INVESTMENT MATERIALS AND

INVESTING TECHNIQUES

Shubha R Joshi

Under guidance of Dr K KumarswamyDr Shalini Dr PraveenDr Dheeraj Dr Srinivasullu Dr. Chandrashekar

Overview• Introduction• History • Review of literature• Types of Investment Materials• Requirements of investments• Composition• Gypsum-bonded Investments• Phosphate-bonded Investments• Ethyl silicate-bonded Investments• Investments for casting titanium based alloys• Investing techniques

• Dental casting investment: A material consisting primarily of an allotrope of silica and a bonding agent. The bonding substance may be gypsum (for use in lower casting temperatures) or phosphates and silica (for use in higher casting temperatures)

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

History

There is a long and rich history associated with investment casting, dating back thousands of years to the production of bronze, copper and gold jewellery, idols and statues as far back as the ancient Egypt and Mesopotamia, the Han Dynasty in China, the Aztecs in Mexico and the Benin civilization in Africa. The earliest known text describing the investment casting process was written by the monk Theophilius Presbyter around 1100 A.D. His writings were used by sculptor and goldsmith Benvenuto Cellini (1500 - 1571), as described in his autobiography, for the production of the Perseus and Head of Medusa sculpture that still stands today in Loggia dei Lanzi in Florence, Italy. In fact, by the mid 1500's the investment casting technique was considered "ordinary procedure" for making bronze statues and other works of art.

The technique re emerged in the late 19th century when dentists began using the technique to make crowns and inlays, following the publication of a paper by Dr. D. Philbrook of Council Bluffs, Iowa in 1897. It was Dr. William H. Taggers of Chicago, however, who spearheaded the use and growth of investment casting as a modern industrial process, following publication of a paper in 1907 that detailed the development of a technique that utilized a wax pattern compound of excellent properties, the development of an investment material and the invention of an air-pressure casting machine.

Review of literature

• Many have worked on dental casting technique• In 1890-Dr Swasey introduced technique for solid

gold inlay • In 1891-Dr Martin used wax for making gold inlay• In 1896-Dr Phil brook introduced pressure casting

technique• In 1907- Dr Taggart presented paper on technique

and material but his castings were small and did not fit the cavities

• In 1910- Dr Van horn promoted idea of thermally expanding wax pattern

• In 1929- Dr Coleman & Weinstein developed cristoballite investments

• In 1936- Dr Schew introduced hygroscopic technique• In 1950- Whip mix corporation developed phosphate

bonded investments• Dr Abrahim weinstein first produced commercially

successful dental gold alloy and porcelain composite • Later improvements in alloy was also carried on • In 1976-Joseph Tuccillo- platinum was eliminated ,gold

was reduced & palladium was increased• In 1978-Paul Cascone-alloy did not contain silver and

platinum• In 1981-Boyajian developed palladium based alloy

containing no gold,platinum or silver & added 5% cobalt & later upto 7.5% but palladium alloy containing more than 7.5% cobalt became eletromagnetic

• In 1985-Cascone-palladium based alloy with 4% cobalt

Types of investment materials available for casting alloys

• GYPSUM-BONDED INVSTMENTS

• PHOSPHATE BONDED INVESTMENTS

• SILICA BONDED INVESTMENTS

• NEWER INVESTMENT MATERIALS FOR TITANIUM AND TITANIUM BASED ALLOYS

Requirements of investments for alloy casting procedures

1.The investment material should be capable of reproducing the shape, size and detail recorded in the wax pattern.

2.The investment should be easily manipulated . Not only should it be possible to mix and manipulate the mass readily and paint the wax pattern easily, but the investment also should harden in a relatively short time.

3.The investment should be able to maintain the integrity at higher temperatures (as the casting is carried out in higher temperatures often as higher than 1000 ºc)

4.On being heated to higher temperatures the investment should not decompose to give off gases that would damage the surface of the alloy.

5.The investment should have a sufficiently high value of compressive strength at the casting temperature so that it can withstand the stresses set up when the molten metal enters the mould

6.The investment material should expand to compensate for the casting shrinkage

7. Investment should be porous enough to permit the air or other gases in the mold cavity to escape easily during the casting

8. Investment should produce a smooth surface and fine detail and margins on the casting.

9. After the casting is complete the investment should break away readily from the surface of the metal and should not have reacted chemically with it.

10.The investment material should be inexpensive.

COMPOSITION

Investment materials consist of a mixture of a

1. REFRACTORY MATERIAL

2. BINDER

3. MODIFIERS

REFRACTORY MATERIAL SILICA (silicon dioxide) is used

as refractory material. It is available in four allotropic forms such as• Quartz • Tridymite • Cristobalite• Fused quartz Quartz and Cristobalite are used

extensively in dental investments.

SiO2

SiO2

1.5%

1.2%

Quartz is a common mineral . Cristobalite occurs naturally as a rare mineral but is normally manufactured by prolonged heating of the quartz at high temperatures to induce the appropriate slow inversion.

Each form of silica exists in two phases.

1. Low temperature phase or alpha phase

2. High temperature phase or Beta phase

High temperature phase is less dense than that of the Low temperature phase

On heating the change between the two phases is rapid and readily reversible on cooling .this change is known as high – low inversion .

• When heated a change in the crystalline form occurs at the transition temperature characteristic of the particular form of silica

Quartz when heated inverts from alpha phase to beta phase at a temperature of 575 ºc

Cristobalite when heated inverts from alpha phase to beta phase at a temperature of 200 to 270 ºc

The beta allotropic forms are stable above the transition temperature and an inversion to the lower or alpha form occurs on cooling. In powdered form the inversion occurs over a range of temperature instantaneously.

• The density decreases when the alpha form changes to beta form with a resulting increase in the new volume. The increase in the volume (or isothermal expansion) is probably due to straightening of the chemical bonds to form a less dense crystalline structure as illustrated in the figure

• The graph shows that the over all thermal expansion and inversion expansion of materials containing cristobalite (A )is greater than that of quartz (B).

The isothermal expansion for the

Cristobalite is 1.3% at 250 ºc

Quartz is 0.6% at 573 ºc

Depending on type of silica used the investment materials are classified as

1. Quartz investments

2. Cristobalite investments

FUNCTIONS

1. It is added to provide refractory during heating which is capable of withstanding very high temperatures during casting without degradation .

2. It regulates thermal expansion.

Binder

It binds the refractory materials together. The nature of the binder characterizes the material

Ex:

• Alpha calcium hemi hydrate for casting gold alloys

• Sodium silicate, ethyl silicate, ammonium sulphate , sodium phosphate for casting cobalt chromium alloys

MODIFIERS• Usually a mixture of refractory materials

and binder is not sufficient to produce all the properties of the investment materials

• Other chemicals such as sodium chloride, boric acid, graphite, copper powder, are often added in small quantities to modify physical properties,.

• The reducing agents are used in some investments to provide a non oxidizing atmosphere in the mold when the gold alloy is cast.

• boric acid and sodium chloride, not only regulate the setting expansion and the setting time, but also prevent most of the shrinkage of gypsum when it is heated above 300 ºC (572° F)

Gypsum bonded investments

• They are the mold materials used in the casting of dental gold alloys with liquidus temperatures no more than 1080 ºc

ADA SPECIFICATION NO2 for casting investments for dental gold alloys encompasses three types of investments .

TYPE1 : [THERMAL EXPANSION TYPE ] employed in casting inlays and crowns

TYPE 2 : [HYGROSCOPIC EXPANSION] type employed in casting inlays and crowns

TYPE 3: for casting complete and partial denture bases

Composition Refractory• Crystalline polymorphs of silica (quartz or cristobalite)• 55-75% Binder• Calcium sulfate hemihydrate (plaster or stone)• 25-45%• In setting, hemihydrate binder combines with water to form

dihydrate(gypsum) Modifiers• Accelerators, retarders, reducing agents or additives that

reduce the thermal contraction of the binder. Coloring agents.

silica

• Silica is added to provide a refractory component during the heating of the investment and to regulate the thermal expansion.

• gypsum shrinks considerably when it is heated, regardless of whether it is set plaster or stone. If the proper forms of silica are employed in the investment, this contraction during heating can be eliminated and changed to an expansion.

The effect of silica content on setting expansion in air (A), in water (B) and thermal expansion(C)

• Setting expansion is increased when interlocking of growing gypsum crystals is inhibited by refractory particles because the crystal growth is directed outward

• Thermal expansion is increased due to summing of

Binder contraction + refractory expansion

• BINDER – Alpha hemi hydrate form of gypsum(30-35%)

it is used as binder for investments used in casting gold containing alloys with melting ranges below 1000 ºc

When this material is heated to the temperature required for complete dehydration and sufficiently high to ensure complete castings, it shrinks considerably and frequently fractures

All form s of gypsum shrink considerably after dehydration between 200- 400 ºc (due to loss of water of crystallization) a slight expansion occurs between 400 ºc and approximately 700 ºc, and then a larger contraction occurs (due to densification by sintering)

This later shrinkage is most likely due to decomposition and release of sulphur gases such as sulphur dioxide.

They not only cause shrinkage but also contaminates the castings with the sulphides of the non noble alloying metals such as silver and copper.

Thus, it is imperative not to heat the gypsum products above 700 ºc. For the gypsum products containing carbon the maximum temperature should be 650 ºc.

MODIFIER - (4-7%)

Used are Reducing agents

Modifying chemicals Coloring matter

Reducing agents : they reduce any metal oxides formed on the metal by providing a non oxidizing atmosphere in the mold when the alloy enters mold.

• Ex– Copper

Modifying chemicals: They regulate setting expansion and thermal expansion and also prevent shrinkage of gypsum when heated above 300 ºc .

• They act by reducing the two large contractions of gypsum binder on heating to temperatures above 300 ºc .

• Ex– Boric acid

Soluble salts of alkali or alkaline earth metals

• Boric acid: • when heated above 150 ºc forms a viscous liquid

which impedes evaporation of last traces of water , delaying the gamma to beta transformation of calcium sulphate.

• This viscous phase also reduces the high temperature contraction that results from sintering because it stabilizes the original contact formed between gypsum crystals and silica during setting .

• Investments containing this boric acid when heated to 670-700 ºc shows increases its compressive strength ranging from 40-50%.

Salts of alkali and alkaline earth metals : ex- sodium chloride

• Reduces first major shrinkage and eliminates second shrinkage of gypsum on heating

• The effect of halide ion is nullified above

650 ºc and rapid contraction occurs (probably the result of accelerated sintering )

• A marked strength decreased on heating to

700 ºc.

• The large high temperature shrinkage of the binder is not observed in gypsum bonded investments containing these modifiers because at a concentration of 50% or more of silica , the silica particles in the set investment form a continuous skeleton that resists over all shrinkage .

• SETTING TIME

• According to ANSI/ADA Specification No. 2 for dental inlay casting investment, the setting time should not be shorter than 5 min or longer than 25 min. Usually, the modern inlay investments set initially in 9 to 18 min.

Types of Expansion:Inlay investments have total expansion in the range of

1.5 – 2.5% .

Purpose: to enlarge the mold to compensate for the casting shrinkage of the gold alloy

• (1) Normal setting expansion: occurs as investment hardens in air

• (2) Hygroscopic setting expansion: occurs as investment hardens while immersed in water

• (3) Thermal expansion: occurs as investment is heated

Normal Setting Expansion:

• Mechanism: silica particles interfere with the interlocking of crystals; the outward thrust of the crystals increases the expansion of investment.

• ADA sp no 2 for type 1 investment permits a maximum setting expansion in air of 0.6% setting expansion of modern investments is 0.4%which can be regulated by accelerators and retarders .

Hygroscopic setting expansion:

• This is one of the methods for expanding the casting mold to compensate for casting shrinkage

• When the gypsum product is allowed to set under or in contact with water and the amount of expansion exhibited is much greater than normal setting expansion

• Expansion range 1.2-2.2%

• The hygroscopic setting expansion may be 6 or more times greater than the normal setting expansion of a dental investment

• The increased amount of expansion is because the water helps the outward growth of crystals

• The investment should be immersed in water before the initial set is complete.

• ADA sp no 2 for such type 2 investments require minimum setting expansion in water of 1.2% and maximum 2.2%.

• The hygroscopic setting expansion is a continuation of ordinary setting expansion because the immersion in water replaces water of hydration and thus prevents the confinement of growing crystals by surface tension of the excess water. Because the diluent effect of the quartz particle, the hygroscopic setting expansion in these investments is greater than that of gypsum binder when used alone

• This phenomenon is purely physical .• The water is drawn between the refractory particles

by the capillary action and thus causes the particles to separate creating an expansion

Factors that increase hygroscopic expansion:

• Composition: more silica, finer particles lead to more outward growth of crystals

• W:P ratio: less water, more powder in mix• Spatulation: more mixing time• Time of immersion: immerse in water before initial

set• Confinement: less opposing force from walls of

casting ring (wetcellulose)• Water: more immersion water• Shelf life: fresher investment

Thermal expansion

• The thermal expansion is directly related to the amount and type of silica present.

• The desirable amount of thermal expansion depends on whether thermal expansion will compensate the casting shrinkage or it will be compensated by hygroscopic setting expansion

• Type 1 investments should have thermal expansion of not less than1% and not greater than 1.6%.

• If hygroscopic setting expansion is used the thermal expansion of 0.5- 0.6% is sufficient .

• The maximum thermal expansion should be achieved at a temperature not greater than 700 ºc as the a breakdown of calcium sulphate binder occurs in presence of carbon ( present as graphite added to the investment as reducing agent or a residue from the burn out the wax pattern) liberating sulphur dioxide

• At high temp, sulfur dioxide gas is released causing discoloration and embrittlement of alloy.

Factors affecting the thermal expansion :1.Effect of water powder ratio:• more powder, less water result in increased thermal

expansion2.Effect of chemical modifiers:• The addition of small amounts of sodium potassium

or lithium chlorides to the investments eliminates the contraction caused by gypsum and increases the expansion without the presence of excessive amounts of silica .

• Silca do not prevent gypsum shrinkage but counter balance it where as chlorides reduce gypsum shrinkage

• Compressive strength: minimum strength is necessary to prevent fracture of investment from the impact of metal entering the mold---more powder, less water increase investment strength.

• According to ADA sp no 2 the compressive strength should not be less than 2.5MPA

Fineness

Fineness affects• Surface roughness of the casting• Hygroscopic expansion

Finer silica is preferrable

Porosity • During the casting process, the molten metal is forced

into the mold under pressure . As the molten metal enters the air must be forced out ahead of it. If the air is not completely eliminated a back pressure builds up to prevent the gold alloy from completely filling the mold.

• Common method for venting the mold is through the pores of the investment.

• More gypsum crystals ->less is the porosity• Lower the hemi hydrate -> greater the amount of

water used to mix the investment ->more the porous is the investment

• Uniform particle size ->greater is its porosity

Storage • Should be stored in air tight and moisture proof

containers• Should be purchased in small quantities• as the investment materials are composed of different

ingredients each of which posses a different specific gravity , these components settle , under a normal vibration that occurs in dental laboratory.

• This separation influences on the setting time and other properties of the investment

• For this reason and as well as to avoid accidental moisture contamination the investment should be purchased in small quantities

Divestment• It is a gypsum bonded material mixed with colloidal

silica• Setting expansion is 0.9%• Thermal expansion is 0.6% when it is heated to 677 ºc• As it is a gypsum bonded material it is not

recommended for high fusing alloys.• Divestment phosphate is a phosphate bonded

investment used as a divestment for fusing alloys.

Rapid heat investments

• some "rapid-heat" invest ments have been introduced, which are placed immedi ately after setting into a furnace preheated to 700°C. The recommended technique is to place the mold, 30 minutes after the pattern is invested, into the pre heated furnace for an additional 30 minutes; the casting is then made.

Setting and thermal expansion

• Measurements on a mold in a lined inlay ring showed that the periphery of the investment mass reached 250°C within 6 minutes of entering the hot furnace, while the center was at only 110°C. The center of the mold did not reach 250°C until 4 minutes later. Both periphery and center had reached a maxi mum of 690°C within the 30-minute heating period.

Advantages

• They save the laboratory time as the furnace is maintained at 700 ºc instead of being repeatedly heated and cooled

• The investment total expansion under these conditions was 1.95% , more than enough to compensate casting shrinkage of ordinary dental alloys

Most palladium and base metal alloys used for partial dentures and porcelain fused to metal restorations have high melting temperatures. They should be cast at a mold temperature higher than 700 ºc.

To withstand these high temperatures molds require different types of binders such as

silicate and

phosphate compounds

INVESTMENT MATERIALS AND

INVESTING TECHNIQUES

Shubha R Joshi

Phosphate bonded investments

• They are used in construction of high melting

temperature dental alloys .• Soldering and porcelain veneering • To make soldering fixtures that hold prosthetic

components in alignment while they are being joined with solders brazing alloys or welding alloys

Classification

Type 1 • For casting of inlays crowns and other

restorations especially for alloys like gold, platinum ,palladium cobalt chromium and nickel chromium

Type 2 • For casting of removable partial dentures

• Composition Binder• Magnesium-oxide (basic) and phosphate (acid,

mono-ammonium) Refractory• Colloidal silica liquid. Increases expansion and

enhances casting surface smoothness Modifiers• Carbon: to produce clean castings and facilitate the

devesting. Don’t use with palladium-containing alloys because carbon embrittles alloy)

CompositionRefractory materials – (concentration of

approximately 80%)silica in quartz , cristobalite or a mixture of two .

Purpose• To provide high temperature thermal shock

resistance • High thermal expansion • To control thermal stresses related to thermal phase

transformation of cristobalite and along with glasses and other metal oxides to provide bulk and help to control the surface finishing of casting

Binder (<20%)• Magnesium oxide (acid) and a phosphate

(base)• Originally phosphoric acid was used but

mono ammonium phosphate has replaced it as it can be incorporated in powder form

• Mono ammonium phosphate which in reaction with water in the presence of calcined magnesium oxide powder provides for binding of particles at ambient temperatures

Modifiers • Carbon is often added .

• It produces clean casting.

• Facilitates easy divesting of casting and mold .

• Generally added when casting alloy is gold.

• When silver palladium or base metal alloys are invested with the investment containing carbon ,it embrittles the alloys even though the investment is heated to the temperature that burn out the carbon.

• The basic binding reactions is the same for all phosphate bonded investments, there are important differences in properties due to composition.

Those used for:

• Casting of high temperature alloys and

• Making dies used in fabrication of porcelain veneers

Contain quartz and cristobalite to achieve expansion to compensate shrinkage

Soldering investments do not require fine powders and are designed without high expansion fillers

It is to keep parts that are to be joined from shifting while they and the surrounding investment is heated to the joining temperature

Graphite is found in some of the investments to render them more permeable after burn out to provide a reducing atmosphere.

• It is available as two component systems • 1- It is a Powder which contains refractory materials

and binders and modifiers• 2- Aqueous solution stabilized with colloidal silica

Because the newer gold containing alloys and other alloys used for metal ceramic restorations have higher melting temperatures their contraction during solidification is also greater . Colloidal silica suspension facilitate greater expansion of the investment which can compensate the casting shrinkage.

Setting reactions

Setting and thermal expansion

• There is a slight expansion and this can be increased by using colloidal silica solution instead of water .

• Thermal expansion of phosphate bonded investments when mixed with water and compared with that of special liquid

• When phosphate investments were mixed with water they exhibited a shrinkage with in a range of 200 -400 ºc

• This contraction is eliminated when colloidal silica solution replaces water

• The early thermal shrinkage of phosphate investments is associated with a decomposition of binder magnesium ammonium phosphate and by evolution of ammonia .

• Expansion can be varied by the proportions of silica and water.

1.More silica and less water – more expansion .

2.Less silica and more water- less expansion.

Liquid can be used as full strength or diluted with water to provide some degree of control over setting or thermal expansion.

Properties• High temperature mold is achieved by

formation of complex silicophosphates( from the reaction of some of the silica with the excess of dihydrogen phosphate)

1 Compressive strength • Type 1- 2.5 mpa • Type 2- 3 mpa2 Thermal expansion• 0.8% when 50:50 mixture of liquid and water

Working and setting timeAffected by1.Temperature Warmer the mix faster it setsThe setting reaction liberate the heat and

accelerates rate of setting2.Mixing time increased mixing time and mixing

efficiency result in faster set and greater rise in temperature

• The more the efficient the the better the casting in smoothness and accuracy

• Mechanical mixing under vaccum is preferred

3.Liquid:powder ratio

• Increase in the liquid:powder ratio increases the setting time.

Advantages

1.They have high fired strength. This make them handle without breaking before they are placed in a furnace for the wax burn out process and strong enough to with stand the impact and the pressure of centrifugally cast molten alloy

2.They also provide high setting and thermal expansion enough to compensate cast metal prosthesis or porcelain veneers during cooling

3.They have ability to with stand the burn out process with temperatures that reach 900 ºc and also 1000 ºc for short period of time (for fabricating porcelain veneers or performing metal joining operations

Disadvantages

1. When used with higher melting alloys those with casting temperatures higher than 1375 ºc they result in mold breakdown and rougher surfaces on casting

2. Their higher strength although an advantage make divesting a difficult and tedious task

3. When higher expansion is required more of silica liquid is used with the result that more dense and less porous mold is produced this results in incomplete casting if a release for trapped gases is not provided

4.When the powder is supplied in bulk form rather than in sealed pre measured packages it can react over time with moisture in air and result in lower expansion during setting or loss of ability to set to a strong mass

ETHYL SILICATE BONDED INVESTMENTS

APPLICATIONS

They are used in construction of high fusing base metal partial denture alloys

COMPOSITIONRefractory material - Silica

Binder –Silica gel or ethyl silicate

Modifier – Magnesium oxide (strengthen the gel)

Ammonium chloride - accelerator

It is supplied as a powder and liquid or two liquids

If supplied as a powder and liquid • Powder consists of refractory

particles of silicas and glasses along with the calcined magnesium oxide and some other refractory oxides in minor amounts

• Liquid contains stabilized alcohol solution of silica gel

If supplied as 2 liquids• One is ethyl silicate Certain types of amines are added

for hydrolysis and gelation to occur simultaneously

• Other is acidified solution of denatured alcohol

SETTING REACTION

When binder silica gel is used

• Silicic acid gel is formed when sodium silicate is added to a acid or acid salt

• MgO added strengthen the gel

When ethyl silicate is used as a binder

• Colloidal silicic acid is first formed by hydrolyzing ethyl silicate in presence of Hcl, ethyl alcohol, and water.

• Si(OC2H5)4 + 4H2O _ Si(OH)4+4C2H5OH

• This silicic acid (sol) is mixed with the silica

• to which MgO is added to render the mixture alkaline

• A coherent gel of polysilicic acids is formed

• The soft gel is dried at a temperature below 168 c

• During the drying process the gel loses alcohol and water to form a concentrated ,hard gel .

• The volumetric contraction accompanies drying which reduces the size of the mold .

• This contraction is known as “GREEN SHRINKAGE” occurs in addition to setting shrinkage

So the mold enlargement with this type of investment must compensate

• Casting shrinkage

• Setting shrinkage and

• Green shrinkage

ManipulationThese investments have a special

particle size gradation and are handled in a different manner

The powder is added to hydrolyzed ethyl silicate liquid, mixed quickly and vibrated into a mold , that has an extra collar to increase the height .

The mold is placed on a vibrator that has a tamping action

This allows the heavier particles to settle while the excess liquid and some of the finer particles rise to the top .

The top of the mold is prone to cracking due to greater drying shrinkage from evaporation of the ethyl alcohol.

The cracks must be removed before the firing process

Otherwise ,when the mold is heated to burn out a pattern and achieve thermally induced expansion the cracks will grow and result in faulty casting.

To overcome this problem a sufficient header of the investment is provided to allow for the removal of the cracked portion by grinding.

In about 30 minutes the accelerator in the powder hardens the settled part, and the excess is poured off (to avoid crack formation).

The liquid powder ratio in settled part is greatly reduced and the setting shrinkage is reduced to 0.1%

Thus distortion is minimized and these investments are well suited for producing large ,precise castings.

The expansion of the investment is all due to thermal expansion.

Properties

1.Compressive strength -1.5Mpa

2.Thermal expansion( linear) –

About 1.5% to 1.8% can be attained between room temperature and 1000 ºc to 1177 ºc

Advantages• High temperature cobalt chromium and

nickel chromium alloys can be casted• Good surface finish is obtained• Low distortion• High thermal expansion• Thin sections with fine detail can be

reproduced (as they are less dense)• Divesture is easier as they have low fired

strength

Disadvantages• Extra precaution needed in handling

the low strength fired molds

• Low strength and high thermal expansion require a more precise burn out process (flammable alcohol is released) and firing schedule to avoid cracking and hence destruction of mold.

Newer investments for castingtitanium based alloys

• Newer investments have been aimed at the casting of titanium or titanium based alloys .

• Conventional phosphate bonded or ethyl silicate bonded investments are deficient for this purpose.

• Molten titanium is highly reactive with the oxygen and is capable of reducing some of the oxides commonly found in the investment.

• Titanium can also dissolve residual oxygen, nitrogen, and carbon from the investments.

• These elements can also harden and embrittle titanium in the solid state

• As a result a modification of the existing refractory formulations and binders or new refractory formulations and binder systems are required.

Composition • According to the source of binder they can be

classified as 1. Phosphate bonded2. silicate bonded3. Cemented Refractories that can be used are1. Silica2. Alumina3. Magnesia4. Zirconia

Objectives for a titanium investment should be

1. To reduce breakdown of the investment

2. To reduce contamination of the titanium.

To reduce breakdown of the investment

• Reducing the reaction with investment is to employ molds that have been expanded by burn out process and then cooled back to near ambient temperature prior to casting process

• This reduces the time that the alloy is in contact with mold at elevated temperatures and over all reactivity is reduced

• To avoid contamination of titanium by oxygen through the reduction of refractory oxides of the investment ,refractory materials that are less easily reduced by titanium should be used

• Some modifications of phosphate bonded investments have been explored for the purpose of rendering them more compatible with molten titanium alloys

• To achieve expansion with out the use reactive powders a phosphate investments that contains both magnesia and alumina as refractories was developed .

• This investments can attain large expansion by the spinel reaction of alumina and magnesia.

• An other approach to obtaining the needed expansion is through the use of spodumen (Li2O – AL2O3, - SiO2,) (Okuda et al, 1991). Spodumen expands irreversibly upon heating through the temperature range of 900°C to 1100°C.

• Reaction of ethyl silicate bonded investments with liquid titanium have been reported to be some what less than that of phosphate bonded investments this is most likely due to use of highly refractory oxides in the powder. Regardless these investments require a more complex procedure for their use

• A more recent development is an investment using magnesia bonded by an aluminous cement which contains a mass fraction of 5% zirconium powder.

• The aluminous cement serves as a binder for the magnesia as a refractory .

• It sets by mixing with water.

• Oxidation of the zirconium powder to zirconia during the burn out process provides irreversible expansion to compensate for shrinkage of the casting during cooling from the solidification temperature.

• Titanium casting from this investment was reported to have smooth surfaces free of contamination from mold reaction

Investing • After the wax pattern for an inlay or other small

restoration, such as a crown or bridge abutment, has been prepared and sprued, and a surface tension-reducing agent has been applied, it then should be invested promptly. The correct water/ powder ratio of the investment mix, a required number of spatulation turns, and a proper investing technique are essential to obtain acceptable casting results, There are two different methods of investing the wax pattern: hand investing and vacuum investing.

• the appropriate amount of distilled water (gypsum investments) or colloidal silica special liquid (phosphate investments) is dispensed. The liquid is added to a clean, dry mixing bowl, and the powder is gradually added to the liquid, using the care and caution to minimize air entrapment. Mixing is formed gently until all the powder has been wet; otherwise, the unmixed powder may inadvertently be pushed out of the bowl. Although hand-mixing is an option, it is far more common to mix all casting investments mechanically under vacuum.

• Vacuum Mixing

• Mechanical mixing under vacuum removes air bubbles created during mixing and evacuates any potentially harmful gases produced by the chemical reaction of the high-heat investments.

• Once mixing is completed, the pattern may be hand-invested or vacuum invested.

• For investing by hand, the entire pattern is painted (inside and out) with a thin layer of investment. The casting ring is positioned on the crucible former, and the remainder of the investment is vibrated slowly into the ring.

Hand-Investing Procedure

• With vacuum investing, the same equipment used to mix the investment is employed to invest the pattern under vacuum.

vacuum-Investing Procedure

• the amount of porosity in the investment is reduced by vacuum investing. As a result, the texture of the cast surface is somewhat smoother with better detail reproduction. The tensile strength of vacuum-mixed investment is also increased.

• Air bubbles that remain in the mix, even with vacuum mixing, can be entrapped on flat or concave surfaces that are not oriented suitably for air evacuation. Tilting the ring slightly aids in releasing these bubbles so that they can rise to the surface. Excessive vibration should be avoided, because it can cause solids in the investment to settle and may lead to free-water accumulation adjacent to the wax pattern, result ing in surface roughness. Excessive vibration may also dislodge small patterns from the sprue former, resulting in a miscast

• If the hygroscopic technique is employed, the filled casting ring is immediately placed in a 37°C water bath with the crucible former side-down. For the thermal expansion or high-heat technique, the invested ring is allowed to bench set undisturbed for the time recommended by the manufacturer.

SUMMARY

Of the various types of investments described the most commonly used investment used is phosphate bonded investment material. The increase in the use of the higher melting alloys resulted in the increase in the use of the phosphate bonded investments.

Gypsum bonded investments cannot withstand temperature higher than 700 ºc and it can be used only with conventional gold alloys .

The processing attention and extra care needed in burn out procedures limited the use of silicate bonded investments

Newer investments are developed for titanium based alloys . Titanium is highly reactive with the oxygen and is capable of reducing some of the oxides commonly found in the investment.

Modification of the existing refractory formulations and binders is done or new refractory formulations and binder systems are developed

REFERENCES• Dental meterials and ther selection-william j o’brien.• Phillip’s science of dental materials-anusavise• Applied dental materials-john f.mccabe• Restorative Dental materials – Craige.• Fundamentals of fixed prosthodontics-herbt

t.shilingburg• Contemporary fixed prosthodontics-rosensteil• J Prosthet Dent 1996;75:211-6. J Prosthet Dent 1999;82:519-21. J Prosthet Dent 2007;97:157-64. J Prosthet Dent 2006;95:42-9.Internet resourses