax pattern should be cleaned of any debris, grease or oil ( a commercially available wax pattern cleaner or a diluted synthetic detergent is used)

Any excess liquid is shaken off and pattern is left to air dry A thin film of cleanser left on pattern reduces surface tension of wax pattern and permit better wetting of investment Liquid and powder are mixed with care to not to incorporate air in the mix. (Vacuum mixing is preferred)

Pattern is painted with layer of investment inside and out.Casting ring is positioned on crucible former and

remaining investment is vibrated slowly in to the ring.Vacuum mixing results the texture of the cast surface

–smoother with better details reproduction.-It also increases the tensile strength of investment.

Vacuum mixing

Investing the wax pattern

Investing the wax pattern

Care should be taken to not to give excessive vibration while

investing because it may cause solids to settle and may lead to free water accumulation adjacent to pattern resulting in surface

roughness.

Investing the wax pattern

Setting expansion

WAX ELIMINATION AND HEATING

Gypsum bonded and phosphate bonded investment are ready for burnout after setting of one hour ( They should be kept in 100% humidity in case if one need to wait).

1. Remove crucible former and any metal sprue. 2. Clean any debris with camel hair brush. 3. Investing rings are placed in a room-temperature furnace and heated to prescribed maximum temperature.

For GYPSUM BONDED-468ºC for hygroscopic technique (low heat technique)

650°C for thermal expansion technique ( high heat technique)

RELATED PROBLEMS Investment decomposition- is seen with gypsum bonded investment therefore careful temperature

setting should be done. Melted wax absorbed in investment-

To prevent this begin burnout when mold is still wet also in high heat technique, high temperature convert

carbon to CO2 or CO which escapes through pores.

Heating is done at 500ºc for 60 minute ( It can be done up to 5 hour or longer with little damage)

Expansion is obtained by- 1. 37ºc water bath expands the wax pattern.

2. Warm water entering investment mold adds some hygroscopic expansion.

3. thermal expansion at 500ºc.

Added expansion for newer noble alloys is obtained by- 1. increasing water bath temperature to 40ºc.

2. using two layers of liners. 3. increasing burnout temperature range of 600ºc to 650ºc.

HYGROSCOPIC LOW HEAT TECHNIQUEFOR GYPSUM BONDED INVESTMENT

This approach depends almost entirely on high heat burnout to obtain the required expansion and at the same time

eliminating the wax pattern.

Additional expansion is obtained from- Expansion of pattern from heat produced during

setting. Warm water entering the mold from wet liner

causing some hygroscopic expansion.

HIGH-HEAT THERMAL EXPANSION TECHNIQUE FOR GYPSUM BONDED INVESTMENTS.

Burning out the wax pattern

… in the furnace

Thermal expansion

Images from Bego Virtual Academy

Mold are placed in furnace at room temperature and slowly heated to 650ºc to 700ºc in 60 minutes and held for

15 to 30 minute.

Related problems-Rapid heating can generate steam which can cause flaking of mold walls. It may also cause cracking of investment because outer layer of investment expands thermally resulting compressive stress in outer layer and tensile stress in inner layer. These crack appear from interior outwardly in form of radial cracks.

Sulfer di oxide production

It occurs rapidly over 700*c with reduction of calcium sulfate by carbon.

CaSo4 + 4C CaS +4CO 3CaSo4 + CaS 4CaO +4SO2

The sulfur dioxide produced contaminates gold castings and makes them brittle.

Therefore casting should be made immediately after casting temperature has been reached to prevent sulfur contamination.

PHOSPHATE BONDED INVESTMENT-

• Usual burnout temperature range is from 750ºc to 900ºc.

• Heating rate is usually slow till 315ºc and is quite rapid thereafter reaching completion after a hold at the upper temperature

1.Expansion of wax pattern because setting reaction raises the mold temperature considerably

2.Setting expansion is higher than in gypsum bonded because of special liquids (silica sol)

used to enhance such expansion.3.Thermal expansion is high because of higher

burnout temperature.

PHOSPHATE BONDED INVESTMENTS OBTAIN THEIR EXPANSION FROM FOLLOWING SOURCE

CASTING MACHINESSeveral types and designs of casting machines are used to make

dental castings. All casting machines accelerate molten metal into the mold

either by 1. Centrifugal force or 2. Air pressure.

Numerous modifications and variations of these methods are used in different machines.

• The selection of the casting and melting techniques is influenced by type of alloy and restoration to be cast.

Phosphate bonded investment material

1 powder

2 debubblizer

3 liquid

12

3

Three types of casting machines are used

• CENTRIFEUGAL CASTING MACHINES• INDUCTION MELTING MACHINES• ELECTRICAL RESISTANCE HEATED CASTING MACHINES

CENTRIFEUGAL CASTING MACHINE

A variety of centrifugal machines are available. • Some spin the mold in a plane parallel to the table top on which the machine is mounted. • Others rotate in a plane vertical to the table top. • Some are spring-driven, and• others are operated by electric power. • An electric heating unit is attached to some machines to melt the alloys before spinning

the mold to throw in the metal. • Others have a refractory crucible in which the alloy is melted by a torch before the casting

operation is completed. All these machines depends on the centrifugal force applied to the molten metal to

cause it to completely fill the mold with properly melted metal

Diagram showing the relation between the ceramic crucible used to melt the alloy and the casting ring in a centrifugal casting machine . Both the crucible and the ring are placed on the casting machine . Once molten , the metal is rapidly driven centrifugally out of the crucible and into the mold in the casting ring . The traverse of the metal takes less than one second .The oxidized elements and flux , which are less dense ,lag behind the molten metal as lag.

• Broken arm casting machine ( Penwalt / Jelenko )• Emesco senior casing machine (J.M. Ney company)

Are the examples of centrifugal casing machine and incorporate a “broken arm ” principal

The melting crucible and mold are positioned at right angle to the spring-propelled beam of the apparatus.

As the beam begins its rotation the melted alloy , by its inertia, is carried directly toward the mold.

As centrifugal force is generated by the spinning of the arm the casting flask and the mold swing into alignment and become subjected to the full centrifugal force.

The metal is melted by torch flame in a glazed ceramic crucible attached to broken arm of crucible.

After the metal has reached the casting temperature the machine is released and spring triggers the rotational motion.

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

Usually the pressure gradient at the moment before

solidification ,begins ,reaches about 0.21 to 0.23 MPa at tip of casting.

Methods of Melting the AlloysGenerally two methods are employed for melting of metal alloy

prior to casting:– Flame / Blow pipe.– Electrical.

A) FLAME The fuel employed is a mixture of oxygen-air and acetylene.The temperature of gas-air flame is influenced by the nature of

the gas and the proportion of gas and air in the mixture. Care should be taken to obtain a non-luminous flame, with

combustion zones clearly differentiated for melting the alloy.

Parts of the flame:

First cone that comes directly from the nozzle Air and gas are mixed before combustion. No heat is present in the zone.

• Second cone is the combustion zone Green and immediately surrounding the inner cone. Gas and air are partially combusted. “Oxidizing zone” andShould be kept away from the metal during fusion (melting).

• Third zone is the reducing zone which is blue. Hottest part of the flame and is just beyond the tip of Green

combustion zone. The area should be on metal during fusion.

B) ELECTRICALThere are two methods by which electricity can be employed to melt the alloy.

• Induction.• Electric Arc (Resistance).Although, both the methods work on same principle i.e. heat energy is

produced when electric current is passed through a conductor depending upon the voltage applied across it.

H = P x tH = V x I x t (P = V x I)H = V2 x R x t (I = V x R) Ohms lawWhere,• H = Heat produced in joules.• P = Powel in watts.• t = Time in seconds.• V = Voltage in volts.• I = Current in amperes.• R = Resistance in ohms. The heat energy utilized for melting the alloy.

INDUCTIONThe apparatus consist of high frequency induction coil across which high voltage is applied. This high frequency coil surrounds the crucible in which the alloy / metal pellets are melted.

The energy required to melt the alloy is supplied by a high frequency generator.

The melting crucible is held in a centrifugal arm, and during the melting process it is surrounded by the high frequency generator coil.

The alloy is melted by two ways depending upon the type of crucible.

• In case of graphite crucible the electric energy of coil is converted to thermal energy which is utilized by means of conduction radiation (Graphite is a good conductor of heat and electricity) by the graphite crucible to melt the alloy.

• Noble metal alloys are generally melted in graphite crucible.

When ceramic crucibles are used the induction coil induces the ‘eddy current’ in the alloy itself which provides energy for the melting of the alloy.

• When the alloy is molten, the centrifugal process is initiated and the induction coil being lowered.

• The molten alloy is then forced into the casting mold located behind the melting crucible by means of centrifugal force

• The ceramic crucibles are generally used for base metal alloys like nickel-chrome, chrome-cobalt etc.

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 alloy 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’.

air pressure technique

• The alloy is melted in in the crucible hollow of the ring followed by applied on the melt.

• Here the alloy is melted by blow-pipe or flame in the hollow left by the crucible former.

• The high surface tension of the liquid metal prevents it from falling down in to the sprue channel.

• Once the metal has been melted to the casting temperature, the air tight piston is applied with 10 to 15 psi air pressure to force the metal into the mold.

ELECTRICAL RESISTANCE HEATED CASTING ELECTRICAL RESISTANCE HEATED CASTING MACHINEMACHINE(High frequency centrifugal casting machine)(High frequency centrifugal casting machine) There is automatic melting of metal in graphite crucible ( this is There is automatic melting of metal in graphite crucible ( this is advantageous for metal-ceramic restoration in which trace advantageous for metal-ceramic restoration in which trace amount of base metals are prevented from oxidation from torch amount of base metals are prevented from oxidation from torch flame flame from overheating.from overheating.

INDUCTION MELTING MACHINE Metal is melted by induction field that develops with in the

crucible surrounded by water cooled metal tubing. Molten metal is forced in to mold by air pressure or both.

Fornax®T• The compact high-frequency induction casting

machine• • • The Fornax® with benchtop design • induction heating (semiconductor generator)

• casting temperature up to 1550 °C: ideal for all dental alloys (except titanium)

• Cooling with BEGO circulation unit or through direct connection to water supply and outlet

• Heating level switch for ceramic or graphite crucible

Fundor T Casting machine (BEGO)

Features:

• Casting of all dental alloys (except titanium).

•High degree of safety, by virtue of cover lock.

•Automatic gas lighter as accessory simplifies handling of

the melting torch significantly.

High Frequency Centrifugal Casting

Machines

• Degutron (Degusa) Degutron (Degusa)

• The energy required here to melt the alloy is supplied by a high frequency generator.

MEGAPULS 3000 induction casting machine.

• The machine operates using high frequency electromagnetic waves and should not be operated by anyone with a heart pacemaker.

VACUUM PRESSURE CASTING MACHINES•Combilador CL-G 77 (Heraeus Kulzer)

Features

• Recommended for casting all types of crown

and bridge work in precious metal alloy.

• Electronic temperature control with digital

display.

• Max.temp. upto 1450°C.

TITEC F210 M (OROTIG)

It is a semi automatic titanium casting machine where by the amount of vacuum, Argon-gas saturation and over pressure levels as well as the delivery of energy to electric arc to melt the metal are automatically performed in a sequence once the machine is started.

The decision to halt the process on the spot and implement corrections before starting the process again are in operators hands.

• The rotation of the melting / casting chamber from horizontal to vertical position for the casting to take place, when the metal (titanium) melting point has been optimally reached, is also in the operators hand.

• Therefore, both the automation of the machine and the operators skill go hand in hand to achieve excellent results constantly.

CASTING CRUCIBLES

Clay – used for high noble and noble metal alloys.

Carbon - used for high noble and higher fusing gold based metal ceramic alloys.

Quartz - recommended for high fusing alloys for any type specially suited for alloys that have high melting

and are sensitive to carbon contamination.

polishing

Common causes of failures of casting involves 1. Distortion

2. Surface roughness and irregularities 3. Porosity

4. Incomplete or missing details.

CASTING DEFECTS

Distortion of pattern and dimensional changes

This occurs due to • High tendency of wax to deform.• High coefficient of thermal expansion:

35×10-6 /°C.• Low softening temperature.• Warping of wax when unrestrained.

Distortion

pre investing:--temperature at which pattern is fabricatedi.e. lesser the temperature at which pattern is

made ,more are the chances of distortion at high temp due to release of stresses.

-more the storage time and temperature , more will be distortion

Pre investing

investing

Prevention• Softening the wax uniformly by heating at 50°C for

atleast 15 mins.• Reducing the time between wax fabrication and

investing (not more than 30 mins)• If more than 30m minutes

– store in refrigerator• Readaptation of the margins after storage.• Careful removal of the wax pattern from the die

- symmetrical forces

DISTORTION DURING INVESTING.

• During pouring of the investment• Expansion of investment material• Movement of investing ring before the

hardening of the investing material.

PREVENTION

• Gentle pouring • No movement of ring before hardening of

investment material.

Roughness is defined as relatively finely spaced surface imperfections whose height, width and direction establish the

predominant surface pattern.

Irregularities refers to isolated imperfection such as nodules that do not characterize the total surface area.

Surface roughness of the casting is invariably greater than that of wax pattern from which it is made.

SURFACE ROUGHNESS AND IRRAGULARITY

Causes1. Prolonged heating leads to

Disintegration of the investmentContaminates the casting producing loss of surface

texture.2. Water powder ratio.

Higher the ratio: the surface of the investment does not record the fine detail.

3. Casting pressure is too high.Causes molten metal moves along the mold wall with

pressure leading to minute loss of investment particles/ fine detail.

4. Foreign bodies.Pieces of investment if get broken , gets incorporated into metal resulting in roughness.

5. Reuse of metal alloy.6. Improper pickling of metal.

Taking gold alloy out from pickling solution with stainless steel tweezer causes plating on the gold alloy with base metal and leads to surface roughness.

Surface irregularities on an experimental casting .

a). Air bubbleb). Water filmc). Inclusion of foreign body.

Caused by:-

NODULES

Small nodules on a casting are caused by air bubbles that becomes attached to the pattern during or subsequent to the investing procedure.

• Removal of nodules on margin or on internal surface might cause marginal discrepancy or alter the fit of the casting, On the occlusal surface it effects the occlusal anatomy.

• Best method to avoid air bubble is to use vacuum investing technique and also by using a wetting agent.

If the investment become separated from the wax pattern , water film may form irregularly over the surface. this type of surface irregularity appears as minute ridges or veins on the

surface Too high L:P ratio also produces these surface

irregularities. Use of wetting agent prevents such irregularities.

WATER FILM

Mold Cracking/Fins on Casting

Too rapid heating - unequal expansion of the investment.

• Outside portion gets heated and expands more in comparison to inner one.

• Leading to cracks in the investment.• which in turns produces fins in casting.• Commonly present in cristobalite investment

which has low inversion temperature.

contd

Mold Cracking/Fins on CastingPattern position1. When many patterns are present in same plane.

– Prevention by investing more than one pattern in different planes.

– Allow 1mm space between the patterns.2. Patterns placed close to ring wall or near to the ring

end. Space patterns 4 to 5 mm from mold wall and end.

Investment not thoroughly set prevention by Spatulate longer, use warmer liquid and allow to

set in warmer room.

FOREIGN BODIES

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

It results in surface roughness and also incomplete casting areas or surface voids.

POROSITYDepending upon location

ExternalInternal

Effects of porosity:- Weakens the alloyRoughness and discoloration If severe can lead to tooth surface micro leakage and

secondary caries.

i. Solidification defects (A) Localized shrinkage porosity

(B) Micro porosity

ii Trapped gases (A) Pin hole porosity

(B) Gas inclusion porosity (C) Subsurface porosity

iii Residual air

TYPES OF POROSITY

Manifests as large irregular void found near the junction of the sprue and casting.It is caused by Incomplete feeding of molten metal during solidification.

Nobel metal alloy contract 1.25% when changes from liquid to solid.

Therefore there must be continual feeding of molten metal through the sprue to make up for shrinkage. If sprue freezes in its cross-section before feeding is completed- a localized shrinkage void will occur in the portion of casting which is last to solidify.

LOCALIZED SHRINKAGE POROSITY-

Prevention Of Localized Shrinkage Porosity

1. Proper size selection of the spruesa). Diameter ≥largest thickness of the pattern

size of the sprues varies from 6- 12 guage– 2.5mm (10 guage)-molars / metal ceramics.– 2mm (12 guage ) for premolars/ partial coverage.b). Length wax pattern should be placed 6 mm-3 mm from the top of the

ring ,depending on the type of the investing materials.

c). Flaring – allows even flow of the metal into mold and therefore less

porosity– also acts as reservoir ,and allows continuous feeding of the

molten alloy.

In X ,the sprue is attached with rounded corners to avoid turbulance and the risk of investment fracture.In Y, the sharp corners of attachment are not desirable.

• Indirect spruing Use of reservoir between wax pattern and sprue

base .recommended in 2 unit /3 unit bridge.

Gate Method Of Spruing Large Wax Pattern

Need for following gate technique• Generally the alloy immediately adjacent to the

walls of the mold solidifies first, • The exposed surface of the button will solidify

second, And• The alloy internal to the mold will cool last.

• Porosity occurs in the portion of the casting that cools last. • For large castings such as multiple unit bridges,

the spruing arrangement provide a bulk (or reservoir) of metal that is external to the dental unit but positioned at the heat center of the investment mold.

• In the gate sprue technique the runner bar will usually be the last part to solidify and will contain porosity.

Gate Method

Large units (e.g., bridges cast as a single unit) sprued by the "gate" method, in which a series of

• Short 8-gauge wax sprue formers are attached to

• Continuous 6- or 8-gauge "runner bar" of wax or (preferably) plastic this runner bar in turn is attached to

• The crucible former by two or more large sprue formers,

Photo pg 211 RM

This method provides (1) a minimum of distortion of wax pattern,(2) a fairly uniform reservoir of metal the entire

casting, and(3) even distribution of the alloy to all parts of the

casting to minimize porosity.

If bulky pontics are placed at the heat center of the mold or are cast without a resrvoir within the investment, porosity will occur in the pontic itself or at the connector between the pontic(s) and the abutments, resulting in a weak bridge

Proper Gate spruing, reservoir is in the heat center of the mold

Chill ventsWhere a dental casting contains both thick and thin elements,

porosity can be minimized by the addition of chill vents that accelerate the cooling of the pontic relative to the parts of the casting external to the dental unit itself. These cast extensions are cut off after retrieval of the casting from the investment.

GATE METHOD

• Care must be taken not to distort the large pattern during handling or by heating and softening the large bulk of spruing wax because shrinkage on cooling stresses the pattern.

• The use of rigid plastic sprue formers ·and runner bars will reduce this problem.

SUCK BACK POROSITYIt is a type of localized shrinkage porosity

which occur due to development of hot spot that is created by hot metal impinging on a point of mold wall. This hot spot causes the local region to freeze last.

It can be eliminated by-1. flaring the point of sprue attachment

1. by reducing the mold melt temperature differentialthat is lowering the casting temperature by about 30°C.

Incorrect solidification sequence that results in suck back porosity. the sprue area freeze before the cusp area of the crown and the metal in the crown had to feed the shrinking alloy, causing a void in the crown.

Correct sequence of solidification of an alloy in the investment mold of a full cast crown .the button should freeze last .this order allows the molten metal to compensate for the shrinkage realized when such increment of metal goes from the liquid to solid state

PREVENTION• “Y” shaped sprue.• Half of the molten alloy enters through

both side,therefore the temperature of the investment does not rise under the sprue.

Present as fine grain in casted alloy

Also occurs from solidification shrinkage but

Is generally when solidification is too rapid for the micro voids to segregate to the liquid pool.

MICRO POROSITY

PreventionBy increasing the mold temperature.

PIN HOLE POROSITY/ GAS INCLUTION POROSITY

It occurs due to entrapment of gas during solidification. Metals such as copper and silver dissolve oxygen in large amount in liquid state. Molten platinum and palladium have strong affinity for hydrogen and oxygen. On solidification absorbed gases are expelled and pin hole porosity result.

Prevention 1.Premelting alloy in charcoal2.Properly adjusted blow pipe

(reducing zone of the flame should be used).

Its exact cause is not known.

It is assumed it may be caused by simultaneous nucleation of solid grains and gas bubbles at the first moment, that the

metal freezes at the mold walls.

Prevention by controlling the rate at which the molten metal enters the mold.

SUB SURFACE POROSITY

ENTRAPPED AIR POROSITY- BACK PRESSURE POROSITY

This is caused by inability of air in mold to escape through the pores in the investment or

By the pressure gradient that displaces the air pocket towards the end of investment via the molten sprue and button.

It is seen on inner surface of casting and can produce large concave depression.

Incidence of air entrapment is increased by 1. Dense modern investment. 2. Dense investment due to vacuum mixing. 3. Clogging of mold with residual carbon

when low heat technique is used.

1.Proper burn out cycle-according to ADA specification no.4, :melted wax at 500ºC should leave no residue in excess of .10%

Incidence of air entrapment can be decreased by- 1. proper burnout. 2. Adequate mold and casting

temperature. 3. High casting pressure. 4. Proper L:P ratio.

2. Adequate mold and casting temperatureTemperature differential between mold and casting should be less.Lesser differences ,allow the alloy to flow easily,

and the gases are displaced easily.3. Sufficient high casting pressure.(.10-.14 MPa)4. Water powder ratio of investment: mixing should be done

according to manufacturer instruction. Too dense investment: Prevents rescuing of the

gases.5. The distance of 3-6 mm for phosphate bonded

investment and gypsum bonded investment respectively.

It occurs when molten alloy is prevented from completely filling the mold.

causes are 1. Insufficient pressure to overcome back pressure.

Therefore sufficient pressure should be applied for at least 4 seconds.

2. Incomplete elimination of wax residue from the mold.

It results in shiny rounded margin which Is caused by strong reducing atmosphere created by carbon mono oxide left

by residue wax.Improper burn out of waxes liberates CO2 ,H2O,N2O

INCOMPLETE CASTING

The formation of gases and their elimination depends on the

• O 2 content,• Burnout temperature• Time of burnout.

PREVENTION1. For 500ºCmold temperature and large wax pattern :Time required 1hr2. For proper circulation of O2 :for half of the time sprue hole is placed downward,

(melted wax drains down).Rest half of the time ,it is placed upward, to allow air circulation in the mold cavity.

Other causes are 3.Improper venting of gas

manifested as round and dull margins.4.Casting temperature too low. i.e. , the alloy is

not melted properly5. Mold temperature too low.

FUSION TEMPERATURE OF VARIOUS ALLOYS

• Chrome-Cobalt 870–900°C (1600–1650°F)• Nickel-Beryllium 815–870°C (1500–1600°F)• Palladium (2–8% Gold) 700–870°C (1300–1600°F)• Silver-Palladium 760–815°C (1400–1500°F)• Ceramic Gold 700–815°C (1300–1500°F) (45% or higher) • Crown & Bridge Gold 575–700°C (1100–1300°F)

PREVENTIONSPRUING: 1. by adding auxillary sprues: which provides a). channel

for leading the molten alloy into the mold cavity.b). Wax and gas elimination .c). creates reservoir for molten metal which compensates for

alloy shrinkage.2. Size of the sprue larger than the thickest part of the wax pattern .

smaller size of the sprue -leads to solidification of the metal in the sprue – don’t compensate for solidification shrinkage.

3. Length of the sprue- 3-6 mm for phosphate bonded and gypsum bonded.

4. Flaring-acts as reservoir,Keeps the metal in the molten state for long.

Factor which also governs the complete casting

1. The amount of metal used.High density noble metal alloys

6gms=(4 dwt or pennyweight) is recommended for premolar and anterior crown9gms=(6 dwt) for molars12gms=(8 dwt)=for pontic

1 metal pellet =6 gms.When casting a restoration that requires 2 dwt of alloy, casting with 8-10 dwt is advisable.

Number of metal pellets required for base metal alloy is less

2.Casting force –In the centrifugal machine the force is dependent on

the number of turns.For Noble Metal casting - 3 turnsBase Metal alloys - 4-5 turns.

Too large castings

Causes• Excessive expansion of wax• More than 2 liners used• Improper positioning of liners.

PREVENTION Single thickness cellulose linerRestrict liner 3.25 mm short of the metal ring (allows controlled longitudinal expansion).

TOO SMALL CASTING

Causes

Use of metal ring without linerReverse pressure causes decreases in dimension Prevention :- use liners • It provides cushioning effect • If wet , allows hygroscopic

expansion.

DISCOLORED CASTING

Black castings are common gypsum bonded investment materialCause : 1. wax not , completely eleminated Carbon residues on the mold wall get deposited on the surface of the casting.2. Disintegration of gypsum bonded investment above 700°C, resulting in release of

SO2 and sulfide layer is formed over the alloy which cannot be removed by pickling.

Prevented byProper burn out,Premelting alloy with charcoal.Properly adjusted torch.

Rough Casting Surfaces Cause Solution

Insufficient spatulation. Spatulate as recommended in instructions to thoroughly complete all setting reactions. Replace worn bowl and paddle -- New equipment improves mixing action.

Rate of burnout too rapid. Decrease heating rate or try two-stage procedure per instructions.

Overheating alloy melt. Review alloy manufacterer’s directions.Defective pattern and/or pattern matieral.

Use only high quality pattern materials. Avoid was contamination.

Wet pattern. Dry pattern thoroughly after application of debubblizer

Investment BreakdownCause Solution

Burnout too rapid Follow burnout procedure as recomended in instructions.

Use of pre-heated furnace at high temperature

If pre-heated furnace is used, initial temperature should be below 500°F (260°C).

Investment not thoroughly set Spatulate longer, use warmer liquid and allow to set in warmer room.

Mold overheated Check furnace for temperature calibration

Insufficient setting time. Lengthen the time of bench setting before mold is placed in oven -- 60 minutes minimum.

Use of uncoated, solid plastic sprues and runner bars

A light coat of wax will allow pattern wax to drain and plastic to expand.

Pits And Nodules On CastingsCause Solution

Inpurities in wax or plastic Be sure that pattern material is free of foreign matter.

Air bubble entrapped Use wetting agent (SMOOTHEX); dry thoroughly. Take care during investing to avoid entrapping air.

Insufficient vacuum during spatulation. A good vacuum of 26-29" Hg (660-740 mm) is recommended for mixing.

Insufficient spatulation Increase spatulation time by 15-20 seconds

Entrapment of loose investment particles Remove loose investment from sprue hole before burnout. Avoid sharp edges in sprue system.

Mold Cracking/Fins on Casting

Cause Solution

Too early and/or rapid burnout

Lengthen the time of bench setting before mold is placed in oven, 60 minutes minimum.

Plastic pattern or sprue materials with high melting point may plug sprue hole during early burnout

Select pattern and sprue materials that melt easily and burn out without difficulty; coat plastic with wax. Hollow sprues are preferred

Too many patterns in one plane.

Too many patterns in one plane.Avoid placing too many patterns in a single plane.

Patterns placed close to ring wall or near to the ring end.

Space patterns 4 to 5 mm from mold wall and end.

Mold is not porous enough for gases to escape.

Scrape the end of the mold to remove top skin before burnout.

Mold Cracking/Fins on Casting

Use of excessive casting pressure.

Reduce pressure (number of turns).

Liner flush with ring end. Leave adequate space (2-3 mm) at both ends of the ring to lock in investment

Air bubble in set mold Avoid air entrapment when investing.

Incomplete CastingsCause Solution

Incomplete elimination of pattern materials

Heat soak longer at the recommended temperature. Clean furnace outlet, recalibrate.

Insufficient heating of the alloy. Increase temperature of alloy. Transfer mold to casting machine and cast at once.

Excessive cooling of mold Casting machine operated with insufficient pressure or too few turns. Increase casting pressure; use more turns.

Misalignment of crucible and sprue hole. Position mold with sprue hole aligned with crucible.

Cause Solution Improper special liquid concentration To increase expansion, increase special

liquid concentration and to decrease expansion, decrease liquid concentration.

Improper liquid/powder ratio Check liquid/powder ratio and accuracy of measurements.

Liquid, mix and room temperatures affect casting dimension

High liquid, room and mix temperatures give larger castings and vice versa. Normal range 70-80°F (21°C).

Dry liner inhibits hygroscopic Use liner that readily absorbs water

Pattern deformation. Handle wax pattern with extreme care.

Inaccurate Casting Fit

Porosity in CastingCause Solution Incorrect sprueing Review sprueing systemIncomplete burnout Increase burnout time or

temperature to eliminate pattern material completely

Entrapment of loose investment particles Remove loose investment from sprue hole before burnout.

Alloy that absorbs gas when melted and releases gas upon solidification, especially when alloy is overheated

Use fresh alloy or change method of melting. Adjust gas mixture. Check directions for melting of alloy

References

• Anusavice “Skinners science of dental materials”. Tenth Edition.• Murrow and Rudd• Shillingburg HT, Jacobi R, Brackett SE. Fundamentals of tooth preparations for cast

metal & porcelain restorations. 1st ed. Carol stream. Qintessence Publishing Co. Inc – 1991.

• Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. 3rd ed. Missouri (CN). Mosby – 2001.

• Craig RG. Restorative Dental Materials. 8th ed.St Louis: CV Mosby Co.1989• Laboratory manual of the requirements used for casting – BEGO, DENTAURUM,

OROTIG,MEGAPULS 3000.• Michael J. Lessiter, Ezra L. Kotzin, Timeline Summer 2002,www.castsolutions.com• http://images.google.co.in/imgres?imgurl=http://www.edwardfeinbergdmd.com/

Short%2520History%2520of%2520Dentistry/William-Taggert-Casting-machine-1907.jpg&imgrefurl=http://www.edwardfeinbergdmd.com/history_of_dentistry.htm&h=256&w=217&sz=16&hl=en&start=32&tbni

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