investment materials

DENTAL CASTING INVESTMENT MATERIALS

andDIE MATERIALS

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

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 α-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 & FPD’s. 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 INVESTMENTS

GYPSUM:- 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 α- hemihydrate, allotropic forms of silica & chemical modifiers.

1) GYPSUM:- α-hemihydrate acts as a binder to hold the ingredients together & to provide rigidity.

- Most investments contain α-hemihydrate because greater strength is obtained which depends on the amount of binder present.

- 25% to 45% of α-hemihydrate is present. - Used in casting gold alloys with melting ranges

below 1000ºC. Above 1000ºC greater shrinkage & frequent fractures takes place.

- All forms of gypsum shrink after dehydration between 200ºC to 400ºC .

- Slight expansion takes place between 400ºC & 700ºC.

- 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 700ºC.

- For gypsum products containing carbon, the max. temp. should be 650ºC 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 α-quartz to β-quartz, at a temp. of 575ºC(1067ºF).

- Cristobalite undergoes an anologous transition between 200ºC & 270ºC from to β form.

- 2 inversions of Tridymite occurs at 117ºC & 163ºC respectively.

- Density decreases as α form changes to β 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 300ºC, 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 300ºC.

- Boric Acid:- When heated above 150ºC, it forms a viscous liquid. This viscous liquid retards the evaporation of water i.e. delay α β 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 100ºC to 175ºC on drying.

2) Between 175ºC - 225ºC 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. - α β inversion of the refractory. - Sintering of the binders.

- These investments are heated to temp. in the range of 670ºC- 700ºC & 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 EXPANSION

Definition:- 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 expansion“in air”of 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 expansion“in air”of 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 EXPANSION

If 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 EXPANSION

The 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 EXPANSION

1) Effect of Composition:- - HSE α silica content. - Finer silica particles HSE. - α- hemihydrate(in presence of silica)

HSE.2) Effect of W:P ratio:- W:P ratio eα 1/HSE.3) Effect of Spatulation:- Mixing time α HSE.4) Shelf life of investment:- 5) Effect of time of Immersion:- 6) Effect of Confinement:-

7) Effect of the amount of added water:- HSE α 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% nor >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 500ºC .

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 700ºC.

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 700ºC, its contraction curve follows the expansion curve during the inversion of the β-quartz or β-cristobalite to its stable α 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 700ºC 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 CONSIDERATIONS

Fineness:- 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 INVESTMENTS

The 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 1504°C. 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 900ºC. They can withstand temp. upto 1000ºC for short periods.

DISADVANTAGESWhen used with higher melting alloys i.e. those with casting temp. greater than 1375ºC, 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 168ºC.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 1090°C to 1180°C 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.

DIE STONE INVESTMENT COMBINATION

In 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 677ºC. 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.

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. 482ºC & 649ºC . 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 482ºC 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 1150ºC - 1200ºC . 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 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 under the 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.

DIE MATERIALS

INTRODUCTION :

Die is the positive reproduction of the form of a prepared tooth in any suitable substance.

A die must be an accurate reproduction of the prepared tooth both in dimensions and surface details.

It must represent all of the prepared surfaces, including the margins and a reasonable amount of the uncut apical position of the tooth.

A die must be made of a material that is

Dense

Hard

Capable of being used in the production of wax patterns and in the fitting and finishing of castings without undue risk of damage to its surface.

Because direct fabrication of

patterns for extracoronal restorations in the

mouth is

inconvenient

Difficult

Time consuming

Virtually impossible, practically all wax

patterns are made in the laboratory with the

indirect technique.

IDEAL REQUISITES OF A DIE MATERIAL :

1. Die material should be compatible with the impression materials.

2. Dimensional accuracy and stability – material should remain dimensiona-lly stable on storage.

3. Reproduction of detail – The ability to reproduce fine detail and sharp margins is essential.

4. Mechanical properties – High strength is important.

5. Color-Good color contrast with other

materials being used.

5. Toughness -To allow burnishing of foil and

resist breakage.

6. Easy and quick manipulation and rapid

fabrication.

7. Non-injurious to health by touch or inhalation.

8. All surfaces must be accurately duplicated and

no bubbles or voids can be accepted.

9. Economical.

CLASSIFICATION OF DIE MATERIALS :

1. Inorganic materials

a. Die stone, based on calcium sulfate hemihydrate Eg. Velmix

b. Dental cement – zinc silicophosphate

c. Ceramic materials, fired at 6000C.

2. Metallic materials

a. Dental amalgam

b. Metal sprayed dies (Bismuth-tin alloy)

c. Electroplated dies

i. Copper plated

ii. Silver plated

3. Polymeric materials

a. Auto-polymerizing acrylic

b. Other resins which polymerize at room temperature.

Eg. Epoxy resins

4. Composite materials

a. Polymer / metal composites with polyester polymer

b. Polymer / ceramic composites based on epimine polymer

GYPSUM

Casts and dies are frequently prepared from dental stone, which has calcium sulfate hemihydrate Ca(SO4)2, H2O (or) CaSO4.

½ H2O as their main constituent, prepared by heating calcium

sulfate dihydrate.

Two sources i) mineral gypsum, ii) by-product of other industries

Calcium sulfate dihydrate : CaSO4.2H2O(Gypsum)

Heat in open vessel 1200C

Heat in autoclave,

steam pressure, 120-

1300C

Heat ground gypsum in autoclave :

sodium succinate additive, 1400C

Heat in boiling 30% solution

of Cacl2 or MgCl2

Calcined calcium sulphate

hemihydrate (-hemihydrate)

Autoclaved calcium sulfate hemihydrate

(-hemihydrate)

Calcium sulphate hemihydrate

(sometimes called densite; similar to

autoclaved material (CaSO4)2 H2O

Calcium sulfate hemihydrate CaSO4. ½ H2O

Heat at < 2000C Heat at > 2000C

Hexagonal calcium sulfate; soluble anhydrite

Orthorhombic calcium sulfate, insoluble anhydrite

Anhydrous calcium sulfate : CaSO4

Dental gypsum products are available in 5 forms (ADA Types I to V)

Type I – Impression plaster

Type II – Model plaster

Type III – Dental stone (Class I stone or Hydrocal)

Type IV – High-strength dental stone (class II stone, densite, or

improved stone)

Type V – High strength, high expansion stone.

The die stone is relatively *Inexpensive *Easy to use *Compatible with all impression materials. Disadvantage * Susceptibility to abrasion during the carving of the wax pattern.

ADVANTAGES

DIE STONE

* Gypsum hardeners, such as aqueous colloidal silica or soluble resin restorations, can be used instead of water during mixing of the stone. * Resistance to abrasion is increased by approximately 100%.

* Disadvantage is the slightly increased setting expansion.

Variations in Die techniques and materials :

* Several means are used to increase the resistance to abrasion.

In this the die material and the investing medium have a comparable composition. A commercial gypsum – bonded material called divestment is mixed with a colloidal silica liquid.

The die is made from this mix and the wax pattern is constructed on it.

Die stone – investment combination

Divestment is a gypsum-bonded material, it is not

recommended for high-fusing alloys.

“Detail reproduction, contact angle and die hardness

of elastomeric impression and gypsum die material

combinations.”

This study compared the surface detail

parameters, interfacial contact angle and die hardness

for some combinations of polyvinyl siloxanes,

polyether, polysulfide and reversible hydrocolloid

impression materials and type IV, Type V and resin

reinforced – type IV stone.

1. Adequate detail reproduction of prepared teeth may be

achieved with the combination of any of the elastomeric

impression materials with any of the die stone materials

tested.

2. When using polyvinyl siloxane or polyether impression

material, type IV-resin reinforced die stone may produce dies

that are less scratch resistant on the surface than the type IV

die stone used in this study.

3. No overall combination of die and impression materials was

identified as superior to another for all of the surface

properties studied.

Electroplated Dies / Electroformed Dies :

Used to overcome the poor abrasion resistance

of gypsum.

Advantages

High strength,

Hardness

Abrasion resistance.

The time required to produce a cohesive film

of metal typically 8 hrs

ELECTROLYTIC CELL

1. Because of their low energies, silicone

impression materials are difficult to

electroplate evenly.

2. Polyether impressions, because of their

hydrophilic nature, imbibe water and

become distorted.

3. Polysulfide polymers can be silver plated,

but it is more difficult to copper plate them.

The

The first step in the procedure is to treat the surface of the impression material so that it conducts electricity.

This process is referred to as METALLIZING. In this process, a thin layer of metal, such as silver is deposited on the surface of the impression material.

Metallizing agents are :

1. Bronzing powder suspended in almond oil

2. Aqueous suspensions of silver powder

3. Powdered graphite

Requirements for electroplating :

1. The impression to be coated is made the cathode.

Anode is the metal to be deposited either silver or copper Anode and cathode holder.Electrolyte is the solution through which the electric current is passed. The ions are deposited from the anode to the cathode.

COMPOSITION OF ELECTROPLATING BATH :

Copper :

Copper sulfate (crystals) – 200g

Sulfuric acid (concentrated) – 30 ml

Phenosulfonic acid – 2 ml

Water (distilled) – 1000 ml

Silver :

Silver cyanide – 36g

Potassium cyanide – 60 g

Potassium carbonate – 45g

Water (distilled) – 1000ml

5. AMMETER :- The current passed is of

10ma / tooth area for 12 hrs. It should

not exceed 50ma.

6. Plating tank – glass or hard rubber with

well fitting cover to prevent

evaporation.

PROCEDURE :

a) Copper plating :

* The surface of the impression is rendered conductive by coating it with fine particles of copper or graphite.

* The coated impression is made the cathode (negative electrode) of a plating bath, with an anode (positive electrode) of copper.

* The electrolyte is an acid solution of copper sulfate (about 250g/l.

* A current is passed, causing slow dissolution of the anode and movement of copper ions from anode to cathode, so plating the impression.

5. Dental stone is then cast into the plated impression.

6. The technique is often not considered suitable for the elastomeric materials.

b) Silver plating :

Polysulfide and silicone impression materials can be silver plated by the same general technique except

1) The impression is coated with silver or graphite powder.

2) The anode is silver.

3) The electrolyte is an alkaline solution of silver cyanide

Problems in metal-forming : Due to

Faulty conduction

Exhausted solution

Over concentrated solution

Metal anode too small

Friable metal deposit

POLYMERIC MATERIALS :

Auto-polymerising acrylic : Although self-cure acrylic polymers are often recommended for use as die materials,

Disadvantages.

The monomer reacts with all except silicone impression materials.

The heat of reaction distorts thermoplastic materials.

There is a large percentage of monomer and the resultant volumetric contraction makes the material unsuitable as a die material.

FILLED POLYMERIC MATERIALS (EG. EPOXY RESINS, POLYESTERS AND EPIMINES)

Presentation : Some combination of liquids, pastes and powders.

Recently fast-setting epoxy materials have been supplied in automixing systems similar to those for automixing addition silicones.

Common brands : Diemet; goldex; impredur; polyroqq

PROPERTIES :

1)Set within an hour to become rigid, abrasion-resistant solids.

2)Show some shrinkage on polymerization .

3)Epoxy resins react with polysulfide impression materials.

4)Water retards the polymerization of resin,.

Disadvantage:

Advantages : Rapid Set More abrasion resistant Not as brittle as die stones

Shrinkage on polymerization may be a source of inaccuracy – fillers reduce this shrinkage.

DIE MATERIALS

Advantages Disadvantages

Recommended use

Precautions

ADA type IV stone

1)Dimensional accuracy

2)Straight forward technique

3)Low cost

4)Straight forward-in-office procedure

1)Will be damaged if not handled carefully

2)Lower abrasion resistance

Most situations 1)Accurate proportioning essential

2) Vacuum mix recommended

ADA type V stone

1) Straight forward technique

2) Low cost

3) Straight forward in-office procedure

4) Harder than type IV

1) Increased expansion

Most situations 1)Accurate proportioning essential

2)Vacuum mix recommended

Advantages Disadvantages Recommended

use

Precautions

Epoxy

resin

1) High strength

2) Good abrasion

resistance

1) Polymerization

shrinkage

2) Time consuming

complex procedure

Complete

ceramic crowns

Not

compatible

with

polysulfide

and

hydrocolloid

Electropl

ating

1) High strength

2) Good abrasion

resistance

1) Time consuming

2) Special

equipment needed

Complete

ceramic crowns

Silver uses

toxic

cyanide.

Incompatibl

e with many

impression

materials.

“Study of the physical properties of type IV gypsum, resin-containing and epoxy die materials .”

JPD 2000 Apr; 83 (4): 466-73.

All gypsum products expanded, whereas the epoxy resin material contracted during setting.

The epoxy resin exhibited much better detail reproduction, abrasion resistance and transverse strength than the gypsum materials.

A conventional type IV gypsum exhibited the highest surface hardness whereas the epoxy resin had the lowest value.

The resin-modified gypsum products were not significantly superior to the conventional type IV gypsum die materials.

Zinc silicophosphate cement : This die material consists of a combination of zinc phosphate and silicate cement

a) Powder – Zinc oxide is the principal constituent

MgO may be present upto 10%

Very small quantities of other oxides or metallic salts (eg. fluorides) may be present.

b) Liquid – An aqueous solution of phosphoric acid, containing about 30 to 40% water.

MANIPULATION :

a) Mixing by hand :

• Use a thick glass slab :

• Steel spatulas should not be used.

• The correct P/L ratio is important. A typical P/L ratio is 1.6g / 0.4ml

• mixing should be complete within 1 min.

The set material has a cored structure consisting of

• Unreacted glass particles.

• These are sheathed by a surface layer of siliceous gel and

• They are embedded in a largely amorphous gel, where Al phosphate is the essential binding agent.

Advantages : Harder than die stone May be used with all impression materialsDetail is good

* Shrinkage on setting, loss of water on standing.

* It is a rather brittle material and readily fractured in thin sections.

Disadvantage

CERAMIC DIE MATERIALS :

Two ceramic die materials are available :

a) To form the dies heating to over 10000C is necessary.

b) Fired at 6000C for 8 minutes to produce a hard strong die.

Common brands : Ceramite H &V, Ducera-Lay,

Properties: * Extremely abrasion – resistant; * Some shrinkage on firing.

Application : The production of dies for porcelain inlays, onlays and veneers.

Metal Spraying : Many alloys or metals can be melted and dispersed in fine droplets with an oxyacetylene or other flame.

These fine particles of molten metal or alloy can be sprayed on to many impression materials without burning.

1)Advantages : > A metal coated die can be obtained rapidly.> Accuracy is good.

Disadvantages :

* The alloy is rather soft; care is needed to prevent abrasion of the die* Special equipment is needed. * A face mask must be worn to prevent inhalation of the fine spray of metal.

AMALGAM :

Silver-tin or copper amalgam may be packed into rigid impression materials such as compound.

ADVANTAGES

Dimensional Accuracy is good.

DISADVANTAGES

There is a delay of some 10-12 hours before the die is sufficiently hard to be used..

FLEXIBLE DIE MATEIRALS :

These are similar to heavy-bodied silicone or

polyether impression materials

Application

Used to make provisional restorations or indirect

composite resin inlays on onlays chairside.

Advantages

More rapidly setting

Ease of removal of the provisional or inlay..

“Comparison of the surface detail reproduction of flexible die material systems”.

JPD 1998 Oct; Vol 80 No:4 pp: 485-9

This study compared the surface detail reproduction of 7 potential flexible die materials when used in combination with 7 elastomeric impression materials.

Impregum F die materials with Extrude light impression material produced better surface detail reproduction than the control dies..

“Marginal adaptation of indirect composite inlays fabricated on flexible dies.”

JPD 2000 Mar;Vol 83 No.: 3 pp: 306-13.

Material used were ;

1) Condensation silicone / poly vinyl siloxane

2) Wash viscosity PVS/medium or heavy viscosity PVS

3) Irreversible hydrocolloid / medium viscosity PVS

4) Wash viscosity PVS / polyether

5) Control system – wash viscosity PVS and type IV stone

die.

Composite inlays made on the

* CS/PVS, * IH/Medium viscosity PVS, * Wash viscosity PVS / PE * Wash viscosity PVS * Type IV stone die had statistically similar mean marginal openings that were 100 m.

Composite inlay made on (PVS/PVS) had mean gingival margin openings > 100 m that were significantly larger than all other systems tested.

Cast or die material Impression material

Gypsum product Compound Zinc oxide eugenol Agar or Alginate,Plaster, if coated with separator Polysulfide rubber base Silicone (all types) rubber base Polyether rubber base.

Electroplated copper Compound Silicone (all types) rubber base.

Electroplated silver Polysulfide rubber base Silicone (addition hydrophobic) rubber base Polyether rubber base.

Epoxy resin Silicone (all types) rubber base (some require sepator) Polysulfide rubber base, if coated with separator, Polyether rubber base.

Compatibility of cast or die materials with impression materials

“Compatibility of impressions and die stone material.”

Oper Dent 1990 May-Jun Vol 15 No. 3 Pp: 82-5.

The purpose of this pilot study is to test the compatibility of four light-bodied polysiloxane impression materials with 11 different die stones.

All 11 die stones and polysiloxane impression combinations performed within the 20m ADA standard.

DIE SYSTEMS :

Available methods of cast and die systems :

2 basic working cast and die system.

1. Working cast with separate die.

2. Working cast with removable die.

FIRST METHOD : WORKING CAST WITH SEPARATE DIE :

(Multiple – Pour Technique)

The working cast with a separate die is the simplest means of fabricating a working cast and die.

Advantages of working cast with a separate die :

• It is a simple technique and ease of fabrication.

• It is slightly more accurate than removable die system.

• It keeps the relationship between the abutments fixed and immovable.

• This is a sure method of accurately orienting the preparation model to each other.

•Because the gingival tissue and other landmarks are intact, it is easier to obtain physiologically harmonious restoration contours when fabricating the wax pattern.

•No requirement of special equipment.

•The use of solid master cast precludes error caused by incomplete seating of a removable die.

Disadvantages of this system :

• The wax pattern must be transferred from one to another cast. In the process there may be destroy some of the internal adaptation of the wax pattern.

• The working cast and the sectional cast for the die can be obtained from by pouring an elastomeric full-arch impression twice or more.

•If a double pour is used, the first cast is usually more accurate than the second is (larger than the first).

•It may be difficult to transfer complex or fragile wax pattern from cast to die.

•This technique can be used only with elastomeric impressions.

IMPRESSION POURING :

High-strength type IV (class II, “Densite”) or high-strength, high-expansion type V stones should be used for fabricating the die.

An impression should be washed under cold running tap water, to remove mucous and saliva that may cover it, before disinfecting it in an appropriate solution.

Stone is added to the impression in small increments above the preparation

Tray is tilted to fill the preparation

Stone is added to the impression so that the base of set stone will be 1 inch thick

DIE PREPARATION :

Carefully separate the poured cast from the impression.

A material such as Super-sep may pained on the surface of the prepared teeth on the cast to guard against surface erosion or etching when the casts are trimmed.

Liquid, prevulcanized latex has also been suggested for this purpose.

Trim the cast from which the die is made on a model trimmer to removal all excess stone around the prepared tooth. The handle of the die should be slightly larger in diameter than the preparation and octagonal in cross section.

Its sides ought to be parallel or slightly tapered toward the base.

The die is first trimmed on a model trimmer

A properly trimmed die handle is slightly larger in diameter than the preparation

Improperly trimmed die with a handle that meets the preparation at an angle

Use a pear-shaped acrylic bur to trim the die “apical” to the finish line of the preparation.

The area “apical” to the finish line should be smoothed and made free of ridges with the discoid end of a Tanner carver.

The handle should be 1 inch long The die trimmed with an acrylic bur

Shaping of the die handle near the finish line is completed with a scalpel

After the die has been trimmed, the finish line should be highlighted with a sharp colorbrite red pencil.

A black, graphite pencil should not be used for this purpose.

Relief should be applied to the preparation area of the die to provide space for cement.

Enamels and lacquers have been used for this purpose.

The die is smoothed below the finish line with the discoid end of a tanner carver

The preparation finish line on the die should be outlined with a red pencil

Die relief agent is painted on the preparation portion of he die

A die hardening agent (cyanoacrylate or

acrylic resin lacquer) can be applied to the finish

line area of a die to prevent abrasion by waxing

instruments during the fabrication of the wax

pattern.

The thickness of cyanoacrylates at the finish

line can range from 1.0 to 2.5 m of thickness.

.

SECOND METHOD WORKING CAST WITH A REMOVABLE DIE

Dies that can be removed from the working cast have become very popular.

They are convenient to use

If removable die system is used, it should satisfy the following requirements.

1. The dies must return to their exact original position.

2. The dies must remain stable, even when inverted.

Advantages of the removable die system :

1) Convenient to use

2) A removable die eliminates discrepancies between a separate die and working cast.

3) A removable die also eliminates discrepancies.

4) No special equipment may be required.

The cast containing the dies must be easy to mount on an articulator

Disadvantages of the removable die system :

1. Risk of introducing an error in the pattern if the die does not reseat accurately in the working cast.

2. Interproximal margins can easily be damaged during the sawing procedure.

STRAIGHT DOWEL PIN :

This means of orienting dies has been in use for a number of years.

The brass dowel pin is one of the most accurate dowel types in terms of resisting horizontal displacement

And the second lowest in vertical deviation of four types of removable dies.

A dowel pin is positioned over each prepared tooth in the impression.

Dowel pins are positioned over the impression with bobby pins

Paper clips are added to nonremovable parts of the unset first pour to provide retention

The stone around the dowel pins is lubricated

A wet paper towel can be used to fill in the open, center portion of the impression

Wax at the ends of the dowel pins is located and removed

Dies are separated from the rest of the cast with a fine saw

Ends of the dowel pins are tapped to loosen the dies from the cast

Base of the die is trimmed with an acrylic bur

Dies are reseated into the cast Mounted casts

CURVED DOWEL PIN :

Curved dowels can be incorporated

into a working cast by fixing the dowels to

the impression before it is poured or by

cementing the dowels into holes drilled in a

previously poured cast.

To install pins before pouring the

impression, use finger pressure to insert a

curved dowel, tip, first, into the large opening

in a positioning bar.

With that bar oriented faciolingually, hold

the assembly so that the head of the dowel

extends 1.0 to 2.0 mm into the impression of

the prepared tooth.

Depressions are made about 2.0 mm deep on either side of the dowels

Thin coat of petrolatum is applied to the stone and dowels

Boxing wax is placed around the impression, with the tips of the dowels sticking through

The completed cast is sawed A segment is removed by pressing on the exposed tip of its curved dowel

The working cast is trimmed to receive cemented dowels

Holes are drilled into the underside of the cast

The cast is keyed with a large acrylic bur

A drop of cyanoacrylate cement is placed into each of the drilled holes

The head of a curved dowel is seated into the cement lined hole

A curved dowel is cemented into each removable part of the working cast

PINDEX SYSTEM :

In the pindex system a reverse drill press is

used to create a master cast with dies that can be

removed and replaced repeatedly with great

precision.

The impression is poured without

positioning and attaching dowel pins beforehand.

The machine accurately drills parallel holes

from the underside of a trimmed cast.

Parts of the pindex machine

Sufficient stone to allow for trimming is added

The bottom of the cast is trimmed on the model trimmer

Cast should sit perfectly flat on a tabletop

its thickness form base to preparation finish line must be a minimum of 15 mm.

If the bottom of the cast is flat, it insures that the pin holes drilled into it will be parallel.

There should be two pins for each die, two for each pontic (edentulous) area, and two pins in each terminal segment containing unprepared teeth.

Use the switch on the side of the machine to turn it on. A red pilot light will indicate that it is running. Place the prepared cast on the worktable and align the first pencil mark with the illuminated dot from the light beam director.

The cast should be 15 mm thick, exclusive of the teeth

The periphery of the cast is trimmed on the model trimmer

The palate / tongue area is trimmed with an arbor band

Location of the pin holes is marked with a pencil

Pencil marks are placed under the illuminated dot

The thumbs are used to stabilize the cast while lifting the handle bar with fingers

Debris is removed from the pin holes

The pin holes are refined with a hand reamer

Cyanoacrylate cement is placed on the pins prior to cementing the pin tips

Shorter pins placed before the long pins

White sleeves are placed on the long pins and gray sleeves on the short pins

The bottom of the cast is lightly coated with petrolatum

The ends of the gray sleeves are blocked with wax

Utility wax is placed on the ends of the long pins

A palatal / tongue filler is made of boxing wax

The filler is placed on the cast The filler is seated to the cast

Boxing wax is applied around the cast The base is poured in microstone

The base former is filled with microstone

Stone is vibrated around the bases of the pins

The cast is seated in the base former Wax over the long pins is removed

The saw cuts are premarked with a pencil

The cast is removed by tapping the pins with an instrument handle

Dies are sectioned from the underside Dies may also be sectioned from the occlusal aspect of the cast

Large condenser can also be used to loosen the dies

The completed cast on the articulator

After the dies are sectioned trim them in

the conventional manner.

Mark the finish lines with a red pencil.

Apply die hardener and die spacer

according to the manufacturers instruction.

Before mounting the cast on the

articulator, evaluate the height of the base.

DIE-LOCKING SYSTEM :

Die Lok System :

This die-relating system has two main components :

A full-arch tray, which is useful for indexing multiple preparations or preparations on both sides of the dental arch,

And a universal quadrant tray, which can be used for isolated or adjacent preparations, such as an inlay of onlay.

The full-arch tray consists of three components : A grooved base : An internally indexed outer arm ; And a posterior locking bar.

The quadrant tray, on the other hand, consists of two components :

A rectangularly indexed compartment with one wall that is made of a single indexed locking arm that can be opened and closed on a hinge.

This locking arm securely holds the individual dies in place and allows for easy access to the die sections.

ACCUTRACK :

This system is similar in design to the full-arch Die-Lok tray with several design modification

APPLICATION

indexing single or multiple die preparations.

The system contains only a full-arch tray that has three main components :

a blue flexible rubber base, which confines the dental stone :

And two independent, indexed, locking arms that securely contain the individual dies for easy access to them.

The cast should be poured in a U shape The lingual side of the cast base is trimmed with an arbor band

Horizontal grooves are cut in the base to give it retention

The cast is placed into stone in the tray Disassembly of the cast

The cast is jarred loose from the tray base by tapping on the front of the base

A saw cut is made on each side of the prepared tooth

The prepared tooth is broken free from the cast by hand

The die is trimmed apical to the finish line with an acrylic bur

The cast and dies are reassembled in the tray

The cast and tray mounted on the articulator

Advantages and disadvantages of die-locking systems


•Extremely accurate die-relative system •The trays are reusable and cost of the system is minimal •Sectional-arch trays are particularly useful for inlay/onlay type of restorations

•Many time-consuming steps are required •The dental arch circumference does not always conform to the universal die-locking trays; in these cases other die-indexing techniques should be used.


•No auxillary pins or dowels are needed •The impression need only be poured once •Magnetic indexing on the Accutrack tray provides for easy and clean mounting •The removable dies have no movement in any direction

* Much cast trimming is necessary to shape the stone to conform to the tray in the Die-Lok technique: the thin stone cast may fracture during this process. •The indexing grooves in the Acctrack system are small and the die stone can easily fracture in these places during die removal.

Dilts W.E. ; Podshadley A.G et al (1971) evaluated removable die systems to determine any vertical shift of the dies from their original position in the casts and any relative horizontal shift after the dies had been repositioned 30 times. Evaluated four removable die systems were :

• Brass dowel pin

• Precision stainless steel dowel pin

• Stainless steel pin dowel

• Di-lok quadrant tray

The result showed an increasing mean vertical deviation of the dies in the following order:

> Quadrant trays, brass dowel pin, precision stainless steel and stainless steel pin dowel.The mean horizontal deviation of the dies increased in this order.

> Brass dowel pin, precision stainless steel dowel pins, stainless steel pindowls and quadrant tray.

Myers M. ; Hembree J.H. (1982) studied the relative

accuracy of four die techniques :

The brass dowel pin

The plastipin

J-pin and

Logix model system.

They found that vertical and horizontal shift revealed no

statistical significance among various dies.

Covo L.M., Ziebert G.J. et al (1988) studied the accuracy of three removable die systems :

• The Accu-trac system ;

• The pindex system ; and

• The conventional brass dowel pin positioning technique.

The result showed no significant difference in the shift of the removable die in a horizontal direction between any of the three systems.

There was no significant difference in the displacement of the removable die in the vertical direction with the Pindex system and the conventional dowel pin technique

Richardson D.W. ; Sanchez R.A. et al (1991) evaluate the positional accuracy of dies with four die tray systems :

- Accu-trac precision die system ;

- Model-tray system ;

- Sterdo split model tray ;

- Tricodent one cast die tray system.

Result showed that all four die tray systems were not significantly different in most measurement categories

Serrano J.G. ; Lepe X. et al (1998) evaluated the 3-dimensional accuracy of 4 removable die systems

• Belle de st. Claire ;

• Pindex ;

• DVA ;

• Conventional brass dowel system.

All 4 systems tested use either 1 or 2 pins as the repositioning device.

They concluded that all 4 die systems provided a similar degree of dimensional accuracy, although pindex showed the least horizontal movement and the brass dowel system produced the least occlusogingival reseating discrepancy.

The ease with which a restoration is fabricated and the accuracy with which it will fit the mouth is materially affected by the casts and dies.

So a die material should be selected that has

Good dimensional accuracy,

Abrasion resistance and

Ability to reproduce fine detail and sharp margins.

CONCLUSION

One of the aim of restorative dentistry is to restore the teeth to an anatomical form that will harmonize with the adjacent teeth and those of the opposing arch.

In order that these requirements may be met in restoration constructed by the indirect techniques, dies must occupy position in a model which reproduce those of the prepared teeth in the dental arch.

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

5) “Comparative evaluation of the influence of cast hardening agents on surface abrasion, surface hardness & surface detail reproduction properties of refractory investment material”

Feb 1998 6) Craig R.G. & Powers J.M.- “Restorative dental materials” 11th edition 2001 7) O’Brien W.J.- “Dental materials & their selection” 3rd edition 20028) Phillips R.W. – “Skinners science of dental materials” 9th edition 19929) Takahashi J.,Okazaki M. – “Non uniform Vertical &

horizontal setting expansion of a phosphate bonded investment” JPD 1999;81: 386-391.