investment materials/ online orthodontic courses

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INDIAN DENTAL ACADEMY

Leader in continuing dental education www.indiandentalacademy.com

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INTRODUCTIONINTRODUCTION

Following the production of wax pattern, the next stage in many dental procedures involves the investment of the pattern to form a mould.

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A sprue former is attached to the pattern and assemblage is located in the casting ring.

Investment material is poured around the wax pattern whilst in a fluid stage.


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When the investment sets hard the wax and the sprue former is removed by softening and/or burning out to leave a mould which can be filled with an alloy or ceramic using a casting technique.


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(GPT-7)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)

Refractory investmentAn investment material that can withstand high

temperatures used in soldering or casting.www.indiandentalacademy.comwww.indiandentalacademy.com

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(CRAIG)An investment may be described as a

ceramic material that is suitable for forming a mold into which a metal or alloy is cast.


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Requirements of investments Requirements of investments for alloy casting proceduresfor 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 it paint the wax pattern easily, but the investment also should harden in a relatively short time.


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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 1000c)

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


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

7. Casting temperature must not be critical. Preferably thermal expansion versus temperature curve should have a plateau of thermal expansion over a range of casting temperatures .

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


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

10. 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.

11.The investment material should be inexpensive.


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Factors involved in selection Factors involved in selection of investment materialof investment material

1. Type of alloy to be cast2. Casting temperature to be

used.


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Types of investment materials Types of investment materials available for casting alloysavailable for casting alloys

• GYPSUM-BONDED INVSTMENTS• PHOSPHATE BONDED

INVESTMENTS• SILICA BONDED INVESTMENTS• NEWER INVESTMENT MATERIALS

FOR TITANIUM AND TITANIUM BASED ALLOYS


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COMPOSITIONCOMPOSITIONInvestment materials consist of a mixture of a

1. REFRACTORY MATERIAL2. BINDER3. MODIFIERS


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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.


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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 phase2. High temperature phase or Beta phase


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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


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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.


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• 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

•


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• It is exhibited by a rapid increase in the linear expansion as indicated in the figure.

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


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The isothermal expansion for the Cristobalite is 1.3% at 250cQuartz is 0.6% at 573cDepending on type of silica used

the investment materials are classified as

1. Quartz investments 2. Cristobalite investments


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FUNCTIONS1. 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. The wax pattern is eliminated from

the mold by heat. During heating the Investment expands which is necessary to compensate partially or totally for the casting shrinkage of the alloy .


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The expansion is accomplished by a combination of simple thermal expansion coupled with a crystalline inversion (isothermal expansion) which results in significant expansion.


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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

Binder


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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,.


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Gypsum bonded investmentsGypsum 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 .


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They are

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


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GYPSUM BONDED INVESTMENTS

INLAY INVESTMENTS

DENTURE INVESTMENT

THERMAL EXPANSION(ISO Type 1)

HYGROSCOPIC EXPANSION(ISO Type 2)

RAPID HEAT

SLOW HEAT


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CompositionComposition• REFRACTORY

MATERIAL – Silica –(60-65%)

Increasing the proportion of silica in the investment powder increases the Manipulation time , Initial setting time ,


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• setting expansion both in air and water and thermal expansion and reduces compressive strength


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The increased manipulation and setting time and reduced compressive strength occur because the particles of the refractory filler interfere with the interlocking of growing gypsum crystals making this less effective in developing a solid structure.


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• 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


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• 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


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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 400c and approximately 700c, and then a larger contraction occurs (due to densification by sintering)

This later shrinkage is most likely due to deposition and sulphur gases such as sulphur dioxide are emitted.


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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.


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MODIFIER - (4-7%) Used are

Reducing agents Modifying chemicals

Coloring matterReducing agents : they reduce any

metal oxides formed on the metal by providing a non oxidizing atmosphere in the mold when the mold alloy enters

• Ex– Copper


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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


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• 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 .


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


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

• A marked strength decreased on heating to 700 c ranging from -50 to -85 %


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• 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 .


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1.Particle size of the powder • Affects the smoothness of the mold cavity

surface • Affects the inherent porosity of the mold• Only the particle size of the refractory filler is of

practical importance as they remain unchanged in the said investment

• The gypsum crystals formed during setting of the binder are much smaller than silica particles

Properties Properties


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• Refractory with the fine particle size– smooth mold surface and smooth casting

• The venting of the mold cavity is normally provided by porosity inherent in set material (density)

• So the refractory powder used in the investment should have uniform fine particle, size no more than 75 m


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2.Manipulation time2.Manipulation time

• Investing the wax pattern must be completed while the mix is still fluid

• Loss of fluidity is indicated by disappearance of glossy surface from the mix.


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3. Setting time3. Setting time

• According to ADA sp no 2 the setting time for dental inlay casting investment should not be shorter than 5 min and not more than 25 min.

• The modern inlay investments set initially in 9 – 18 min .


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4.Expansion4.Expansion

• Inlay investments have total expansion in the range of 1.5 – 2.5% .

• Purpose of setting expansion is to aid in enlarging the mold to compensate partially for casting shrinkage.

• Setting expansion of three types 1. normal setting expansion2. hygroscopic expansion3. thermal expansion


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Normal setting expansionNormal setting expansion

• Mixture of silica and gypsum hemi hydrate results in greater setting expansion than that of gypsum products when it is used alone.

• The silica particles probably interfere with intermeshing and inter locking of the crystals as they form.


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• Thus the thrust of crystals is outward during the growth and they increase expansion .

• 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 .


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Hygroscopic setting expansionHygroscopic 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


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• 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%.


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Factors affecting hygroscopic Factors affecting hygroscopic setting expansionsetting expansion

1.Effect of composition:• Finer the particle size of silica greater the

hygroscopic expansion. • Alpha hemi hydrate produces greater

hygroscopic expansion than beta hemi hydrate in presence of silica.

• Higher the silica content greater the expansion and at least 15%of binder is necessary to prevent drying shrinkage


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2.Effect of water powder ratio The higher the water powder ratio of original investment water mixture the less the hygroscopic setting expansion

3.Effect of spatulation:• The shorter the mixing time the less is

the hygroscopic expansion 4.Effect of shelf life:• The older the investment the

lower is the hygroscopic expansion.


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5.Effect of time of immersion:• The immersion before the initial set causes

greater expansion .6.Effect of confinement:• The confining effect of the opposing forces such

as the walls of the container in which the investment is placed or the wall of the wax pattern is much more pronounced on the hygroscopic expansion than the normal setting expansion

• The effective hygroscopic setting expansion is likely to be less in proportion than in the normal setting expansion.


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7.Effect of amount of added water:• Magnitude of hygroscopic expansion is

in direct proportion to the amount of water added during the setting period until a maximum expansion occurs

• No further expansion is evident regardless of any amount of water added

• Once the setting starts the later water is added to the investment the less is the hygroscopic setting expansion because part of crystallization has already started in normal fashion.


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• Some of the crystals have intermesh and inhibit further crystal growth after the water is added

• On the same basis the less water that is added the lower is the expansion


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8.Effect of casting ring liner:• Asbestos ring liner is used for lining the

casting ring • This liner makes additional water available

to the setting investment and causes an increased setting expansion.

• Even when the mold sets in the air as in thermal expansion technique some hygroscopic setting expansion occurs.


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• The investments used in the thermal expansion technique have relatively high silica content, so increase in setting expansion produced by exposure to water is high .As this high setting expansion is uncontrollable an isotopic a dry water proof asbestos ring liner is used

• The combination of a low water powder ratio and a wet liner considerably increases the investment setting expansion


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The hygroscopic setting expansion is a continuation of ordinary setting expansion because the immersion 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


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• 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

• The effect is not permanent after the water is evaporated unless a binder is present


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• The term hygroscopic is an mis inomer• This hygroscopic setting is as normal a

phenomenon as that which occurs during normal setting expansion

• The water is drawn into setting material by capillary action and not by hygroscopy


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Thermal expansionThermal expansion

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

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

• 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


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

• If only a thermal expansion is used with normal setting expansion then it should be 1 – 1.6%.

• 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 .


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• The sulphur dioxide formed causes sulphide alloy formation and gold alloy casting resulting in discoloration and embrittlement of the alloy

• Quartz has a low thermal expansion than cristobalite hence additives are added to investments containing quartz .


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Factors affecting the thermal expansion :1.Effect of water powder ratio:• More the amount of the water used for mixing less

is the 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 . Silcas donot prevent gypsum shrinkage but counter balance it where as chlorides reduce gypsum shrinkage


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5.Strength5.Strength • According to ADA sp no 2 the

compressive strength should not be less than 2.5MPA

• Alpha hemihydrate increases the compressive strength

• Chemical modifiers increase the strength• More water during mixing less is the

strength


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• Heating the investments above to 700 c increase or decrease strength as much as 65% depending on composition

• Greatest reduction in strength is found upon heating in investments containing sodium chloride

• As the investment sets to room temperature strength decreases considerably because of fine cracks that formed during cooling


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6.Fineness6.FinenessFineness affects• Setting time • Surface roughness of the casting• Hygroscopic expansionFiner silica is preferrable


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7.Porosity7.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.


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• 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


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Storage 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.


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• 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


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DivestmentDivestment

• 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.


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Rapid heat investmentsRapid heat investments

• Investments based on a cristobalite refractory require slow heating while the alpha to beta inversion is occurring some rapid heat investments have been introduced which are placed immediately after setting into a furnace pre heated to 700 c .


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CompositionComposition

• Cristobalite a form of silica is used as refractory material.

Technique :• Place the mold 30 min after the pattern is

invested into the preheated furnace for a n additional 30 min, the casting is then made .


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Setting and thermal expansionSetting and thermal expansion

• Setting expansion measured under ordinary conditions is still occurring rapidly at 30 min it is not complete until 2 hours after mixing and measures 1%.

• The rapid rate of expansion at 30 min means the precise timing of placement of the mold in the furnace is critical if reproducible mold expansion is to occur


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• This drastic heating program could be expected to cause severe thermal cracking in an ordinary cristobalite investment.

• Measurements on a mold in a lined inlay ring showed that the periphery of the investment mass reached 250 c within 6 min of entering the hot furnace while the centre was at only 110 c until 10 min.

• Both periphery and centre reached a maximum of 690 c within 30 min heating period.


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• The expansion caused by the inversion of cristobalite shown on the graph beginning at 110 c and finishing at 170 c took place then enough of the outer parts of the specimen reached 250 c to produce a volume change.


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Advantages 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


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Gypsum bonded investments

cannot withstand temperatures greater than 700c

A large contraction occurs when gypsum bonded investments are heated above 700c.

The later shrinkage is due to decomposition by interaction of silica with calcium sulphate to liberate sulphur trioxide gas.

CaSO4+SiO2 -> CaSiO3+SO3


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Another reaction which may take place is on heating is that of between calcium sulphate and carbon (May be derived from the residue left after burning out wax pattern or may be present as graphite in the investment):

CaSO4+4C -> CaS+4CO further reaction can occur liberating

sulphurdioxide 3CaSO4+CaS -> 4CaO +4SO2This decomposition not only causes shrinkage but

also contaminates the casting with the sulphides of the non noble alloying elements .

So gypsum should not be heated above 700c


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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 700c.

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

silicate andphosphate compounds.


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As suggested by skinner (1963) “ the definitive advantage of this type of investment is

that there is less chance for the contamination of the gold alloy during casting…. So far as is known at present such contamination is avoided with phosphate bonded investments . On this basis , I am inclined to predict that the dental investment of the future may be phosphate bonded not gypsum bonded”. As predicted the phosphate bonded investments are widely used.

PhosphatePhosphate bonded bonded investmentsinvestments


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APPLICATIONS• 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


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ClassificationClassification

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


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CompositionCompositionRefractory 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 www.indiandentalacademy.comwww.indiandentalacademy.com

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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


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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.


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• 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 veneersContain quartz and cristobalite to achieve

expansion to compensate shrinkage


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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.


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• 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.


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Setting reactionsSetting reactions


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Setting and thermal expansionSetting and thermal expansion

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


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

• This contraction is eliminated when colloidal silica solution replaces water

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


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• 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.


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PropertiesProperties• High temperature mold is achieved by

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


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Properties Properties • Casting investments 1 Compressive strength • Type 1- 2.5 mpa • Type 2- 3 mpa• Setting expansion within 15% of manufacture

stated value • Full strength liquid .4%2 Thermal expansion within 15% of manufacture

stated value • 0.8% when 50:50 mixture of liquid and water3 Modulus of rupture- .1to .5 mpa


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• Refractory die stones1. Compressive strength 13 Mpa2.Setting expansion within 30%of

manufacture stated value 3.Thermal expansion within 15% of

manufacture stated value


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Working and setting timeWorking 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


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• 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.


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Advantages 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


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


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Disadvantages Disadvantages

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

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


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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


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ETHYL SILICATE BONDED ETHYL SILICATE BONDED INVESTMENTSINVESTMENTS

APPLICATIONSThey are used in construction of

high fusing base metal partial denture alloys


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COMPOSITIONRefractory material - SilicaBinder –Silica gel or ethyl silicateModifier – Magnesium oxide (strengthen the

gel) Ammonium chloride - accelerator


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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


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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


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SETTING REACTIONWhen binder silica gel is used • Silicic aid gel is formed when sodium

silicate is added to a acid or acid salt• MgO added strengthen the gelWhen ethyl silicate is used as a binder• Colloidal silicic is first formed by

hydrolyzing ethyl silicate in presence of Hcl, ethyl alcohol, and water.


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• 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


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• 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


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So the mold enlargement with this type of investment must compensate

• Casting shrinkage• Setting shrinkage and• Green shrinkage


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ManipulationManipulationThese 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 .


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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.


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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.(or)


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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.


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PropertiesProperties

1.Compressive strength -1.5Mpa2.Thermal expansion( linear) – Within 15% of manufacturers value.

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


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AdvantagesAdvantages• 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

strengthwww.indiandentalacademy.comwww.indiandentalacademy.com

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DisadvantagesDisadvantages• 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.


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Newer investments for castingNewer investments for castingtitanium based alloystitanium 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.


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• 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.


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Composition 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


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Properties Properties • Standards do not exist for investments for

the casting of titanium and its alloys so mechanical properties for the conventional phosphate and ethyl silicate based binders would apply here as well


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Objectives for a titanium investment Objectives for a titanium investment should be should be

1. To reduce breakdown of the investment

2. To reduce contamination of thetitanium.


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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


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• Lowering of the mold temperature requires that either non reversible expanders such as metals that expand by oxidation at elevated temperatures be used or that the temperature of the mold be kept just above the temperature where a reversal of expansion due to crystalline phase changes take place


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• 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


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• The GIBBS free energy of formation per mole of oxygen

• Titanium oxide at 1727 c is -716 kJ/mol of oxygen

• Titanium dioxide at 1727 c is -580 kJ/mol of oxygen

• Cristobalite and quartz are -550 and -549kJ/mol respectively

• From this it is clear that titanium may be expected to be oxidized by silicon dioxide which is reduced


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

• One investment consisting of phosphate binder ,magnesia and quartz was developed under the hypothesis that quartz would not be as reactive as silica


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• This investment was recommended for use of room temperature mold to reduce reaction with titanium but the contamination of castings by reaction with the investment was still encountered


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• To make the use of setting expansion of phosphate binder , alumina and magnesia, both of which are good heat resistance, can be used as refractories: however the thermal expansion is low .

• If either is of the powders are mixed with silica to raise expansion some contamination with silica again becomes as a problem


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• 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.


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• 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


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• 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.


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• 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.

• The zirconia formed is highly stable as it has an FEFof -728kJ /mol of oxygen and it should not contaminate titanium

• Titanium casting from this investments were reported to have smooth surfaces free of contamination from mold reaction


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SUMMARYSUMMARY

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.


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Gypsum bonded investments cannot withstand temperature higher than 700c 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.


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

The applications of the investments available are summarized below in the table:


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GYPSUM BONDED INVESTMENTS(700c)PHOSPHATE BONDED INVESTMENTS(900-1000C)

SILICA BONDED INVESTMENTS(1090 -1180c)NEWER INVESTMENTS FOR TITANIUM

Mold for gold casting alloys

Mold for base metal and gold casting alloys ; mold for cast ceramics and glassesRefractory die for ceramic build upMould for base metal casting alloys

Mould for titanium containing alloys


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