DENTAL CASTING INVESTMENT MATERIALS

CONTENTSIntroductionEvolutionDefinitionProperties of an ideal investment materialCompositionGypsum bonded investments - Classification - Composition - Setting time - Normal setting expansion - Hygroscopic setting expansion - Thermal expansion - Control of expansion - Thermal contraction - Strength - Other gypsum investment considerations

Phosphate bonded investments - Composition - Setting reaction - Setting & thermal expansion - Working & Setting time - Advantages & disadvantages Ethyl silicate bonded investments - Composition - Advantages & disadvantages Lost wax technique Ringless casting system Die stone investment combination Casting ring liner Hygroscopic thermal gold casting investments Soldering investments Investments for all- ceramic restorations Investment of titanium & titanium based alloysReview of Literature Summary & Conclusion References

INTRODUCTIONThe adoption of casting practice in dentistry for making gold inlays, crowns, bridges & other restorations represents one of the major advances in restorative dentistry.In recent years, base metal alloys have been cast into:- Crowns, bridges & removable partial denture restorations

All such casting operations involve :- 1) A wax pattern of the object to be reproduced. 2) A suitable mold material known as investment, which is placed around the pattern & permitted to harden. 3) Suitable furnaces for burning out the wax patterns & heating the investment mold.

EVOLUTIONIt is remarkable that the art of lost wax casting was so widely known in ancient times. It is not an easy process & calls for considerable skills in its execution.The Aztec gold-smiths of pre-Colombian Mexico used lost wax process to create much of their elaborate jewellery. In the city of Benin,now a part of Nigeria, the brass smiths continue to produce lost wax castings using a method passed down through the ages from one generation to the next. The brass cutters begin with a core of clay kneaded into a mass. They shape the clay into the approximate size & shape of the article to be made. These cores are then dried in sun for several days. The brass smith creates a pattern for the casting by covering one of these cores with beeswax.

After finishing the wax, it is covered in a thick coating of clay. The 1st layer is applied as a very fine slip. Before the pattern is fully sealed in the coating, a thin roll of wax is added to form a channel into which the molten metal will be poured. The thicker layer of clay is added for investing the form completely, creating a mold. This tech. was largely ignored by modern industry untill the dawn of the 20th century when it was rediscovered by the dental profession for producing crowns & inlays. The true significance of the use of investment casting in dentistry was not realised untill the research of Dr. William Taggart of Chicago was published in 1907. He described a tech., formulated a wax pattern compound of excellent properties, developed an investment material & also invented an air pressure casting machine.

In a solid mold technique, a wax sprue was placed in a steel casing & surrounded by a setting slurry. Drawbacks were, extremely long pre-heat, size limitations & poor dimensional tolerances.In 1538, molds for large statues were made which, if one desired to make them of bronze, are first made of wax by the ordinary procedure. In ordinary procedure, the the original model was created in wax. The image was then coated with the milky slurry of plaster, building up successive layers untill a strong shell enveloped the wax. Alternatively, the image could be dipped or invested in a bucketful of freshly mixed plaster which set up rock-hard in a very short time. After melting the wax & casting molten metal into the void, a perfect duplicate of the original pattern was created.

DEFINITIONINVESTMENT :- It can be defined as a ceramic material that is suitable for forming a mold into which a metal or alloy is cast. The operation of forming a mold is described as investing . The investment should be capable of reproducing the shape, size & details recorded in the wax pattern.The investment mold should be capable of maintaining its shape & integrity at elevated casting temperatures. The investment should have a sufficiently high value of compressive strength at the casting temp. so that it could withstand the stresses set up when the molten metal enters the mold.

PROPERTIES REQUIRED FOR AN IDEAL INVESTMENT:

1) Easily Manipulated 2) Sufficient strength at room temperature 3) Stability at higher temperature 4) Sufficient Expansion 5) Beneficial casting temperature 6) Porosity 7) Smooth surface 8) Ease of Divestment 9) Inexpensive

COMPOSITIONIn general, an investment is a mixture of 3 distinct types of materials:- 1) Refractory Material:- This material is usually a form of silicon dioxide such as:- - Quartz - Tridymite - Cristobalite or a mixture of these.

2) Binder Material:- Since the refractory materials alone do not form a coherent solid mass, some kind of binder is needed. The common binders used for dental casting gold alloy is -CaSO4 hemihydrate. Phosphate, ethyl silicate & other similar materials also serve as binders for high- temperature casting investments.3) Other Chemicals:- Chemicals such as NaCl, boric acid, potassium sulfate, graphite, copper powder or MgO are often added in small quantities to modify various physical properties.

In general, there are 3 types of investments materials available: 1) GYPSUM BONDED INVESTMENTS :- For conventional casting of gold alloy inlays, onlays, crowns & FPDs. 2) PHOSPHATE BONDED INVESTMENTS:- For metal ceramic restorations, for pressable ceramics & for base metal alloys. 3) ETHYL- SILICATE BONDED INVESTMENTS:- For casting of removable partial dentures with base metal alloy.

1) GYPSUM BONDED INVESTMENTSGYPSUM:- is a mineral that is mined in various parts of the world. Chemically the gypsum that is produced for dental purposes is nearly pure calcium sulfate dihydrate.

CLASSIFICATIONThe gypsum bonded investments are classified by the ISO(1990) as:- TYPE 1:- Thermal expansion type. For casting inlays & crowns. TYPE 2:- Hygroscopic expansion type. For casting inlays & crowns. TYPE 3:- For casting complete & partial denture bases with gold alloys.

COMPOSITIONEssential ingredients of the dental inlay investment employed with the conventional gold casting alloys are - hemihydrate, allotropic forms of silica & chemical modifiers. 1) GYPSUM:- -hemihydrate acts as a binder to hold the ingredients together & to provide rigidity. - Most investments contain -hemihydrate because greater strength is obtained which depends on the amount of binder present. - 25% to 45% of -hemihydrate is present. - Used in casting gold alloys with melting ranges below 1000C. Above 1000C greater shrinkage & frequent fractures takes place.

- All forms of gypsum shrink after dehydration between 200C to 400C .- Slight expansion takes place between 400C & 700C.- A large contraction then occurs

.- Second shrinkage is most likely caused by:-DecompositionThe release of sulphur gases, such as SiO2. - This decomposition causes shrinkage & also contaminates the castings with the sulphides of non-noble alloying elements, such as silver & copper. - Therefore, it is imperative that gypsum investments should not be heated above 700C. - For gypsum products containing carbon, the max. temp. should be 650C to obtain proper fit & uncontaminated alloys.

2) SILICA:- Present 55% to 75% & acts as a refractory material during the heating of the investment & regulates the thermal expansion. - If the proper form of silica is used in the investment, the contraction of gypsum during heating can be eliminated & changed to an expansion. - Silica exists in 4 allotropic forms:- - Quartz - Cristobalite - Tridymite - Fused quartz - When quartz, cristobalite or tridymite is heated, a change in crystalline form occurs at a transition temp. characteristic of the particular form of silica.

- eg. When quartz is heated, it inverts from -quartz to -quartz, at a temp. of 575C(1067F). - Cristobalite undergoes an anologous transition between 200C & 270C from to form. - 2 inversions of Tridymite occurs at 117C & 163C respectively. - Density decreases as form changes to form, with a resulting increase in volume exhibited by a rapid increase in linear expansion. - Fused quartz is an amorphous & glasslike in character, & it exhibits no inversion at any temp. below its fusion point. It has low coefficient of thermal expansion & is of little use in dental investments.

- Quartz, cristobalite or a combination of the 2 forms may be used in a dental investment. - On the basis of the type of silica employed,dental investments are classified as:-Quartz investment Cristobalite investment

EFFECT OF VARYING COMPOSITION:- - Increasing the proportion of silica in the investment powder increases the manipulation time, initial setting time, setting expansion both in air & water & thermal expansion & decreases the compressive strength. - The rate of setting reaction is unchanged. - The increase in manipulation & setting time occurs because the particles of the refractory filler, interfere with the interlocking of the growing gypsum crystals & making this less effective in developing a solid structure. The compressive strength of the investment is reduced for the same reason.

- The setting expansion is increased when interlocking of the growing gypsum crystals is inhibited by the refractory particles, because more of the crystal growth is directed outwards. - The thermal expansion is increased, since increasing the proportion of the expanding component increases the observed expansion.

3) MODIFIERS:- Consists of:- a) Modifying Agents:- They regulate the setting expansion & setting time & also prevent most of the shrinkage of gypsum, when it is heated above 300C, eg. Boric acid, NaCl. b) Coloring agents c) Reducing agents:- They are used in some investments to provide a non- oxidizing atmosphere in the mold when the gold alloy is cast. eg. Carbon, powdered graphite or powdered copper.

EFFECTS OF MODIFYING AGENTS:- In 1982 & 1986 studies were done by Mori T. on the thermal behaviour of gypsum binder in dental casting investments & effect of boric acid on the thermal behaviour of this binder was found & he said that All of these act mainly by reducing 2 large contractions of gypsum binder that occur on heating to temp. above 300C. - Boric Acid:- When heated above 150C, it forms a viscous liquid. This viscous liquid retards the evaporation of water i.e. delay transformation of CaSO4. The presence of the viscous liquid phase, also reduces the high temp. contraction that results from sintering.

- The presence of modifiers added to increase the thermal expansion also affects the strength changes of the investment that occur on heating, because of their effect on the CaSO4 binder. Ohno et. al. has done a study in 1982 on the effect of phase transformations of silicas & CaSO4 on the compressive strength of gypsum bonded investments at high temp. & they said that:- 1) On heating GBI without these additives, show a rapid increase in compressive strength of about 100% in the range of 100C to 175C on drying. 2) Between 175C - 225C there is decrease in compressive strength, because of the dehydration reaction.

3) Relatively minor strength fluctuations occur during subsequent heating to higher temp. because of:- -Further phase changes in the binder. - inversion of the refractory. - Sintering of the binders. - These investments are heated to temp. in the range of 670C- 700C & show compressive strength changes ranging from +10% to -40%. - Investments containing boric acid, when heated to the same temp. range show increase in compressive strength ranging from +40% to +50%.

SETTING TIMESetting time measured & controlled in the same manner as plaster.According to ADA/ANSI specification no. 2 for dental inlay casting investment, the setting time should not be < 5 min. or >25 min.Usually, the modern inlay investments set initially in 9 to 18 min. Sufficient time should be allowed for mixing & investing the pattern before the investment sets.

NORMAL SETTING EXPANSIONDefinition:- The volumetric or linear increase in physical dimensions of an investment caused by chemical reactions that occur during hardening to form a rigid structure.PURPOSE:- To aid in enlarging of the mold, to compensate partially for the casting shrinkage of the mold. Regardless of the type of gypsum product used, an expansion of the mass can be detected during the change from hemihydrate to dihydrate.A mixture of silica & gypsum hemihydrate results in setting expansion greater than that of the gypsum product when it is used alone.The ADA Specification no. 2 for Type 1 investment permits a maxi. setting expansionin airof only 0.6%. The setting expansion of modern investments is 0.4%.

The ADA Specification no. 2 for Type 1 investment permits a maxi. setting expansionin airof only 0.6%. The setting expansion of modern investments is 0.4%. The setting expansion of an investment with a comparatively high gypsum content is more effective in enlarging the mold than with a low gypsum content.

FACTORS AFFECTING NORMAL SETTING EXPANSIONIf the pattern has thin walls, the effective setting expansion is somewhat greater, than for a pattern with thicker walls.The softer the wax, the greater is the effective setting expansion. If a wax other than the Type 2 inlay wax is used, the setting expansion may cause a serious distortion of the pattern.The lower the W:P ratio for the investment, the greater is the effective setting expansion.

HYGROSCOPIC SETTING EXPANSIONThe hygroscopic setting expansion was 1st discovered in connection with an investigation of dimensional changes of a dental investment during setting. If the setting process is allowed to occur under water(slurry), the setting expansion may be more than double in magnitude because of the additional crystal growth permitted.ADA Specification no. 2 for Type 2 investments requires a mini. setting expansion in water of 1.2% & maxi. expansion permitted is 2.2%.

PURPOSE:- To expand the casting mold to compensate for the casting shrinkage of the gold alloy The HSE differs from the NSE in that, it occurs when gypsum is allowed to set under or in contact with water & that it is greater in magnitude than NSE.The HSE may be 6 or more times the NSE of a dental investment.

FACTORS CONTROLLING THE HYGROSCOPIC EXPANSIONEffect of Composition:- - HSE silica content. - Finer silica particles HSE. - - hemihydrate(in presence of silica) HSE.Effect of W:P ratio:- W:P ratio e 1/HSE.Effect of Spatulation:- Mixing time HSE.Shelf life of investment:- Effect of time of Immersion:- 6) Effect of Confinement:-

7) Effect of the amount of added water:- HSE amount of water added8) Water bath Temperature:-9) Effect of Particle size of Silica:- Particle size affects the smoothness of the mold cavity surface & the inherent porosity of the mold. Finer particles of silica ensures smooth mold surface & smooth casting.

THERMAL EXPANSION DEFINITION:- It is the increase in dimension of a set investment due to temp. increase during burnout. The expansion of a gypsum bonded investment is directly related to the amount of silica present & to the type of silica employed. A considerable amount of quartz is necessary to counterbalance the contraction of the gypsum during heating.The thermal expansion of quartz investment are influenced by:- - The particle size of the quartz. - Type of gypsum binder - W:P ratio

Since greater expansion occurs during the inversion of the crystobalite, the normal contraction of the gypsum during heating is easily eliminated.According to ADA/ANSI Specification no. 2 for Type 1 investment which rely on the thermal expansion for compensation, the thermal expansion must not be 1.6%.For Type 2 investment, which rely on hygroscopic expansion for compensation of the contraction of the gold alloy, the thermal expansion be between 0% & 0.6% at 500C .

FACTORS AFFECTING THERMAL EXPANSION 1) W:P ratio:- The magnitude of thermal expansion is related to the amount of solids present. W:P ratio thermal expansion.

2) Effect of Chemical Modifiers:- - A disadvantage of an investment that contains sufficient silica to prevent any contraction during heating is the weakening effect of silica. - The addition of small amount of Na, K or Lithium chloride to the investment eliminates the contraction caused by the gypsum & increases the expansion. - Boric acid similar effect. It hardens set investment, but, it disintegrates during the heating of the investment & a roughened surface of the casting may result. - Silicas do not prevent gypsum shrinkage but counterbalance it, whereas chlorides reduce gypsum shrinkage below temp. of 700C.

CONTROL OF EXPANSION 1) Composition:- By the choice of refractory & binder & by the use of certain additives. 2) W:P ratio:- W:P ratio Setting expansion & Thermal expansion. 3) Period of exposure to water:- In hygroscopic expansion techniques, additional control can be obtained by varying the length of time the setting investment is exposed to an aqueous environment.

THERMAL CONTRACTION/ COOLING OF THE INVESTMENTWhen an investment is cooled from 700C, its contraction curve follows the expansion curve during the inversion of the -quartz or -cristobalite to its stable form at room temp. The investment contracts to less than its original dimension, because of the shrinkage of gypsum when it is first heated.If the investment is reheated, it expands thermally to the same, maxi. reached when it is 1st heated.In practice, the investment should not be heated a second time, since internal cracks may develop which affect the quality of the casting.

STRENGTHThe strength of an investment increases rapidly as the material hardens after initial setting time. The free water content of the set product affects its strength.2 strengths of a gypsum product are:- 1) Wet Strength 2) Dry Strength The dry strength is 2 or more times the wet strength.The strength of an investment must be adequate to prevent fracture or chipping of the mold during heating & casting of the gold alloy.The strength of an investment is measured in terms of its compressive strength which should not be too high.

The compressive strength is increased according to 1) Type & amount of gypsum binder present 2) Use of chemical modifiers.According to ADA Specification no. 2, the compressive strength for the inlay investment should not be less than 2.4MPa when tested 2hrs after setting.

FACTORS AFFECTING STRENGTH 1) W:P ratio:- W:P ratio Porosity CS & TS. 2) Heating the investment to 700C may / the strength as much as 65%, depending on the composition. 3) After the investment has cooled to room temp., its strength decreases considerably, because of the fine cracks that form during cooling. 4) The addition of an accelerator or retarder lowers both the wet & dry strength.

OTHER GYPSUM INVESTMENT CONSIDERATIONSFineness:- Finer the investment, the smaller are the surface irregularities on the casting.Porosity:- More gypsum crystals present in the set investment, less porosity. More uniform the particle size, greater is its porosity. Storage:- The investment should be stored in airtight & moisture proof containers.

2) PHOSPHATE BONDED INVESTMENTSThe rapid growth of the use of metal ceramic restorations & use of cast removable partial dentures & the increased use of higher melting alloys have resulted in an increased use of phosphate bonded investments materials.Phosphate bonded investments are of 2 types:- 1) Type 1- For crowns, inlays & other fixed restorations. 2) Type 2- For partial denture & other cast removable restorations.

COMPOSITION Consists of :- 1) Filler:- The filler is silica, in the form of Cristobalite, Quartz, or a mixture of the two and in a concentration of approx 80%. - Purpose:- To provide high temp. thermal shock resistance & a high thermal expansion. 2) Binder:- It is less than 20%. -The binder consists of magnesium oxide and a phosphate that is acid in nature. - MgO reacts with the filler at room temp.

3) Colloidal Silica Liquid Suspensions:- - The colloidal silica suspensions are used with the phosphate investments in place of water to compensate for the greater contraction of the high fusing alloys during solidification, as it increases the setting expansion of the investment. - For base metal alloys, a 33% dilution of the colloidal silica is required.

4) Carbon:- - It is often added to the powder to produce clean castings to facilitate the divesting of the casting from the mold. - This is appropriate if the casting alloy is gold. - The latest evidence indicates that palladium does react with carbon at temperature above 1504C. Thus, if the temperature exceeds this temperature during casting, a carbon free investment should be used.

SETTING REACTION - Magnesium ammonium phosphate formed is polymeric. Although the stoichiometric quantities are equal molecules of magnesia and monoammonium phosphate, an excess of magnesia is usually present, and some of it is never fully reacted. - On heating, the binder of the set investment undergoes thermal reactions.

SETTING & THERMAL EXPANSIONS- Theoretically, the reaction should entail a shrinkage,but in practice there is a slight expansion by using acolloidal silica solution instead of water.- The early thermal shrinkage of phosphate investments is associated with the decomposition of the binder, magnesium ammonium phosphate, and is accompanied by evolution of ammonia.- Some of the shrinkage is masked because of the expansion of the refractory filler especially in the case of cristobalite.

WORKING & SETTING TIME - Phosphate investments are affected by temperature. - The setting reaction itself gives off heat, and this further accelerates the rate of setting. - Increase in mixing time and mixing efficiency results in faster set & a greater rise in temp. - Increase in w:p ratio, increases the working time

ADVANTAGES They have both green strength & fired strength.This makes them easy to handle without breaking before they are placed in a furnace for wax burnout & strong enough afterwards to withstand the impact & pressure of centrifugally cast molten alloy. They can provide setting & thermal expansion high enough to compensate for the thermal contraction of cast metal prosthesis or porcelain veneers during cooling. They have the ability to withstand the burnout process with temp. that reach 900C. They can withstand temp. upto 1000C for short periods.

DISADVANTAGESWhen used with higher melting alloys i.e. those with casting temp. greater than 1375C, these investments are at a disadvantage. These temp. coupled with high mold temp., result in mold breakdown & rougher surkace on castings. The high strength of these investments can make removal of the casting from the investment difficult. When higher expansion is required, more of the silica liquid is used with the result that a more dense & less porous mold is produced. This can result in incomplete castings if a release of trapped gases is not provided.

3) ETHYL SILICATE-BONDED INVESTMENT Losing popularity - complicated and time consuming procedures involved.Consists of powder & liquid. - Powder contains refractory particles of silica & glass along with calcined MgO & some other oxides in minor amount. - 2 Liquids one is ethyl silicate & the other an acidified solution of denatured ethyl alcohol.

Binder is a silica gel that reverts to silica on heating. Several methods may be used to produce the silica or silicic acid gel binders:- - When the pH of sodium silicate is lowered by the addition of an acid or an acid salt, a binding silicic acid gel forms. - An aqueous suspension of colloidal silica can be converted to a gel by the addition of an accelerator, such as ammonium chloride. - Another system for binder formation is based on ethyl silicate. A colloidal silicic acid is formed by hydrolyzing ethyl silicate in the presence of hydrochloric acid, ethyl alcohol and water. Si(OC2H5)4 + 4H2O Si(OH)4 + 4C2H5OH

A polymerised form of ethyl silicate is used, to form a colloidal sol of polysilicic acid. The sol is then mixed with the quartz or cristobalite, to which is added a small amount of finely powdered MgO to make the mixture alkaline.A coherent gel of polysilicic acid then forms, accompanied by a setting shrinkage. The soft gel is then dried at temp. below 168C.During drying process the gel loses alcohol & water to form a concentrated, hard gel.

A volumetric contraction accompanies the drying which reduces the size of the mold.This contraction is known as green shrinkage and it occurs in addition to the setting shrinkage. Investments of this type are designed to reduce the layer of silica gel around the particles. This type of investment can be heated between 1090C to 1180C and is compatible with the higher fusing alloys.

ADVANTAGESThe investments have the ability to cast high temp. cobalt- chromium & nickel- chromium alloys & good surface finish, low distortion & high thermal expansion.They are less dense than PBI & thin sections with fine details can be reproduced.The low fired strength makes removal of casting from investment easier than with PBI.

DISADVANTAGE Added processing attention & extra precaution needed in handling the low strength fired molds. The low strength & high thermal expansion requires a more precise burnout process & firing schedule to avoid cracking.

LOST WAX TECHNIQUE Introduced by TAGGART in 1907. Elimination of the wax pattern from the mold of set investment material is reffered as Burnout. It should not be started until investment is set for minimum of 1hr. If preheated, causes cracks in investment. Ideally it should be kept in oven when mold is still little wet as water trapped prevent absorption of wax & when water vaporises, it flushes out wax.

If the burnout procedure is delayed for several hrs., it should be kept in humidor to avoid excessive drying, as molten wax during burnout may get absorbed by dry investment. If the rubber crusible former is used it is removed To facilitate a rapid & clean burnout, the ring can be placed on a raised object within the oven, allowing free release of molten wax & gases.

RINGLESS CASTING SYSTEMTo provide maxi. Expansion of investment, a ringless system is available commercially. The system called powder cast ringless system, consists of 3 sizes of rings & formers, preformed wax sprues & shapes, investment powder, & a special investment liquid.The tapered plastic rings allows for the removal of the investment mold after the material has set. This system is suited for the castings of alloys that require greater mold expansion than traditional gold based alloys.

DIE STONE INVESTMENT COMBINATIONIn this the die material & the investing medium have a comparable composition. A commercial GB material called Divestment is mixed with colloidal silica liquid. The setting expansion of the material is 0.9% & thermal expansion is 0.6%, when heated to 677C. Since divestment is a GB material, it is not recommended for high fusing alloys, as used for metal ceramic restorations.The PBI for divestment is used in the same manner as the Divestment material & is suitable for use with high fusing alloys.

CASTING RING LINERSWith the use of solid metal rings or casting flasks, provision must be made to permit investment expansion. The mold may actually become smaller because of the reverse pressure resulting from the confinement of the setting expansion. The most common technique to provide investment expansion is to line the walls of the ring with a ring liner. Earlier, asbestos was the material of choice, but it can no longer be used because its carcinogenic potential makes it a biohazard. 2 types of non-asbestos ring liner materials are:- - An aluminium silicate ceramic liner - A cellulose paper liner.

4) HYGROSCOPIC THERMAL GOLD CASTING INVESTMENT It is designed for use with either hygroscopic or thermal type of casting technique. Thermal expansion of this investment takes place in the range bet. 482C & 649C . This expansion is high enough to use the investment with the thermal casting tech. without water immersion. But when it is immersed in a water bath, then the investment expands hygroscopically. With the hygroscopic tech. the investment only needs to be heated to 482C to provide appropriate expansion.

5) SOLDERING/BRAZING INVESTMENTS When soldering the parts of a restoration, such as clasps on a RPD, the parts must be surrounded with a suitable ceramic or investment material before the heating operation. The assembled parts are temporarily held together with sticky wax until they are surrounded with the appropriate investment material, after which the wax is softened & removed. The portion to be soldered is left exposed & free from investment to permit wax removal & effective heating before it is joined with solder.

2 types of brazing investments are:- - Type 1- Gypsum bonded dental brazing investments - Type 2- Phosphate bonded dental brazing investments.The investment for soldering is similar to casting investment. They are designed to have lower setting & thermal expansions than casting investments, a feature that is desirable so that the assembled parts do not shift in position during the setting & heating of the investment.

6) INVESTMENT FOR ALL-CERAMIC RESTORATIONS 2 types of investment materials have been developed recently for producing all- ceramic restorations:- - Type 1 Used for the cast glass technique composed of phosphate bonded refractories. - Type 2 Refractory die type of material, used for all- ceramic veneers, inlays & crowns.Refractory dies are made by pouring the investment into impressions. When the investment is set, the die is removed & is heated to remove gases that may be detrimental to the ceramic (degassing) A refractory die spacer may be added to the surface.

Then, the porcelain or other ceramic powders are added to the die surface & fired.These materials must accurately reproduce the impression, remain undamaged during the porcelain firing & have a thermal expansion compatible with that of ceramic, otherwise the ceramic could crack during cooling. These materials are also phosphate bonded & they generally contain fine grained refractory fillers to allow accurate reproduction of detail.

7) INVESTMENT OF TITANIUM & TITANIUM BASED ALLOYS In 1993 a study was done by Togoxa T.& Maixazaki T & Tamaki X. on the selection of investment for improving fits of Ti castings & they said that the castings of Ti should not be done with conventional PBI or SBI, because, Ti is highly reactive with oxygen & is capable of reducing some of the oxides commonly found in these investments. Ti can also dissolve residual oxygen, nitrogen & carbon from the investment. These elements can harden & embrittle Ti in the solid state.

Hence, modification in the existing refractory formulation & binder or new refractory formulations & binder systems are required.COMPOSITION:- - These investments can be classified as:- 1) Phosphate bonded 2) Ethyl silicate bonded 3) Cemented REFRACTORIES:- - Silica(SiO2) - Alumina(Al2O3) - MgO - Zirconia(ZrO2)

PROPERTIES:- - The properties are at most same as conventional PBI or SBI. - The main objective of the different refractory compositions & binders is to reduce the breakdown of the investment & the contamination of Ti. - One approach of reducing the reaction of Ti with the investment by employing models that have been expanded by the burnout process & then cooled back to near ambient temp. prior to casting process. - This reduces the time that the alloy is in contact with the mold at elevated temp., & the overall reactivity is reduced. In order to avoid the contamination of Ti by oxygen through the reduction of refractory oxides of the investment, refractory materials that are less easily reduced by Ti should be used.

PHOSPHATE BONDED TITANIUM INVESTMENT:- - To achieve expansion without the use of reactive powders, a PBI that contains both magnesia & alumina as refractories was developed. - This investment can achieve large expansion by the reaction of alumina & magnesia, when it is burned out at 1150C - 1200C . ETHYL- SILICATE BONDED INVESTMENT:- - Reactions of ethyl silicate bonded investments with the liquid Ti has been reported to be somewhat less than that of PBI. - This is due to the use of highly refractory oxides in the powder. But these investments require more complex procedure for their use.

CEMENTED TITANIUM INVESTMENT:- - This investment use magnesia bonded by an aluminous cement(CaO-Al2O3) & contains 5% zirconium powder by weight. - The aluminous cement serves as binder for the magnesia refractory & it sets by mixing with water. - Oxidation of the zirconium powder to zirconia during the burnout process provides irreversible expansion to compensate for the shrinkage of the casting during cooling from the solidification temp. - The zirconia formed is highly stable & it does not contaminate Ti. Ti castings from this investment had smooth surface, free of contamination fron the mold reaction.

REVIEW OF LITERATURE K. Asgars, D.B. Mahler & F.A. Peyton, 1955, investigated a hygroscopic technique for inlay casting using controlled water additions. A technique & equipment for this particular technique was described by them. They concluded that:- - The av. Deviation of expansion values resulting from controlled water additions was significantly less(0.1%) than that for complete immersion(0.3%). - The hygroscopic expansion for an av. mix was higher than for a thinner mix & the expansion reduced with the no. of spatulation turns within limits. - The expansion for a new investment was significantly higher than for an aged one.

A study conducted in our department under the able guidance of DR. N.P.PATIL Sir, in 1998, evaluated the influence of cast hardening agents on surface abrasion, surface hardness & surface detail reproduction properties of refractory investment materials. It was concluded that:- - Surface abrasion resistance improved significantly using hardening agents. - However the surface reproducibility was reduced with both treated & untreated samples when compared with a steel model.

Junzo Takahashi, Masayuki Okazaki,1999, conducted a study with the purpose of measuring the internal setting expansion of PBI & assessed the effect that different pattern materials may have on internal setting expansion. They concluded that vertical setting expansion was higher than the horizontal setting expansion & regardless of the type of pattern material, a PBI caused non uniform setting expansion, especially in horizontal direction, which lead to the distortion of the pattern.

C.L. Chew, M.F. Land, 1999, conducted a study with the purpose of evaluating & comparing the compressive strength characteristics of phosphate bonded v/s gypsum bonded investments. They also investigated if these values changed as a function of time & temp. It was concluded that at elevated temp., all materials approximated peak strength 2hrs after initial mixing.There was no significant difference in their strengths at room temp. However, the PBI exhibited significantly increased compressive strength as a function of time & temp.

A study conducted in our department under the able guidance of DR. N.P.PATIL Sir, in 2001 evaluated the fit of commercially pure titanium cast copings using 3 different investment materials (Titec, Rematitan plus, Tycast) . It was concluded that the percentage thermal expansion was significantly higher for Tycast resulting in a corresponding increase in discrepancy in marginal fit & internal surface adaptation. It was hence recommended that Titec & Rematitan should be preffered over Tycast.

SUMMARY & CONCLUSION Of the three main types of casting investment materials, the phosphate bonded products are becoming most widely used. Silica bonded materials are rarely used now a days due to the fact that they are less convenient to use than the other products & that the ethanol produced in the liquid can spontaneously ignite or explode at elevated temperatures. The investment which is best able to retain its integrity at the casting temp. & able to provide the necessary compensation for casting shrinkage is chosen.

REFERENCES 1) Anusavice K.J. Phillips science of dental materials 11th edition, 2003 2) Asgars K., Mahler D.B. Hygroscopic technique for inlay casting using controlled water additions JPD Sept. 1955; 711-724 3) A comparative evaluation of the fit of commercially pure titanium cast copings using different investment materials. Sept. 2001 4) Chew C.L., Land M.F. Investment strength as a function of time & temperature J. Dent. 27(1999); 297-302

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