INVESTMENT MATERIALS AND INVESTING TECHNIQUESUnder guidance of Dr K Kumarswamy Dr Shalini Dr Praveen Dr Dheeraj Dr Srinivasullu Dr. Chandrashekar

Shubha R Joshi

OverviewIntroduction 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 procedures1.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 c) 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.

COMPOSITIONInvestment 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.

SiO2a b2

SiO

1.5% 1.2%

ab

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 c Cristobalite when heated inverts from alpha phase to beta phase at a temperature of 200 to 270 c 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 c Quartz is 0.6% at 573 c 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.

BinderIt 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 C (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 c 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 c 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 c (due to loss of water of crystallization) a slight expansion occurs between 400 c and approximately 700 c, 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 c. For the gypsum products containing carbon the maximum temperature should be 650 c.

MODIFIER - (4-7%)Used areReducing 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 c . They act by reducing the two large contractions of gypsum binder on heating to temperatures above 300 c . Ex Boric acid Soluble salts of alkali or alkaline earth metals

Boric acid: when heated above 150 c 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 c shows increases its compressive strength ranging from 40-50%.

Salts of alkali and alkaline earth metals : exsodium chloride Reduces first major shrinkage and eliminates second shrinkage of gypsum on heating The effect of halide ion is nullified above 650 c and rapid contraction occurs (probably the result of accelerated sintering ) A marked strength decreased on heating to 700 c.

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 c 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 expansion 2.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 c 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" investments have been introduced, which are placed immediately after setting into a furnace preheated to 700C. The recommended technique is to place the mold, 30 minutes after the pattern is invested, into the preheated 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 250C within 6 minutes of entering the hot furnace, while the center was at only 110C. The center of the mold did not reach 250C until 4 minutes later. Both periphery and center had reached a maximum of 690C within the 30minute heating period.

Advantages They save the laboratory time as the furnace is maintained at 700 c 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 c. 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 highmelting 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

ClassificationType 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. Dont use with palladium-containing alloys because carbon embrittles alloy)

CompositionRefractory materials (concentration ofapproximately 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 (